PROVEN TECHNOLOGY - F135 JSF ENGINE CHARACTERISTICS

CONVENTIONAL TAKE OFF AND LANDING SHORT TAKE OFF AND VERTICAL LANDING CTOL / CV ENGINE DESIGN STOVL PROPULSION SYSTEM DESIGN

Maximum Thrust (in pounds): 43,000 (191.3 kN) Maximum Thrust (in pounds): 43,000 (191.3 kN)

Intermediate Thrust (in pounds): 28,000 (128.1 kN) Short Takeoff Thrust: 38,100 (169.5 kN)

Length: 220 inches (5.59 meters) Hover Thrust 39,400 (175.3 kN) Main Engine: 15,700 Lift Fan: 20,000 Inlet Diameter: 46 inches (1.17 meters) Roll Post: 3,700

Maximum Diameter: 51 inches (1.30 meters) Length: 369 inches (9.37 meters)

Bypass Ratio: 0.57 Inlet Diameter 46 inches (1.17 meters) Main Engine: 50 inches (1.27 meters) Lift Fan: Overall Pressure Ratio: 28

Maximum Diameter: 51 inches (1.30 meters)

Bypass Ratio 0.56 Conventional: 0.51 Powered Lift:

Overall Pressure Ratio 28 Conventional: 29 Powered Lift:

Conventional Take Off and Landing Short Take Off and Vertical Landing CTOL/CV Engine Design STOVL Propulsion System Design

Maximum Thrust 43,000 lbs (191.3 kN) Maximum Thrust Class 41,000 lbs (182.4 kN)

Intermediate Thrust 28,000 lbs (128.1 kN) Intermediate Thrust Class 27,000 lbs (120.1 kN)

Length 220 in (5.59 m) Short Take Off Thrust Class 40,740 lbs (181.2 kN)

Inlet Diameter 43 in (1.09 m) Hover Thrust 40,650 lbs (180.8 kN)

Maximum Diameter 46 in (1.17 m) Main Engine 18,680 lbs (83.1 kN)

Bypass Ratio 0.57 Lift Fan 18,680 lbs (83.1 kN)

Overall Pressure Ratio 28 Roll Post 3,290 lbs (14.6 kN)

Length 369 in (9.37 m)

F135 Specs Charts Main Engine Inlet Diameter 43 in (1.09 m) Updated September 2012 Main Engine Maximum Diameter 46 in (1.17 m) Lift Fan Inlet Diameter 51 in (1.30 m)

Lift Fan Maximum Diameter 53 in (1.34 m)

Conventional Bypass Ratio 0.56

Powered Lift Bypass Ratio 0.51

Conventional Overall Pressure Ratio 28

Powered Lift Overall Pressure Ratio 29

“Then we used a variety of brain- a jet engine is already transferring around Joint Strike Fighter storming and creativity exercises to 75,000 horsepower.” come up with a new concept,” Bevilaqua A lift fan concept involves two STOVL- PERSPECTIVES continues. “ The technique that worked related problems at once. “The lift fan Code One Magazine July 1996 Vol. 11 No. 3 broke the problem down into its funda- system efficiently transfers thrust from Paul Bevilaqua Lift-Fan System Inventor mental elements. Since modern fight- the back of the airplane to the front,” Paul Bevilaqua could claim that he has ers have a thrust-to-weight ratio greater Bevilaqua explains. “At the same time, it been working on the Marine and Royal than one, the basic problem is to get half increases the total thrust of the engine Navy variant of the Joint Strike Fighter of the thrust from the back of the air- because it increases the bypass ratio from since 1985, when he began researching plane to the front. The simplest solution a relatively low one associated with fighter short takeoff and vertical landing tech- is to duct it there, but ducting makes the engines to a high one for vertical flight. In nologies on a NASA project at the Skunk airplane too wide to go supersonic. So other words, it makes the airplane more Works. His subsequent work led to a pat- we looked for other ways to extract ener- like a helicopter in the vertical mode. ent in 1990 for the lift-fan concept used in gy from the back, transfer it to the front, “The Harrier makes a similar ap- the Lockheed Martin STOVL variant. and produce lift. proach,” Bevilaqua continues. “It has a “The goal of those early studies was a “We generated a lot of wild ideas in- large fan to augment the thrust of a small supersonic STOVL aircraft,” Bevilaqua ex- volving energy beams and superconduc- engine core. But the airplane has to live plains, “but at that point, we were design- tivity,” Bevilaqua says. “but none worked with that fan in the cruise mode. Because ing airplanes, not inventing propulsion out until we looked at a variable-pitch tur- the fan is so large, the airplane can’t go systems. Several companies were con- bine to extract power from the jet ex- supersonic. ducting similar studies. Everyone was re- haust. From that point, everything just “Our lift fan approach is like taking working old concepts or looking at new started falling into place.” that one large fan on the Harrier’s engine, concepts that didn’t provide any real ad- From these ARPA studies, the Skunk breaking it into two smaller fans, and vantage. NASA was disappointed in the Works recommended two STOVL ap- turning off one of the smaller fans when lack of innovation.” proaches: a gas-driven fan and a shaft- the airplane converts to the cruise mode,” As these studies ended, the Ad- driven fan. ARPA liked both of them. “We he explains. “The concept doesn’t com- vanced Research Projects Agency asked thought the shaft-driven fan was the bet- promise the other JSF variants. Our the Skunk Works if it could come up with ter concept,” Bevilaqua says. “However, STOVL concept requires twin inlets, any new ideas. “We started from the be- the gas-driven fan was perceived as being what we call bifurcated inlet ducts, to ginning,” Bevilaqua recounts. “First, we less risky. Propulsion engineers are famil- create the space needed for the lift fan. looked at all the old ideas that hadn’t iar with ducting gases through an airplane. That is the only design requirement. worked and tried to understand why they But the idea of shafting 25,000 horsepow- And bifurcated ducts have low-observ- hadn’t worked. From that study, we made er was new. People were uncomfortable able and performance advantages that a list of requirements for an ideal super- with the magnitude of the number. But improve all of our JSF variants.” sonic STOVL propulsion system. there’s really little to fear. The shaft inside http://www.codeonemagazine.com/images/C1_V11N3_SM_1271449318_7528.pdf Genesis of the F-35 It would have to be variable- [4], so that it would be neces- pitch, so that it could be feath- sary to close the cruise nozzle Joint Strike Fighter ered during cruise. Another down to keep the engine from http://pdf.aiaa.org/getfile.cfm?urlX=- %3CWI’7D%2FQKS%2B%2FRP%23IW%40%20 driveshaft could be run from over speeding. On the other %20%0A&urlb=!*0%20%20%0A&urlc=!*0%20%20 %0A&urld=!*0%20%20%0A%3CWI’7D%2FQKS%2B%2FRP %23IW%40%20%20%0A&urlb=!*0%20%20 the added turbine stage hand, if the lift fan was con- %0A&urlc=!*0%20%20%0A&urld=!*0%20%20%0A through the engine to a lift fan: nected to the turbine at the Paul M. Bevilaqua | Nov-Dec 2009 Rolls-Royce was already build- same time that the bypass air “....It became apparent that the ing three spool engines. The was diverted to the wings, the best way to extract power from lift fan provides one lift post. lift fan would absorb the extra hot high-pressure exhaust gas Vectoring the cruise nozzle turbine power and keep the is with a turbine, the best way down would create another lift engine from speeding up. Then to get the power forward in an post. Shifting power between varying the nozzle area would aircraft is with a driveshaft (it the lift fan and cruise nozzle shift power back and forth for is light and does not increase would provide control in pitch. pitch control. the cross-sectional area of the Similarly, engine bypass air When the lift fan was disen- fuselage), and the best way to could be ducted off to nozzles gaged for cruise, the bypass produce vertical thrust is with in the wings and thrust could flow would be returned to the a fan (increasing mass flow is be shifted from one wing to cruise nozzle. This would match the best way to increase thrust the other to provide roll control. the nozzle area to the cruise per horsepower). But ducting off the bypass power requirement again. In Therefore, the best solution air would effectively increase fact, it would not be necessary to the problem of producing the nozzle exit area for the to add another turbine stage. thrust ahead of the center of core flow and lower the back The existing turbine would gravity would be to add anoth- pressure on the turbine sec- move off its design operating er turbine stage to extract tion. That would increase the point to provide shaft power for power from the exhaust gases. power produced by the turbine hover and back to its design operating point for cruise. The To summarize, the solution into additional shaft power, it existing driveshaft for the was to extract some of the is necessary to consider the engine fan could just be length- energy from the engine exhaust changes in the static pressure ened to power the lift fan. jet by changing the operating of the air as it flows through Because the lift fan is not point of the turbine, move the engine. The variation of connected to the engine during it forward with a shaft, and total energy (top) and static cruise flight, the engine oper- turn it into additional thrust pressure (middle) through an ates like a conventional mixed- by adding it to a larger mass engine are shown in Fig. 4. The flow turbofan engine during flow of air with a fan. The lift pressure rises through the cruise. For STOVL operations, fan is attached to a driveshaft compressor (2–3), remains the lift fan is connected to the extending from the front of the constant through the combus- cruise engine by engaging a cruise engine, as shown in Fig. tor (3–4), and then drops clutch on the driveshaft. The 3, and bypass air for the roll through the turbine section cruise engine nozzle is simul- jets is tapped off from behind (4–5) and nozzle (5–6), in two taneously opened, increasing the cruise engine fan. Thinking steps. As the pressure drops the pressure drop across the about how to extract power through the turbine section, engine’s turbine section. This from the back of the airplane the flow accelerates. The causes it to extract additional and transfer it to the front resulting thrust of the jets shaft power, which is used to resulted in a flash of insight that from the turbine nozzles spins drive the lift fan. The engine produced the dual-cycle-engine the turbine disk that powers then operates in hover like a concept as the solution for the the driveshaft....” [7.7Mb PDF] separate-flow turbofan with a STOVL Strike Fighter. http://pdf.aiaa.org/getfile.cfm?urlX=- higher bypass ratio. This dual- %3CWI’7D%2FQKS%2B%2FRP%23IW%40%20 %20%0A&urlb=!*0%20%20%0A&urlc=!*0%20 cycle operation is the novel Principle of Operation %20%0A&urld=!*0%20%20%0A%3CWI’7 D%2FQKS%2B%2FRP%23IW%40%20%20 feature of the engine in the To appreciate how this dual- %0A&urlb=!*0%20%20%0A&urlc=!*0%20%20 F-35 [5]. cycle engine turns jet thrust %0A&urld=!*0%20%20%0A

Liftfan is effectively an engine turned on its end,’ said Neil Mehta, The walk marked the end of the Harriers’ 41-year career and closed an programme director. ’This means the air has to turn through 90° illustrious chapter in British aviation history. The iconic aircraft has before entering into the aircraft, causing huge distortions in airflow.’ become one of the country’s greatest technical achievements, being the only military jet that could hover above the ground and fly in areas Counter-rotating blisks are A normal aircraft engine is usually other fighter aircraft were unable to reach. manufactured using hollow cleared to around a 30-knot cross wind. The F-35B, however, has to blades joined to the disc deal with winds up to 300mph through linear friction (483km/h). ’To put that into context, Hurricane Katrina Despite this capability, the Harrier jets fell victim to the cuts outlined in welding measured at about 75mph at its the government’s strategic defence and security review. The UK has no max, so we’re getting an awful lot Inner workings:schematic shows how the system will be positioned plans to replace the Harrier and many remain concerned for the loss of more distortion,’ he said. To address this, the team has used military capability. But while the UK may be losing the Harrier, it is not computational fluid dynamics to model airflow behaviour and For the blisks, Rolls-Royce is using hollow titanium blades. The losing the engineering expertise to develop the technology. aerodynamic performance. technology, derived from civil turbofans, cuts weight by around 40 per cent. Mehta said the current version of the LiftSystem is about 320kg At an aero engine test base in Bristol, Rolls-Royce engineers are ’Our programme for one cycle of the simulation uses as much lighter than the original demonstrator but is also stronger and more working on a new power system for the F-35 Lightning II Joint Strike processing power as a PlayStation running for 14 years,’ said Mehta. reliable. Fighter (JSF), the latest generation of combat aircraft. Dubbed the ’We’ve done more than 1,000 of those simulations.’ LiftSystem, the technology will enable short take-off and vertical He added: ’This time last year, we did our first hover and first vertical landing operations for the F-35B variant of the JSF programme, which landing. Those two events were the start of proving that all our work is planned to enter service with the US Marine Corps in 2012. the preceding year was accurate. We did vertical landings and nothing untoward happened. We’ve since done 58 vertical landings, 79 hovers, Rolls-Royce's LiftSystem for 90 slow landings and 95 short take-offs, and nothing has gone wrong.’ “Our programme for one Orders for the LiftSystem are the Joint Strike Fighter cycle uses as much power expected to total more than 600, and the US Marine Corps and the 28 March 2011 | By Ellie Zolfagharifard as a PlayStation running for Italian Navy have already acquired the system. There are still several         R  14 years” years of flight tests remaining, and        NEIL MEHTA, ROLLS-ROYCE shipborne trials are due to take place later this year. Production of the system for use in training aircraft is under way and Rolls-Royce is Landing:a runway is not needed delivering one LiftSystem per month.

Designed around the airflows are two counter-rotating bladed discs, or Mehta said the UK is in a strong position to continue developing the ’blisks’, manufactured using individual hollow blades joined to the disc technology. ’We developed the Pegasus engine that goes in the Harrier, through linear friction welding. The blisks take air from the top of the and we’ve now developed the LiftSystem,’ he said. ’There are no other fuselage and blast it through a vane box at the bottom of the craft. Western countries with that kind of expertise.’ Each vane box can be directed independently, so the air can be generated fore and aft. !" 

# At the rear of the aircraft, thrust is produced from a rotating nozzle known as the three-bearing swivel module (3BSM). In normal flight, 20,000lb cold thrust Two-stage counter-rotating fan the nozzle points rearwards, propelling the aircraft forwards at speeds of up to Mach 1.6. When vertical thrust is required, the nozzle swivels Uses hollow blisk technology downwards in less than two seconds, creating a vertical thrust of up to   $% & 20,000lb, equalling the force generated by the Liftfan. Directs 20,000lb of thrust from the main engine Stabilising this force are two small ’roll posts’ in the wings. These ducts Rotates 95° in less than two seconds each direct 2,000lb of thrust during short take-off and vertical landing. Compact flap mechanics regulate the amount of thrust produced by '   each of the roll posts. During short take-off, the roll posts are opened Directs 2,000lb bypass thrust from the main engine Rotating fan: situated behind the cockpit and the clutch is engaged, the 3BSM is swivelled downwards and power from the main engine is increased to produce enough thrust Hydraulically actuated nozzles during short take-off operations The LiftSystem will allow a fighter aircraft to achieve both vertical lift from the Liftfan. Looking up: the F-35B will achieve vertical lift and supersonic capabilities. Like the Harrier, the F-35B will be able to Provides aircraft roll control and lateral stability land without a runway and take off like a helicopter, while performing ’One of the big challenges for the LiftSystem is that we only use it for    When, in December, Britain’s Harrier jets landed at RAF Cottesmore for as a fighter aircraft. At the heart of the system is a component known short take-off and landing,’ said Mehta. ’In a normal mission, that is the final time, the sombre mood weighed on all those present. In a as the Liftfan. This a 50in (127cm), two-stage, counter-rotating fan only around five per cent of the total activity. So, at that point, we Lockheed Martin F-35B Lightning ll ritual known as the ’walk of honour’, the pilots disembarked from their capable of generating 20,000lb (9,000kg) of thrust. Situated just have to make sure the system is low weight, otherwise it’s not earning aircraft and walked away without taking a single look back. behind the cockpit, it produces the aircraft’s forward vertical lift. ’The its keep in there.’

The Shaft Driven Lift Fan Propulsion System ratio for the STOVL and subsonic mission segments and a low bypass ratio for the supersonic mission segments. for the Joint Strike Fighter Segment Short Subsonic Supersonic Vertical The purpose of this paper is to describe an Take Off Cruise Combat Acceleration Landing innovative, dual cycle propulsion system that increases Paul M. Bevilaqua the bypass ratio of the cruise engine for short take off and Wing Jet High Low High Low Low High Low Reaction High Low Need T ASTOVL Program Manager Lift Lift T/W SFC T/W IR Drag sp SFC Control T/W HGI vertical landing, without oversizing the engine or creating Lockheed Martin Skunk Works an unacceptable footprint. As shown in Figure 3, the

Feature Jet Vector High Mixed Hot Low Thrust High Cold effective bypass ratio is increased by installing a lift fan Palmdale, California Flap Thrust BPR Flow Parts BPR Balance Transfer BPR Jets Aft behind the cockpit. For STOVL operations, the lift fan is connected to the cruise engine by engaging a clutch on a Variable Bypass Ratio drive shaft extending from the front of the engine. The Abstract large wing, while the acceleration time drives the design engine operating point is simultaneously changed to Analysis and testing are used to show the feasibility toward a small wing. There is an intermediate wing size convert some of the jet thrust to shaft horsepower. For that simultaneously satisfies all of the conventional of an innovative shaft driven lift fan propulsion system cruise flight, the lift fan is disconnected and the engine for supersonic STOVL aircraft. Dual cycle operation of performance goals, as shown in the Figure. Although this operating point is changed to produce jet thrust, rather the cruise engine makes it possible to convert some of the design point requires somewhat more thrust than a design Figure 2: Analysis of Propulsion System Features than shaft horsepower. The engine then operates as a jet thrust to shaft horsepower for driving the lift fan. that satisfies any single goal, both the short take off and and devise a vertical lift system that would meet them. conventional mixed flow turbofan. Operation of the propulsion system is described and it is vertical landing goals require even greater thrust. The requirement to perform STOVL strike and close air The result is a dual cycle propulsion system with a analytically shown that the designs of the engine, drive Because the STOVL performance goals require more support missions was derived from top level military much higher bypass ratio in the vertical flight mode than shaft, and clutch are within the state of the art. A thrust than the combat goals, some form of thrust campaign strategies [1]. Each of the required missions in the conventional flight mode. This system provides demonstrator engine and lift fan were assembled from augmentation is necessary. Increasing vertical thrust by was then subdivided into its mission segment tasks. Each high levels of thrust augmentation with a cool, low available components and operated for almost 200 hours afterburning is not a satisfactory solution, due to the high segment task was further decomposed into needed pressure footprint, excess control power, and minimal in a full size airframe model. Testing proved the temperatures and velocities of the lift jets generated by performance capabilities, in order to reach a level where effect on the design of the airframe. Since the cruise feasibility of changing the engine cycle to drive the lift this approach. Increasing the size of the cruise engine to specific design features could be identified to meet the engine is optimized for conventional flight, the fan, and of rapidly transferring thrust back and forth provide sufficient vertical thrust is not an optimal mission objectives. performance of the propulsion system is not penalized for between the engine and lift fan to provide pitch control. solution either, because the engine would then be larger its STOVL capability. The durability of the mechanical drive system and flight than necessary for conventional flight. This would A simplified version of this analysis for the weight gearbox were also demonstrated. impose a weight penalty, because the inlets, nozzle, and propulsion system is shown in Figure 2. For example, In this paper, the results of analysis and testing will airframe would also be larger than necessary. Also, when the subsonic cruise mission segment requires low specific be used to show the feasibility of this shaft driven lift fan this oversized engine is throttled back for cruise, fuel fuel consumption, which requires a high bypass ratio propulsion system. In the next section, the dual cycle Introduction consumption would be increased. engine. The actual fan diameter and pressure ratio would operation of the cruise engine will be described, and be determined at the next level of decomposition. At the analysis will be used to show that the design of the The next generation strike fighter will need to The method of functional analysis was used to level shown, the Figure highlights the essential design engine, drive shaft, and clutch are within the state of the combine the short takeoff and vertical landing systematically analyze the conflicting performance goals problem: the STOVL, cruise, and combat segments art. The results of propulsion system tests performed capabilities of the AV-8B Harrier with the supersonic require high bypass ratios, while the supersonic segment with a lift fan and demonstrator engine assembled from performance of the F-16C Falcon, while providing requires a low bypass ratio. The ideal solution would be available components will be presented in the section greater range and increased survivability. This 1.3 a variable cycle propulsion system, with a high bypass after that. The Allison Advanced Development Company combination of vertical and supersonic performance Direct Lift Design Point demonstrated the performance of the lift fan, gearbox, requirements means that engine design, always an 1.2 STO and shaft for the high power levels of a production important part of any new airplane program, is a VL propulsion system. The Pratt & Whitney engine 1.1 Thrust to CTOL particularly significant factor in the development of a Design company assembled a dual cycle F100-PW-229-plus Weight Point new strike fighter. The engine must provide enough Ratio 1.0 engine from components of existing engines. It was (TSL / WTO ) vertical thrust for short takeoffs and vertical landings, but Transonic Acceleration operated in the conventional mode and then used to drive must not be so large that it increases supersonic drag or .9 the Allison lift fan to demonstrate operation in the fuel consumption during cruise. STOVL mode. Sustained Turn Instantaneous Turn Figure 1 shows the maneuver and acceleration .8 constraints on the thrust to weight ratio, T/W, and wing Dual Cycle Thermodynamics 0 loading, W/S, of a representative strike fighter. The 0 2030405060708090 The energy to drive the lift fan is extracted from the curves are obtained by setting thrust equal to drag for Wing Loading (WTO / S) air that flows through the cruise engine. The change in each of the performance goals. To minimize engine size, the energy of the air as it passes through an engine the maneuver goals drive the airframe design toward a Figure 1: Performance Constraints on T/W and W/S operating in a conventional turbojet cycle is shown in Figure 4. Energy is added to the air as it passes through ______Presented at the American Helicopter Society 53rd Annual Forum, Virginia Beach, Virginia, April 29 - May 1, 1997. the fan and compressor, as seen in the Figure. The Copyright 1997 by the American Helicopter Society, Inc. All rights reserved. magnitude of the added energy is representative of the Figure 3: Shaft Driven Lift Fan Propulsion System 12/ station 5 to drop to atmospheric pressure. The larger Mv ª M º = pressure drop across the turbine produces more shaft « » 80,000 mV m horsepower, while reducing the thrust of the core flow. ¬ ¼ Stall Limit Turbine Power 70,000 100, 000 HP Engaging the clutch at the same time as the nozzle area is Fuel Burn 200, 000 HP STOVL Nozzle Power so that the jet thrust increases with the square root of the increased transfers the additional power to the lift fan, so Energy 80, 000 HP 60,000 (HP-hr) mass flow ratio. Efficiency that the speed of the engine does not increase. This is 50,000 25000 .93 Waste Heat Shaft SHP .92 shown by the two points in Figure 6. The lower point is Compressor Work 120, 000 HP To actually transfer the energy to a larger mass of 100, 000 HP Horsepower .91 .90 the conventional operating point and the upper point is air, another turbine stage is added to the engine. The 40,000 .89 .88 2345 6 .87 used when the lift fan is engaged. Nearly 25,000 CTOL .86 energy extracted by this power turbine is used to drive an 30,000 .85 horsepower can be extracted before the turbine section additional fan, as shown in Figure 5. The power turbine Ideal 20,000 Operating reaches its stall limit. The thrust that can be produced and fan are mechanically independent of the rotating Line with this much power depends on the diameter and components of the basic gas generator. Because energy 10,000 40 60 80 100 120 140 160 180 200 pressure ratio of the lift fan. For example, with a fan is extracted from the hot primary flow, the thrust of the Normalized RPM (N / Rt) diameter of 40 in to 50 in, the thrust is on the order of engine core exhaust flow is reduced. However, there is a 15,000 to 20,000 pounds. net thrust increase for the complete turbofan system. Figure 4: Energy Changes through a Turbojet Engine Figure 6: Turbine Performance Map Because the net pressure drop from the turbine entry The shaft driven lift fan propulsion system increases to the nozzle exit does not change, the static temperature newest generation of engines. This energy appears as an thrust in a similar way; that is, energy is extracted from and the engine speed stabilizes. Because its speed is of the exhaust jet is also unchanged. Therefore, increase in both the pressure and temperature of the air. the hot turbine exhaust flow and transferred to a larger higher, the engine produces more thrust. The locus of extracting power from the gas stream does not change Following compression, additional energy is added to the mass of air by the lift fan. However, the power to drive steady state matching conditions defines the engine the waste heat, which is the excess static temperature of air by burning fuel in the combustor at constant pressure. the lift fan is not obtained with a separate power turbine, operating line, which is the diagonal running from the the exhaust jet. However, the total temperature of the jet but by changing the operating point of the turbine that bottom left to the top right in Figure 6. The engine and Energy is then extracted from the hot, high pressure is reduced. The equation for the change in turbine power drives the engine fan. This can be understood by compressor are designed so that the turbine power and gas by the turbine section. Since the turbine drives the is compressor, the amount of energy extracted by the examining the performance map of a typical turbine compressor power match near the point of maximum section, as shown in Figure 6. At any point on the map, efficiency at every speed. turbine is the same as the amount added by the ''SHP mc p T0 the power produced by the turbine is given by, compressor. This energy loss is seen as a drop in the If the clutch connecting the engine to the lift fan is so that extracting 25,000 shaft horsepower reduces the pressure and temperature of the gas. The energy J 1 engaged at the same time as the fuel flow is increased, the ª º total temperature of the jet approximately 250 degrees remaining in the hot gas that leaves the turbine section is Turbine Power = mc T«1 () P P J » additional power can be used to accelerate the lift fan, converted to thrust by expanding it through a nozzle. The p 04 5 4 Fahrenheit in this engine. ¬« ¼» instead of the engine. By selecting the fuel flow to match temperature and pressure of the gas drop as it expands the power produced by the turbine to the power required The energy equation for the exhaust jet can be through the nozzle. Although the static pressure of the in which T04 is the stagnation temperature of the gas to drive the lift fan, the engine speed can be held written in the form gas returns to atmospheric pressure, the temperature of constant. The process is similar to depressing the gas entering the turbine section, and P5 /P4 is the pressure 2 the exhaust jet remains higher than the atmospheric drop across the turbine section. pedal in an automobile with a manual transmission. With TTvc0  / 2 p temperature, so that this energy is lost as waste heat. the clutch disengaged, stepping on the gas causes the Increasing the fuel flow produces more turbine Since the exhaust velocity is determined by the jet thrust The thrust of the engine can be increased if the engine to accelerate. Engaging the clutch at the same power by increasing T04. The additional power and mass flow rate, vFm , the energy equation for the energy in the exhaust jet is transferred to a larger mass of time as you depress the gas pedal transfers the power to accelerates the engine until the power absorbed by the the drive wheels, so that the engine does not accelerate. air, rather than simply being expanded through the compressor matches the power produced by the turbine, nozzle. The following simple analysis illustrates this However, in a STOVL aircraft the requirement is to phenomenon. If the thrust of the exhaust jet is mV , then increase the maximum thrust of the engine. But at 2 its kinetic energy flux is m V / 2 . Transferring this maximum thrust, the turbine inlet temperature, T04, is quantity of energy to a larger mass of air reduces the already at the material limit of the turbine section. As a velocity of the exhaust jet. This can be written, result, the gas temperature can not be increased to provide the energy to drive the lift fan. In the dual cycle 1 2 1 2 engine, the additional power is obtained by increasing the 2 Mv = 2 mV pressure drop across the turbine section. As seen in so that the jet velocities are inversely proportional to the Figure 7, the pressure rises through the compressor, square root of the jet mass flows, remains constant through the combustor, then drops

12/ through the turbine section and nozzle, in two steps. This v ª m º Turbojet Engine Turbofan Engine is shown by the solid line in the Figure. = « » V ¬ M ¼ Increasing the nozzle exit area reduces the pressure drop across the nozzle, causing a corresponding increase The ratio of the thrust of the two jets is obtained by in the pressure drop across the turbine. For example, substituting this equation for the velocity ratio in the increasing the nozzle area so that A = A , as shown by expression for the thrust ratio, Figure 5: A Power Turbine Drives the Engine Fan 6 5 the dashed lines in Figure 7, causes the static pressure at Figure 7: Nozzle Area Controls Turbine Power exhaust jet can be written in terms of the nozzle thrust as, 13 ª º 2 « 16 u SHP » Fm d TT  « 4 » 3000 0 2c «SZV 1  f » p ¬ ¼ Therefore, the reduction in total temperature appears as a Figure 10 shows how the diameter of an aluminum 2000 reduction in thrust at the nozzle exit. However, it shaft transmitting 25,000 horsepower varies with the Horsepower reappears as a reduction in stagnation temperature when speed of the engine. The lower limit is a solid core shaft 1000 the jet impinges on the ground during vertical landings, and the upper limit is a thin wall shaft with a wall and this reduces heating of the surface material. thickness equal to 5% of the shaft diameter. The solid 5000 RPM 3500 RPM shaft is smaller, but the thin wall shaft is lighter. The 0 The net effect of changing the turbine operating 010203040 point is to transfer thrust from the engine exhaust jet at design of a shaft is more complicated than this, because it Time (seconds) the back of the aircraft to the lift jet at the front of the depends on the number of support bearings and the aircraft. The relative magnitude of the energy transferred natural frequencies of the shaft, but an aluminum shaft several inches in diameter can transmit the power is illustrated in Figure 8. It is a fraction of the total Figure 11: Clutch Size Decreases with Engagement Figure 9: Hybrid Fan Engine required to drive the lift fan. energy available in the engine, and less than the power Time being extracted to drive the engine fan and compressor. meant that these engines produced less thrust in the The clutch that connects the drive shaft to the lift fan Although the low pressure spool of the engine does have vertical mode than in the cruise mode. The tandem fan has two functions. The first is to reduce the shock of The horsepower absorbed by the clutch during the to be redesigned to handle the additional power, the engines can be classified between the single cycle engagement by slipping during the period of engagement. engagement period decreases as the engagement period energy levels are not extraordinary. The technology Pegasus engine and the dual cycle shaft driven lift fan The second is to efficiently transmit the torque of the increases, required to convert an existing military engine to drive a propulsion system. driveshaft to the lift fan, when the clutch is engaged. The lift fan is comparable to converting the military engine to simplest clutches use the friction between two surfaces to IZ 2 2/ a high bypass ratio commercial engine. It is well within accomplish both functions. However, the number of HP the state of the art. t Mechanical Drive Components disks and the size of the springs required to press them The shaft driven lift fan propulsion system is a together in order to transmit 25,000 HP would result in a 2 Power is the product of force times velocity. in which the IZ 2/ is the rotational kinetic energy of development of the Tandem Fan propulsion system [2] heavy clutch. Therefore, the horsepower transmitted by a drive shaft is the lift fan after it is connected to the engine, and t is the and the Hybrid Fan engine [3]. The Hybrid Fan engine is equal to the product of torque and angular velocity, time of engagement. As seen in Figure 11, the knee of shown in Figure 9. Both of these engines combined a To reduce the size of the clutch, friction is only used the curve is near 10 seconds at low engine speeds. This low bypass ratio cruise cycle with a high bypass ratio SHP W uZ to accelerate the lift fan from rest to the relatively low two step engagement strategy permits the design of a STOVL cycle. In the STOVL cycle, the flow from the idle speed of the engine before take off, or during relatively lightweight clutch. The shaft must be designed to resist the torque. For a engine fan was diverted from the engine core to nozzles approach to the landing area. Less than full power is given horsepower, the torque decreases as the rotational at the front of the aircraft. Therefore, the engine fan transmitted under these conditions, so that a small multi- velocity of the shaft increases, Propulsion System Demonstration became the lift fan. However, the operating point of the disc friction clutch can be used to perform this Although analysis had shown that the designs of a turbine was not changed to produce additional power to SHP engagement. Once the speed of the lift fan matches the drive the lift fan. In addition, the loss of the W engine speed, a mechanical lockup is engaged. This is dual cycle engine, drive shaft, clutch and lift fan are supercharging effect of the engine fan on the core flow Z used to transmit full power when the engine speed is within the state of the art, there were practical concerns increased for short takeoffs and vertical landings. regarding the development of such a propulsion system. The high rotational speeds typical of jet engines, around For example, there were concerns about the weight and 10,000 rpm, make it possible to transmit large amounts of 250 efficiency of the gearbox that drives the lift fan, and power with relatively small driveshafts. In terms of the questions regarding the ability of the engine control Turbine Power maximum unit shear stress of the drive shaft material, V , Fuel Burn 100,000 HP system to synchronize the change in nozzle area with the 200,000 HP and the polar moment of inertia of the shaft, Ip, the 200 Energy operation of the lift fan, and the ability to rapidly transfer Nozzle Power (HP-hr) Lift Fan Power torsion formula for round shafts gives for the diameter of 55, 000 HP thrust from the engine to the lift fan for pitch control. To 25, 000 HP the shaft, Waste Heat 150 demonstrate the feasibility of the shaft driven lift fan Compressor Work 120, 000 HP 100, 000 HP Shaft Diameter propulsion system, the Allison, Pratt & Whitney, and 2VI (Inches) p Rolls Royce engine companies built and tested a 2345 6 d 100 W .05 Thin Wall demonstrator engine. To minimize the costs of this demonstration and show the relatively low risk associated For hollow round shafts, the polar moment of inertia is Solid Core 50 with developing a dual cycle engine, the demonstrator 44 engine and lift fan were assembled from existing engine Sdf 1  components. I 0 p 0 123 4 5 6 7 8 9 10 32 RPM (x1000) in which f is fraction of the shaft diameter that is hollow. Figure 10: Driveshaft Diameter Decreases with RPM Figure 8: Energy from the Jet Drives the Lift Fan The formula for the diameter of the shaft becomes, The successful completion of these tests demonstrated the 0.6 Summary feasibility of building a lightweight lift fan and gearbox 04 8121620 Time (Sec) The shaft driven lift fan provides a solution to many at the design power levels. of the problems associated with the development of a The Pratt & Whitney engine company combined the supersonic STOVL strike fighter. It provides high levels fan and core of a F100-PW-220 engine with the low Figure 14: Response to Pitch Control Commands of thrust augmentation, with a relatively cool, low pressure turbine from the more powerful F100-PW-229 pressure footprint. The aircraft is balanced in hover engine to create the dual cycle PW-229 plus engine. The because thrust is transferred from the rear of the aircraft fan drum rotor was modified for attaching the shaft to to the front, without increasing frontal area. Pitch and drive the lift fan, and the fan duct was modified so that roll control power are also obtained by transferring thrust the bypass air could be diverted to the ducts that supply around the aircraft without changing total lift. Since the the roll control jets. The digital electronic engine control cruise engine is optimized for conventional flight, the software was modified to control fuel flow and nozzle performance of the engine is not penalized for STOVL area on the STOVL operating line of the turbine map. capability. Removing the lift fan creates a conventional Rolls Royce, Ltd. built the variable area thrust vectoring strike fighter with little penalty for commonality. lift/cruise nozzle and the offtake ducts and nozzles for the The feasibility and mechanical integrity of the shaft Figure 12: Two Stage Lift Fan with Gearbox roll control jets. and reduces the engine thrust. Opening the inlet guide driven lift fan propulsion system has been successfully The engine was first run without the lift fan demonstrated. Analysis has shown that up to 25,000 The production lift fan system is shown in Figure 12. It vanes produces more lift fan thrust. If the movement is connected to demonstrate operation in the cruise mode. shaft horsepower can be extracted from the exhaust jet of consists of a two stage counter rotating fan section, with coordinated, thrust is transferred from the aft nozzle to Then the lift fan was connected to demonstrate operation a modern turbofan engine to drive a lift fan. Due to the variable inlet guide vanes to modulate the thrust of each the lift fan, while the total thrust remains constant. This in the STOVL mode. The primary objectives of these high rotational speed of the engine, this power can be stage at constant rotational speed. This arrangement of provides a large pitching moment which can be used to tests were to prove the feasibility of changing the transmitted with a drive shaft less than 10 inches in the fans permits the use of two driven gears, which control the aircraft in hover. Since thrust transfer is operating point of the turbine section to provide power diameter. A relatively lightweight clutch can be reduces the load on each gear tooth in half. This keeps accomplished without changes in engine speed, high for driving the lift fan, and the ability to rapidly transfer employed by engaging the lift fan at low engine speeds, the power at a level similar to that currently being used response rates are achieved. In addition, the pitch control thrust from the cruise engine to the lift fan and back, in and using mechanical lockup to transmit the full engine on heavy lift helicopters. Spiral bevel gears will be used loop is decoupled from the total thrust control loop which order to provide pitch control power. The dual cycle power at high speeds. to accommodate the speed and torque requirements of the is used to command changes in sink rate. operation of the engine was successfully demonstrated by system. The nozzle consists of two telescoping hood Practical concerns regarding the ability of the engine connecting the lift fan to the engine, and then increasing Figure 14 shows the response of the propulsion segments to deflect the lift fan thrust aft during takeoff. control system to synchronize the operation of the lift fan engine speed to full power along the STOVL operating system to a command to rapidly cycle the thrust between and cruise engine were addressed by testing a The demonstrator lift fan shown in Figure 13 line [4]. maximum nose up and maximum nose down moments. demonstrator propulsion system. The Allison Advanced represents one stage of the production system. However, Six complete cycles were performed in 16 seconds. The Pitch control is obtained by coordinating the area Development Company demonstrated the performance of the single fan of the demonstrator system operates at the response rate is excellent. The larger variation in the change on the cruise engine nozzle with the movement of the fan, gearbox, and shaft under the high power gear same power level as one stage of the production system. peak to peak thrust split is due to differences in the the lift fan inlet guide vanes. At constant engine speed, loading of the production system. The Pratt & Whitney The first stage fan and inlet guide vanes from the Pratt & nozzle discharge coefficient between the small scale increasing the nozzle area produces more turbine power engine company assembled a dual cycle F110-PW-229 Whitney F119 engine were used for the lift fan. The nozzles which were used to design the nozzle schedule plus engine from existing components, and Rolls Royce Allison Advanced Development Company had gears and the full size nozzles. This illustrates the differences provided a variable area thrust vectoring nozzle. Testing manufactured using two different processes. The first set that can be expected when small scale data is used to of the assembled components proved the feasibility of had teeth generated and hard finished on the same design full size hardware. The control schedules could changing the engine cycle to produce jet thrust or shaft machine to provide high quality gears with a parallel be corrected to eliminate the overshoot. However, this horsepower, and of quickly transferring enough thrust to depth tooth form. The second set were finish ground on a would not change the dynamic response, and therefore control the aircraft in hover. A total of 200 hours of Weiner grinder so that a more sophisticated tooth contact was not felt to be necessary. trouble free operation were achieved during testing of the pattern could be produced. Both designs were This demonstration of the shaft driven lift fan propulsion system successfully manufactured. However, the hard finished propulsion system was highly successful. More than 40 gears were used in the demonstrator system, because they hours of static testing were accomplished with no were the first set completed. Since the ground tooth gear problems in the operation of the engine, mechanical drive set were also completed successfully, they are preferred system, or lift fan. The test proved the feasibility of for the operational aircraft. changing the cycle of the cruise engine to provide power Allison demonstrated the performance of the to drive the lift fan, and demonstrated the capability to complete lift fan, gearbox, and drive shaft at the high rapidly transfer thrust back and forth from the cruise power gear loading of the operational aircraft [4]. The engine to the lift fan to provide pitch control. As a result power transfer efficiency of the gear set was measured. of these tests, the propulsion system was installed in a Vertical operation of the lubrication system and the oil full size airframe model and operated for another 160 cooling system were demonstrated, and the ability of the hours to study jet effects in hover and transition [5]. inlet guide vanes to modulate the fan thrust was shown. Figure 13: Single Stage Demonstrator Lift Fan The distortion limits of the lift fan were also measured.

and other engine manufacturers developed compact turbojet engines specifically for such applications. Various combinations  History Of The F-35B Swivel Duct of numbers and locations of engines were built and flown on several VTOL prototypes and experimental aircraft. Three-bearing swivel nozzle designs were studied by virtually all of the engine companies in the mid 1960s. The US Patent By Posted 12 June 2014 Office received applications for many variations of the 3BSD from Pratt & Whitney, General Electric, and even from Boeing Kevin Renshaw Military Aircraft of Wichita, Kansas.

By the late 1960s, Pratt & Whitney was designing and testing a three-bearing swivel nozzle for use on the Convair Model 200 Sea Control fighter. Design drawings dated 1967 show detail design layouts. The first nozzle was built and tested on a Pratt & Whitney JT8D in the mid 1960s. The tests included operating the nozzle in full afterburner with the nozzle deflected ninety degrees. The test rig was positioned to exhaust upward to avoid heating the ground under the test stand, though subsequent tests positioned the nozzle downward at the ground to assess the effects of ground proximity back pressure on nozzle performance.

The Convair Model 200 was proposed in June 1972 to respond to the US Navy request for designs for a fighter/attack aircraft $FULWLFDOFRPSRQHQWRIWKH)%VKRUW takeoff/ for the Sea Control Ships. The VTOL aircraft would have used a PW401 engine with an afterburning 3BSD plus twin Allison vertical landing, STOVL, variant of the Lightning II XJ99 lift engines located behind the cockpit for added vertical lift forward of the center of gravity to balance the aft nozzle is the three-bearing swivel duct nozzle, or 3BSD. thrust. To deal with ground environment generated by the combination of the afterburning rear nozzle and the high The nozzle, mounted at the back of the aircraft, temperatures and pressures of the lift engines, the ships would be equipped with special vertical landing areas with metal allows the thrust from the Pratt & Whitney F135 grates to allow the hot air flow to pass through. engine to be vectored from straight aft for conventional flight to straight down for STOVL This same request for designs led to the development of the Rockwell XFV-12 ejector augmented lift design. The Rockwell operations. The 3BSD can move through ninety- design was selected for prototyping but proved unable to produce enough thrust for vertical flight. Some have reasoned that five degrees of motion seamlessly with no change the Navy selected the ejector design knowing that it would fail, thus eliminating a potential threat the smaller Sea Control in engine operation. The nozzle also provides yaw Ships posed to the large Nimitz class carriers with conventional catapult and trap equipment. Whatever the case, the three- control during hover and during transitions to bearing swivel nozzle design was relegated to the file cabinets at Convair in San Diego, California. hover. DARPA ASTOVL And Beyond

Origins Studies continued through the 1970s and 1980s on STOVL fighters to replace the Harrier. The studies usually added Interest in designs for vertical takeoff and landing, supersonic performance and multimission avionics and radar that the Harrier did not have in its original design. or VTOL, fighter aircraft began in the 1960s at the The United States and the United Kingdom collaborated on studies of propulsive lift systems for the next generation of VTOL height of the Cold War when NATO bases were and STOVL aircraft. The Defense Advanced Research Projects Agency, or DARPA, started the STOVL Strike Fighter studies seen as vulnerable to preemptive attacks. Such in the late 1980s. Lockheed, General Dynamics, McDonnell Douglas, and Boeing all developed concepts. The studies led to aircraft, secured in hardened shelters, could still the Advanced STOVL competition that Lockheed won. In 1993, Lockheed purchased the General Dynamics Fort Worth take off and land from bases with damaged operation, which by then was the only GD division involved with aircraft manufacturing and design. runways. The DARPA program later evolved into the tri-service Joint Advanced Strike Technology, or JAST effort, which evolved into The United States, United Kingdom, Germany, the Joint Strike Fighter concept, then the X-35B prototype, and finally to today’s F-35B. and France all built and tested multiple VTOL fighter designs. However, only the British The DARPA program included construction and wind tunnel tests of a Large Scale Powered Model, or LSPM, used to Kestrel/Harrier family made it into service. measure the aerodynamics and propulsion interaction of the shaft-driven lift system developed by Lockheed. Meanwhile, only the Yak-38, which used a similar This view shows the motors that turn the segments of the The original design for the primary nozzle on the LSPM was a two-dimensional Single Expansion Ramp Nozzle, called SERN. engine and nozzle arrangement to the Harrier, duct and the loop in the fluid lines almost identical to the saw service on the Russian side. On this design, one nozzle flap is longer than the other. The nozzle vectors the primary thrust by deflecting the upper flap 1972 design. The X-35 and F-35 nozzles use jet fuel as through at least ninety degrees. To control the nozzle exit area in hover, the lower flap was designed as a sliding panel that US Navy studies in the 1960s evaluated Sea the working fluid to drive the nozzle motion – hence the would retract as needed to adjust the backpressure on the engine – a critical control needed to make the shaft-driven lift fan Control VTOL aircraft designed to operate from turbine work. Rolls-Royce was contracted to build the LSPM nozzle to run behind a Pratt & Whitney F100 engine and to ships with smaller decks than from decks on term “fueldraulics." The engine-mounted fuel pumps design the X-35 prototype nozzle. traditional aircraft carriers. These proposed provide the pressure for the fuel to drive the actuators. As Lockheed began small-scale wind tunnel tests of the nozzle under the DARPA program and as Rolls-Royce began building fighters would take off vertically with full loads. Then the fuel goes into the engine to be burned. The Navy’s concept of operations would also the LSPM hardware, the shortcomings of the design became more apparent. require these aircraft operate as traditional carrier-launched fighters, which necessitated afterburning engines. However, trying to turn the flow with the upper flap and getting the flow to turn around the sharp lower lip produced a poor This dual operational approach led to larger, heavier aircraft designs that needed more vertical thrust than could be provided thrust coefficient. In effect, the engine flow was running into a wall (the upper flap in the deflected position) and separating by just the primary engine or engines. The most popular solution was to add small lift engines just aft of the cockpit to provide across the lower lip. The nozzle was also gaining weight. The flat sides and large upper flap did not make a good pressure vertical thrust forward of the aircraft center of gravity. These designs were called Lift Plus Lift/Cruise. Allison, Rolls-Royce, vessel. Thicker material and significant amounts of external stiffening were needed to hold the nozzle shape and to permit the flaps to seal. Moving a six-foot-long upper flap against full engine thrust required a very large and heavy actuator. Throughout the 1970s and 1980s, the Soviet Navy wanted a supersonic STOVL fighter to operate from its ski jump equipped carriers. At what point the Yakovlev Design Bureau became aware of the multi-swivel nozzle design is not known, but the The tests of the LSPM in the NASA-Ames wind tunnel and on a hover test stand proved the shaft-driven lift fan system could Soyuz engine company created its own variant of it. The Yak-41 version of the nozzle, from published pictures, appears to be operate. A better solution was needed for the nozzle of the X-35B, however. a three-bearing swivel duct with a significant offset “kink.” The Yak-141 also used two RKBM RD-41 lift engines – an almost At this same time, Lockheed was integrating parts of the former General Dynamics team into the ASTOVL effort. Engineers identical arrangement to the Convair Model 200 design. The aircraft was also re-labeled as a Yak-141 to imply a production from Fort Worth had access to the archives from Convair that were transferred to Fort Worth when the San Diego operation version, but no order for follow-on series came from the Russian Navy. closed. The Yak-141 was flown at the Paris Airshow in 1991. The flight displays of the Yak were suspended when the heat from the Included in these archives were the Model 200 documents – particularly the description of the nozzle. In October 1994, Pratt & lift engines started to dislodge asphalt from the tarmac. At the 1992 Farnborough show, the Yak was limited to conventional Whitney funded the Lockheed Fort Worth team to perform a study of the 3BSD for the ASTOVL configuration. This effort takeoffs and landings with hovers performed 500 feet above the runway to avoid a repeat performance of asphalt damage. evaluated ground clearance of the nozzle in vertical lift position, calculated aft body drag of the nozzle, and predicted overall But the Yak-141 does deserve credit for being the first jet fighter to fly with a three-bearing swivel nozzle – twenty-five years performance of the installed propulsion system. after it was first designed in the United States.

The results of that study showed that the 3BSD design was significantly lighter than the SERN nozzle. Moreover, the design During the early days of the JAST effort, Lockheed (accompanied by US government officials from the JAST program office) also showed superior propulsion performance in all modes. The 3BSD was subsequently included in the ASTOVL visited the Yakovlev Design Bureau along with several other suppliers of aviation equipment (notably also the Zvezda K-36 Configuration 141 – the original canard delta design of what evolved into the X-35. ejection seat) to examine the Yakovlev technologies and designs.

The 3BSD was scaled to match the PW611 engine being designed for the X-35. The weight saved by incorporating the 3BSD Yakovlev was looking for money to keep its VTOL program alive, not having received any orders for a production version of in place of the square SERN was estimated to be more than 1,800 pounds. Moreover, the weight savings occurred at the far the Yak-141. Lockheed provided a small amount of funding in return for obtaining performance data and limited design data aft end of the aircraft and thereby helped the overall balance of the X-35 design. Furthermore, the 3BSD provided built-in yaw on the Yak-141. US government personnel were allowed to examine the aircraft. However, the 3BSN design was already in capability that the SERN did not have. The original ASTOVL design would have incorporated yaw vanes in the lift fan at some place on the X-35 before these visits. additional weight. Their position at the bottom of the lift fan would have produced unwanted rolling moments when yaw was The 3BSD was invented in America in the 1960s, proposed by Convair to the US Navy in the 1970s, first flown by the commanded. Russians in the late 1980s, re-engineered from the 1960 Pratt & Whitney design for the X-35 in the 1990s, and put into The 3BSD provides yaw control through the first swivel bearing. The resulting yaw thrust force is applied through the production for the F-35 in the 2000s. Sometimes a good idea has to wait for the right application and set of circumstances to centerline of the engine – very near the overall aircraft vertical center of gravity. In this position, the thrust force results in no come along. One moral of this story is not to throw out good work done in the past. It just might be needed later on. added rolling moment. The axisymmetric nozzle provided better thrust coefficients in both horizontal flight and vertical lift mode. The 3BSD moved the vertical thrust location of the nozzle farther forward relative to the SERN, resulting in a better hover balance between the forward lift fan and the rear nozzle. Kevin Renshaw served as the ASTOVL Chief Engineer for General Dynamics and was later the deputy to Lockheed ASTOVL Chief Engineer Rick Rezabek in 1994 when the 3BSD concept was incorporated into the X-35B design. Renshaw continues The 3BSD was then combined with low-observable, or LO, axisymmetric nozzle designs that had been recently flown on the to work in the Advanced System Development branch of Skunk Works where he is currently working on flight demonstration US Air Force F-16 fighter. Pratt engineers also dug into their archives and found much of the original design and test data on of the DARPA ARES VTOL UAV program. the 1960s development of the 3BSD. They also found designs for moving duct liner cooling air across the bearings.

The Lockheed ASTOVL/JAST team formally changed from the SERN nozzle to the 3BSD in 1995 with a compact axisymmetric convergent/divergent nozzle for the STOVL version and a longer set of nozzle flaps for better performance on the CTOL and CV variants. Other changes included planform trades (canards versus aft tails), inlet designs (caret inlets versus a diverterless bump inlet), landing gear arrangement, and weapons integration.

But a critical ingredient of meeting the STOVL weight and performance had already been put in place by combining a thirty- year-old vectoring approach with a modern engine and LO nozzle. The axisymmetric nozzle provided predictable back pressure control that worked cleanly with the Shaft Driven Lift Fan system. The pieces were falling into place.

Pratt & Whitney and Rolls-Royce built and flight qualified the 3BSD for the X-35 prototype using many of the design concept drawings from the earlier P&W work. The prototype included the liner that directs bypass cooling air through the swivel joints at all deflections, even in afterburner (though afterburner was and is not used in hover on the X-35B and F-35B).

The first flight of the conventional takeoff and landing X-35 occurred in October 2000 with the STOVL X-35B flying in June 2001. The prototype and production engine nozzles closely resemble the designs from the Convair installation. All three variants of the X-35 flew with the short nozzle flaps designed for the STOVL variant. The longer LO nozzle flaps for the production F-35A variant and the F-35C variant were developed later under a production System Design and Development effort.

Russian Swivel Nozzle Designs A great deal of misinformation has appeared on the Internet regarding the relationship of the Soviet Yak-41 (later Yak-141), NATO reporting name Freestyle, to the X-35 and the rest of the JSF program. The Pratt & Whitney 3BSD nozzle design predates the Russian work. In fact the 3BSD was tested with a real engine almost twenty years before the first flight of the Yak. Powering the Lightning II aviation blog cites Warren Boley, President Maintainability of Pratt & Whitney Military Engines, as say- From the outset the F135 has been designed April 2012 Chris Kjelgaard ing the F135 weighs 1,500lb (680kg) more for maintainability, building on the experi- than the F119. This would put the F135’s dry ence Pratt & Whitney gained with the F100 “…The LiftFan (one of three major compo- weight at around 5,400lb (2,450kg). How- for the F-15 and F-16 and then with the F119 nents of the Rolls-Royce LiftSystem, which ever, the F135 may have a higher thrust-to- for the F-22. (When designing the F119, the provides the F-35B’s hover capability) is not weight ratio than the F119 (the F119’s over- company brought in US Air Force mechanics engaged while in normal forward flight and all pressure ratio is 26:1 compared with the to help design its engine-mounted controls does not feature at all in the F-35A CTOL F135’s 28:1) and so the 5,400lb figure might and accessories for maintainability). In the and F-35C CV conventional take-off and be high. Boley has also suggested the F135 F135, all controls affixed to the casing are landing variants of the Lightning II. However, has an uninstalled wet-thrust capability of ‘single-deep’ – no control units are mount- from the outset the specification for the approximately 51,000lb (226.86kN). If this ed on top of each other – and the nuts and F-35’s engine called for “tri-variant com- reads across to an installed basis – in which bolts which attach them to the engine casing patibility”: the engine powering an F-35A bleed air and shaft horsepower would be ex- are encapsulated in the control assemblies is identical to that powering an F-35B or tracted to power aircraft systems – it should themselves, so nuts and bolts stay with the an F-35C. Nevertheless, the engines are provide a comfortable operating margin over control units when these are removed. This designated differently: the F-35A power- the 43,000lb [?] (119.27kN) of wet thrust re- greatly minimizes the risks of nuts and bolts plant is the F135-PW-100; the engine for quired by the spec…. being lost and causing foreign-object debris the F-35C is the F135-PW-400; and the …Another key feature of the F135 is its (FOD) damage. F-35B engine is the F135-PW-600. augmentor, or afterburner system. While Similarly, all engine clamps and blocks Since the F-35B powerplant needs an available details of the augmentor are stay on the engine casing when an F135 is extra LPT stage to provide the power neces- sketchy, the F135 is known to employ multi- removed for maintenance and the engine sary to turn the driveshaft (which, through zone (probably three-zone) fuel injection aft uses no safety wire, eliminating another po- a clutch and gearbox, drives the LiftFan), of the afterburner’s pilot light. These zones tential source of FOD damage. All controls F135s built to power other F-35 variants inject fuel independently, so that the after- and accessories are mounted on the bottom have the second LPT stage as well. “The en- burner does not act in an all-or-nothing way of the engine, making it easier for mechan- gine was designed to support that severe but instead provides a variable range of ad- ics to get to them; and these assemblies are STOVL requirement,” says O’Donnell. For en- ditional, smoothly transitioning wet thrust modular so that, say, a mechanic could eas- gines powering CTOL F-35As and F-35Cs, the at the pilot’s command. Also, like the F119 ily remove the electronics or valves or re- additional turbine stage offers a substantial augmentor, the F135 augmentor is stealthy: lays for an F135 fuel control unit as entire extra power margin, allowing for potential The design of the two engines’ augmentors modules…. F-35 weight growth. Since the engine isn’t places multi-zone fuel injection into curved …So seriously did P&W take the job of heavily taxed in many CTOL missions, its vanes which eliminate conventional spray making the F135 highly maintainable that maintainability is improved too…. bars and flame holders and block the line it tried to design the engine to require only …P&W also won’t confirm the dry weight of sight to the turbine when looking into the a single hand tool, clamped to the engine of the F135, but a source commenting on an engine from behind. when not in use, for all line-maintenance 1 jobs. P&W couldn’t quite achieve that ideal flow from the LiftFan, as commanded by the are situated in the lower part of each inner but did succeed to the point where only six pilot through the aircraft’s FADEC units. wing section and act to provide roll con- hand tools are required…. When the F-35B is hovering, the drive- trol for the F-35B while it is in hover mode. shaft delivers 28,000 shaft horsepower to In order to do this, the roll-post ducts di- …The Rolls-Royce LiftSystem the LiftFan’s clutch-and-bevel-gear sys- rect bypass air from the engine to the roll One of the most remarkable features of the tem so that the LiftFan provides 20,000lb posts, which drive the air out through the F-35 programme is that when the STOVL (124.55kN) of downward thrust as a column bottom of each wing. In the F-35B, 3,700lb F-35B is hovering, its propulsion system of cool air. (In the F-35B’s hover mode the (16.46kN) of thrust in the form of bypass produces very nearly as much thrust with- coupled F135-driveshaft arrangement acts air is directed out to the two roll posts while out afterburner as the engine does in for- exactly like a turboprop engine, except that hovering. [Total 39,400lb of thrust] ward flight with its afterburner fully lit. The most of its power output is used to drive Each roll-post assembly features a pair F-35B’s engine has to produce 39,400lb air vertically rather than horizontally, so the of flap-type doors in the bottom of the wing, (176kN) of vertical thrust without after- F135 is actually the world’s most power- controlled by the FADEC. Jones says these burner in hover mode, while in conventional ful turboprop engine when installed in the titanium doors are controlled by rotary actu- flight it produces 28,000lb (124.55kN) of dry F-35B.) ators which allow fully variable opening, pro- thrust and 43,000lb [LM says 40,000lb] In hover mode another 15,700lb viding a degree of thrust variability and di- (191.27kN) with full afterburner. (69.84kN) of thrust exits the engine exhaust rectionality so that the pilot can control roll The F135-powered F-35B relies on two as hot gas and is directed downwards at the while hovering. He says Lockheed Martin’s systems to achieve the high level of verti- rear of the aircraft by the aircraft’s three- original X-35 concept demonstrator featured cal thrust. First is its full authority digital en- bearing swivel module (3BSM). This remark- doors between the engine casing and the gine control (FADEC) unit – computers made able piece of equipment consists of three ar- roll-post ducts which could be closed when by BAE Systems and attached to the en- ticulated sections of nozzle casing, each of the aircraft was not hovering, but in produc- gine, but running on Pratt & Whitney pro- which is made from titanium. Each section is tion aircraft there are no such doors and by- prietary FADEC software. In hovering flight, joined to the other sections by and driven by pass airflow is constantly sent to the ducts. the FADEC computers make the engine work its own ring bearing. When the F-35B hovers, The only way to control roll-post thrust is via harder, allowing it to increase dry thrust the FADEC commands the 3BSM – which can the flap-doors in the bottom of the wing. from 28,000lb to 39,400lb without using direct air through a 95-degree range from The demand for very high power dur- afterburner…. 5° forward to horizontally back – to swiv- ing hover requires that the engine receive a …Below the LiftFan, the variable area el downwards to direct hot engine exhaust high amount of airflow, so Lockheed Martin vane box (VAVB) provides an exit path for air in the same direction as the direction of designed the F-35B with a pair of auxiliary the cool air driven downwards vertical- the cool air produced by the LiftFan near the air inlet (AAI) doors in the upper surface of ly by the LiftFan. Rolls-Royce produces the front of the aircraft. The 3BSM can swivel the fuselage behind the big inlet door for the VAVB, which is made of aluminium and con- fully from horizontal to vertical orientation in LiftFan. These AAIs provide additional inlet tains louvred vane doors. These can be an- 2.5 seconds…. air for the F135 engine, not the LiftFan….” gled all the way from 45° back, through fully …According to Jones, the roll posts http://militaryrussia.ru/forum/ vertical to 5° forward to provide variable di- themselves are variable-area nozzles which rectionality for the downward cool-air download/file.php?id=28256 2 JOURNAL OF AIRCRAFT The purpose of this paper is to describe how the technical and Vol. 46, No. 6, November–December 2009 2009 WRIGHT BROTHERS LECTURE program challenges involved in the creation of the F-35 Joint Strike Fighter were met. It will show how multiple service and mission requirementswereincorporatedintoasingleaircraftdesign.Analysis, Genesis of the F-35 Joint Strike Fighter design, ground-test, and experimental flight-test information will be presented. The first section of this paper describes the conceptual Paul M. Bevilaqua design of the original STOVL Strike Fighter for the Marines. Its Lockheed Martin Aeronautics Company, Palmdale, California 93599 development into the Air Force and Marine Common Strike Fighter will be discussed in the next section. The addition of the Navy and DOI: 10.2514/1.42903 overseas partners to create the International Joint Strike Fighter will Functional analysis has been used to design a common aircraft to replace the U.S. Air Force F-16s, Navy and be described in the section after that. The last section summarizes Marine Corps F/A-18s, and Marine AV-8s. The technical and program challenges involved in developing a common the current status of the program and plans for the production and aircraft for all three services were met by designing three highly common, but not identical, variants of the same deployment of the F-35 Lightning II aircraft. aircraft. The key elements of this commonality are an innovative propulsion system that can be switched from a fl turbofan cycle for conventional ight to a turboshaft cycle for vertical takeoff and landing and a basic structural Marine STOVL Strike Fighter arrangement that can accommodate the substitution of stronger parts in the Naval variant to absorb the greater In 1980, the Navy completed the Sea Based Air Master Study [1] takeoff and landing loads of carrier operations. Fig. 2 Evolution of VTOL aircraft. on the future of Naval aviation. An essential conclusion was that an all-STOVL Naval air force designed around then-current tech- Introduction greeted with considerable skepticism, largely because the joint develop a VTOL aircraft. However, tailsitter aircraft had limited nologies would cost more than an equivalent conventional carrier- Tactical Fighter Experimental (TFX) program of the 1960s had not range/payload performance due to the weight limits imposed by based force. Given this result, the Navy began the construction of HE Wright Brothers Lectures commemorate Wilbur and Orville two new nuclear aircraft carriers. NASA took on the challenge of ’ fi succeeded as a joint program. The TFX program was intended to save vertical takeoff and no ability to increase lift off weight with a short T Wright s success in developing the rst practical airplane. But developing technologies for reducing the cost of supersonic STOVL in solving the problems of manned flight, they also demonstrated the several billions of dollars in life cycle costs by using a common ground roll when there was a runway available. In addition, tailsitters airframe and engines to meet both the Navy’s fleet air defense were difficult for pilots to land because they had minimal control aircraft and began the Advanced Short Take Off and VerticalLanding value of their approach to problem-solving. The key elements of their ’ fi (ASTOVL) program. Between 1980 and 1987, NASA funded studies approach were a skeptical study of the relevant literature, to identify requirement and the Air Force s requirement for a long-range ghter power in hover, and the pilot could not see over his shoulder to bomber. The Navy withdrew from the TFX program when the determine how high he was above the ground, or how fast he was at all of the major aircraft companies to devise innovative concepts for errors as well as valid data; innovative thinking, which led to the idea a supersonic successor to the AV-8B Harrier, and the British Ministry that an airplane can be controlled with wing warping; constructive aircraft became too heavy for carrier operations. The Air Force was descending. left with an F-111 too small to be an effective bomber and not of Defence conducted similar studies in the United Kingdom. debate, to work the flaws out of their ideas; systematic testing, Therefore, the second generation of VTOL aircraft, including the ’ maneuverable enough to be a competitive fighter. Mirage III-V and XV-4, were designed with lift engines installed Lockheed s ASTOVL concept was based on the tandem fan engine progressing from wind tunnels to kites to gliders to airplanes; and advocated by Rolls-Royce [2,3]. teamwork, including the contributions of their mechanic Charles In addition, developing a supersonic, vertical takeoff and landing vertically in the fuselage, so that the aircraft could take off and land in (VTOL) fighter was considered a significant technical challenge by The tandem fan engine would have been created by lengthening Taylor, who actually implemented many of their ideas. Lockheed a conventional horizontal attitude. This enabled the pilot to see the fi itself. The stages in the evolution of VTOL aircraft are illustrated in a cruise engine to move the rst stage of the engine fan forward. Martin used a similar approach in developing the Joint Strike Fighter ground and judge his sink rate. However, the lift engines took up too fi Fig. 2. The first attempts to build a vertical takeoff and landing fighter much space in the fuselage and were dead weight during cruise, In the STOVL cycle, the rst stage of the engine fan was to have been (JSF), and so the subject of this paper seems appropriate for a lecture converted to a lift fan by diverting its exhaust flow to nozzles at the commemorating their accomplishments, and it is an honor to have were the tailsitters of the 1950s, including the XFV-1,the XFY-1,and whereas the cruise engines were dead weight during hover. As a the X-13. Because the thrust-to-weight ratio of fighter aircraft was result, the range/payload performance of these aircraft was also front of the aircraft. An auxiliary inlet would be opened to provide air been invited to present it. to the engine core. By moving some of the cruise thrust forward in the The U.S. Air Force F-16 Falcon, U.S. Marine Corps AV-8B already close to 1, designers thought that it would be a simple matter unsatisfactory. In addition, the hot exhaust gases of the lift engines of standing a fighter on its tail and increasing the thrust a little to damaged the airframe and caused ground erosion, and reingestion of vertical mode, this innovative engine concept enabled designers to Harrier, and U.S. Navy F/A-18 Hornet, shown in Fig. 1, are fourth- balance the airplane while hovering. However, diverting the flow of fi these hot gases caused the lift engines to stall and lose lift. generation strike ghters. They will all need to be replaced with new the front fan from the engine core meant the loss of its supercharging aircraft at about the same time, beginning in the next decade. Before The third generation of VTOL aircraft, such as the VJ-101, used fl swiveling lift/cruise engines that were rotated from a vertical position effect on the core ow. Therefore, the tandem fan engine produced the end of the Cold War, each of the services had begun programs slightly less thrust in thevertical cycle than in the cruise cycle, despite to develop fifth-generation replacement aircraft. Fifth-generation for hover to a horizontal position for cruise. However, these air- fi fl the increased mass flow. As a result, the tandem fan engine had to be successor aircraft will incorporate stealth, operate in a net-centric craft were dif cult to transition from hover to cruise ight, or back, and they also suffered from hot-gas ingestion and ground-erosion sized for the hover thrust requirement. This made it somewhat environment, and have greater range. However, it soon became oversized for cruise, which increased fuel consumption. Also, the lift apparent that there would not be enough money in the reduced problems. Further, because the engines had to be sized for hover, they fi fan did not develop sufficient thrust to balance the thrust from the defense budget to fund three separate replacement aircraft programs. were larger than optimal for cruise. The resulting inef ciencies reduced range/payload performance. cruise nozzle, and so the engine had to be moved forward over the A common replacement aircraft was an attractive solution that center of gravity of the aircraft. This concentration of wing, fuel, appealed to some in government and industry. In the latest and most successful generation of VTOL aircraft, the fi thrust of the cruise engine is simply vectored down. The AV-8 uses payload, and engine volume at the center of gravity made it dif cult However, the idea that multiple service and mission requirements to design an aircraft that was slender enough to achieve supersonic could be incorporated into a single aircraft design was initially thrust vectoring of a single lift/cruise engine with a high bypass Fig. 1 Fourth-generation strike fighter aircraft. ratio having enough thrust for vertical landing. In routine operations, speeds. it is flown from any available runway as a short takeoff and vertical When these airframe studies were completed in the summer of landing aircraft. With a short takeoff run, the AV-8 has range/payload 1986,aU.S./U.K.governmentreviewpanelconcludedthatnoneofthe proposed concepts offered a clear advantage in cost or performance. Paul M. Bevilaqua is an Aeronautical Engineer at the Lockheed Martin Aeronautics Company. He earned a B.S. in performance comparable with other lightweight fighters. However, However, the panel did identify four propulsion concepts, including aerospace engineering from the University of Notre Dame. After receiving a Ph.D. from Purdue University for his the fan diameter is too large to enable the aircraft to achieve super- the tandem fan, which seemed promising. They recommended contributions to the theory of shear flow turbulence, he served on active duty as a U.S. Air Force officer assigned to the sonic speeds. The VAK 191 and Yak 38 were hybrid concepts that developingtechnologies thatwouldimprovetheperformanceofthese Aerospace Research Laboratories at Wright-Patterson Air Force Base. He used his understanding of turbulence to vectored the thrust of the cruise engine, but also incorporated lift four concepts, and this work continued until 1991. develop hypermixing nozzles and ejectors for a U.S. Air Force VSTOL (vertical short takeoff and landing) transport engines to increase thrust for hover. In these aircraft, the engines also aircraft. Following his military service, he became Manager of Advanced Programs at Rockwell International’s U.S. took up internal volume and created hot-gas ingestion and ground- Navy aircraft plant, where he led the design of U.S. Navy VSTOL interceptor and transport aircraft. He subsequently erosion problems. Invention of the Dual-Cycle Propulsion System joined Lockheed Martin as Chief Aeronautical Scientist of the Lockheed Advanced Aeronautics Company and Although the short takeoff and vertical landing (STOVL) AV-8and At the same time, NASA was also working with the Lockheed became Chief Engineer of Advanced Development Projects in the Lockheed Martin Skunk Works®. He played a Yak-38 were operational aircraft, they were not capable of supersonic Skunk Works to study the installation of lift engines in the F-117, leading role in creating the Joint Strike Fighter program. He invented the Lift Fan Propulsion System that made it speeds. The fundamental problem was that a propulsion system that to identify the technologies needed to build a stealthy STOVL possible to build a stealthy supersonic VSTOL Strike Fighter, and he proposed designs for conventional and Naval provided enough thrust for hover was too large and required too much Strike Fighter (SSF). In the fall of 1986, the Defense Advanced variants, to share development costs between the U.S. Air Force, U.S. Navy, and U.S. Marine Corps. He subsequently fuel to enable the design of a slender supersonic airframe. During Research Projects Agency (DARPA) expanded the scope of the led the engineering team that demonstrated the feasibility of building short takeoff and vertical landing, the 1970s, in the decade before the start of the Joint Strike Fighter NASA studies when it awarded the Skunk Works a nine-month-long conventional, and Naval variants of this aircraft. program, both the VAK 191 and XFV-12A supersonic demonstrator exploratory study contract to see if a supersonic stealthy SSF could aircraft were unsuccessful, and neither became operational. To be developed for the Marines. This aircraft would have to perform fi Presented as Paper 1650 at the 47th Aerospace Sciences Meeting, Orlando, FL, 5–8 January 2009; received 23 December 2008; accepted for publication 2 July summarize, the development of VTOL ghter aircraft proceeded the air superiority missions of the F/A-18 as well as the close air 2009. Copyright © 2009 by Lockheed Martin Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of along a path to simplification: first the aircraft were tilted, then the support missions of the AV-8. This combination of supersonic and this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood engines were tilted, then the engines werevectored, until it was finally vertical performance requirements meant that the engine must not Drive, Danvers, MA 01923; include the code 0021-8669/09 and $10.00 in correspondence with the CCC. recognized that the only thing that had to be vectored was the thrust. only provide enough vertical thrust for short takeoffs and vertical landings, but must also be small enough that it would not increase When the lift fan was disengaged for cruise, the bypass flow would occurs across the turbine nozzles, increasing the thrust of the jets T ˆ V2A (3) supersonic drag. The propulsion system would be the key component be returned to the cruise nozzle. This would match the nozzle area to from the turbine nozzles and producing more shaft horsepower, in the development of this new strike fighter. the cruise power requirement again. In fact, it would not be necessary while reducing the thrust of the exhaust flow. In general, the effect of where is the air density and A is the fan area. Ideally, a VTOL aircraft has a thrust-to-weight ratio of about 1.2 to to add another turbine stage. The existing turbine would move off its opening the exhaust nozzle is to decrease its thrust while increasing Solving this thrust equation for velocity and substituting in Eq. (2) provide thrust margins for vertical acceleration and control. A design operating point to provide shaft power for hover and back to the thrust of the turbine nozzles. yields horsepower as a function of thrust for cylindrical ducted fans: conventional F/A-18 has a usual takeoff weight of around 37,000 lb its design operating point for cruise. The existing driveshaft for the The power produced by the turbine section of a turbojet engine is and dry thrust of 22,000 lb, giving a thrust-to-weight ratio of only engine fan could just be lengthened to power the lift fan. given by the equation hp ˆ T3=A†1=2 (4) 0.60 in dry power, increasing to just 0.95 in afterburner. AVTOL F/ Because the lift fan is not connected to the engine during cruise A-18 would require about 44,000 lb of dry thrust (1:2  37; 000 lb). flight, the engine operates like a conventional mixed-flow turbofan g1†=g As previously noted, the lift fan must develop approximately turbine power ˆ mc_ T04‰1  P5=P4† Š (1) Comparing a conventional F/A-18 with a VTOL F/A-18 illustrates engine during cruise. For STOVL operations, the lift fan is connected p 22,000 lb of thrust to balance an aircraft the size of an F/A-18. If the the basic problem: there is not enough thrust, and it is all at the back. to the cruise engine by engaging a clutch on the driveshaft. The cruise lift fan has the same 4 ft diameter as the cruise engine, then approxi- _ fl fi AVTOL F/A-18 requires an additional 22,000 lb of dry thrust ahead engine nozzle is simultaneously opened, increasing the pressure drop where m is the mass ow through the turbine, cp is the speci c heat at mately 30,000 shp will be required, according to Eq. (4). To the of the center of gravity for balance and to provide the necessary thrust across the engine’s turbine section. This causes it to extract additional constant pressure per unit mass of air, g is the gas constant, T04 is the accuracy of this analysis, there is sufficient power available from margin. The problem became devising a way to double the engine shaft power, which is used to drive the lift fan. The engine then stagnation temperature of the gas entering the turbine section, and the engine to drive the lift fan. thrust and move half of it to the front of the airplane. Posing the operates in hover like a separate-flow turbofan with a higher bypass P5=P4 is the pressure ratio across the turbine section. The usual This power must be transmitted by the driveshaft. The horsepower problem this way turned out to be the key to the solution. ratio. This dual-cycle operation is the novel feature of the engine in method of increasing turbine power is by increasing the fuel flow, transmitted by a driveshaft is equal to the product of torque and Skunk Works engineers tried a number of brainstorming tech- the F-35 [5]. which increases T04. The additional power of the turbine accelerates rotational velocity. Therefore, for a given horsepower transmitted, niques, but the one that proved most useful was the method of forced To summarize, the solution was to extract some of the energy from the engine until the power absorbed by the compressor matches the the necessary torque decreases as the rotational velocity of the shaft associations. This is a technique for inventing something new by the engine exhaust jet by changing the operating point of the turbine, power produced by the turbine and the engine speed stabilizes. increases. The shaft must be sized to transmit this torque. The torsion generating arbitrary combinations of existing mechanisms. The move it forward with a shaft, and turn it into additional thrust by Because the rotational speed of the engine has increased, the engine formula for hollow round shafts gives, for the diameter of the shaft, technique required making a list of all the ways to extract power from adding it to a larger mass flow of air with a fan. The lift fan is attached pumps more air and produces more thrust. the hot high-pressure exhaust gases at the back of the engine (for to a driveshaft extending from the front of the cruise engine, as shown The performance map of the turbine section in a typical modern d ˆ‰16  shp=! 1  f4†Š1=3 (5) example, turbines, scoops, heat pipes, magnetohydrodynamics, etc.), in Fig. 3, and bypass air for the roll jets is tapped off from behind the fighter engine is shown in Fig. 5. The locus of steady-state matching making another list of all of the ways to transfer power from one cruise engine fan. Thinking about how to extract power from the back conditions defines the engine operating line, which is the diagonal where ! is the rotational speed of the shaft, is the maximum unit point in the aircraft to another (gas ducts, driveshafts, chain drives, of the airplane and transfer it to the front resulted in a flash of insight running from the bottom left to the top right in the figure. The engine shear stress of the driveshaft material, and f is the fraction of the shaft superconducting wires, energy beams, etc.), and making a third list that produced the dual-cycle-engine concept as the solution for the and compressor are designed so that the turbine power and com- diameter that is hollow. This formula gives the stress due to torsion of all the ways to use power to generate thrust (fans, pulse jets, STOVL Strike Fighter. pressor power match near the point of maximum efficiency at every only; it neglects other loads, such as those due to bending and explosions, piezoelectric pumps, etc.). The procedure is to arbitrarily speed. However, at maximum thrust, the turbine inlet temperature T04 vibration. Figure 6 shows how the diameter of a 0.05 thin-walled pick one mechanism from each list and figure out how they might be is already at the material limit of the turbine section. As a result, the aluminum shaft transmitting 30,000 hp varies with engine rpm due to made to work together to solve the problem. This technique led to Principle of Operation gas temperature cannot be increased to provide the power to drive the torsion loads. The high rotational speeds typical of jet engines, the invention of some truly innovative concepts: for example, using To appreciate how this dual-cycle engine turns jet thrust into the lift fan. Instead, during VTOL operation, the additional power more than 10,000 rpm, make it possible to transmit large amounts of the energy of the exhaust gas to pump a gas laser, then beaming the additional shaft power, it is necessary to consider the changes in the to drive the lift fan is obtained by increasing the pressure drop across power with an aluminum shaft just a few inches in diameter. fl energy forward, and then using it to explode the air in a pulse jet static pressure of the air as it ows through the engine. The variation the turbine section, P4–P5. The additional power is shown by the two The size of the clutch depends on both the rotational kinetic energy engine. of total energy (top) and static pressure (middle) through an engine points in Fig. 5. of the fan, I!2=2, and the period of engagement, t. The horsepower But none of these concepts were really practical. It became are shown in Fig. 4. The pressure rises through the compressor (2–3), The lower point is on the conventional operating line, and the that must be absorbed by the clutch during engagement decreases as apparent that the best way to extract power from hot high-pressure remains constant through the combustor (3–4), and then drops upper point is obtained when the pressure drop across the turbine is the time for engagement is increased according to the relation exhaust gas is with a turbine, the best way to get the power forward in through the turbine section (4–5) and nozzle (5–6), in two steps. As increased. In this case, nearly 30,000 hp can be extracted before the 2 an aircraft is with a driveshaft (it is light and does not increase the the pressure drops through the turbine section, the flow accelerates. turbine section reaches its stall limit. There is enough residual power hp ˆ I! =2t (6) cross-sectional area of the fuselage), and the best way to produce The resulting thrust of the jets from the turbine nozzles spins the in the exhaust flow to generate significant thrust from the cruise vertical thrust is with a fan (increasing mass flow is the best way to turbine disk that powers the driveshaft. nozzle during hover. Engaging the clutch while increasing the nozzle The knee of this curve is near 10 s at low engine speeds. increase thrust per horsepower). At every engine speed, the static pressure at the inlet to the turbine area transfers the additional power to the lift fan, so that the speed of The jet pressure ratio can also be estimated from the thrust Therefore, the best solution to the problem of producing thrust section is equal to the pressure rise across the compressor. The the engine does not increase. equation. Because the static pressure in the lift jet returns to ambient – ahead of the center of gravity would be to add another turbine stage to pressure drop across the turbine (P4 P5) plus the pressure drop pressure behind the fan, then – extract power from the exhaust gases. It would have to be variable- across the exhaust nozzle (P5 P6) must therefore equal the pressure rise across the compressor (P3  P2). The distribution of the pressure 1 2 ˆ  pitch, so that it could be feathered during cruise. Another driveshaft Analytical Estimates 2 V Ptotal Patmospheric (7) drops is controlled by the engine exhaust nozzle. Increasing the could be run from the added turbine stage through the engine to a lift The horsepower needed to drive a lift fan can be estimated using fan: Rolls-Royce was already building three spool engines. The lift exhaust nozzle exit area reduces the pressure drop across the exhaust – basic momentum-energy considerations: horsepower hp is the This equation can be solved for the fan pressure ratio PR ˆ fan provides one lift post. Vectoring the cruise nozzle down would nozzle (P5 P6), and so the pressure drop across the turbine nozzles – product of thrust T and velocity V: Ptotal=Patmospheric and yields create another lift post. Shifting power between the lift fan and cruise (P4 P5) must increase to compensate. ˆ nozzle would provide control in pitch. Similarly, engine bypass air For example, increasing the nozzle exit area so that A6 A5,as PR ˆ 1 ‡ 1 2 hp ˆ 2V =Patmospheric (8) could be ducted off to nozzles in the wings and thrust could be shifted sketched in Fig. 4, causes the static pressure at the turbine exit, P5,to TV (2) from one wing to the other to provide roll control. drop to atmospheric pressure, P6. The entire pressure drop then But ducting off the bypass air would effectively increase the nozzle and thrust is the product of mass flow and velocity. If the duct of the fl 25 exit area for the core ow and lower the back pressure on the turbine lift fan is assumed to be cylindrical, so that the exit area of the duct section. That would increase the power produced by the turbine [4], equals the fan area, then thrust equals so that it would be necessary to close the cruise nozzle down to keep the engine from over speeding. On the other hand, if the lift fan was 20 connected to the turbine at the same time that the bypass air was diverted to the wings, the lift fan would absorb the extra turbine 15 power and keep the engine from speeding up. Then varying the Shaft nozzle area would shift power back and forth for pitch control. Diameter (inches) 10

5

0 0 2000 4000 6000 8000 10000 12000 Shaft Speed (rpm) Fig. 3 Shaft-driven lift fan propulsion system. Fig. 4 Variation of pressure through a turbojet. Fig. 5 Turbine performance map. Fig. 6 Driveshaft diameter depends on rpm. DLI CAP CAS

Super Transonic Subsonic Climb Acceleration Acceleration Loiter Agility Ride cruise Maneuver Maneuver

Maneuver G’s High High Low High High

Survivability Sweep W/b T/W W/S Sweep Fig. 7 Original sketch of the JSF propulsion system. where Patmospheric is the ambient atmospheric pressure. Solving the thrust equation for the dynamic pressure at the fan face gives, for the dynamic pressure, High Low Low Low T/W W/S W/b Sweep 1=2V2 ˆ T=2A (9) Fig. 11 Functional analysis was used to flow requirements down to the aircraft design. Therefore, for a 4 ft lift fan developing 22,000 lb of thrust, the Speed pressure ratio is approximately 1.4, which is about the same as the Fig. 9 Achieving survivability with speed and maneuver. This was a new way of designing an aircraft and it required a different design point was selected at the knee of the curve, at the 80/20 point, pressure ratio of the lift jets of the AV-8 Harrier. approach to trade studies. which was judged to give the best value. This resulted in an aircraft This first-order analysis suggested that it might be possible to only a couple of dozen people at each of the participating companies Skunk Works engineers used functional analysis to systematically with about the same performance as an F-18C, but which was almost double the thrust of an existing F-119 engine with a dual- worked on these contracts. analyze the conflicting performance requirements of the design more survivable because it was stealthier and capable of extended cycle shaft-driven lift fan the same diameter as the engine. Such a There were three design missions: Close Air Support, Combat missions and then used constraint analysis and tactical air combat supersonic cruise. However, it was necessary to project a 15% weight variable-cycle propulsion system would provide high levels of Air Patrol, and Deck Launched Intercept. However, there were no simulations to devise the most cost-effective combination of aircraft savings through the use of composites to achieve the required weight thrust augmentation in the STOVL mode, with a cool low-pressure specified speed, maneuver, signature, or other requirements. The capabilities. Functional analysis is a technique for deriving aircraft of 24,000 lb. footprint, ample control power, and minimal effect on the design of only explicit requirement was that the empty weight of the aircraft be design features from mission requirements. Each of the required Although the aircraft in the initial sketch shown to DARPA the airframe. By placing the lift fan in line with the cruise engine, the less than 24,000 lb, which is about 5% more than the empty weight of missions is subdivided into mission segments. Then each mission resembled an F-117, highly swept wings produce an unstable pitch bypass ratio would be increased without increasing the engine an F/A-18C. In other words, the weight of the STOVL equipment segment is decomposed into functions that the aircraft must per- up, even at moderate angles of attack, and were quickly abandoned. diameter. And because the cruise engine can be optimized for was to be about the same as the typical weight increment for the form to accomplish that segment. Finally, each function is analyzed The initial design of the STOVL Strike Fighter had a delta/canard conventional flight, its performance is not penalized for its STOVL navalization of a conventional aircraft. to determine the specific design features needed to perform the planform, as shown in Fig. 13. The active canard was moved like capability. This use of weight as an independent variable was a novel program function. This flowchart is often called a Willoughby template [7]. A a weather vane during subsonic cruise and maneuver, so that it management tool used by DARPA to control the cost of the SSF. In simplified version of this analysis is shown in Fig. 11. It highlights provided no lift and little drag, but it was adjusted to provide lift for DARPA Conceptual Design Contract Awards the past, the Pentagon would release a set of specific performance the conflicting requirements for wing loading, thrust loading, span trimming the nose-down moments that were produced when the flaps To illustrate the installation of such a propulsion system in a requirements. The airframe contractors would then design the loading, and sweep. were deflected and when the center of lift moved aft at supersonic supersonic SSF, an airframe resembling an F-117 without facets was lightest and therefore most affordable airplane that would meet all of Constraint analysis was used to select compromise values for these speeds. The active canard has less trim drag than a horizontal tail [8]. sketched for DARPA. The airframe was not faceted because these requirements. Figure 8 is a typical carpet plot showing the design parameters. Figure 12 shows the sensitivity of the design point The desired performance required an afterburning engine. Because computational speeds had increased in the decade since the F-117 effect of speed and maneuverability on weight. In this case, weight is to varying the speed and maneuver constraints that drove the design. the jet flap nozzles could not accommodate an afterburner, the jet was designed, so that it was now possible to analyze smooth the dependent variable; it depends on the specified M ˆ 1:5 speed The design point is above the speed and sustained maneuver flap was similarly abandoned. The aircraft carried two long-range contours. In this original sketch, shown in Fig. 7, the axis of the lift and the specified 7.5 g maneuver. Of course, weight also depends on constraint lines and to the left of the instantaneous maneuver AIM 120 missiles and two short-range AIM 9 missiles in internal fan was aligned with the axis of the cruise engine, and rotating signature, range, payload, etc., which are other dimensions of the constraint lines. The design point was selected by balancing the cost weapons bays. Models of the aircraft were tested in the wind tunnel nozzles such as those on the Harrier were used to vector the fan thrust. carpet plot. of improving performance against the cost of increasing combat and on the radar range to verify the predictions of both the aero- The core thrust of the supercruising engine was vectored over a jet However, there are often several ways to meet a top-level mission losses if performance was not improved. dynamic forces and the radar cross section. flap [6]. requirement. For example, the same level of combat survivability can The cost savings obtained by improving all of the performance Paul Shumpert, the Skunk Works’ lead propulsion engineer, used DARPA was interested in pursuing the concept further. In be achieved with different combinations of aircraft speed and parameters fell on curves of diminishing returns, similar to those in the software engine simulator provided by Pratt and Whitney (P&W) January 1988, it awarded the Skunk Works a follow-on contract to maneuverability, as illustrated schematically in Fig. 9. Specifying a Fig. 9, which meant that 80% of the optimum performance could be to show that dual-cycle operation of the Advanced Tactical Fighter develop the conceptual design of an aircraft incorporating this 24,000 lb empty weight limit, shown in Fig. 10, was intended to obtained for 20% of what the optimum cost. In other words, the last engine was feasible and that sufficient power could be extracted to dual-cycle propulsion system; McDonnell Douglas and General enable the designers to propose the most effective combination of 20% of performance drove 80% of the costs. Therefore, the initial drive the lift fan. Both Pratt and Whitney and General Electric (GE) Dynamics were given similar contracts to design stealthy versions of speed, maneuver, signature, etc., for an aircraft of specified cost, their ASTOVL aircraft concepts. These were not major programs; without having to get government approval to change requirements. 1.4

1.2 Mach 1.4 1.5 1.6

1.0

0.8 Sustained G's

Ratio 6 0.6

Thrust / Weight Thrust / 5 0.4 4 Instantaneous G's 9 8 7 0.2

0.0 20 30 40 50 60 70 80 90 100 Wing Loading Fig. 8 Weight as a dependent variable. Fig. 10 Weight as an independent variable. Fig. 12 Constraints that determined the design point. Fig. 13 First STOVL Strike Fighter design iteration. then worked with Skunk Works engineers to optimize their to secure funding for the SSF technology maturation and risk- industry in August 1992 for conducting critical technology demon- Advanced Tactical Fighter engine cycles to power a lift fan. The lift reduction effort. This led Gerry Cann, the Assistant Secretary of the strations of shaft-driven and gas-driven lift fan systems and for fan was installed with its axis vertical, because this maximized hover Navy for Research, Development, and Acquisition, to task the Naval the conceptual design of what was called the Common Affordable thrust. The Allison Engine Company designed an innovative lift fan Research Advisory Committee (NRAC) in early 1992 with assessing Lightweight Fighter (CALF). The RFP requested proposals for other with two counter-rotating fan stages. This configuration directs half the feasibility and desirability of developing a STOVL Strike Fighter. novel lift systems as well. Because this was the first public disclosure of the power to each stage of the fan system, which reduces the gear of the DARPA program, some consider this RFP to be the start of the loads in half. With this system, the power through each gear set is JSF program. similar to that used in current heavy-lift helicopters. Beginnings of Jointness Allison also designed a similarly innovative two-stage clutch to In April 1992, Brig. Gen. George Muellner, who was then Deputy connect the lift fan to its driveshaft. A multidisk friction clutch is used Chief of Staff for Requirements at Air Combat Command, visited the DARPA Technology Demonstration and Maturation Contracts to reduce the shock of engagement by slipping while the lift fan is Skunk Works to review recent developments. The STOVL Strike General Electric’s proposal to the Skunk Works for demonstrating accelerated from rest to the engine speed. Once the speed of the lift Fighter was put on the agenda. However, the Air Force was not likely the dual-cycle propulsion system was $5 million less than P&W’s fan matches the engine speed, a mechanical lockup is engaged. This to be interested in a STOVL aircraft, because it had less need for such proposal, and all the other aircraft companies gave subcontracts to transmits the full power required for short takeoff or vertical landing. capabilities than the Navy and it had similar concerns about the GE. However, the Skunk Works chose P&W because the Air Force However, because the dual-cycle propulsion system concept was increased cost. In fact, the Air Force had begun thinking about a had selected the P&W engine over the GE engine for the F-22 Fig. 15 Revised STOVL and conventional Strike Fighter variants. new and unproven, Skunk Works engineers also designed a variant of conventional takeoff and landing (CTOL) MultiRole Fighter (MRF) program and it would be the only engine available when our this aircraft with a gas-driven lift fan, as a fallback option. In the to replace the F-16, although this program had no funding. Therefore, demonstrator aircraft would need an engine. In exchange, P&W and forth for pitch control by synchronizing the operation of the lift gas-driven variant, some of the engine exhaust gases were ducted it was decided to brief Gen. Muellner on a stealthy conventional agreed to work exclusively with the Skunk Works on the develop- fan with the changes in engine nozzle area. And there were also forward, around the engine, and used to spin a turbine that drove the takeoff and landing strike fighter, instead. One of the secrets of the ment of the dual-cycle shaft-driven lift fan concept. Since the shaft- questions about the weight and reliability of the driveshaft, clutch, lift fan, something like a turbocharger. This variant did not develop as Skunk Works is that it was not necessary to deal with miles of red tape driven lift fan concept had been invented under DARPA contract, the and gearbox that powered the lift fan. much vertical lift, required more internal volume for the gas ducts, and endless approval chains to make this decision. Ben Rich system was actually available to any American aircraft company for The demonstrator propulsion system was built and tested to and was therefore heavier and slower than the shaft-driven variant. described this Skunk Works management philosophy in his 1988 government programs and McDonnell Douglas proposed that they address these concerns and prove the feasibility of the dual-cycle However, it appeared that it would be a satisfactory supersonic Wright Brothers Lecture [10]. perform an “apples to apples” comparison of both the shaft-driven engine and drive system. The demonstrator engine and lift fan were successor to the Harrier and it might be less expensive to develop than The conventional variant was quickly created by simply and gas-driven lift fan systems for $60 million. However, in March constructed, like a hot rod, with components from existing engines. the shaft-driven system because it did not require modifying the removing the lift fan and vectoring nozzle from the SSF and 1993 the Skunk Works was awarded a $33 million contract to mature The first-stage fan and inlet guide vanes from the Pratt & Whitney cruise engine. substituting a fuel tank and a more conventional cruise nozzle. technologies for a shaft-driven lift fan and McDonnell Douglas YF119 engine were used for the lift fan. This fan ran at the same This reduced the empty weight of the aircraft by about 15%, while received a $28 million contract for a gas-driven lift fan. power level as one stage of the production lift fan, so that the loading DARPA Naval Study Contract Awards improving its range and reducing its cost. The weight of the fuel A year later, in March 1994, the U.S. Congress appropriated an on the drive gears was the same as in the production gearbox. The In the fall of 1989, DARPA arranged for all three contractors tank and one-half of a tank of fuel turned out to be about the same as additional $6 million to study designs based on a lift/cruise engine demonstrator engine was assembled by joining the fan and core to present their concepts to the Naval Air Systems Command the weight of the lift fan. As a result, both variants had the same concept, which was considered to have less risk because it had of the relatively-low-bypass-ratio P&W F100-PW-220 engine to the (NAVAIR). All three subsequently received follow-on contracts to midmission maneuver performance. The canard was to be used for been shown to be successful in the AV-8 Harrier. The Boeing turbine section from the higher-bypass-ratio F100-PW-229 engine. refine their designs and investigate the feasibility of using stealth in trim at other points in the mission, as the fuel was burned. These Company agreed to match that amount with its own funds and This bigger turbine could provide enough power to drive the lift fan the Naval environment. These studies were completed by the end of aircraft are shown in Fig. 14. received a DARPA contract to design a lift/cruise engine concept. as well as the engine fan. Two holes were cut in the engine case so 1990. After reviewing the results, the Marines expressed interest in At the end of the presentation on the conventional aircraft, Gen. The following year, the U.S. Congress appropriated an additional that the bypass air could be diverted to the pair of roll control jets, and fi conducting a technology maturation effort that would enable them to Muellner was briefed on the Marine STOVL variant and it was $10 million for the lift/cruise concept, which was again matched by the engine fan rotor was modi ed so that the driveshaft could be fl choose between the shaft-driven and gas-drivenvariants of the lift fan suggested that developing a Common Strike Fighter might be an Boeing. attached. Avariable-area thrust-de ecting nozzle was mounted at the system. This prompted the Skunk Works to apply for a patent on the affordable way for both services to get the aircraft they wanted. All three contractors were required to design both operational and rear of the engine, and the digital engine control software was fl fi shaft-driven lift fan and dual-cycle engine concept. The patent was Because the Navy, Marines, and Air Force had all own the F-4 demonstrator aircraft and to perform large-scale powered-model modi ed to run in both cruise and STOVL cycles. awarded three years later [9]. Phantom II, a joint program had previously been successful, despite demonstrations to reduce risk. These tests were intended to validate In December 1994, the assembled lift fan, gearbox, and driveshaft However, in December 1990, then Defense Secretary Cheney the F-111 experience. It seemed to me that if we built it, they would the propulsion concepts, to show that hot-gas ingestion would not be were demonstrated at the Allison facility in Indianapolis, Indiana. fi canceled the Marine’s V-22 program for the second time. The come, to paraphrase the movies. General Muellner requested follow- a problem, and to demonstrate that there was suf cient control power The power transmission losses in the gear set were measured, and fi Marines explained that they were a small service and could only on brie ngs by the Skunk Works to his staff at Langley Air Force for transition from hover to cruise. Large models were used due to operation of the lubrication and oil cooling system in the vertical support one aircraft development program at a time, and they had to Base. Then he met privately with DARPA and the Marines in the uncertainties about scaling the temperature and turbulence effects of position were demonstrated. The distortion limits of the fan were fi focus on the V-22. A few weeks later, in January 1991, Cheney Pentagon. DARPA then arranged brie ngs for Gen. McPeak, the Air the lift jets from small models. The Skunk Works created a new SSF established and the ability of the inlet guide vanes to modulate the fan terminated the troubled A-12 program for default, and the Secretary Force Chief of Staff; Adm. Dunleavy, the Assistant Chief of Naval baseline. This was nominally the same as the original SSF design, thrust was shown. The success of these demonstrations showed the fi fi fl of the Navy directed NAVAIR to begin work on the A/F-X, a new Operations for Air Warfare; and the Of ce of the Secretary of a delta canard con guration with a vertical lift fan and internal feasibility of building a ight-weight lift fan and gearbox for the stealth aircraft intended to replace the A-6. Most members of the Defense (OSD), who then advanced the idea to the service weapons bays. However, the aerodynamic performance estimates required power levels. Lockheed SSF design team were then reassigned to the A/F-X secretaries. In the summer of 1992, the NRAC endorsed the fea- were supported by data from the F-22 program [11]. The principal The lift fan was then shipped to the P&W facility in West Palm fi program. sibility of the SSF and recommended that the Navy work with the Air differences from the F-22 con guration were that the SSF design had Beach, Florida. During February 1995, it was connected to the Force to support the development of designs and technologies for a single engine and a canard. demonstrator engine and operated in both cruise and STOVL cycles, highly common Air Force and Marine MultiRole Strike Fighters. The addition of four new ground-attack missions from the MRF which demonstrated that an engine turbine could be switched from Common Affordable Lightweight Fighter With the support of OSD and the Pentagon, Congress appropriated program changed the design emphasis from a fighter with some strike providing jet thrust to providing shaft power to run the lift fan. The During 1991, DARPA and the Skunk Works continued to brief the $65 million for DARPA to begin a joint STOVL/CTOL Strike capability to a strike aircraft with some air-to-air defensive capability. ability to rapidly transfer thrust back and forth from the cruise engine Pentagon and the staffs of the U.S. Congressional budget committees Fighter program. DARPA released a Request for Proposal (RFP) to The development of stealth and long-range air-to-air missiles had to the lift fan to provide pitch control was also shown. changed the nature of air combat, and so the emphasis was on When these tests were complete, the propulsion system was achieving a first-look, first-kill capability and reducing the need to installed in a full-size airframe model made of fiberglass and steel. dogfight at close range. For these reasons, the two AIM 9 missiles This model is shown in Fig. 16. This model was mounted in the were removed and the aircraft was designed to carry two 2000 lb outdoor hover test facility at the NASA Ames Research Center. The bombs in the internal weapons bays, in addition to the two AIM 120 jet-induced downloads out of ground effect were measured to be less missiles. This increased the aircraft’s frontal area and wave drag. The than 3% of the jet thrust, and the jet fountain and lift improvement Air Force variant was derived, as before, by removing the lift fan and devices were shown to be successful in limiting the induced thrust-vectoring nozzles and substituting a fuel tank and conven- downloads to less than 7% at wheel height. These are very low tional cruise nozzle. These aircraft are shown in Fig. 15. numbers. No hot-gas ingestion was detected over a wide range of Although analysis and computer simulation had shown that it was pitch and roll angles while the aircraft model was suspended 1 ft off theoretically possible to extract enough energy from the exhaust of the ground. the F-119 engine to drive the lift fan, there were practical concerns The transition characteristics of the model were then measured in regarding the operation of such a dual-cycle propulsion system. In the NASA 80  120 ftwind tunnel. Drag polars obtained for a range particular, there were concerns about the thrust losses associated with of flap angles and tunnel speeds were used to show that the aircraft the large swirl angles induced in the engine exhaust flow when the could take off and land on a Wasp-class assault carrier with a 20 kt turbine operating point was changed. There were other questions wind over the deck without using a catapult or arresting gear and that Fig. 14 Conventional and STOVL Strike Fighter variants. about the ability of the engine controls to rapidly transfer thrust back it would have a wide corridor for transition from hover to wingborne programs and to develop technologies for a Joint Attack Fighter that changing the basic structural arrangement. For example, on the Air secretaries in February 1996, John White approved the plan to would replace the AV-8, F-16, and F-18 when they were retired Force and Marine variants, the bulkhead that takes the main landing develop a Joint Strike Fighter. A month later, before the tests of the 1 in fi beginning in 2010. This effort was called the Joint Advanced Strike gear load is made of aluminum and is approximately 2 : thick. The large-scale aircraft models were completed, the JAST program of ce Technology program. General Muellner was appointed the JAST same bulkhead on the Naval variant is made of titanium and is about released an RFP to industry for the design and flight test of the fi 3 in director in December 1993. The rst JAST concept-exploration 4 : thick. This technique was adapted from the F-16 production demonstrator aircraft. The proposals were submitted in June of that contracts were awarded in May 1994, more than a year into the line, in which cousin parts were used to create variants of the same year. The contractors were to propose their own demonstration test DARPA program. The JAST studies did not initially include a basic airframe for different customers who preferred different objectives. Lockheed Martin proposed three principle objectives: Marine STOVL variant, pending the results of the DARPA subsystems. first, to demonstrate that it is possible to build highly common demonstrations that were expected about October 1995 [16]. Because the shaft-driven lift fan propulsion concept was new and CTOL, STOVL, and Naval variants of a Joint Strike Fighter; second, However, in October 1994, the U.S. Congress directed that therefore considered the riskiest of the alternative propulsion to demonstrate STOVL performance and supersonic speed on the the DARPA program (and, specifically, the STOVL variant for the systems, it was decided to reduce the perceived risk of our aircraft same flight, as this had never been done before; and third, to Marines) be the focus of the JAST program. Thereafter, all the design by replacing the canard with a more conventional aft tail. This demonstrate the handling qualities and carrier suitability of the Naval contractors worked on developing Air Force, Navy, and STOVL was easily done, as one of the advantages of the lift fan concept was variant, because Lockheed Martin had never built a Naval fighter Marine variants of a single aircraft, although not all of the JAST the ability to rebalance the aircraft with relatively small changes in before. contractors had CALF contracts. Figure 17 is a timeline showing the size and location of the fan. The three JAST variants are shown in The Skunk Works proposal was to build two aircraft. One would the various programs that were integrated into the Joint Strike Fig. 18. be devoted to STOVL testing, because this had always been Fighter program. The dashed lines identify programs that never In May 1995, Lockheed Martin gave the Yak Aircraft Corporation perceived as the greatest challenge. The other would be first flown as actually awarded any study contracts to industry. A more complete a contract to provide an independent assessment of our STOVL the Air Force variant and then be modified by replacing the wing flaps Fig. 16 Large-scale wind-tunnel model. history covering the period up to 1994 is presented in [17]. propulsion system and airframe concept and also to provide lessons and slats to become the Naval variant. Both aircraft would be built The primary requirement for the Naval variant was the ability to learned from their STOVL aircraft development programs. They with the Naval structure. To reduce the cost of the demonstration, take off and land on a carrier in 300 ft or less with a 20 kt wind were provided with copies of everything regarding the competing available components were used for subsystems that were not critical flight. Measurements also showed that there was sufficient control over the deck. Lockheed Martin considered three alternative CALF concepts that had been published in the open literature, to the test objectives. For example, these aircraft used the nose gear power for acceleration and deceleration during transition and for approaches. The first alternative was for the Navy to operate the same including a copy of the U.S. patent [9] on the Lockheed dual-cycle from the F-15 and modified main landing gear from the A-6. The yaw control in crosswinds up to 20 kt. This technology maturation STOVL aircraft being developed for the Marines; this was certainly propulsion system. Drawing on their own experience developing increased weight of these off-the-shelf components was offset by not program [12,13] demonstrated the feasibility of the dual-cycle lift the easiest solution, but this aircraft would have less range/payload STOVL aircraft, Yak engineers provided us with predictions including mission avionics and weapons bays on the demonstrator fan propulsion system and reduced risk to Technology Readiness performance than a conventional Naval aircraft. of the STOVL performance, including ground effects, of all three aircraft. Level 5. The second alternative was to remove the lift fan and adapt the roll competing aircraft concepts. They also provided a risk assessment control jets to blow the wing flaps. This would increase the wing lift, of each concept. In addition, they provided useful design and per- reducing the aircraft takeoff and landing speeds and enabling it to use formance information for the lift systems of the Yak STOVL aircraft. Concept Demonstrator Contract Awards Joint Advanced Strike Technology Program the carrier catapult and arresting gear. However, the blown flaps on Their final report was very complimentary of our design and gave us In May of 1996, Undersecretary of Defense for Acquisition and In February 1993, at the same time that the first CALF contracts the F-4 Phantom had proved difficult to maintain and Lockheed confidence that it was the right concept. Technology Paul Kaminski changed the program to an acquisition were awarded, the U.S. Department of Defense began a Bottom Up Martin did not feel the Navy would favor this approach. Instead, it At the end of this phase of the program, all three contractors had category 1D program and renamed it the Joint Strike Fighter Review (BUR) of American military forces and modernization was decided to increase the wing area by enlarging the flaps and slats designed demonstrator and production aircraft. The Lockheed program, reflecting the greater scope and cost of the next phase of plans. One of the main objectives was the rationalization of the five and adding a wingtip extension. The increased lift of the larger wing Martin and McDonnell Douglas designs were very similar con- development and making it clear to U.S. Congress that JSF was an tactical aircraft development programs then going on: the Air Force also reduced the takeoff and landing speeds and enabled use of the ventional wing/body/tail configurations, whereas Boeing had a aircraft development program. In November 1996, Boeing and F-22 and MRF programs, the Navy F/A-18E/F and A/F-X programs, catapult and arresting gear. An additional benefit of the larger wing is tailless delta configuration. Lockheed Martin had demonstrated the Lockheed Martin were selected to build concept demonstrator and the DARPA CALF program. The Air Force and Navy made a that it gives the Naval variant greater range than either the Marine dual-cycle shaft-driven lift fan concept at large scale in hover and aircraft. The Marines did not select the McDonnell Douglas lift joint presentation to the BUR task force in which they suggested or Air Force variants, both by reducing the induced drag and by transition. Boeing had tested their large-scale lift/cruise model in engine concept based on concerns regarding the logistics of developing a highly common MultiRole Fighter based on the SSF, providing additional volume for fuel. hover only. After testing the gas-driven lift fan propulsion system, maintaining two different engines in the same aircraft and reports called the Joint Attack Fighter. The Naval variant was envisioned as a Because the carrier arresting system imposes greater loads on the McDonnell Douglas approached Lockheed Martin for permission to about the Russian experience with the Yak-38 and Yak-141. conventional carrier-based aircraft. However, Marine Col. Durham at landing gear and airframe than a conventional landing, the landing work with Pratt & Whitney on a shaft-driven lift fan system of their McDonnell Douglas subsequently merged into Boeing, and BAE OSD [14] and Air Force Lt. Gen. Croker at Air Combat Command gear of the Naval variant was redesigned for a 25 fps vertical velocity, own, but were turned down. They switched to a lift engine concept; Systems and Northrop Grumman, which had been teamed with [15] argued that the Naval variant should be the STOVL aircraft rather than 10 fps used for the conventional and STOVL variants. however, they did not perform a large-scale demonstration of this McDonnell Douglas, joined the Lockheed Martin team. being developed by DARPA. Similarly, the nose gear was redesigned for catapult launches. The system. At this point, Lockheed Martin had become the low-risk Because the planforms of both the Lockheed Martin and the The results of the bottom-up review were announced in additional airframe loads were handled through the use of cousin alternative. Boeing aircraft were relatively conventional, and the F-22 had September 1993. It was decided to cancel the MRF and A/F-X parts, which are stronger parts that replace conventional parts without demonstrated that unfaceted fighter airframes could have reduced signatures, the competition was between the STOVL propulsion Joint Strike Fighter Program fl 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 system concepts. Thrust being the product of mass ow and velocity, In September 1995, not long after he was sworn in as Deputy Lockheed Martin’s approach to achieving the necessary high thrust- Secretary of Defense, John White was briefed by his staff on the to-weight ratio was to use a large mass flow of air at low velocity, ASTOVL shortfall in tactical aviation that was forecast to begin about 2010 and whereas Boeing’s approach was to use a smaller mass flow of air at a on the JAST program created to address the problem. After the higher velocity. The mass flow of the lift fan system was approxi- briefing, he directed Undersecretary of Defense for Acquisition and mately 2.5 times greater than in Boeing’s lift/cruise system, and the Technology Paul Kaminski to create a plan for developing a new joint lift jet velocity was more than one-third lower. A-6 A-12 A/F-X aircraft from the JAST program. At a meeting with all the service The need to reduce fabrication costs of the demonstrator aircraft and the success of the STOVL wind-tunnel tests at NASA Ames enabled Lockheed Martin to change its commonality demonstration. It was decided to devote one aircraft to the demonstration of carrier fi fl JAF JAST handling qualities, and the other aircraft would rst be own as the Air Force variant and then be converted to the STOVL variant by removing the fuel tank and installing the lift fan. The X-35A conventional variant was the first to fly. Its first flight was on 24 October 2000 from the Lockheed plant in Palmdale, California, to F-16 MRF Edwards Air Force Base, a distance of just over 20 miles. It averaged a flight a day for the next 30 days, demonstrating fighterlike maneu- ver performance and supersonic speed. It met or exceeded all of its flight-test objectives. AV-8 F/A-18 SSFSSF1 SSF2 CALF JAST Joint Strike Fighter The test program achieved such high productivity because the airplane had been approved for air-to-air refueling on the basis of qualification in a flight simulator. This was another first, because Fig. 17 Timelines of the programs that were integrated into the JSF program (JAF denotes the Joint Attack Fighter). Fig. 18 Commonality of the three JAST aircraft variants. some new aircraft have taken more than a year of flight tests to be approved for aerial refueling. Boeing was not able to use air-to-air and the Pentagon credited the performance of the lift fan propulsion replace the Air Force F-16s, Navy and Marine F/A-18s, and Marine References fl refueling during its ight-test program. In a very unusual step at this system with the win, and the Lockheed Martin JSF team was AV-8s. All of the JSF variants have essentially the same airframe, [1] Tyler, T., Chambers, C., and Perkins, R., “An Assessment of the Sea early stage in an aircraft development program, this aircraft was subsequently awarded the 2001 Collier Trophy for the development engine, avionics, and subsystems. By spreading the development and Based Air Master Study,” AIAA Aircraft Systems Meeting, AIAA flown by American and British military test pilots in addition to the and demonstration of the lift fan propulsion system. support costs of these components over a larger number of aircraft, Paper 80-1820, Anaheim, CA, Aug. 1980. Lockheed Martin and BAE Systems test pilots. The developmental aircraft have a strong resemblance to the each variant becomes more affordable. In addition, the Air Force and [2] Ward, B., and Penrose, C., “The Hybrid Fan Vectored Thrust Engine,” During December and January, the conventional X-35A was demonstrator aircraft. The planform of the airframe is the same, and Naval variants will provide greater stealth and range than the aircraft Society of Automotive Engineers, Paper 831496, Oct. 1983. converted into the STOVL X-35B by installing the lift fan and the layout of the engine, lift fan, and nozzles is also retained. they will replace, and the Marine variant will combine the supersonic [3] Driggers, H., Powers, S., and Roush, R., “Study of Aerodynamic thrust-vectoring nozzle. During the spring of 2001, the aircraft was However, the prototypes incorporate mission equipment, including performance of the F/A-18C with the short takeoff and vertical Technology for Single Engine VSTOL Fighter/Attack Aircraft,” NASA tethered to a deflector grid that diverted the lift jets to minimize weapons bays, mission avionics, and low-observable coatings. Off- landing performance of the AV-8B. CR 166271, Feb. 1982. [4] Heiser, W. H., “Thrust Augmentation,” Journal of Engineering for ground effects. The operation of the propulsion system (engine, the-shelf subsystems used in the demonstrators have been replaced The technical challenges involved in designing a single aircraft for Power, Vol. 89, No. 1, Jan. 1967, pp. 75–82. lift fan, nozzles, and reaction control system) was checked and with new designs to reduce weight. Similarly, the ram air cooling all three services were met by designing three highly common, but [5] Bevilaqua, P., “Joint Strike Fighter Dual Cycle Propulsion System,” measured. On 23 June 2001, the aircraft was untethered and BAE systems used on the demonstrator aircraft were replaced by liquid not identical, variants of the same aircraft. The STOVL variant, Journal of Propulsion and Power, Vol. 21, No. 5, Sept. 2005, pp. 778– Systems test pilot Simon Hargreaves advanced the throttles to take cooling systems, such as those on the F-22. The wing span of the which was designed first, incorporates a shaft-driven lift fan in a bay 783. weight off the wheels to check the response of the control system in F-35A/B was increased slightly to improve maneuver and range between the inlet ducts and a thrust-vectoring cruise nozzle. The doi:10.2514/1.15228 this case. The airplane rose straight up to a height of 20 ft, under performance. The rudder and horizontal tails were also enlarged to airframe was designed to Air Force specifications, so that the [6] Davidson, I. M., “The Jet Flap,” Journal of the Royal Aeronautical complete control, before Hargreaves settled it back to the grid. This increase control power. The weapons bay doors on the STOVL conventional takeoff and landing variant was developed by removing Society, Vol. 60, Jan. 1956, pp. 25–50. flight is shown in Fig. 19. variant were designed to open during vertical landing to capture the the lift fan and vectoring nozzles from the STOVL variant and [7] Willoughby, W., “Best Practices,” U.S. Government Printing fi Over the next month, the aircraft made 38 flights from the runway fountain created by the lift jets and counter suckdown in ground substituting a fuel tank and a conventional cruise nozzle. The Naval Of ce, Washington D.C., Dept. of the Navy, NAVSO P-6071, fi March 1986. at Edwards Air Force Base during which the STOVL and transition effect. Because this bene t had been demonstrated on the large-scale variant was similarly developed from the conventional variant by [8] Burns, B., “Advanced Aerodynamic Design for Future Combat performance were validated. Then, on 20 July 2001, the X-35B, model at NASA Ames, weapons bay doors had not been included on increasing the wing area, designing stronger landing gear, and using Aircraft,” Proceedings of the International Council of the Aeronautical flown by Marine Maj. Art Tomassetti, became the first aircraft in the demonstrator aircraft. The lift fan inlet and nozzle were also stronger cousin parts to handle the larger airframe loads associated Sciences, 1982, pp. 23–33. history to make a short takeoff, fly supersonically, hover, and land changed. with carrier takeoffs and landings. Both the STOVL and Naval [9] Bevilaqua, P.,and Shumpert, P., “Propulsion System for a Short Takeoff vertically. Boeing’s X-32 aircraft were not able to demonstrate this The cockpit of the prototype aircraft is considerably more variants are about 15% heavier than the conventional variant. and Landing Aircraft,” Lockheed Corp., Calabasas, CA, U.S. Patent Mission X. Lockheed Martin pilot Tom Morgenfeld ferried the advanced than in the demonstrators. The controllers in the X-35B The program challenges were met by having a credible technical No. 5,209,428, 11 May 1993. aircraft back to Palmdale on the aircraft’s final flight on were similar to those in the Harrier, with a control stick, throttle, and a solution and by creating a Joint Program Office, staffed by members [10] Rich, B., “The Skunk Works’ Management Style: It’s No Secret,” 1988 Wright Brothers Lecture, AIAA Paper 1988-4516, 1988. 6 August 2001. The aircraft was refueled six times in the air and separate nozzle lever. In the F-35B, nozzle vectoring is controlled of all three services. The positions of Acquisition Executive and “ ” the flight lasted 3.5 h, ending with six touch-and-go landings. automatically by the stick commands. There is also a voice command Program Manager were rotated between the services. This program [11] Mullin, S., The Evolution of the F-22 Advanced Tactical Fighter, 1992 Wright Brothers Lecture, AIAA Paper 1992-4188, 1992. The second aircraft, configured as the X-35C Naval variant, made system for noncritical functions, such as controlling radio frequency. office developed a joint operational requirements document, which fi fl [12] Bevilaqua, P., Eshleman, J., Falabella, D., Post, M., Quayle, B., and its rst ight on 16 December 2001. Lockheed Martin pilot Joe The cockpit instrumentation in the X-35 included a head-up display freed the airframe contractors from the need to satisfy multiple Tait, P., “ASTOVL Aircraft Technology Demonstration Program,” Sweeney ferried it to Edwards Air Force Base. During 33 h of flight- (HUD) and two small color displays from a C-130 on the instrument customers or mediate between them. Lockheed Martin Skunk Works, Rept. TR 96-1, Palmdale, CA, testing at Edwards Air Force Base, it successfully demonstrated the panel. The cockpit of the F-35 includes a virtual HUD projected onto The three F-35 variants will initially replace at least 13 types May 1996. use of a side-stick controller in simulated carrier approaches. In the visor of the pilot’s helmet and a single large instrument display of aircraft for 11 nations, making the Lightning II the most cost- [13] Eshleman, J., “Large Scale Testing of the Lockheed Martin JSF February 2001, the X-35C was flown from Edwards Air Force Base panel that the pilot can divide into several different screens. effective fighter program in history. Lockheed Martin is developing Configuration,” Proceedings of the International Powered Lift in California to the Patuxent River Naval Air Station in Maryland, The 24,000 lb weight limit and Weight as an Independent Variable the F-35 with its principal industrial partners, Northrop Grumman Conference, SAE P-306, Society of Automotive Engineers, becoming the first X-Plane in history to make a coast to coast flight were not used for the design of the production aircraft. As a result, the and BAE Systems. Two interchangeable engines are under devel- Warrendale, PA, 1996, pp. 319–340. “ across the United States. Another 33 h of flight-testing were desire to improve performance and to reduce manufacturing costs opment: the Pratt & Whitney F135, which powered the first aircraft, [14] Morrocco, J., Joint Attack Fighter Eyed to Replace A/F-X, MRF, and F-22,” Aviation Week and Space Technology, May 1993, pp. 21–22. completed at Patuxent River. The X-35C also achieved supersonic began to add weight to the airframe. For example, a gun was added and the GE Rolls-Royce F136. “ ” fi 9 fi [15] Croker, S., ASTOVL Technology Still Possible for JAF, Defense speeds and accomplished more than 250 eld carrier landing practice and the maneuver limit was raised from 7.5 to g; the wing structure The success of the Wright Brothers in building the rst practical Daily, 5 May 1993. demonstrations. These showed the carrier suitability of the Naval was redesigned to include pylons for external weapons, and the airplane was due to their approach to solving the problems of [16] Walsh, E., “The JAST Program and Beyond,” SEAPOWER Magazine, variant. number of wing attach points was reduced to simplify assembly; the manned flight. The key elements of their approach were team- June 1994, pp. 27–30. Flight-testing of the three X-35 variants reduced the risk of the JSF airframe structure was redesigned to accommodate subsystems and work, constructive debate, innovative thinking, systematic testing, [17] Aronstein, D., and Hirschberg, M., “An Overview of the US/UK airframe and propulsion systems to Technology Readiness Level 6. facilitate access, etc. By January 2004, weight had increased by more and a skeptical study of the relevant literature. Lockheed Martin’s ASTOVL Programs, 1983–1994,” International Powered Lift Confer- The X-35A/B is in the permanent collection of the Smithsonian than 3000 lb. To get the weight back out, a design stand-down was success in developing the Joint Strike Fighter is further evidence of ence, AHS International, Paper IPLC-2000-00039, 2000. Institution and was placed on display at the Udvar-Hazy Center. The declared on 7 April 2004 and the entire team shifted their emphasis the value of the Wright Brothers’ approach. X-35C is on display at the Naval Air Museum in Patuxent River. to weight reduction. Lockheed Martin offered a $100 bonus to employees for every weight-saving idea and awarded $500 for every fi F-35 Lightning II Program pound removed. More than 2000 ideas were submitted on the rst day and more than 2700 lb were removed from the airframe by the In November 2000, the JSF Program Office requested proposals end of the year. Lockheed Martin awarded more than $1.35 million to from both teams for the manufacture and test of 22 developmental its employees for their weight-saving ideas. aircraft: 8 ground-test aircraft and 14 flight-test aircraft. The pro- On 19 February 2006, the first Air Force F-35Awas rolled out at the posals were submitted in February 2001, six months before the end Lockheed Martin plant in Fort Worth, Texas. After a series of ground of flight-testing. On 26 October 2001, the JSF Program Office vibration tests, it was unveiled in a public ceremony on 7 July 2006, announced that Lockheed Martin had won the competition. Boeing when the Air Force announced that it would be called the Lightning II. The first flight of the F-35A was on 15 December 2006. The first STOVL F-35B was unveiled a year later, on 18 December 2007, and made its first flight on 11 June 2008. It will be flown using conventional takeoffs and landings through the end of 2008 and is scheduled to make short takeoffs, then hover, and finally make vertical landings during 2009. In addition to the two aircraft currently in flight test, one is in ground test, five more flight-test aircraft are in final assembly, and another 14 are in various stages of completion on the production line. The first aircraft are expected to become operational with the U.S. Marine Corps in 2012, with the U.S. Air Force in 2013, with the British Royal Navy and Air Force in 2014, and with the U.S. Navy in 2015.

Conclusions The Joint Strike Fighter will achieve significant savings in aircraft Fig. 19 First hover flight of the X-35B. production and life cycle costs by providing a common aircraft to against three different initial flight WASP Display System ASID EMD Cockpit conditions, nominally with two - 360 Deg FOV Display JSF Live Fire Test— - Configure to EMD Geometry - Virtual HUD for Engineering Development iterations each— - Active Flight Controls ³ - Helmet Mounted Display 20Kft, M0.8, straight and level flight Pilot-in the Loop Simulator Testing - Touch Screen Displays Simplified Cockpit Station ³ 30° dive from 18,000 ft, M0.7 with - Voice Recognition & Sound System - MIC Station with Side Control Panels 4-G pull-up to 15° (minimum altitude - Active Flight Controls - Helmet Mounted Display of about 2,000 ft @ M0.92) by JeffreyJeffrey AndruAndruss - Touch Screen Displays ³ 20Kft, M0.8, 4-G wind-up turn - Voice Recognition & Sound System Fixed Base System Each of the 31 common cases was judged on the basis of the following criteria— Simulation Host Computer ³ The F-35 Joint Strike Fighter (JSF) program has taken a new approach to achieving a clearer - Common Processors with Team How well did the predictions for the understanding of the system’s vulnerability while improving the efficiency of Live Fire Test and - Common Operating Systems with Team results of the case match the data from the simulation event? Evaluation (LFT&E) program. In a pioneering effort, the JSF LFT&E program implemented Man-in- VS Federated Models ³ - Engine & FADEC Was the pilot given ample warning, the-Loop (MTL) simulator testing during the test series designated XG-SV-LF-07C (LF-07C.) The - Air Data System and was he/she able to safely eject from - Electro-Hydraulic Actuators the aircraft? - Utilities & Subsystems VS Hotbench ³ purpose of this test series was to examine the response of the pilot and F-35 aircraft to a series of VISF Handware Systems - Entity Simulations Was the aircraft controllable such that - Engineering Test Stand - Electrical Power System - Many others VS Triplex Computer the pilot could return to the forward failures that represent possible damage modes associated with encounters with ballistic threats. - Flight Control Actuators (EHAs) w/Dynamic Loads - RIO Engineering Test Stand line of threat (FLOT)? In other words, - Engine Hotbench w/Engine FADEC/RIO Units and the Electrical Power System (EPS) are During LF-07C testing, the VIF and VSIF - Hydraulic Power Generation System w/loads could the pilot control the aircraft closely coupled within the Integrated represented a real-time, airplane-level - STOVL Door and Effector Actuators enough to change direction and Power Package (IPP); the electrically response to specific ballistically - Weapon Bay Door Actuators maintain altitude? ³ driven Flight Control System (FCS) is representative interrogations. These two Figure 1 VSIF Functional Diagram Was it possible to conduct evasive interdependent with the EPS. These are facilities provide integration of real maneuvering if required for survival? some of the critical subsystems that were aircraft hardware and aircraft OFPs with center of the room consisting of the Wherever possible, previously conducted ³ Was it possible to land the aircraft? In tested during the LF-07C test series. a simulated atmospheric environment. In Vehicle Management Computer (VMC) failure mode and effect testing (FMET) most cases, the actual landing was not an earlier LFT series, the LM Engineering Test Station (VETS) and the was used to supply answers to some of attempted (due to time and expense), A key ingredient in understanding an vulnerability team conducted a wind Remote Input/Output (RIO) Engineering the failure scenarios in the test matrix. but a judgment call was made based aircraft’s vulnerability is the pilot or tunnel test to evaluate F35 aerodynamic Test Station (RETS). In the center of this Where no FMET series existed to address on the nature of the failure and the operator’s situational awareness; performance with partially or totally cluster is the VETS/RETS switch rack the question, a special live fire test case handling qualities. included in this is the pilot’s ability to missing control surfaces. An aerodynamic that allows the VSIF to switch from was developed. determine if the aircraft has been hit and model was developed to represent these three standalone facilities to a single The STOVL-unique cases were similar, what systems may have been conditions and was incorporated in the integrated facility. A total of 40 different test cases were but refined slightly to adequately capture compromised. In the few moments after LF-07C test series. Both facilities had the conducted as part of LF-07C, 31 that various landing procedures associated The LF-07C testing was conducted under damage, the pilot needs to determine if: unique ability to represent these control The VIF system is similar to VSIF, but were common to all variants of the with a STOVL aircraft— the direction of the Lockheed Martin 1) the aircraft is controllable, 2) it will surface damage scenarios in the with less hardware in the loop; much of F-35, and nine that were unique to the ³ How well did the predictions for the (LM) vulnerability group at the Vehicle stay that way, 3) he/she can get home, 4) aerodynamic model; if a portion of the the actual hardware in the VSIF is short take-off vertical landing (STOVL) results of the case match the data from Systems Processing/Flight Control System he/she can complete the mission. A surface were to be blown off from emulated in the VIF. The VIF is divided variant. Each test case was evaluated the simulation event? Integration Facility (VIF) and the Vehicle fundamental question that previous JSF ballistic impact, the aero model into three system areas that consist of a Systems Integration Facility (VSIF) at in testing has not answered is whether the represented that. motion base (MB) simulator, a fixed base Ft. Worth, TX. pilot has sufficient information to make (FB) simulator, and a VETS area. The this assessment and information warning The VSIF, having the most hardware in remaining F-135 engine hotbench room, Testing the Integrated Design of of impending catastrophic failures. the loop, is the most difficult to operate, F-136 engine hotbench room, and display/ F-35: LF-07C Test Motivation and so was used only when necessary. conference room are ancillary to these The JSF LFT&E program is committed Flight Simulator Test Capabilities Consequently, most of the test cases three areas. Only the FB VIF was used to verifying performance of the critical The F-35 program developed several were conducted in the VIF; the VSIF during the LF-07C tests. systems on the aircraft, evaluating the MTL simulation facilities for a variety of was necessary in nine of the cases where built-in redundancies, and understanding purposes, including pilot training, real hardware was required, such as Test Methodology the interdependencies of integrated operational flight program (OFP) electrohydraulic actuators (EHA), Before testing began, the F-35 Live Fire subsystems from a ballistic impact development, subsystem integration, electronic units (EU), and converter/ Team developed a list of test cases to be standpoint. Many questions arise missions systems integration, concept of regulators (C/R.) addressed. The criteria for defining concerning the reaction of complicated operations (CONOPS) development, and required testing was based on issues systems when impacted by ballistic combat evaluations. LF-07C made use of Figure 1 shows a functional diagram of identified in the JSF Live Fire Test and penetrators, and the F-35 is no exception. two of these facilities: the VIF and the the VSIF facility. The facility is equipped Evaluation Master Plan. The matrix of The only feasible method (short of VSIF. Both of these facilities utilize MTL, with dynamic electro-hydraulic actuator test cases was subsequently used to shooting a flying aircraft) is to use varying amounts of aircraft hardware, (EHA) load fixtures, drive stand, EPS, manage and address each question; each Spring 2010 Spring advanced flight simulation that either real aircraft OFPs, and software that and cockpit rooms. Each room’s level of • test case within the test matrix had a includes or models subsystem simulates the flight environment. The integration is controlled by the VSIF different objective with the overall performance with a pilot at the controls. result is a very realistic flight simulation control room. The control room is objective of LF-07C to gather sufficient that closely approximates a flying F-35. separated into three areas for each data to determine the response of Many F-35 critical subsystems are highly integrated product team (IPT)—FCS, production F-35 to failures that integrated. For example, the Power and EPS, and HUA. The “core” of the simulate combat damage to specific

Thermal Management System (PTMS) control room layout is a cluster in the Survivability Aircraft components on the aircraft. Figure 2 Engineering Page Display the basis for the flight test “scripts” that STOVL results from LF-07C refined the the three F-35 variants. Simulator testing will be run during and after actual test matrix for the upcoming STOVL examines the response of the pilot and ballistic impact on the FUSL test article. Propulsion System Live Fire Testing F-35 aircraft to failures that represent Real control surface load data, as (LF-19C.) STOVL simulations showed the possible damage modes associated with recorded in the simulator, has dictated the STOVL propulsion system to be tolerant encounters with ballistic threats. This appropriate structural loading for the of minor changes in roll post thrust due to objective would not be achievable in a control surface structural shot in LF-19D. damage on one side of the aircraft. This purely ballistic test environment, LF-19D will close the loop in regard to eliminated the need to conduct testing that whether it be from cost limitations, matching ballistic damage to the would yield similar results. limited test article lifespan (a full-up test scenarios conducted in LF-07C. If the article can only be “full-up” for so many aircraft subsystems respond differently The use of MTL simulator testing at this stage shots), or pilot exposure to ballistic from ballistic impact, these damage in the test program has proved valuable as the threats. The LF-07C testing also identified scenarios will be retested in the simulator JSF Live Fire Team starts a complex and the best candidate test points on very to provide the complete aircraft response. thorough test program for expensive and complex test articles. Q

Figure 3 ICAW Page on MFD ³ Was the pilot given timely warning, Selected parameters from the BUS traffic and was he/she able to safely eject from recorded in the DREC files included the the aircraft? FCS surface positions (commanded and ³ Was a conventional landing possible? actual), flight parameters found on the ³ Was a rolling vertical (short) landing HUD, and some specific parameters for possible? each test case. ³ Was a vertical landing possible? Test Conclusions Conventional, rolling vertical, and The team conducted and documented vertical landings were not attempted in the results of the 40 test cases, a total of every test case. A landing attempt 213 simulation runs during LF-07C. In depended on the handling qualities addition, 20 FMET cases were reported by the pilot and the nature of the documented in order to answer specific failures inserted. Where there was a issues. Another 49 test cases were question on the capability, a landing was examined and addressed based on attempted. The pilot used the Cooper- similarity with other tests. Archived Harper Handling Qualities Rating Scale data include video records and digital to quantify aircraft handling during and data files of each event. after each test case. Test results were compared to Recorded Data predictions. From the 40 test cases, Each event in the VIF and VSIF was predictions matched 26 times, or 65% documented by visual records as well as of the time. There were 11 cases digital data recorded as a function of (27.5%) where pre-test predictions were time. The visual records consisted of more conservative than the tests. In videos of the pilot “Heads-Up” display other words, where an undesirable event (HUD), the Left Multi-Function Display was predicted, the aircraft turned out to (MFD), and the Engineering Page, a be more capable than predicted. In screen set up to capture pertinent three cases (7.5%), pre-test predictions information in a graphical format. Audio assumed more capability than the data from each event was combined with aircraft had. All cases where the the Engineering Page and was found to be predictions did not match the test will very helpful in recalling critical be further analyzed for potential information concerning the event. changes to future vulnerability analyses.

Integrated Cautions, Advisories, Results from LF-07C testing were used in Warnings (ICAW) as seen on the MFD refining the test matrix for the upcoming were recorded. In addition to the visual LF-19D full-up system level (FUSL) records, digital data was recorded from testing on an F-35 aircraft. FCS data the VIF and VSIF. This “DREC” file recorded during simulator testing, in contains hundreds of parameters, particular during pilot’s reaction to recorded many times per second. simulated threat engagements, provides The STOVL propulsion system test installed with ramps to duct the live fire components easily accessible by ballistic OCCUPANTF135 PROPULSION SYSTEM required additional consideration of the air and hot engine exhaust away from the threats. A key part of this test series was mission scenario to define the test test article. This configuration allowed to evaluate the control system’s ability to conditions. The majority of an F-35B full engine operability in the conventional accommodate damage and to provide a CASUALTYLIVE FIRE TEST M&S (LFT) mission is spent in the up-and-away, and vertical thrust modes. loss of capability indications to the pilot. wing-born mode (i.e., flying as a conven- Testing also addressed the potential of Test Platform STOVL Propulsion tional aircraft where the wings are System Test Stand fire initiation on fuel system components. / SPRING http://jaspo.csd.disa.mil by Charles Frankenberger AS Journal 14 providing the lift); therefore, it follows One vulnerability reduction technology As part of the F-35 LFT Program, the LFT team recently conducted a series of LFTs to assess the Pratt that most of the flying and fighting will assessed during this test series was a & Whitney F135 propulsion system against ballistic damage. The F-35 LFT Master Plan includes a be done in the up-and-away mode. In this fueldraulic fuse that was installed on the mode, the lift fan is static, the lift fan Test Platform Partially Removed to Expose Tunnel and Defectors series of propulsion system tests designed to better understand the capabilities and vulnerabilities of clutch is disengaged, 3BSM is horizontal, Tunnel Under Test Platform the F135, once damaged, and to address assumptions used in the F-35 vulnerability assessment. Three and the roll post nozzles are closed. F135 test series were conducted: Ballistic testing was conducted with the Figure 2 STOVL Test Layout, WSL LFT Site propulsion system in the up-and-away f Short Takeoff and Vertical Landing (STOVL) Propulsion System Test—Designed to address the mode. Then, with the system in a

unique aspects of the F135 propulsion system, specifically related to the STOVL capability. damaged state, the propulsion system Figure 4 F135 Ballistic Test Setup f Engine Ballistic Test—Aimed at better understanding the advance engine control system and the was commanded to transition from wing- capabilities of the main engine with gas path damage. born to jet-born propulsion mode. If the convergent nozzle fueldraulic system. transition was successful, a vertical f Fuel Ingestion Test—Conducted to assess the engine’s fuel ingestion tolerance. landing script was run to assess the The test article used in this test series STOVL propulsion system capability was a STOVL ground test engine. The All testing was conducted at the Weapon CTOL/CV $POWFOUJPOBM
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The test article used in this series was an early production flight test engine. The test was conducted with a representative F-35 inlet. Overall, 41 steady flow events Figure 5 F135 Ballistic Test Events (Fuel System Component and Fan Case) and 32 quick dump events were con- ducted at various conditions. The engine was taken to its limit, resulting in hot streaks during steady flow testing and engine stalls during quick dump events (see Figure 8). The engine showed a high tolerance to ingested fuel.

CONCLUSIONS Figure 6 Fuel Leak and Fire with Self-Extinguish Overall, the test results were favorable and in many cases the propulsion system performed better than predicted. Damage to blades and vanes in both the lift fan and main engine did not result in the catastrophic corn-cobbing often seen when gas path components are dam- Figure 7 Fuel Ingestion Test Setup Figure 8 Quick Dump Ingestion, Stall Event aged. The control system is very capable One vulnerability reduction technology '
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