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.

San Diego's ultrafast airliner designs

From 1946 to 1963, the San Diego branch of General Dynamics' Convair division built a fair spree of short-, medium-, and long-range airliners, including the CV-240 and CV-340 feederliners as well as the 880 and 990 jet airliners. Although production of the Convair 880 and 990 ended in the early 1960s and production of future supersonic aircraft by General Dynamics would be exclusively undertaken in Fort Worth for the rest of the Cold War, GD's Convair San Diego division seized the chance in the 1960s to look at designs for commercial aircraft designed for speeds of Mach 3 or more. 

Drawing of Convair's Mach 3 supersonic airliner proposal of 1961 from the project documents

In 1961,Convair San Diego pitched a design study to the Federal Aviation Administration (FAA) for a supersonic airliner with a top speed of Mach 3, two years before the FAA itself initiated the National Supersonic Transport (NST) competition. This proposal, which bore no company designation, was a delta wing aircraft similar to the North American XB-70 Valkyrie in having canards near the nose, a pair of vertical stabilizers, and wingtips which could fold downwards to generate compression lift at high speed. It would have a wingspan of 114 feet 11 in (35 meters) with the wingtips in horizontal, a range of 4,028 miles (6,482 km), and a seating capacity for 130 passengers, and power was provided by four Pratt & Whitney STF 102 H turbofans situated between the vertical stabilizers in a nacelle below the wing center section. Convair anticipated a market for 150 examples of its Mach 3 SST proposal in the 1970-1975 timeframe, and the cost of each unit was estimated at $14.1 million. Convair, however, seems to have passed on an opportunity to submit its proposal for the NST competition for a Mach 3 supersonic airliner when that requirement was initiated by the FAA in 1963. 

Top: Drawings of Convair San Diego baseline designs for turboramjet-powered hypersonic airliners. The delta wing designs were selected for technical study by Convair.
Bottom: Three-view drawing of Convair San Diego baseline design for a scramjet-powered hypersonic airliner.

In September 1965, NASA awarded GD Convair San Diego a contract to undertake design studies for a hypersonic airliner fueled by liquid hydrogen to be operational in the 1985-2000 interval. Throughout the course of late 1965, for Phase I of the study contract, Convair devised five baseline configurations for a hypersonic airliner with seating capacity for 200 passengers and a range of 5,000 miles (8,047 km). Four configurations were powered by four turboramjets and were designed to travel at Mach 6. One was a delta wing design with a conventional tail empennage that measured 345 feet (105 meters) long with a wingspan of 102 feet (31 meters) and gross takeoff weight 537,040 lb (243,597 kg), and the swing-wing design with the conventional tail empennage had the same length as the conventional delta wing baseline design but had a wingspan of 200 feet (61 meters) (120 feet [36 meters] when the wings were backswept at 70 degrees) and a takeoff weight of 602,483 lb (273,282 kg). The blended body/delta wing iteration had a double delta wing and single vertical stabilizer, and it was 300 feet (91 meters) long with a wingspan of 130 feet (39.6 meters) and a takeoff weight of 543,797 lb (246,662 kg), and the blended body/swing-wing configuration had a sharply swept delta wing planform, a single vertical stabilizer, and variable-geometry wings that were situated ahead of the wing flaps when swept and which would be unswept during low-speed flight. The scramjet-powered baseline design was powered by four scramjet engines and had a top speed of Mach 8, and while similar in overall appearance to the turboramjet-powered blended body/double delta wing baseline iteration, it differed in being 79 feet (24 meters) in height (compared to 76 feet [23 meters] for the turboramjet-powered blended body/double delta) and having a higher gross takeoff weight of 846,927 lb (384,159 kg).

Drawings of the final configurations of the Convair hypersonic airliner designs selected for Phase II technical studies.

After comparing the performance advantages and drawbacks of its baseline hypersonic airliner designs, in 1966 Convair chose the turboramjet-powered conventional delta wing and blended body/double delta wing configurations for its Phase II technical studies because of their lower operating cost, sonic boom intensity, and gross takeoff weight, while the conventional swing-wing layout was retained for limited Phase II studies on abort and subsonic hold. Convair found out that turboramjet engines arranged in a podded layout had a lower engine cooling/thrust fuel flow than when housed in a buried installation, The final design of the blended body/double delta wing iteration was 317 feet (97 meters) long with a wingspan of 124 feet (37.8 meters) and it had a takeoff weight of either 636,851 lb (288,870 kg) with engines in podded layout or 512,300 lb (232,375 kg) with the engines housed in a buried installation below the rear fuselage. On the other hand, the final design of the conventional delta wing iteration was 386 feet (117 meters) long with a takeoff weight of either 1,022,621 lb (463,853 kg) with the engines podded or 750,837 lb (340,574 kg) with the engines housed in a buried configuration.

In December 1966, just months after Convair finished its design studies for a hypersonic airliner, the FAA declared the Boeing 2707 the winner of the NST competition for a Mach 3 airliner. Convair, like Lockheed, North American and McDonnell Douglas, knew firsthand that making hypersonic engine technologies mature enough for a hypersonic airliner to become feasible required engine tests, and so the Convair hypersonic airliner project was destined to never leave the design phase, like the company's Mach 3 SST proposal of 1961.  

For more on Convair's hypersonic airliner studies and Mach 3 SST concept, see the following links:

https://up-ship.com/blog/?p=3224 

https://web.archive.org/web/20100521055526/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720065553_1972065553.pdf

Reference:

Wise, C.E., and Wood, N. (March 2, 1967). "On to Mach 12." Machine Design 39 (5):84-89.

Unseen hypersonic airliners from the Los Angeles area, part 3: the Lockheed CL-1725

The Lockheed company undoubtedly built the fastest-ever military aircraft, including the Archangel-12 and SR-71 Blackbird reconnaissance aircraft, the YF-12 interceptor derivative of the Archangel-12, the air-launched D-21 reconnaissance drone, and the Q-5/AQM-60 Kingfisher target drone (a derivative of the company's X-7 ramjet-powered experimental aircraft). As I've mentioned before, Lockheed had made its unlikely decision to look at the notion of a hypersonic airliner with the CL-500 VTOL airliner project of the late 1950s even before the X-15 had begun flight testing, but the company rightly chose not to give the go-ahead for building the CL-500 due to the immaturity of hypersonic air-breathing technology. By the late 1970s, Lockheed chose to revisit the hypersonic airliner idea, this time confident that ramjet tech was slowly but steadily becoming mature enough for application to an intercontinental airliner.

A table of the five CL-1725 HYCAT design configurations studied by Lockheed. The HYCAT-1A was derived from the HYCAT-1 iteration shown in the topmost row. 

In early 1979, under contract from NASA, Lockheed-California initiated the Hypersonic Cruise Aircraft Propulsion Integration Study to investigate the feasibility of a long-range hypersonic airliner capable of traveling at Mach 6 over a distance of 5,754 miles (9,260 km) with a seating capacity of 200 passengers and powered by a turbojet/scramjet propulsion system. Five designs, collectively called "HYCAT", were investigated by Lockheed, and the internal designation CL-1725 was applied to the HYCAT designs, all of which were to be fueled by liquid hydrogen. The first design, the HYCAT-1, had a long, slender fuselage, a single vertical stabilizer, and delta wings with forward swept trailing edges running along the length of the rear fuselage and terminating just ahead of the tail empennage, and it had six turbojets situated below the fuselage with tandem turbojet and scramjet inlets. The HYCAT-2 was a double-deck iteration which was powered by four turbojets fed by air flowing into intakes mounted atop the rear fuselage ahead of the vertical stabilizer and had scramjet inlets below the rear fuselage, and the HYCAT-3 design was also a double-decker but featured high-mounted backswept wings with twin vertical stabilizers at the wingtips and four symmetric vectoring turbojets below the rear fuselage, with scramjets mounted on the sides of the rear fuselage. The HYCAT-4 design featured a semi-blended wing body hypersonic airliner with an area-ruled fuselage featuring a double-deck seating layout, a conventional tail empennage, and four turbojet engines fed by air passing through air intakes atop the wings along with scramjets below the wings. The HYCAT-5 design, like the HYCAT-2, was a double-deck hypersonic airliner powered by four turbojet engines housed in air intakes atop the rear fuselage ahead of the vertical stabilizer and four scramjets below the rear fuselage, but it differed in having a compound delta wing and canards near the nose for low-speed trim. The HYCAT-1 to HYCAT-5 were truly gigantic aircraft designs, even bigger than the swing-wing Boeing 2707-100 and -200, with the HYCAT-1 measuring 395 feet 4.8 in (120.5 meters) long with a wingspan of 114 feet 4.92 in (34.87 meters) and a gross weight of 677,649 lb (307,382 kg), and the HYCAT-4 measuring 340 feet (103.6 meters) in length with a wingspan of 146 feet 9 in (44.73 meters) and a gross weight of 959,426 lb (435,196 kg). 

A three-view drawing of the HYCAT-1A design marrying the HYCAT-1's wing planform with the HYCAT-4's tail empennage

After carefully analyzing the operational, technical, and aerodynamic advantages and trade-offs of the five HYCAT designs, Lockheed decided to adopt a refined iteration of the HYCAT-1, the HYCAT-1A, as the baseline HYCAT design. The HYCAT-1A had the wing planform of the HYCAT-1 but differed in having a stretched rear fuselage and the tail empennage of the HYCAT-4, and Lockheed judged these characteristics essential to provide trim for relative change in the center of pressure through the speed range and enabled the use of drooped ailerons for low-speed lift. The HYCAT-1A featured a passenger  cabin with two mid-fuselage decks as in the HYCAT-2, and it was powered by four 75,000 lb (34,019 kg) thrust turbojet engines housed below the rear fuselage and ten scramjets. The baseline HYCAT-1A design was 344 feet 10.8 in (105.12 meters) long with a wingspan of 98 feet 1.92 in (29.92 meters) and a gross weight of 600,000 lb (272,155 kg), and it would use the turbojets at speeds of up to Mach 4.5, after which the turbojet inlets would be closed and the scramjets would be ignited to allow the aircraft to reach Mach 6. The final general arrangement for the HYCAT-1A had a slightly longer fuselage 389 feet 4.32 in (118.68 meters) meters, and it featured a wingspan of 109 feet 2.76 in (33.29 meters), and a gross weight of 773,700 lb (350,944 kg). Both separate inlet turbojet/scramjet and common variable-geometry inlet turbojet/ramjet propulsion systems were investigated for the HYCAT-1A point design, and the latter propulsion arrangement was seen as creating increased inlet air flow and thrust after the turbojets shut down as well as reduced fuel consumption during acceleration and subsonic cruise. The design of the HYCAT-1A would form the basis of the Lockheed CL-2103 hypersonic bomber project envisaged in 1980, which was bigger than any of the CL-1725 HYCAT designs.

For more info on Lockheed's HYCAT designs, see the following links:

https://www.secretprojects.co.uk/threads/lockheed-hypersonic-transports-hycat.1950/

https://ntrs.nasa.gov/api/citations/19800006815/downloads/19800006815.pdf

https://ntrs.nasa.gov/api/citations/19800006816/downloads/19800006816.pdf

Unseen hypersonic airliners from the Los Angeles area, part 2: liquid hydrogen designs from Inglewood and Long Beach

What did Douglas and North American Aviation have in common in the 1950s and 1960s? They both built research aircraft for investigating supersonic and hypersonic flight, with the D-558-2 Skyrocket becoming the first airplane to reach Mach 2 and the North American X-15 entering the history books as the first manned aircraft to fly at hypersonic speeds. Also, the two companies conceived designs for a supersonic airliner in the early 1960s, with Douglas working on the Model 2229 and North American envisaging the NAC-60, the latter which was submitted for the Federal Aviation Administration's National Supersonic Transport (NST) competition but eventually rejected by the FAA in favor of the Boeing 2707 and Lockheed L-2000 as one of the finalist designs for the NST contest. With this rich experience in mind, Douglas and North American decided to take the unorthodox next step in high-speed aircraft development by looking at the notion of hypersonic air travel. 

Left: Cutaway artwork of a 1967 concept by North American for a Mach 10-14 hypersonic airliner
Right: Artwork from 1967 of two North American Mach 6 hypersonic airliners (cutaway view provided for one of them)

In the 1950s and 1960s North American Aviation made huge strides in developing superfast aerospace vehicles with tests of the SM-64 Navaho supersonic cruise missile in the 1950s and flight tests of the XB-70 Valkyrie prototype Mach 3 strategic bomber and X-15 hypersonic research aircraft, the latter which became the first aircraft to reach Mach 6. Fortified by prior experience with the XB-70 and X-15 in addition to having worked on the NAC-60 supersonic airliner project, in early 1967 North American unveiled two concepts for airliners capable of traveling at hypersonic speeds. One hypersonic airliner concept envisaged was a highly swept delta wing design with two outward canted vertical stabilizers on a flattened rear extension aft of the trailing edges of the delta wings, and it would have had a seating capacity for 130 passengers with the crew of two. Power came from four turboramjets fueled by liquid hydrogen or liquid methane stored in fuel tanks below the interior, and the aircraft would have had a top speed of Mach 6 and cruising altitude of 80,000-100,000 feet (24,384-30,480 meters). Another concept involved a design with a semi-conical fuselage, backswept wings which had pentagon-shaped vertical fins at the wingtips, seating capacity for 136 passengers, and a top speed of Mach 10 to 14. It was powered by four liquid hydrogen-fueled scramjet engines and had a large expansion ramp at the rear of the fuselage for the scramjet ducts, and it would cruise at altitudes of 110,000-140,000 feet (33,528-42,672 meters). In both concepts, the jet turbines would be ignited at speeds of up to Mach 3, and the air inlets for the turbo-compressors would be closed as the aircraft cruised at hypersonic speeds. Given that heat friction is generated by speeds beyond Mach 3, it seems reasonable to assume that these two proposals would have been constructed from heat-resistant alloys such as Inconel X and Beta-21S. No company designations are known for North American's hypersonic airliner concepts, but presumably those designs bore internal designations within the D-400 to D-434 numerical sequence because the company's D-435-1-4 proposal for a delta-wing modification of the X-15A-3 and D-436 and D-458 proposals for the initial study phase of the Advanced Manned Strategic Aircraft (AMSA) program were conceived shortly after the hypersonic airliner projects were unveiled.

Company artwork of the giant Douglas DC-2000 hypersonic airliner concept

In 1973, the Long Beach division of McDonnell Douglas conceived its own proposal for a hypersonic airliner for operational use by the year 2000. Designated DC-2000 by the company, it was 475 feet (144 meters) long with a wingspan of 170 feet (52 meters), a gross weight of 875,000 lb (396,893 kg), a top speed of Mach 6, and a range of more than 5,000 miles (8,047 km). The DC-2000 would carry 500 passengers, and power was provided by a combination of four liquid-hydrogen fueled turbojets and four ramjet engines, with the turbojets being used for speeds of up to Mach 3.5 and the ramjets providing thrust at speeds in the hypersonic flight regime. In addition to having a slender fuselage, it had a single vertical stabilizer and clipped delta wings with backswept trailing edges. The passenger capacity being envisaged for the DC-2000 was quite unusually high compared to that of the earlier North American hypersonic airliner design studies yet reflected the ambitions of McDonnell Douglas for advanced long-range air travel by the end of the 20th century.

Early 1970s proposal by North American Rockwell for a hypersonic airliner

As a side note, although North American's 1967 hypersonic airliner designs did not progress beyond the design phase, in late 1972, the Space Division of North American (which by then had become part of Rockwell International) worked out a design for a hypersonic airliner with a seating capacity for 200 passengers and a top speed of Mach 6. The proposal, for which no company designation is known, was 300 feet (91.4 meters) long with a wingspan of 112 feet (34 meters), and a gross weight of 481,400 lb (218,359 kg), and it had a single vertical stabilizer and delta wings with a forward swept trailing edge running along the length of the rear fuselage and extending beyond the spine of the vertical stabilizer to point ahead of the rudder. It would have a range of 4,600 miles (7,000 km) and power was provided by four 58,000 lb (260 kN) turbojets and nine 157,000 lb (698 kN) thrust scramjets situated below the rear fuselage and fueled by liquid hydrogen.

For more on North American and Rockwell International's hypersonic airliner proposals, see the following links:
https://up-ship.com/blog/?p=20863

https://www.aerospaceprojectsreview.com/blog/?p=299

https://ntrs.nasa.gov/api/citations/19730023221/downloads/19730023221.pdf

References:

Ingells, D.J., 1979. The McDonnell Douglas Story. Fallbrook, CA: Aero Publishers.

Wise, C.E., and Wood, N. (March 2, 1967). "On to Mach 12." Machine Design 39 (5):84-89.

Unseen hypersonic airliners from the Los Angeles area, part 1: the Lockheed CL-500

In the late 1950s, aircraft manufacturers in the US with experience building passenger aircraft, including Boeing, Convair, Douglas, and Lockheed, began planning for the day when airliners capable of supersonic speeds would become US commercial aviation's wave of the future in the 1970s and beyond, presaging design of the Boeing 2707, Convair 58-9, Douglas 2229, Lockheed L-2000, and North American NAC-60. However, years before the Federal Aviation Administration (FAA) initiated the competition in 1963 for a supersonic transport that would produce the Boeing 2707 and Lockheed L-2000 designs, Lockheed was already thinking about the far-fetched notion of an airliner optimized for hypersonic speeds, and from the late 1950s to 1970s, aircraft manufacturers in the Los Angeles basin unveiled a flurry of design studies for hypersonic airliners. Therefore, I'm beginning my overview of hypersonic airliner designs conceived in the Los Angeles area with a discussion of Lockheed's idea of a VTOL hypersonic airliner.

Cutaway view of the baseline Mach 4 iteration of the CL-500 VTOL hypersonic airliner from the company documents.

In 1959, Lockheed conceived a design for an intercity airliner not only capable of speeds beyond Mach 3 but also optimized for vertical takeoff and landing (VTOL). This proposal, designated CL-500, was a very unorthodox design in that it resembled a rectangular slab with sharply swept delta wings and a vertical stabilizer whose base extended to a point on the airframe aft of the overhead main entrance door. The front nose section of the aircraft accommodated twelve turbojets fed by air intakes at the front of the nose, and lift jets were housed in efflux slots along the chines of the CL-500, with ramjet/reheat burners placed at the rear of the fuselage. The lift jets would be used during vertical takeoff, and after making a transition to forward flight, the CL-500 would use its jet engines at speeds of up to Mach 3, after which the turbojets would operate as ramjets for hypersonic flight and the supersonic ram inlet would be shut. The CL-500 had a seating capacity for 60 passengers and a flight station behind the forward fuel compartment for the jet engines for two crewmembers, and because it had no windows on the sides of the fuselage, it featured a television and periscope visual display to enable the passengers and crew to guide the aircraft in forward flight and during takeoff and landing. The baseline 60-passenger CL-500 design had a top speed of Mach 4, and other variants of the CL-500 studied by Lockheed would have traveled at Mach 7. To withstand heat friction above Mach 3, the CL-500 was to be manufactured from titanium and a few other heat-resistant metal alloys.

Unsurprisingly, the notion of a VTOL hypersonic airliner encapsulated by the CL-500 was not only a rather whacky concept but also was sure to be ridiculed by the airline industry as too far-fetched given the insurmountable hurdles to adapting the CL-500's VTOL engine nozzles to immense heat generated by flight at hypersonic speeds.