JSF PROGRAM // X-32 & X-35 Back in 2000, the most THE valuable fi ghter fl y-offs in modern times took place between the Boeing X-32 and Lockheed X-35, as Gerard MATCH Keijsper explains

T DOES NOT seem so long ago, The fi nancial situation was tight successful supersonic but it is almost 30 years since because of the peace dividend – the STOVL fi ghter. For this reason, the Defense Advanced Research Berlin Wall had fallen just a couple of DARPA assigned an experimental ’X’ Projects Agency (DARPA) issued years earlier – and no one wanted to designation for the prototype: the X-32. a Request for Proposals for the put budgetary pressure on the F-22 General Dynamics had been testing a Advanced Short Take Off and program. DARPA pointed out that the full-scale E-7 ejector propulsion model in IVertical Landing (ASTOVL) aircraft USMC had a similar problem, but there NASA’s 80ft x 120ft wind tunnel but, technology demonstration program in could be co-operation if the extra fuel in the end, it off ered a fan-in-wing 1992. The aim was to build a supersonic tank was replaced by a lift fan to off er the propulsion concept similar to the Ryan STOVL fi ghter with an empty weight of supersonic STOVL capability desired by XV-5A. McDonnell Douglas went in with 24,000lb, similar to the F/A-18C Hornet, the service. The USAF showed interest, an off er for both a SDLF (shaft driven to keep the weight and cost in check. asking DARPA to provide the technology lift fan) and GDLF (gas driven lift fan) by the end of the 1990s. propulsion system. Northrop off ered a Financial constraints similar propulsion solution as the Russian The USMC was having trouble getting STOVL testing Yakovlev Yak-38: the lift + lift-cruise Above: The Boeing political support for the Bell Boeing V-22 Many agencies had tried to develop a system. Boeing off ered the Harrier X-32A performed Osprey, with then-US Vice President Dick supersonic STOVL fi ghter and the history propulsion system called ’direct lift’. up-and-away fl ight Cheney one of its fi ercest critics, and books are full of examples of experimental The Lockheed and McDonnell Douglas demonstrations for the US Navy, as well they could not aff ord to fund another failures, including the Hawker Siddeley designs could remove the lift fan for the as fi eld carrier high-profi le program on their own. USAF P.1154; Dassault Balzac V; EWR VJ-101 (tilt conventional fi ghter and Northrop could practice landings Brig Gen George Muellner, then Deputy engines and lift engines); Rockwell do the same with the lift engine. Boeing Boeing Chief of Staff for Requirements at Air International XFV-12A and British didn’t have this advantage, so selected Combat Command, Langley Air Force Aerospace (BAe) P.1216, but problems a delta wing as these could hold a larger Right: X-35A mostly Base (AFB), received a presentation from such as hot gas ingestion, suck-down fuel volume. A requirement set by DARPA did up-and-away fl ights for the Lockheed/DARPA about a replacement forces, and the fountain forces made was that a full-scale model had to be USAF requirement. for the F-16, but with more range. them unsuccessful. Hot gas ingestion, developed to prove the design could Lockheed managed a This was the kind of range the USAF the transition from conventional fl ight to carry its weight in the STOVL fl ight very aggressive fl ight schedule, putting craved, even if it didn’t have the funding hover and managing to lift its own weight regime to prevent a repeat of the Boeing under pressure to mount another project, owing to the at the same time had proven to be the Rockwell International XVF-12A, which Lockheed Martin cost of the F-22A Raptor. three biggest problems in developing a could not lift its own weight.

90 August 2021 // www.Key.Aero Right: A 1996 wind with a full-scale model prepared for testing. tunnel model of the X-35. Note the McDonnell Douglas had selected the Skunk Works and General Electric YF120 for their GDLF. X-32 stickers above the model. The inlet design was that of The dream team the F-22 NASA Lewis McDonnell Douglas and BAe teamed Research Center up with Northrop Grumman to form the dream team, as all western operational STOVL and recent naval STOVL experience was now under one roof. The McDonnell Douglas team abandoned the GDLF concept, much to the chagrin of NASA, and adopted the lift + lift cruise propulsion system that Northrop had pursued. The dream team did not bother with a full-scale model for In March 1993, Lockheed’s SDLF point coming when General Dynamics the lift + lift system as they reasoned they propulsion system and McDonnell Fort Worth missed out on a possible F-16 were developing a proven propulsion Douglas’ GDLF system were selected, replacement and was subsequently sold capable of lifting its own weight. At with General Dynamics, Boeing and to Lockheed. the same time as changing propulsion Northrop losing out. Boeing rallied their Two years before the Advanced systems, it changed its propulsion US Senate representatives, arguing Tactical Aircraft (ATA) program, General provider, as they now wanted to off er a that the sole proven concept had been Dynamics’ A-12 Avenger II project had lower risk and a proven engine: the Pratt & rejected. Through the Senate, Boeing got been cancelled. The F-16 production was Whitney F119. Both Boeing and Lockheed an agreement with DARPA to continue expected to come to a halt soon as the had selected the F119 engine from the but they had to fund 50% of the costs USAF had opted to stop buying them. outset and, as such, there was never a themselves; Northrop continued with With General Dynamics losing out on this competitive engine off ered for the JSF. its own money. DARPA contract, it was expected that Fort Lockheed was already in talks with Worth could soon be out of business. Joint Strike Fighter General Dynamics over acquiring the McDonnell Douglas was co-operating The DARPA program was eventually Fort Worth division, with the tipping with BAe in an exclusive deal on the GDLF, absorbed by the Joint Advanced Strike

www.Key.Aero // August 2021 91 JSF PROGRAM // X-32 & X-35

replace the entire US Inventory of fighter Left above: X-32A aircraft. Indeed, no new fighter designs seen here with a gaping hole through were expected for another 30 years. the middle allowing McDonnell Douglas had once been the imagination a major company on the civil side of to work out the size of the engine aviation, but that had been diminished by in proportion of competition from Boeing and Airbus. So, the aircraft. The soon after, McDonnell Douglas was sold demonstrator aircraft would have a to Boeing. BAe and Northrop Grumman minimum of mission decided to join Lockheed. systems Boeing The prototypes (or concept demonstration aircraft, as they were officially called) had to prove that the design would be stealthy, could replace the Harrier by landing vertically, have a short take-off run, and be launched Left below: The large- from a conventional aircraft carrier while scale powered model seen here right after having a fighter performance equal to roll-out at the Skunk the F-16. In short, they should fulfil all Works in Palmdale. Its the requirements of the three US armed designation was the X-32 and it was used services and many exports were expected at the NASA Ames to follow automatically. Outdoor Aerodynamic Research Facility’s wind tunnel. This The Lockheed effort model can now be The task at hand was not an easy one found at a scrapyard but Lockheed’s Advanced Development at Fort Worth right beside the HALO Company – better known as Skunk model of the F-35 Works – was on the job with innovative Lockheed Martin proposals. For the lift concept, new SDLF technology from Skunk Works was met with some optimism as it seemed to tackle most of the issues. The SDLF design was refined by engineer Paul Bevilaqua’s team and patented in 1993. To reduce cost, the Technology (JAST) program, which meant Strike Fighter (JSF) program. initial design was borrowed from the F-22. the two downselected contractors would Both Boeing and Lockheed Martin were get the chance to build a demonstrator downselected on November 16, 1996, The Boeing effort aircraft. As a result, the X-35 designation and received a contract to build and test Boeing was not accepted by DARPA as was added. The fact that the demonstrators two prototypes on what was to become it was not offering “new technology”. Its were to be built under JAST meant the JSF. McDonnell Douglas supplied propulsion unit was a modern version of they would become more than simply the fighter powerhouse, the legacy of the Harrier system. Boeing reasoned that technology development – they would be the F-4 Phantom II, F-15 Eagle and the their offer was less risky as it had already an actual aircraft demonstration. To reflect F-18 Hornet, but had no other work in proven itself and, as such, only had to this, the name was changed to the Joint the future as the F-22 and the JSF would solve the hot gas ingestion problem. The

Left: The X-35B with the hat-trick tail was a sign of the confidence the Skunk Works team had in their aircraft. It would be the first to perform a short take-off, transfer to conventional flight, go supersonic in level flight and convert to a hover and perform a vertical landing Lockheed Martin

92 August 2021 // www.Key.Aero Above: Boeing propulsion system selected required that air that prevented hot air from going diverted to the vectoring nozzles at the proved its weapon the engine was located in the centre of forward – located between the lift nozzles centre of the aircraft. Once the fl ow switch bays on the X-32A because the company the aircraft, close to the centre of gravity, and the two pitch nozzles in the nose had been operated, the X-32B was in wanted to repeat making the design stubby looking. provided additional pitch control when STOVL mode, after which deceleration was the success of the A British company called Fairey the jet screen was moved. The system was controlled by vectoring the nozzles. Lockheed/General Dynamics/Boeing Hydraulics Ltd (FHL) was responsible for a activated by a switch on the instrument YF-22 demonstration new development known as the Attitude panel that would see the doors of the lift Maiden fl ights program, where they Control System (ACS). This took care of the system opened simultaneously. A built-in The Boeing X-32A CTOL took off on its fi red an AMRAAM missile during the conversion from conventional wing-borne test check of the ACS actuators then took fi rst fl ight on September 18, 2000, from fl ight Boeing fl ight to jet fl ight. The ACS was formed place, before the fl ight controls kicked in. Palmdale, piloted by Fred Knox, making it by eight variable-area nozzles: two pitch It was a complicated process. the fi rst of the JSF concept demonstrator nozzles positioned in the nose and two in To complete the conversion, the pilot aircraft (CDA) to fl y. Boeing was fl ying the tail, along with two yaw nozzles on the had to push the fl ow switch control button for about a month without competition, Below: The Boeing X-32B in a hover at tail and two roll nozzles in the wingtips. on the throttle. The ACS had to close the resolving small technical problems and NAS Patuxent River, The jet screen – an exhaust of cooler main exhaust, so that thrust would be learning that they would have to make do Maryland. The Boeing X-32B achieved what the Harrier was ON JUNE 23, 2001, THE X-35B LIFTED FROM THE GROUND never was able to do: supersonic fl ight. This was made possible by VERTICALLY FOR THE FIRST TIME, WITH BAE SYSTEMS the raw power of the engine Boeing PILOT SIMON HARGREAVES AT THE CONTROLS

www.Key.Aero // August 2021 93 JSF PROGRAM // X-32 & X-35

without aerial refuelling. On October 24, 2000, Boeing test pilot Dennis ’Irish’ O’Donoghue experienced an accident. After he noticed the X-32A’s brakes were not working, he was forced to land on a lakebed and let the aircraft slide to a standstill. This caused the X-32A to be grounded for three weeks. The same day saw Tom Morgenfeld taking the X-35A for its first flight from Palmdale to Edwards AFB, a distance of 35 miles, taking the aircraft to 10,000ft with the landing gear down. In a short period, Lockheed managed to

qualify the X-35A for aerial refueling and Above: The X-32B in ticked off all the test points one by one at hover mode. Note the missing landing gear impressive speed. doors and inlet cowl. The Boeing X-32A rejoined the flight The lift improvement program one week before Lockheed device trapped the exhaust fountain completed the tests for the X-35A by under the fuselage performing a supersonic flight and Boeing achieving a 20º angle of attack (AoA) and Left: The lift fan is an altitude of 34,000ft. Boeing, it seemed, lowered in the X-35A, had some catching up to do, but had the which would be advantage of not needing to convert one transformed in the X-35B. The people in of their test aircraft as Lockheed had done. the image indicate The X-35A was now being reconfigured the size of the lift fan so that it could function as the X-35B. It Lockheed Martin had a ‘drop-in’ cylinder with ballast inside it. When the X-35 was converted from a CTOL variant to a STOVL variant, the cylinder was removed and replaced with the lift fan, which had the same mass. The Boeing X-32 continued its flight test program at an aggressive pace. Below left: The The X-35C carried out its maiden flight General Dynamics E-7 on December 16, 2000, with Joe Sweeney based on the F-16 seen here in the wind at the controls. This aircraft featured more tunnel at NASA Ames control surfaces and a reinforced structure in 1988. The ejector to ensure it could cope with the stresses propulsion system had been tried during of carrier landings, which have a higher the ASTOVL program, impact on the structure of the aircraft. but General Dynamics After completing aerial refueling went in with a fan-in-wing design qualification, the X-35C was flown based on the Ryan from Edwards AFB to the Lockheed XV-5B NASA Ames THREE REASONS THAT LOCKHEED PURSUED SDLF 1. The extra weight incurred as a result of the lift fan and the shaft was insignificant compared to the extra thrust gained by this new technology. 2. As the lift fan used no combustion, the exhaust was colder, making the ground environment better suited for operations. 3. STOVL thrust requirements can be disconnected from the main engine, so that the engine can be conventionally sized to the requirement of up-and-away thrust that’s needed to achieve transonic acceleration.

94 August 2021 // www.Key.Aero The X-35A shown Fort Worth division in Texas and, a day The aircraft was technically adjusted, proceeded to level fl ight and achieved powering up its later, completed its transfer to Naval Air which meant the landing gear doors and supersonic speed. After completing the engine prior to a fl ight test, paving the way Station (NAS) Patuxent River in Maryland, the inlet cowl were removed from the jet supersonic dash, he landed the aircraft for the fi rst fl ight becoming the fi rst X-plane in history to to reduce weight. The fear that the aircraft back vertically, performing exactly the Lockheed Martin complete a cross-continental fl ight. was too heavy was a real one, but Boeing mission profi le it was anticipated the Both the X-32A and the X-35C pointed out that their preferred weapon PWSC would have to fulfi l. To prove a performed up-and-away fl ight tests, many system concept (PWSC) was lighter and point that it was no accident, Lockheed fi eld carrier landing practices (FCLPs), would not incur this problem. repeated the feat again on July 26, 2001, bolters and wave off s before completing Boeing did not want to repeat the this time with Hargreaves at the controls. demonstration fl ights. embarrassment of the old experimental Two days later, on the last fl ight test STOVL planes that could not lift their day and in answer to Lockheed Martin's Major milestones weight. On June 23, 2001, the X-35B took X-mission, Boeing X-32B – fl own by USMC On December 29, 2000, a major milestone off from the ground vertically for the Maj Jeff Karnes – executed a short take was passed by adapting the X-35A to fi rst time, with BAE Systems pilot, Simon off , transitioned into conventional fl ight, the X-35B with the lift fan installed. The Hargreaves, at the controls. The capability broke the sound barrier, then transitioned cockpit was equipped for this part of the of hovering and vertical take-off and back to the STOVL mode in such a fl ight test program with a new control unit landing was proven at Palmdale. manner that he was still airborne due to situated beside the throttle and the stick, As soon as the X-35B was at Edwards AFB aerodynamical forces, then made a known as a thrust vector lever (TVL). the testing was geared to transitioning low-speed landing. A vertical landing was It was by operating the TVL in the X-35B, from normal fl ight to hovering. The not risked. For this fl ight, the inlet cowl that the pilot could vector the exhaust Boeing X-32B made its fi rst vertical and landing gear doors were reinstalled. of the main engine from 22º to 95º and, landing with O’Donoghue in control. Both the X-32B and the X-35B had The fi rst operational similarly, vector the thrust of the lift fan Amid the demonstration fl ights, the X-32B exceeded the weight limit set by DARPA. western STOVL fi ghter nozzle through a range of 34ºs to 95º. encountered the problem of hot gas Lockheed Martin had proven that the with the technology demonstrator of The business end of the program arrived ingestion during vertical landings, causing X-35B had excess power in hover, whereas its replacement, with the fi rst fl ight of the X-32B on March a pop stall on one occasion. Boeing was on the edge of its capability. although some 29, 2001. The jet fl ew some conversion For future growth capability, the decision at the outset had to be persuaded fl ights at Edwards, getting the speed Making aviation history to select the Lockheed Martin F-35 was a that designing a down to 140 knots, before transferring to USMC test pilot, Maj Art Tomassetti, made no-brainer. The lift fan was successful, but supersonic STOVL NAS Patuxent River. aviation history on July 20, 2001, when the F-35 became signifi cantly heavier than fi ghter was a worthy DARPA program As NAS Patuxent River was at sea level, he became the fi rst pilot to complete a the originally intended 24,000lb, resulting Lockheed Martin things would be easier here for Boeing. short take-off using the lift fan. He then in a substantial cost increase.

www.Key.Aero // August 2021 95