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Home , Aerion, Boom Technology, Concorde, Quiet Spike, Sound barrier, Subsonic aircraft

Supersonic Commercial

J. Gabriel Funtanilla1 University of Colorado - Boulder, Boulder, CO, 80302

This paper describes the birth of supersonic and its subsequent rise, fall, and revival in commercial transport through the past 50 years. Supersonic flight owes its beginnings to a joint effort between NACA and the U.S. Air Force in the second half of the twentieth century. Since then NACA’s successor, NASA, has spearheaded the effort to understand and improve the flight of such aircraft. In order to survive today’s market, a viable must effectively maneuver itself beyond various noise, emission, and cost constraints. The following describes design, with an emphasis on engine related issues, by examining what has been done and what is to come.

Nomenclature

BPR = CAEP = Committee on Environmental Protection CD = convergent-divergent CO = carbon monoxide CO2 = carbon dioxide FAA = Federal Aviation Administration g0 = acceleration of gravity on Earth HSCT = high speed civil transport HSR = High Speed Research (Program) L/D = -to- ratio LPP = lean premixed, prevaporized mf, mi = aircraft takeoff and landing mass NOx = nitrogen oxide PMC = polymeric matrix composites QueSST = Quiet Supersonic RQL = rich burn, quick mix, lean burn s = range of aircraft, as determined by the Breguet Range Equation SBJ = supersonic SFC = specific fuel consumption SOx = sulfur oxides SST = supersonic transport TAPS = twin-annular pre-mixing swirler V = cruise velocity

I. Introduction N 1947, NACA broke the with its Bell X-1 experimental aircraft, ushering in a new era of flight. I Only a decade after the dawn of supersonic flight were proposals made for a supersonic transport (SST). The less heralded of the two SSTs to ever operate is the ’s Tu-1441, which was first flown in late 1968, a few months before its more nostalgic counterpart, the .

1 Graduate Student, Engineering Sciences, University of Colorado - Boulder 1 American Institute of Aeronautics and Astronautics

Figure 1. Bell X-1. The first supersonic aircraft.

Early development of the Tupolev project was positive, with the Tu-144 becoming the first commercial aircraft to exceed Mach 2. However, due to a crash in the 1973 and budget restrictions, the Tu-144 was delayed into passenger flight until 1977 (well after the Concorde). Its lifetime as a SST was cut short by another test flight crash and the project was permanently cancelled in 1983. In the early 1990’s, the Tu-144 was resurrected by NASA for use in supersonic research with its final flight occurring in 1999. The Anglo-French Concorde2 outlived the Tu-144 and was flown commercially until 2003. The Concorde entered service in 1976 and continued to carry passengers for the next 27 years with few hiccups. A fatal crash in July 2000 halted operation, but after careful investigation of the cause, modifications were set in place and the Concorde resumed flight in November 2001. Its eventual retirement was due to the many challenges SST faces, in this case namely noise and economic profitability.

Figure 2. Tu-144 and Concorde. left: Tu-144, right: Concorde.

Although enthusiasm for supersonic commercial aircraft was initially high after the demonstration of supersonic flight, the only cases of SST today remain the Concorde and Tu-144. This can be attributed to the overwhelming challenges engineers face in providing a supersonic aircraft design acceptable to federal regulation agencies like the FAA, , and passengers alike. Supersonic flight is dramatically different than subsonic flight. To this day the fundamental problem of supersonic flight remains noise. Noise includes the so called “community noise”—noise during takeoff, approach, and landing due to the fan speed and high exhaust velocities of engines optimized for supersonic cruise—and of course the inevitable emitted by vehicles traveling at supersonic speeds. Since the initial flight of the X- 1 and the original sonic boom, restrictions have limited supersonic commercial flight to trans-oceanic routes. This is the reason the Concorde was relegated to flying over the Atlanic. This restriction then, is related to eligible routes of SST. Routes traveled must consist mainly of over-water flight and have enough passenger traffic demand to be profitable. Hence, the Concorde’s trips from to London. Range, then, becomes an issue for supersonic vehicles. The lift-to-drag ratio (L/D) of supersonical vehicles is much lower than the standard 18-20 of subsonic commercial aircraft, such as the -320. Therefore the range is negatively impacted, as shown by the Breguet Range Equation:

(1)

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where s is the range, V is the cruising velocity, SFC is the specific fuel consumption, g0 is the acceleration of gravity on Earth, mi is the initial mass at takeoff, and mf is the final landing mass. A low lift-to-drag ratio requires higher velocities, lower specific fuel consumption (SFC), and/or low structure weight to maintain equivalent range to . The previous generation of SST were heavy, gas guzzlers requiring high exhaust velocities to maintain flight range. To obtain high cruise velocities, but minimize ram drag supersonic engines are invariably engines or low bypass ratio (BPR) . These engines consume more fuel per thurst, which along with the increasing prices of fuel in the early 2000’s was another contributing factor to the Concorde’s retirement. The last big hurdle for supersonic engine design deals with atmospheric pollution. Emissions of gas engines include greenhouse gases, water vapor and carbondioxide (CO2), -affecting nitrogen oxides (NOx), carbon monoxide (CO), sulfur oxides (SOx), unburned hydrocarbons (UHC), and soot. The most significant of these pollutants are CO2 and NOx. Of the two, NOx has the bigger influence on supersonic engine design. Because supersonic vehicles, especially those that cruise above Mach 2.0, fly in the , the interaction between NOx and ozone at these altitudes results in the depletion of the ozone layer. And because NOx production rate increases exponentially with temperature, with the exact amount dependent on the gas’s residence time, for the best engine performance the combustor must simultaneously raise the turbine entry temperature to the material limits and quench the hot gas as quickly as possible. If supersonic transport is going to be successful, it is paramount that the combustor technology be able to meet NOx emission regulations. Alluded to earlier, NASA began a project in the 1990’s to overcome the challenges faced by the Tu-144 and Concorde. NASA’s High Speed Research (HSR) Program3 sought to develop a new generation of commercial high speed civil transport (HSCT). The goals of the program were to diminish supersonic deficiencies in noise, emissions, service life, weight, range, and payload for an HSCT that would carry 300 passengers 5,000 nm at Mach 2.4. the scope of the study also included defining three notional SST classes: a (SBJ), an overland SST, and the aforementioned HSCT. After about a decade of design study and research the following conclusions were made by the team. 1) design techniques could minimize the sonic boom 2) The use of a low bypass ratio was suggested, even though many supersonic aircraft are powered by , this is to keep SFC as low as possible 3) Need an advanced combustor that emits ultralow levels of NOx, possible designs include lean premixed, prevaporized (LPP) and rich, quick mix, lean (RQL) combustors 4) High temperature materials are needed due to the heating of the “skin” of the airframe when traveling at Mach numbers upwards of 2.0, investigated aluminum and alloys as well as polymeric matrix composites (PMC); these materials need to be light weight to maintain range 5) Aerodynamic experiments were done with the intention of improving L/D while minimizing ; possible 10-15% increase in L/D from Concorde and Tu-144 design Eventually, the technical challenges for the project were deemed to difficult to overcome and the project was ended. It concluded that the focus on achieving Mach 2.4 was too aggressive and that a smaller, lower speed, vehicle, such as a supersonic business jet cruising around Mach 1.6 would be more feasible4. It also suggested that a vehicle cruising at Mach 2.0 may have a net productivity similar to that of a Mach 2.4 SST. The inception of a supersonic aircraft into the commercial sector was still wishful thinking at the approach of the 21st century. Fast forwarding to today, there have been many advances in technology that have made SST attractive again. Techniques for mitigating the sonic boom, the supersonic commercial aircraft’s fundamental problem, using shaping have been physically demonstrated beginning with NASA’s flight of an F-5E Tiger II with a lengthened in 20035. NASA then moved on to the “”5, a 24ft telescopic lance like structure attached to the nose of an F-15B. The spike created three small shock waves that remained separate rather than coalescing into one large sonic boom. Another concept to reduce the sonic boom is a biplane configuration in which the interaction between the two reduce or deflect skyward the shock waves6. NASA is currently teaming with ’s Skunk Works in its design of a Quiet Supersonic Technology (QueSST) demonstrator aircraft7 to perform human testing and obtain data on sonic boom mitigation. Overall these techniques can reduce the effects of the boom felt on the ground and testing results may convince the Federal Aviation Administration (FAA) to lift restrictions of supersonic commercial flight over land.

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Figure 3. F-5E Tiger II with lengthened fuselage. Demonstrator for the vehicle shaped sonic boom test (NASA Photo)

Figure 3. F-15B equipped with "Quiet Spike". A Collaboration between Gulfstream Aerospace and NASA

Further revival of supersonic transport can be seen in ’s proposal of a supersonic business jet and BOOM’s recent unveiling of a one-third scale demonstrator for its proposed 40-seat, supersonic airliner8. As technology continues to advance maybe there is finally a place for supersonic commercial aircraft.

II. Supersonic Aircraft Design Supersonic flight’s high speeds inherently cause problems in aircraft design, for both the airframe and engine. First and foremost, require a long, slender airframe in order to reduce drag and make flight efficient. As mentioned before, flight at high Mach numbers increase the temperature on the airframe due to friction and obviously shock formation occurs. Because supersonic vehicles travel at high speeds and altitudes, engines must provide more thrust than a commercial aircraft while minimizing NOx emissions in order to be viable. Engine inlets must be aerodynamically efficient and must be designed to handle the required mass flow at supersonic cruise and subsonic speeds. Often times a variable geometry inlet is required, especially when cruising above Mach 1.6. The complexity in design leads to high costs of manufacturability, which compounded with fuel costs, drives ticket prices for supersonic travel somewhere near the 5-10 thousand dollar price. The following describes what is being done to overcome design challenges faced when designing a supersonic aircraft.

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A. Airframe Design To mitigate drag at supersonic velocities, vehicles are designed to be long and slender with a limited span. Variable-geometry wings have been used to optimize flight for supersonic vehicles at both supersonic and subsonic flight velocities. One example is the F-14 Tomcat.

Figure 4. F-14 Tomcat. An example of variable- geometry wing design.

Another technique is the delta-wing, utilized by the Concorde and Tu-144, which has the advantage of using a high at low speeds to generate a vortex on the upper surface and increase lift. Frictional heating is another problem and is caused by high velocity air flowing over the aircraft. Most supersonic designs use aluminum alloys such as Duralumin. The downfall of such aluminum alloys are their limited lifecycles when subjected to high temperature. This limits the speed of supersonic cruise vehicles to around Mach 2.0 as suggested by the HSR Program. Another aerodynamic task is the integration of the engine and airframe. The design must limit drag and must produce the mass flow rates needed for various flight conditions. This is usually achieved by side-mounted intakes with variable inlet mechanisms. Nozzle consideration should be included with inlet design. A convergent-divergent (CD) nozzle is needed for supersonic flight and thus adds mass to the total structural weight.

Figure 5. SR-71 Blackbird. Used axial adjustable Figure 6. F-22 Raptor. Shows engines mounted to the conical nozzle to vary mass flow. fuselage underneath the wing.

Finally, and most importantly, is designing the aircraft to mitigate the effect of the sonic boom. Previously, mentioned were vehicle shaping techniques that result in a reduced boom. NASA plans to perform tests with its

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QueSST aircraft with the hope that they will be able to convince the FAA to loosen their restrictions on future supersonic ’ flight over land.

Figure 7. QueSST main design features. How NASA plans to build a supersonic transport without the boom.

B. Engine Design At the dawn of supersonic flight, engines or turbojet with were needed to provide enough thrust to break . Since that initial X-1 flight, development of turbofan technology has made low bypass ratio engines a viable option for the powering of supersonic vehicles. Although turbofans’ larger frontal area cause greater ram drag than a turbojet, low BPR turbofans can lower the engine’s overall specific fuel consumption, especially for operation around Mach 1.5. The possibility of variable cycle engines4, which allows for the varying of the BPR, would make the propulsion system optimal for different flight conditions could also improve SFC. Although fuel consumption is poor for aircraft traveling at supersonic speeds, engines are capable of providing enough thrust to power a supersonic vehicle. Further constraints lie in emission regulations and to some degree limiting “community-noise” as much as possible. The way forward is to make the engines more efficient, meaning advancing the combustor technology. High temperature combustion increases the efficiency of the engine, however, current nickel-alloy materials limit how hot the combustor can burn. The HSR Program deemed matrix composites (CMC) a promising combustion chamber liner, but after nearly two decades CMC technology is still largely untested. However, CMC is not without its own issues, its brittle nature and high cost could prevent its eventual use in combustor design. On the flip side of the coin, increasing combustion chamber also increases NOx production. To limit this negative effect, hot gases need to be quenched rapidly to decrease residence time. Today’s technology is aimed at achieving this goal. The General Electric twin-annular pre-mixing swirler (TAPS) combustor, used on the GEnx, is producing NOx with a 30% margin to current Committee on Aviation Environmental Protection (CAEP/8) standards10. Other advanced combustor designs, specifically tailored to supersonic commercial aircraft, are proposed in the Smith, et al. paper “Advanced Combustor Concepts for Low Emissions Supersonic Propulsion”11. These concepts focus on fuel-lean gas-phase combustion as an approach to low NOx emissions.

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Figure 8. General Electric - TAPS Combustor (GE photo)

III. The Concorde Early plans for the design of the supersonic transport vehicle began in 1957, with the program actually starting in January 1962. A joint effort between Aerospatiale and Aircraft Corporation (BAC), the aircraft was initially envisioned to carry 80 passengers over a range of 4500 km at a of Mach 2.2 (maximum). During design study it was discovered that a highly swept was needed for acceptable aerodynamic characteristics. Thus, the foundation of a beautiful aircraft, known today as the Concorde, was born.

Figure 9. Concorde viewed from below.

The choice of engine for the Concorde was made when supersonic flight was still in its infancy. The engine needed would have to be capable of providing high thrust for takeoff and acceleration to supersonic speeds, while limiting fuel consumption at supersonic cruise conditions. Eventually the selection of the Rolls-Royce/SNECMA Olympus 593 was made. It was determined that four engines were needed and would need to be additionally furnished with afterburners to produce enough thrust for takeoff. A revolutionary variable geometry nozzle was implemented to optimize thrust for all conditions. During the Concorde’s ten year development period, the engine was improved to meet design requirements. The resulting production engines include a seven stage low-pressure and seven stage high-pressure compressor delivering a mass flow rate of 186 kg/s with a pressure ratio of 15.5. The main combustor was annular with 16 fuel injectors. The turbine was comprised of a single high-pressure stage as well as a single low-pressure stage.

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Figure 10. Cutout view of twin Olympus 593 engines with variable geometry divergent block, (courtesy of SNECMA).2

The Concorde also utilized variable geometry for its inlets to adapt its mass flow intake depending on flight condition. Even though variable geometry is more easily implemented with rectangular shaped inlets, the technology used on the Concorde was state-of-the-art at the time. a) b)

c) d)

e)

Figure 11. Inlet geometry corresponding to various Mach numbers and modes of operation. a) geometry at M < 1.3, b) geometry at M > 1.3, C) critical flow regime under cruise conditions, d) subcritical regime under cruise conditions, and e) supercritical regime under cruise conditions (adapted from the Concorde flight manual).2

Though the Concorde was designed nearly 50 years ago, it is still unmatched by any other vehicle. The aircraft relied on revolutionary concepts for its time2: 1) An ogival wing planform with low thickness-to ratio and a sharp wing apex inducing an intensified vortex that provides additional lift at low speed and high angle of attack

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2) Variable geometry engine inlets including bleed doors and variable ramps that allow boundary-layer control and flow bleeding for mass flow rate adjustment 3) Variable geometry with bucket deflectors accommodating low and high expansion ratios, as well as reverse flow for aircraft deceleration 4) Fly-by-wire controls 5) Fuel transfer for longitudinal stability tuning allowing trim drag reduction under cruise conditions 6) First application of full authority control Though its technical achievements were great, it could not meet the same economical success. With its final flight in 2003, the Concorde, the last and only supersonic commercial aircraft, relinquished its attempt to become a viable form of transportation slipping into hibernation.

Table 1. Concorde Characteristics Capacity: 92-120 passengers Max Takeoff Weight: 408,000 lbs (185,000 kg) Max Zero Fuel Weight: 203,000 lbs (92,080 kg) Max Fuel Weight: 26,400 gal (95,680 kg) Operating Empty Weight: 173,500 lbs (78,700 kg) Max Payload: 29,500 lbs (13,380 kg) Length: 202 ft 4 in (61.66 m) Height: 40 ft (12.2 m) Wingspan: 84 ft (25.6 m) Wing Area: 3,856 ft2 (358.25 m2)

Maximum Speed: Mach 2.04 Cruise Speed: Mach 2.02 Range: 3,900 nmi Service Ceiling: 60,000 ft (18,300 km) L/D: low speed – 3.94, approach – 4.35 Mach 0.94 – 11.47, Mach 2.04 – 7.14

Powerplant: 4 Rolls-Royce/SNECMA Olympus 593 Mk 610

Table 2. Olympus 593 Characteristics Type: Turbojet Diameter: 47.75 in (1.212 m) Length: 13 ft 3 in (4.039 m) Dry Weight: 7,000 lbs (3,175 kg)

Max Thrust: wet – 38,050 lbf (169.2 kN), dry – 31,350 lbf (139.4 kN) Overall Pressure Ratio: 15.5:1 Mass Flow of Air: 186 kg/s (410 lb/s) SFC cruise: 33.8 g/kN-s (1.195 lb/lbf-h) SFC sea level: 39 g/kN-s (1.39 lb/lbf-h) Thrust-to-Weight Ratio: 5.4:1

IV. BOOM XB-1 In late 2016, unveiled a one-third scale realization of Boom’s XB-1 supersonic civil aircraft, fondly called the “Baby Boom”. It will demonstrate the key for practical supersonic travel. Boom’s XB-1, currently under construction with flight testing set for late 2017, might be described as the heir to the Concorde. Its goal is to achieve an economically profitable, trans-oceanic, Mach 2.2 cruising aircraft—making New York to London just 3 hours and 15 minutes—while keeping ticket prices affordable. Boom claims the XB-1 features three major aerodynamic advances from its predecessor. First, an area ruled fuselage featuring a gentle tapering in the aft cabin. Second, a chine or wing extension, that stretches toward the

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nose to accommodate a center of lift shift as the aircraft accelerates. And third, a refined delta wing that will reduce supersonic induced drag and help quiet the sonic boom. The XB-1 will implement carbon composite technologies into its airframe design. This is a response to the heating and weight constraints of supersonic aircraft and allow the aircraft to handle the high temperatures produced by skin friction while simultaneously decreasing the operating empty mass. In the scaled model, the advanced propulsion system that drives the aircraft is powered by three GE J85-21, 3500 lb thrust, turbojet engines. Like the Concorde, the XB-1 will utilize variable geometry intakes and nozzles (see Fig. 12). The intricately designed inlet utilizes oblique shock waves to compress the supersonic air to the subsonic speeds required by the engine. Digitally controlled movable surfaces are capable of precisely positioning the shock waves to achieve ideal deceleration for a wide range of flight conditions. Overall, Boom’s version of a SST is similar to that of the Concorde, although boosted by technological advances, it still seeks a niche market of over-water operation. This may be step in the right direction for reinvigorating the SST model, but it lacks any real punch. The only supersonic transport that will hold weight with regulation agencies, engineers, airliners, and the general public is one that has solutions to the still unanswered noise, fuel consumption, emissions, range, and affordability questions. Does this mean developing even more advanced propulsion systems and materials, creating odd shaped vehicles to suppress sonic booms, relaxing regulations? I don’t know, but at least the ball is rolling again.

Figure 12. XB-1 Propulsion System. (Boom Technologies photo)

Figure 13. Boom Technologies XB-1 and demonstrator "Baby Boom". (Boom Technologies photo)

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V. Conclusion The past 60 years has seen the rise and fall of supersonic commercial aircraft. Challenges of supersonic flight seemingly reduced supersonic travel to wishful thinking. But, while SST was on the backburner, NASA’s High Speed Research, among others, continued the advancement of supersonic technology and today we are equipped with tools well beyond the infancy of Concorde’s time. Now a revival of supersonic transport aircraft is starting to be seen in companies like Boom Technologies, and Aerion with its supersonic business jet collaboration with Airbus. Along with these independent companies, NASA continues to push on that sound barrier with projects like QueSST working towards diminishing the sonic boom. As technology continues to advance, and man’s thirst to break the boundaries of what is possible gets stronger, maybe it will be possible to fly from Seattle to Tokyo, conduct your business, and fly home, perhaps not. The biggest challenge for supersonic transport may not be muffling the sonic boom or creating a magically efficient engine. No, breaking the sound barrier was the easy part, making it profitable is the real challenge.

Figure 13. Depiction of Lockheed Martin and NASA's QueSST Aircraft. (NASA photo)

Acknowledgments J. G. Funtanilla would like to recognize Dr. Lakshmi Kantha for sharing his enthusiasm for and vast knowledge of all things propulsion with the students of his Aircraft Propulsion course.

References 1 “144 (TU-144). First in the world supersonic passenger production aircraft,” Tupolev. Web. 11 Dec. 2016. http://www.tupolev.ru/en/aircrafts/tu-144 2Candel, S., “Concorde and the Future of Supersonic Transport,” Journal of Propulsion and Power. Vol. 20, No. 1, Jan-Feb. 2004.

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3 “U.S. Supersonic Commercial Aircraft: Assessing NASA’s High Speed Research Program,” Committee on High Speed Research., Aeronautics and Space Engineering Board., Commission on Engineering and Technical Systems., National Research Council., 1997. 4 “Commercial Supersonic Technology: The Way Ahead,” Aeronautics and Space Engineering Board., 2001. 5Mason, W. H., “Supersonic Aerodynamics,” AOE 4124 Configuration Aerodynamics, Tech, 2016. 6Kusunose, K., Matsushima, K., Goto, Y., Yamashita, H., Yonezawa, M., Maruyama, D., and Nakano, T. C., “A Fundamental Study for the Development of Boomless Supersonic Transport Aircraft,” AIAA Meeting Papers, AIAA, Reno, NV, Jan. 2006. 7Bennett, J., “How NASA Wants to Build a Supersonic Plane Without the Boom,” Aviation Week, 13 June. 2016. 8 “XB-1 Supersonic Demonstrator,” Boom Technologies. Web. 11 Dec. 2016. http://boomsupersonic.com/xb-1/ 9 “NASA Moves to Begin Historic New Era of X-Plane Research,” NASA. Web. 11 Dec. 2016. https://www.nasa.gov/aero/nasa-moves-to-begin-historic-new-era-of-x-plane-research 10 “GE9X’s New TAPS Combustor to Maintain Its Cool Under Fire,” GE Aviation. Web. 11 Dec. 2016. http://www.geaviation.com/press/ge90/ge90_20141120.html 11Smith, L. L., Dai, Z., Lee, J. C., Fotache, C. G., Cohen, J. M., and Hautman, D. J., “Advanced Combustor Concept for Low Emissions Supersonic Propulsion,” Journal of Engineering for Gas and Power. Vol. 135, May. 2013. 12Kantha, L., “Testing and Regulatons,” Clash of the Titans, University of Colorado – Boulder, 2016.

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