McDonnell Douglas studies for the High Speed Civil Transport program

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).

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.

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.

For more on the McDonnell Douglas HSCT designs, see the following links:

https://www.secretprojects.co.uk/threads/us-supersonic-transport-sst-program-post-1971.4786/page-4

https://ntrs.nasa.gov/api/citations/20050123580/downloads/20050123580.pdf

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.

The McDonnell Douglas D-3135: the first commercial blended wing body from Long Beach

It's been known to me ever since I was a teenager that beginning in the late 1980s McDonnell Douglas investigated designs for a blended wing body (BWB) airliner, viewing a BWB concept as boasting more fuel efficiency than a tube-and-wing airliner. Notwithstanding the fact that Boeing continued work on a large BWB aircraft after acquiring McDonnell Douglas and built the subscale X-48 vehicle to test the flight characteristics of a BWB with not just commercial but also military applications, the BWB designs from McDonnell Douglas were not the first commercial designs for blended wing body aircraft that the company designed. In the 1970s, McDonnell Douglas first toyed with the idea of a blended wing body when it designed a so-called spanloader aircraft as a potential rival to the Boeing 747's hold on the commercial air freight market.

Artwork of the McDonnell Douglas D-3135 spanloader freighter with containers being loaded into the wing (the conventional D-3133 "Nation Builder" design is shown in the background). 

In October 1975, under contract from NASA, McDonnell Douglas began undertaking concept studies for a spanloader airplane to be used for long-haul commercial freight, and the company designation D-3135 was applied to the McDonnell Douglas spanloader commercial freighter design studies. The D-3135 was 202 feet 6 in (61.72 meters) in length with a wingspan of 285 feet 5 in (87 meters), a height of 73 feet 8 in (22.45 meters), a wing area of 18,314 square feet (1,701.4 m²), and a gross takeoff weight of 1,350,000 lb (612,350 kg). It resembled the Boeing Model 759-100 and 759-121 spanloader proposals in marrying a straight wing having 20% percent thickness with a conventional tailed layout and a slim fuselage, and it featured wingtips canted outwards at 18 degrees. Power came from six individually podded 58,000 lb (258 kN) thrust Pratt & Whitney JT9D turbofans situated above and ahead of the wing's leading edge, and the D-3135 was to carry 637,000 lb (288,940 kg) of freight housed in 42 intermodal containers, which were loaded into the wing via the wingtips. To compartmentalize its gross weight with tarmac infrastructure, the D-3135 had 16 main landing wheels below the center fuselage and two sets of four outrigger landing wheels below the outer wing sections.

Alternate McDonnell Douglas flying wing spanloader freighter design (courtesy of NASA)

In tandem with the D-3135, McDonnell Douglas also worked out a flying wing spanloader freighter similar in appearance to the flying wing iterations of the Boeing 759. It was 225 feet (68.6 meters) long with a wingspan of 311 feet 8.16 in (95 meters), a height of 67 feet 3.08 in (20.5 meters), a wing area of 14,896 square feet (1,384 m²), and a gross takeoff weight of 1,115,746 lb (506,102 kg). The thick wings protruded from an abbreviated center section including the cockpit, and a pair of vertical stabilizers with top-mounted vertical stabilizers were situated along the wingtips. The flying wing spanloader design would carry 600,000 lb (272,155 kg) of cargo housed in 32 intermodal containers, and power was provided by six 40,000 lb (177.9 kN) thrust turbofans situated above and ahead of the leading edges of the wings. As with the D-3135, the flying wing spanloader would have four sets of four main landing wheels below the center section and two sets of outrigger landing gear with four wheels each.

Despite offering greater freight capacity than the 747 cargo versions, none of the McDonnell Douglas spanloader designs progressed beyond the design phase, and even if the D-3135 had been built, its outrigger landing wheels would not been compatible with narrow-gauge runways, in which case the design probably might have needed to take off from airfields without long, narrow runways. Today, the JetZero company headquartered at the city in California where the D-3135 was designed is developing the Z5 blended wing body airliner project and testing a prototype for the Z5 concept, and if flight tests of the subscale and full-scale technology demonstrators for the JetZero design are successful, it is not implausible that the Z5 could potentially be adapted into a purpose-built commercial freighter about the same wingspan as the D-3135.

References:

Cox, G., and Kaston, C., 2020. American Secret Projects 3: U.S. Airlifters Since 1962. Manchester, UK: Crécy Publishing.

Gunston, B., 1991. 

Giants of the Sky: The Largest Aeroplanes of All Time. Sparkford: Patrick Stephens Limited.

A-X and A/F-X designs from the Los Angeles area

After the cancellation of the A-12 Avenger II naval stealth bomber in January 1991, the US Navy still found itself in need of a replacement for the A-6 Intruder, and thus Secretary of Defense (and future vice president) Dick Cheney asked the Secretary of the Navy to initiate a new effort at shopping for an A-6 replacement, the A-X (not to be confused with the A-X ground attack aircraft competition won by the A-10). Like the ATA program for which the A-12 had been designed, the A-X program called for a stealthy, two-seat attack aircraft with all-weather/day/night capability and advanced, integrated avionics and countermeasures, but it also stipulated that the aircraft have greater operating range and multirole combat capabilities. A Request for Proposals (RFP) was issued for the A-X program on July 1991, with an October 29, 1991 deadline set for aircraft manufacturers to submit initial A-X designs, and the Navy planned to fund up to five A-X studies at a cost of $20 million each, with follow-on plans for separate demonstration, development, and production stages by late 1992. The US Air Force also took part in the A-X program, hoping to someday deploy a stealthy successor to the F-15E Strike Eagle, which was replacing the F-111 Aardvark as the USAF's frontline fighter-bomber, and also potentially the F-117 Nighthawk.

Northrop's flying wing (left) and blended wing (right) proposals for the Navy's A-X program. (courtesy of Tony Chong via the Secret Projects Forum)

Northrop responded to the A-X requirements with three subsonic designs (flying wing, a performance-driven aircraft, and a blended wing body) and a few supersonic designs in the first half of 1991. The flying wing proposal resembled Northrop's losing design for the ATA competition in seating the pilot and navigator/bombardier in tandem in the cockpit but differed in having a sawtooth trailing edge of the center wing section like that of the B-2 and the engine inlets placed near the wing leading edges and adjacent to the crew compartment, and it was 40 feet 9 in (12.4 meters) long with a wingspan of 76 feet (23 meters) and a speed of Mach 0.85. Power was provided by either two 24,164 lb (107.49 kN) thrust General Electric F404s or alternate engines from Pratt & Whitney. The blended wing body design also had a tandem-seat cockpit and engine inlets below the wings but featured a spearhead-shaped wing with a tail empennage incorporating backswept horizontal stabilizers with slight dihedral, and it was 57 feet 9 in (17.6 meters) long with a wingspan of 68 feet (20.7 meters) and a top speed of Mach 0.95. The powerplant (including potential options) for the blended wing body design was the same as that for the flying wing design. The performance-driven iteration had clipped diamond-shaped wings, all-moving trapezoidal canards, and a butterfly-shaped tail with two armpit inlets just aft of the wing leading edges, and it was  60 feet 8 in (18.5 meters) long with a wingspan of 54 feet 9 in (16.69 meters), a top speed of Mach 0.95, and two 27,322 lb (121.5 kN) thrust Pratt & Whitney PW7000 turbofans. The supersonic  design studies, which resembled Northrop's designs for the cancelled Naval Advanced Tactical Fighter (NATF) program, had speeds of up to Mach 1.8, and one proposal, the Advanced Strike Fighter, was 60 feet 5.4 in (18.42 meters) long with a wingspan of 54 feet (16.46 meters), featuring folding wings with an angled trailing, two outwardly canted vertical stabilizers, a pair of swept canards along the nose, and two supercruise turbofans (probably based on the Pratt & Whitney F119). Armament of the Northrop proposals consisted of AIM-120 air-to-air missiles and laser-guided bombs carried in internal weapons bays. Northrop judged the subsonic designs to have a better chance of fulfilling the A-X performance parameters, but it neglected to continue with its subsonic A-X proposals, presumably because of its preoccupation with the B-2 Spirit stealth bomber and AGM/MGM-137 TSSAM stealth cruise missile (which played a role in the company's YF-23 losing the ATF competition to the F-22), and in mid-October 1991 it joined an industry team formed by General Dynamics and McDonnell Douglas which in July had offered for the A-X program a derivative of the A-12 Avenger II with slightly higher aspect ratio wings whose outer wing sections had cranked leading edges.

Left: Artist's conception of three Lockheed/Boeing/General Dynamics AFX-653s in flight near a carrier task force
Right: The Rockwell International/Lockheed proposal for the A-X/A/F-X program

About the same time that Northrop was fleshing out its A-X concept studies, Lockheed and Boeing put forward a design for a two-seat aircraft which had delta wings with backswept trailing edges, a pair of outwardly canted triangular vertical stabilizers, and two supercruise turbofan engines. When the RFP for the A-X program was issued in July 1991, this proposal gave way to a joint proposal by Lockheed, Boeing, and General Dynamics for a derivative of Lockheed's F-22 derived swing-wing proposal for the cancelled NATF program. The Lockheed/Boeing/General Dynamics proposal was 61 feet 8 in (18.80 meters) long with the wings spanning 67 feet 8 in (20.62 meters) when in forward position or 37 feet 2 in (11.33 meters) when full swept, and it differed from the navalized F-22 in having a slightly shorter nose, straight leading edges of the wing roots, slab-shaped horizontal stabilizers, and two 27,322 lb (121.5 kN) thrust Pratt & Whitney PW7000 turbofans. The pilot and bombardier/navigator were seated in tandem in the cockpit, and armament consisted of AIM-120s and laser-guided bombs housed in four internal weapons bays. In the same month that they jointly conceived their swing-wing A-x proposal, Lockheed and Boeing co-partnered with Grumman to develop a clean-sheet A-X proposal, for which few details are known. In late October 1991, Rockwell International and Lockheed jointly proposed an an A-X design which seated the pilot and bombardier/navigator in tandem and had swing wings like the Lockheed/Boeing/General Dynamics concept but featured a platform shaped like an isosceles triangle. The Rockwell International/Lockheed design was powered by two PW7000 turbofans fed by inlets below the fuselage near the leading edges of the center section and situated at the rear of the fuselage between two outwardly canted vertical stabilizers. 

On December 30, 1991, the Navy awarded $20 million Concept Demonstration/Evaluation contracts to the McDonnell Douglas/LTV, Lockheed/Boeing/General Dynamics, Grumman/Boeing/Lockheed, Rockwell International/Lockheed, and Northrop/McDonnell Douglas/General Dynamics industry teams for their A-X proposals. The Demonstration/Validation (Dem/Val) proposals were to be offered by September 1992, and one of the industry consortium designs would be selected for prototyping in 1994, after which the first flight of the whichever A-X proposal was selected was to take place by 1996, with plans to deploy the A-X in 2005. However, in September 1992, several people in Congress demanded that the A-X Dem/Val phase also involve prototype aircraft to be evaluated in a fly-off contest, and the Navy delayed the Dem/Val phase by two years and planned A-X deployment to 2007, but it rejected the idea of a fly-off competition as too expensive. The cancellation of the NATF program in the spring of 1991 also meant that the Navy and Air Force added an air-to-air capability to the A-X requirements in late 1992, leading to the A-X program being renamed A/F-X to fully reflect its combined attack/fighter capabilities. To reflect this change, the Lockheed/Boeing/General Dynamics A-X design was refined to have cranked chines like those of the F-22 and thus became known as AFX-653 or "A/F-22X", even though it still shared only 20 percent parts and 50 percent technology commonality with the F-22. 

The F/A-18E/F Super Hornet, which ended up becoming the Navy's successor to the F-14 and A-6 after the A/F-X program was canceled.  

In early 1993, before the Navy could prepare to evaluate the design submissions for the A/F-X program, the Pentagon initiated a Bottom Up Review of existing military aircraft programs in early stages of development, including the A/F-X, amid defense budget cuts after the end of the Cold War that came to be known as the "peace dividend". On September 1, the A/F-X program was cancelled along with the Grumman ASF-14 derivative of the veteran F-14 Tomcat and the F/A-18E/F Super Hornet derivative of the F-18 Hornet proposed in 1991 as a low-cost alternative to the A-X program was instead selected for full-scale development as an interim replacement for the F-14 and A-6, carrying out its first flight on November 29, 1995.

For more on the design studies for the A-X and A/F-X programs from southern California, see the following link:

https://www.secretprojects.co.uk/threads/navy-ax-and-a-f-x-projects.2150/

References:

Chong, T., 2016. Flying Wings & Radical Things: Northrop's Secret Aerospace Projects & Concepts 1939-1994. Forest Lake, MN: Specialty Press.

Friedmann, N., 2022. U.S. Navy Attack Aircraft 1920-2020. Annapolis, MD: Naval Institute Press.

Taylor, J.W.R., 1995. Jane's All the World's Aircraft 1995-1996. Coulsdon, UK: Jane's Information Group.

Thomason, T., 2009. Strike From the Sea: U.S. Navy Attack Aircraft from Skyraider to Super Hornet 1948-Present. Forest Lake, MN: Specialty Press.