MX-1554 designs from the Los Angeles Basin, part 2: the North American D-103, Douglas 1245, and Northrop N-65

As I mentioned previously, the Lockheed L-205 (aka Model 99) was one of the three winners of the MX-1554 announced by the US Air Force in July 1951, but it eventually was canceled without ever entering full-scale development due to weight issues and budget constraints. However, Lockheed was not the only company based in Los Angeles County to propose a design for the MX-1554 requirement. Three more aircraft manufacturers headquartered in Los Angeles County worked out with their own interceptor designs for the MX-1554 requirement, and paradoxically, they all had prior experience with design and development of all-weather interceptor fighters, putting them in a nominal position to come out with cutting-edge interceptor designs in response to MX-1554.

Desktop models of the twin-engine (top) and single-engine (bottom) North American D-103 interceptor designs.

North American Aviation's design work for the MX-1554 competition deserves discussion first with regards to MX-1554 proposals that didn't make the cut when it came to being chosen by the USAF for full-scale development. Although a handful of publications noted that North American envisaged and submitted two designs for MX-1554 in January 1951, until the 2010s, only photos of desktop models of these proposals were known in published literature (e.g. Buttler 2007). However, technical data for the two designs unearthed from North American Aviation company documents by Buttler (2013) has shed new light on those proposals, especially dimensions and performance. Judging from North American Aviation's list of Preliminary Design Designations, the company designation D-103 was allocated to the North American designs for the MX-1554 contest. Although it is would be wasteful to replicate the detailed account of the D-103 designs provided by Buttler (2013), these two proposals had slightly clipped high-mounted delta wings and  mid-fuselage horizontal stabilizers with slight dihedral. One of these designs was powered by a single turbojet (exact type unknown) fed by a large chin intake, and the other proposal had two side-by-side turbojets (exact type again is unknown) on the sides of the fuselage with their air intakes protruding from the wing roots. The armament for both North American proposals consisted of cannons in the forward weapons bay, Falcon air-to-air missiles in the center weapons bay below the centerline, and 2.75 in forward-firing air-to-air rockets in the rear weapons bay, and the twin-engine iteration was much heavier than the single-engine design. Despite being heavier than the single-engine D-103 proposal, the twin-engine D-103 iteration had a greater climb rate, with an estimated climb to 45,000 feet (13,716 meters) in 2.98 minutes compared to the single-engine D-103 being estimated to reach the same altitude in 3.60 minutes.

Three-view drawing of the Douglas Model 1245 (courtesy of National Archives)

In the same month that North American conceived the D-103 designs, the Santa Monica division of Douglas came out with a design submission for the MX-1554 of its own, designated Model 1245 by the company. Like a handful of aircraft projects conceived by Douglas Santa Monica, the Model 1245 bore some similarity to the Douglas X-3 Stiletto supersonic research aircraft in having the tail empennage situated above the exhaust pipe for the jet engine, but it differed from the X-3 in having a Wright J67 turbojet with air fed through a pair of air intakes ahead of the wing's leading edge, wings backswept at 35 degrees, a shorter nose, and horizontal stabilizers with 25 degree dihedral just below the base of the vertical stabilizer. Unlike the D-103, however, the Model 1245 would have no internal weapons bay, instead featuring six outboard hardpoints on which the Falcon missiles would be carried, while two drop tanks would be carried below the innermost pylons. The pilot accessed the cockpit of the aircraft through a hatch in the floor, and the Model 1245 was to be 66 feet 1 in (20.14 meters) long with a wingspan of 37 feet 6 in (11.43 meters), a wing area of 400 ft² (37.20 m²), and a top speed of 1,071 mph (1,723 km/h). The design philosophy of the fuselage, air inlets, and wings of the Model 1245 is rather reminiscent of that of the left fuselage nacelle of the proposed Douglas Model 1265 supersonic parasite bomber.

Left: Northrop N-65 proposal with diamond-shaped wings and horizontal stabilizers (drawing number PD-1170-1)
Right: Design iteration of the N-65 with an underslung TJ-15 turbojet (drawing number PD-1173-3)

Now this brings me to Northrop's forgotten interceptor designs for MX-1554. Northrop had worked on designs for a supersonic interceptor under the company designation N-53 back in 1949, but when the MX-1554 requirement was issued, in late June 1950 it shelved work on the N-53 to begin undertaking new supersonic interceptor studies under the designation N-65. Some initial N-65 designs resembled the N-53, but one early concept (drawing number PD-1168-6) resembled a manned version of the Northrop XSSM-A-5 Boojum supersonic cruise missile project with two General Electric J47 turbojets at the wingtips and armed with six air-to-air missiles carried within an internal weapons bay. Later N-65 configurations utilized the Wright TJ-15 turbojet (probably a variant of the Wright XJ61-W-3 turbojet), either in single- or twin-engine layout. One TJ-15 powered N-65 iteration, which bore the drawing number PD-1170-1, was 63 feet 4 in (19.3 meters) long with a wingspan of 45 feet (13.72 meters), and it sported high-mounted diamond-shaped wings with two TJ-15 turbojets situated the wing roots on the sides of the fuselage, and diamond-shaped horizontal stabilizers halfway up the vertical stabilizer; armament comprised four Falcon missiles that would be fired from forward-facing launch tubes in the belly and 16 2.75 in folding-fin unguided rockets to be carried within the outer rims of the turbojets. Another iteration, drawing number PD-1173-3, had a single TJ-15 housed in a ventral inlet below the fuselage and resembled one of Northrop's N-53 design studies in the wing planform, and it measured 58 feet (17.9 meters) long with a wingspan of about 34 feet (10.5 meters), with armament consisting of four Falcon missiles and 16 folding-fin unguided rockets housed in pop-out shoulder weapons bays aft of the cockpit. Other N-65 concepts included a design powered by two Pratt & Whitney J57 turbojets and an unmanned aircraft (drawing number PD-1188) similar in planform to the PD-1173-3. The latter was 52 feet 3.6 in (15.94 meters) long with low-mounted wings spanning 24 feet 9.6 in (7.56 meters), and it had one Westinghouse J46 turbojet in the rear fuselage fed by air intakes on the sides of the fuselage, while a single conventional warhead would be housed in the forward fuselage. 

In end, Northrop axed design work on the N-65 by October 1950 because of its growing preoccupation with the F-89 Scorpion all-weather fighter program, so no N-65 design was submitted to the US Air Force for consideration. The North American D-103 and Douglas Model 1245 that were submitted would be passed on in favor of the F-102 and XF-103. North American by then was undertaking production of the F-86D Sabre Dog (originally F-95) interceptor version of the F-86 Sabre that entered service in 1951, and the Douglas company was pretty busy with production of the AD (A-1) Skyraider, A3D (A-3) Skywarrior, F4D (F-6) Skyray, and DC-6, so it would almost certainly have not have had the resources to develop a long-range interceptor for the USAF.

References:

Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.

Buttler, T., 2013. Early US Jet Fighters: Proposals, Projects, and Prototypes. Manchester, UK: Hikoki Publications.

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

MX-1554 designs from the Los Angeles Basin, part 1: the Lockheed L-205

In June 1950, the US Air Force initiated the MX-1554 requirement (aka 1954 Interceptor) for a long-range supersonic interceptor to defend the US from the Soviet Union's recently deployed Tupolev Tu-4 strategic bomber (a reverse-engineered copy of the B-29) and still-in-the works gas turbine-powered Tu-16, Tu-95, and M-4. One of the winning designs for the MX-1554 contest, the F-102 Delta Dagger built in San Diego, has been extensively covered in numerous publications, and some people, myself included, know that the unbuilt Republic XF-103 design powered by both a turbojet and a ramjet was also a winning entrant in the MX-1554 competition, although it never reached the hardware phase. However, what has been heavily overlooked is that Lockheed entered its own design into the MX-1554 competition and was awarded a development contract, only for it to be subsequently canceled before that design could be given a military designation, and equally forgotten are designs by North American and Northrop for the MX-1554 requirement. Therefore, this and the next post will cover designs by Lockheed, North American, and Northrop devised for MX-1554.

A desktop model of the Lockheed L-205 (Model 99) interceptor. The similarity of the dorsal air intake to that of the F-107 is apparent.

As early as 1949, Lockheed had been working on design studies for a lightweight jet interceptor, known by the company designation L-190, which covered proposals with straight, delta, and variable-geometry wings. None of the L-190 studies never materialized, but after the Request for Proposals (RFP) for the MX-1554 competition was released in June 1950, Lockheed immediately returned to the concept of a purpose-built interceptor when it conceived a design for an all-weather interceptor, designated L-205 by the company, by the beginning of January 1951. The overall L-205 design submission had straight wings with tapered leading and trailing edges like those on the F-104 Starfighter but nevertheless retained the tail empennage of the XF-90 prototype penetration fighter. It measured 63 feet 9 in (19.4 meters) long with a wingspan of 30 feet 4 in (9.2 meters), a wing area of 300 ft² (27.9 m²), and a gross weight of 32,125 lb (14,572 kg), and power was provided by one 15,000 lb (66.7 kN) thrust General Electric J53 turbojet fed by air flowing through a dorsal air intake behind the cockpit (similar to that seen on the North American F-107). Unlike those of the F-104, however, the L-205's wings were low-mounted and lacked anhedral. The L-205 would be armed with six Falcon air-to-air missiles housed in a mid-lower fuselage bay and twenty 2.75 in (7 cm) folding-fin unguided air-to-air rockets placed on the sides of the bay, and it would climb to 40,000 feet (12,192 meters) in 1.6 minutes, while the service ceiling was to be 63,000 feet (19,202 meters) and operating range would be 1,760 miles (2,834 km).

On July 2, 1951, the US Air Force declared the Convair Model 8 along with the Lockheed L-205 and Republic AP-57 (the latter one of the three Republic designs proposed for the MX-1554 requirement) the winners of the MX-1554 competition, and these designs were given the go-ahead to proceed to the full-scale mock-up phase under a Phase I development contract. After being awarded a Phase I contract, the L-205 received the Basic Model Number 99, hence it being also called Model 99. However, the US Air Force couldn't afford to fund all three interceptor designs, and the L-205/Model 99 also was heavier than the Convair design and thus would have had consequent reductions in performance, so the USAF canceled the L-205 project by late 1951, even as the Convair Model 8 and Republic AP-57 were given the designations YF-102 and XF-103 respectively.

Although the L-205 project ended up becoming a loser-turned-winning design for fiscal reasons in spite of having been selected for development along with the F-102 and XF-103, Lockheed would apply a few features of the L-205, namely the tapered straight wings, to design of its future F-104 Starfighter, the first American supersonic lightweight jet fighter to be built.

References:

Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.

Buttler, T., 2013. Early US Jet Fighters: Proposals, Projects, and Prototypes. Manchester, UK: Hikoki Publications. 

Lockheed's first gas turbine-powered Connie: the L-151

The Lockheed Constellation family was unquestionably one of the finest airliner dynasties to come of age after the end of the World War II, spawning evolutionary developments such as the improved L-649 and L-749 versions of the original Constellation, the L-1049 Super Constellation, the L-1649 Starliner, and the EC-121 Warning Star, but also the turboprop-powered L-1249 (R7V-2/YC-121F) and the unbuilt XB-30 heavy bomber. The L-1249 that was derived from the Super Constellation constituted the first instance of any variant of the Constellation family being built with turboprop engines, although it never advanced beyond the prototype stage. However, what has been overlooked in books on the Constellation is the fact that even as World War II was winding down, Lockheed looked at fitting the baseline Constellation with gas turbine powerplants.

Left: Three-view drawing of the Lockheed L-151-1 jet airliner
Right: Three-view drawing of the L-151-2 along with a side view of the L-151-3 (upper right)

In 1945, the development of jet engines was destined to make the Lockheed Constellation and many other large piston-powered American transport aircraft designs envisaged during the years of US involvement in World War II slowpokes compared to jet or turboprop aircraft designs being worked on by US aircraft manufacturers in the 1943-1945 timeframe. Lockheed had developed America's first jet engine design, the L-1000 (military designation XJ37), to power its L-133 jet fighter project, but when the L-133 itself was deemed too advanced for its time and the initial L-1000 design turned to be more complex than imagined by engineer Nathan Price, it ended up being redesigned as a simple jet engine comprising two sixteen-stage axial compressors connected by an intercooling stage. Capitalizing on flight testing of the P-80 (later F-80) Shooting Star jet fighter, Lockheed in early 1945 conducted design studies to equip the Constellation airliner with gas turbine engines under the company designation L-151. The first design, called L-151-1, replaced the L-049's Duplex Cyclones with six L-1000 turbojets in two underwing nacelles (three engines per nacelle), and judging from what little technical data has survived, the L-151-1 had the same length and wingspan as the commercial L-049 but would have a slightly higher gross weight of 100,000 lb (45,359 kg). Two turboprop-powered L-151 iterations were worked out, the L-151-2 with four General Electric TG-100 turboprops and the L-151-3 with four Westinghouse turboprops. These designs had same gross takeoff weight as the L-151-1 and compared to the L-049, they would have greater top speed, with the L-151-2 being designed to cruise at 335 mph (539 km/h) and the L-151-3 having a cruising speed of 370 mph (595 km/h). The L-151-2 and L-151-3 also had a higher service ceiling and rate of climb at 18,000 feet (5,486 meters) than the L-049, with L-151-2 having a climb rate of 610 ft/min (3.10 m/s) and capable of reaching a 28,000 ft (8,534 meter) altitude and the L-151-3 featuring an estimated climb rate of 1,060 ft/min (5.348 m/s) and an altitude of 31,000 feet (9,449 meters). Nevertheless, the one performance pitfall of the L-151-2 and L-151-3 designs was operating range. The L-151-2 and L-151-3 would have had operating ranges of 3,500 miles (5,633 km) and 3,100 miles (4,989 km) respectively, well below the L-049's maximum range of 4,400 miles (7,081 km). The dearth of specifications for the L-151-1 makes it unclear whether the L-151-1's range would have been equivalent to or exceeded that of the Constellation, but the marginally lower operating range of the L-151-2 and L-151-3 compared to that of the L-049 meant that Lockheed did not pitch those designs to the airlines. In the meantime, the L-1000 engine which the L-151-1 would have used ended up being cancelled by the late 1940 without leaving the development phase, and the L-151-1 also would never reach the hardware phase.

Front views of the different L-151-5 variations accompanied by side and bottom views of different auxiliary turbojet placements proposed for the L-151-5 (far-right)

This was not the end of the L-151 story, however. In 1950, Lockheed took advantage of its development of the L-1049 Super Constellation with a stretched fuselage by envisaging a spree of design studies for an L-1049C with two 4,200 lb (18.7 kN) thrust auxiliary jet engines, designated L-151-5. The proposed L-151-5 was quite akin to the B-36 Peacemaker being fitted with four auxiliary General Electric J47s in two podded pairs for extra speed beginning with the B-36D version, and a number of L-151-5 schemes were proposed as follows:

• Mounting the auxiliary turbojets below underwing pylons (as with the B-36)
• Placing the auxiliary turbojets at the rear of the outboard R-3350 Duplex Cyclone radial piston engines to provide minimum drag for jet engine out condition
• Placing the auxiliary turbojets on the wingtips to provide minimum drag when all engines were operating (with or without auxiliary fuel tanks pyloned below the outboard wing sections)

Lockheed calculated the performance of the L-151-5 design and determined that when compared with the L-1049C, the L-151-5 offered 25 percent greater block speed and a shorter takeoff distance of 3,750 feet (1,143 meters) along with a slightly lower operating cost. Still the operating range of the L-151-5 would be slightly lower than that of the L-1049C due to the drag generated by the jet engines when they were not in operating condition. Even though the L-151-5 would have been easy to create by equipping an L-1049C airframe with auxiliary turbojets because it required no substantial structural modifications compared to the L-151-2 and L-151-3, no airline expressed an interest in the L-151-5 at all.

Notwithstanding the fact that the L-151 was used by Lockheed as a catchall company designation for a variety of proposals to fit the Constellation with gas turbine engines, one good question arises: would the L-151-1 have been an outstanding and profitable airliner with respect to its impact on passenger air travel in the late 1940s and early 1950s had it been built? There's no question that US industry was developing more advanced jet engines with high power/weight ratios by the time that World War II ended, including the Allison J35 and General Electric J47, but since the L-1000 engine had offered the L-133 project far greater estimated top speed than that of the Shooting Star or Airacomet, in my own opinion, the L-151-1 would have carried out passenger flights at much shorter flight times, although it have somewhat slower than the Boeing B-47, B-52, and 707 due to its straight wings.

References:

Slayton, B., 1999. "The Lockheeds That Never Were, Part II." AAHS Journal 44 (2): 102-113.

Lost Broncos from Burbank and El Segundo

Much has been written about the Light Armed Reconnaissance Aircraft (LARA) competition of the mid-1960s for an aircraft designed to carry out counter-insurgency (COIN) operations, which produced the North American OV-10 Bronco (which won the LARA competition) and the rival, prototype-only Convair Model 48 Charger. However, the Model 48 Charger, despite being manufactured in my home state, was not the only LARA design conceived in southern California. Two additional companies based in southern California put out design bids for the LARA competition, one emanating from Burbank and another coming from the design offices of the El Segundo Division of Douglas. 

Company artwork of the Douglas D-855

Following the issuance of the tri-service LARA requirement in late 1963, nine companies submitted bids for the LARA competition by March 1964, of which three (Convair San Diego, Douglas El Segundo, and Lockheed) were based in southern California. The Douglas proposal, designated D-855, took the form of a high-wing airplane with a T-tail empennage, a rear loading door, a crew of two seated in tandem, a length of 35 feet (10.67 meters) long, and a wingspan of 29 feet (8.83 meters). The D-855 would carry six fully-equipped troops or a 6,000 lb (2,721 kg) payload in the fuselage, and various weapons loads (e.g. folding fin unguided air-to-ground rockets and bombs) were carried below five hardpoints, one situated below the fuselage centerline, two below the wing roots, and two positioned just inboard of the wingtips. Top speed of the D-855 was to be 230 mph (370 km/h), and power would be provided by either two United Aircraft of Canada T74 or Garret AiResearch T76 turboprops. The El Segundo Division of Douglas informally referred to the D-855 as the "Skyraider II", a testament to its rich experience with attack aircraft design.

Full-scale mockup of the Lockheed CL-760 with external weapons carried onboard or individual displayed on the ground. 

The Lockheed submission for the LARA competition, the CL-760, shared the high-wing layout and the  tandem seating arrangement for the two crewmembers with the D-855, but it had a conventional tail empennage whose horizontal stabilizers formed a triangular planform. It measured 40 feet 3.5 in (12.28 meters) long with a wingspan of 30 feet (9.14 meters), an empty weight of 5,106 lb (2,316 kg), a gross weight of 9,270 lb (4,205 kg), and a top speed of 325 mph (523 km/h). Accommodations were provided for either eight fully equipped troops or six paratroopers in the cargo hold, and power was to come from two Garrett T76 turboprops, while the retractable main landing gear and four 7.62 mm machine guns would be housed in two fuselage blisters. The CL-760 had four racks below the wingtips (two below each wingtip) and a fifth pylon below the fuselage centerline for carrying various weapons, including folding-fin unguided air-to-ground rockets, air-to-surface missiles, and bombs. When configured for reconnaissance, the CL-760 itself would carry advanced cameras and sensors to allow it to loiter over positions and movements of enemy insurgents in various terrains such as dense forests and open fields at low altitudes. 

In August 1964, the US Navy declared North American NA-300 was declared the winner of the LARA competition (although the US Air Force and US Marine Corps had preferred the rival Convair Model 48 design, of which a prototype was under construction at the time of the announcement of the outcome of the LARA competition), receiving the designation OV-10 and the official name Bronco. Because the Douglas El Segundo Division and Lockheed lost the LARA competition to the NA-300, the Model 48  would be the only competing LARA design from southern California to be built and flown, making its first flight on November 25, 1964, eight months before the OV-10 took to the skies.

References:

Francillon, R., 1987. Lockheed Aircraft Since 1913. Annapolis, MD: Naval Institute Press. 

Ginter, S., Auten, H., Knebel, J, and Fink, J., 1997. Convair Model 48 Charger (Naval Fighters No. 39). Simi Valley, CA: Ginter Books.

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.