In late 1986, NASA initiated the High Speed Civil Transport (HSCT) program to investigate the feasibility of a new-generation high speed commercial aircraft to enter service in the early 2000s, and two-year study contracts were awarded to Boeing and McDonnell Douglas for analysis of various design configurations for a high-speed passenger airliner. Given that the Boeing 2707 which had won the National Supersonic Transport (NST) competition of the 1960s was doomed to cancellation as a result of environmental issues without being built, the HSCT program would take environmental, operational, and other non-vehicle factors into account that would decide which high-speed airliner configuration would be commercially acceptable and develop new engine technology which potentially could reduce nitrous oxide emissions which harm the Earth's ozone layer. Design requirements called for the HSCT to carry 250-300 passengers over a distance of 7,500 miles (12,070 km).
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| Mach 3.2 (left) and Mach 5 (right) airliner designs studied by McDonnell Douglas under the company designation D-3235 during the initial phase of the High Speed Civil Transport (HSCT) program in 1986-1988. |
After being awarded a two-year study contract from NASA for the HSCT program, McDonnell Douglas initiated Phase I of HSCT design studies by investigating a spree of concepts for advanced high-speed airliners under the company designation D-3235, including a kerosene-fueled Mach 2.2 airliner, a twin-fin Mach 4 airliner using either kerosene, liquid hydrogen, or methane fuels, and a Mach 6 airliner with twin vertical stabilizers using methane or liquid hydrogen. Upon initiating Phase II of the HSCT study contract, McDonnell Douglas picked three HSCT designs for study, a Mach 2.2 airliner, a kerosene-fueled Mach 3.2 airliner derived from the company's Advanced Supersonic Transport (AST) project of the late 1970s, and a methane-fueled Mach 5 aircraft. The first two concepts were tailored to meet the 700,000 lb (317,514 kg) gross weight goal, while the Mach 5 concept was to weigh nearly 1,00,000 lb (453,592 kg). The Mach 3.2 and Mach 5 designs were chosen for further refinement and evaluation because they were seen as offering different configurations, cruise altitudes, and fuel types potentially leading to relative advantages in terms of environmental characteristics. The baseline Mach 3.2 design conceived as part of Phase III, the D-3235-3.2-3A, was 315 feet (96 meters) long with a wingspan of 121 feet 2.7 in (36.9 meters) and four Pratt & Whitney duct burning turbofans. It had a double-sweep arrow wing design and a conical taper single-lobe fuselage, and seating capacity varied from 239 to 392 passengers. Due to concerns about the effects of the sonic boom from supersonic cruise, a derivative of the baseline Mach 3.2 iteration, the D-3235-3.2-4B, was conceived with the wing planform modified to reduce the leading edge sweep of the outer wing panels, in which case it had a wingspan of 144 feet 8.968 in (44.12 meters). Another Mach 3.2 iteration with a less intense sonic boom, the D-3235-3.2-5, had sharply backswept wings with backswept trailing edges and lacked horizontal stabilizers. The Mach 5 concept, the D-3235-5.01-15A, borrowed technologies from the National Aero Space Plane (NASP) program, including high-temperate lightweight metal alloys, elevons, and variable-cycle engines for hypersonic flight, and it took the form of a delta winged vehicle measuring 292 feet 8 in (89.2 meters) long with a wingspan of 136 feet 8.6 in (41.67 meters). The D-3235-5.01-15A had a spatula-shaped nose to help reduce sonic booms by providing blunted-lift distribution, and it featured a pair of vertical stabilizers next to the pitch and roll surfaces. Power was provided by four 72,183 lb (321 kN) thrust General Electric variable-cycle engines housed in an engine ramp nozzle below the rear fuselage, and the variable-cycle powerplant would operate as a turbofan at speeds of up to Mach 3, after which the inlet for the turbofan core engine was closed and a ram air bypass duct around the engine would allow the powerplant to function as a ramjet in the Mach 3 to 5 flight regime.
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| Artwork of the ultimate McDonnell Douglas HSCT design, the D-3235-2.4-7A |
In 1989, NASA began the High-Speed Research (HSR) program to develop “enabling technologies” to meet the requirements outlined for the HSCT program. The HSR program was divided into two phases, Phase 1 and Phase 2. The former was to focus on developing technological concepts for environmental compatibility, and Phase 2, which was planned to begin in 1994, was to demonstrate the environmental technologies as well as define and demonstrate selected, high-risk technologies for economic viability. By 1993, Phase 1 was completed and NASA began Phase 2 when it refined HSCT requirements insofar that a 300 passenger supersonic airliner with a cruising speed of Mach 2.4, an altitude of 60,000 feet (18,288 meters) and an operating range of 5,754 miles (9,260 km) was agreed upon, with the resulting concept aircraft embodying these parameters dubbed the Technology Concept Airplane (TCA), which featured four 49,500 lb (220 kN) mixed-flow turbofans. The mixed-flow turbofan concept would reduce jet noise by mixing low-energy air with engine high-energy exhaust flows during takeoff. Following the start of Phase 2, McDonnell Douglas conceived a new supersonic airliner design with a top speed of Mach 2.4, designated D-3235-2.4-7A. It measured 334 feet (101.8 meters) long with a wingspan of 128 feet (39 meters), a height of 56 feet (17.1 meters), and a takeoff weight of 753,000 lb (342,000 kg), with power provided by four mixed-flow Pratt & Whitney or General Electric turbofans. Although similar to the earlier D-3235-3.2-3A in having a double-sweep arrow wing design planform, the D-3235-2.4-7A design differed in having the underwing turbofans closely spaced and a swept cruciform tail empennage akin to that of the Boeing 2707-300. Like the rival Boeing Model 1080-924 design, the McDonnell Douglas D-3235-2.4-7A itself was to utilize an "external vision" system replacing cockpit windows with computer-generated graphics accessible to pilots on cockpit displays. A first flight was planned for 2003, with certification scheduled for 2005-2006 and service entry planned for 2007, and McDonnell Douglas also foresaw a market for 500-1,500 HSCTs.
Long before Phase 2 of the HSR program was begun, in 1989 NASA acquired the two prototypes of the F-16XL fighter-bomber variant of the F-16 Fighting Falcon for use in testing aerodynamic properties and sonic boom characteristics of the HSCT, modifying them with a laminar flow wing. The F-16XL-1 conducted flight tests of laminar flow technology for for the HSCT from May 3, 1990 and to September 1992, and the F-16XL-2 was tested with the supersonic laminar flow configuration from October 1995 to November 1996. Beginning in 1995, the F-16XL was used by NASA for investigating the takeoff performance, engine noise characteristics, and sonic boom phenomena of the Boeing and McDonnell Douglas HSCT designs, and NASA also used one SR-71 as part of the Low-Boom SR-71 Modified Signature Demonstration Program begun in 1993 for testing the sonic boom characteristics of the HSCT, flying in tandem with the F-16XL. Although the F-16XLs were able to achieve laminar flow at supersonic speeds, they fell short of attaining laminar flow characteristics at speeds and altitudes which were planned for the HSCT, but NASA deemed the results a success.
Nine months after the F-16XLs completed tests of the supersonic laminar flow configuration and sonic boom properties for the HSCT, on August 1, 1997, McDonnell Douglas was acquired by Boeing, and thus work on the D-3235-2.4-7A design was shelved as Boeing was the only company left participating in the HSCT program. By January 1999, NASA canceled the HSCT program altogether due to budget constraints and Boeing's decision to abandon further development of the Model 1080-924 design for the HSCT program because it was increasingly concentrated on the subsonic airliner market. Today, Boom Aerospace is developing the Overture supersonic airliner, which won't be as fast or big as the ultimate HSCT design from McDonnell Douglas but will use environmentally-friendly fuels (not to mention that it will be powered by Boom Aerospace's own turbofan engine, the Symphony), and if the Overture flies and enters airline service, it will finally provide the US airlines with a native supersonic airliner after the failure of the Boeing 2707 and HSCT programs to reach the completion/flight test phase.
References:
Conway, E.M., 2008. High-Speed Dreams: NASA and the Technopolitics of Supersonic Transportation, 1945–1999. Baltimore, MD: John Hopkins University.
Taylor, J.W.R., 1995. Jane's All the World's Aircraft 1995-1996. Coulsdon, UK: Jane's Information Group.
Taylor, M., 1996. Brassey's World Aircraft & Systems Directory 1996/97. London, UK: Brassey's Ltd.
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