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. 

Southern California's UCLASS and CBARS drone designs

In March 2010 the US Navy initiated the Unmanned Carrier-Launched Airborne Surveillance and Strike (UCLASS) program for an unmanned combat air vehicle (UCAV) with tactical strike and intelligence capabilities, issuing for Request for Information (RFI) for the program that month. Since the early 2000s, the Navy had made efforts to develop a UCAV, beginning with the Naval Unmanned Combat Air Vehicle (UCAV-N) program initiated in mid-2000 that produced the Northrop Grumman X-47A Pegasus and the Unmanned Combat Air System Demonstrator (UCAS-D) program (a rebranding of the Joint Unmanned Combat Air Systems (J-UCAS) program after the US Air Force quit in early 2006) that led to development and flight tests of the X-47B full-scale UCAV technology demonstrator. Although the X-47B was still undergoing taxi tests and had not yet flown by the time the UCLASS program was started, the Navy was sorely lacking a purpose-built replacement for the long-retired A-6 Intruder given that the stealthy A-12 Avenger II flying wing attack aircraft which was to have replaced the A-6 ended up being canceled in January 1991 after cost overruns and developmental problems before any aircraft could ever be completed. The UCLASS concept aircraft was to be a stealthy strike platform with a weapons load comprising twenty-four 250 lb (113 kg) GBU-39 SDB bombs and able to utilize a suite of modular and/or federated ISR sensors with all weather capability as well as EO/IR, multi-mode radar, and ESM. The Navy's roadmap for the future of its combat aviation capabilities called for deployment of the UCLASS by 2018.

Left: Computer-generated image of the General Atomics Sea Avenger derivative of the company's MQ-20 Avenger (originally Predator C). (courtesy of General Atomics)
Right: Mock-up of the Lockheed Martin Sea Ghost in early 2013. (courtesy of Lockheed Martin)

In response to the RFI issued for the new UCLASS program, three companies in southern California conceived designs for the UCLASS program that were based on existing designs. General Atomics, which is based near San Diego and has developed unmanned aircraft since the late 1980s, proposed a navalized version of the MQ-20 Avenger (originally called Predator C) as the Sea Avenger, which had an internal weapons bay, a retractable electro-optical/infrared sensor, and folding wings along with an arrestor hook, drag devices, carrier suitable landing gear, and other carrier compatible provisions. For its part, Northrop Grumman offered a derivative of the X-47B, being well-positioned to do so because of its experience with development of the X-47A and X-47B. In July 2012, Lockheed Martin unveiled a carrier-based derivative of the RQ-170 Sentinel stealthy unmanned tactical reconnaissance flying wing, the Sea Ghost, which would have the range and flight endurance of the A-6 Intruder but carry a smaller weapons load while utilizing open architecture avionics to enable new sensors or mission systems to be carried onboard over time. Like the F-35C naval variant of the F-35 Lightning II, the Sea Ghost was to incorporate sea spray optimized stealth materials, something that Lockheed Martin was most probably  mindful of when it undertook design of the unbuilt A/F-117X naval derivative of the F-117 Nighthawk in the mid-1990s.

On December 19, 2012, the Defense Department's Joint Requirements Oversight Council (JROC) had the UCLASS requirements revised to make the program heavily favor permissive airspace intelligence, surveillance, and reconnaissance (ISR) capabilities because the Pentagon expected plans for a strike capability for the UCLASS to be constrained by its planned submission of its budget for FY2014. The payload capacity specified in the original requirement was revised to 3,000 lb (1,360 kg) comprising 1,001 lb (454 kg), and stealth requirements were sharply reduced to lower costs. The US Navy initially planned to issue a Request for Proposals (RFP) for the UCLASS program in late 2012, but this was delayed to April 17, 2014 due to disagreements over the UCLASS concept aircraft's degree of stealth, in-flight refueling ability, and ability to survive in contested airspace. While the Sea Ghost and the proposed derivative of the X-47B relied on stealth, the Sea Avenger and Boeing's proposed adaptation of the Phantom Ray (itself a reincarnation of the X-45C) preferred to emphasize range and payload rather than stealth. Even before the RFP was issued, on August 14, 2013, the Navy awarded $15 million development contracts to Boeing, General Atomics, Lockheed Martin, and Northrop Grumman to develop their UCLASS airframe designs. 

Not too long after Boeing, General Atomics, Lockheed Martin, Northrop Grumman received contracts to refine their UCLASS airframe designs, in December 2013, the UCLASS concept aircraft evolved into a "heavy-end" strike/ISR aircraft with a length of 68 feet (21 meters), a weight of 70,000 to 80,000 lb (32,000 to 36,000 kg), an endurance of up to 14 hours, and greater room for weapons and sensors, making it comparable in size to the F-14 Tomcat and bigger than the X-47B and F/A-18E/F Super Hornet. The original UCLASS requirement parameter calling for providing daily ISR coverage from an aircraft carrier remained fixed, and one role explored for this revised UCLASS concept aircraft was an aerial refueling platform to refuel fighter aircraft. These revisions to the UCLASS concept aircraft were again due to cost constraints, but the four companies which had proposed designs for the UCLASS program pushed back against the revised requirements on the grounds that they required a drone far more capable than the Sea Ghost, Sea Avenger, and operational derivatives of the X-47B and Phantom Ray. The UCLASS concept was changed again on July 17, 2014, this time focusing on an unmanned aircraft whose missions initially were to include permissive airspace ISR and strike before eventually expanding to contested littoral and coastal ISR and strike missions and attacking enemy surface ships, and the designation ZRAQ-25A was allocated to the UCLASS program. The US Navy in December 2014 stipulated that E-2 Hawkeye squadrons would be given control over the ZRAQ-25A during aerial operations, and the Navy's FY 2016 budget request had the planned operational deployment of the UCLASS further delayed to 2022-2023 and an RFP for the revised UCLASS concept aircraft originally scheduled to be issued in September 2014 postponed due to an ongoing review of what roles would be performed by the ZRAQ-25A. 

Left: Computer-generated image of the Lockheed Martin Sea Ghost refueling an F-35C (courtesy of Lockheed Martin)
Right: Computer-generated image of the proposed adaptation of the General Atomics Sea Avenger submission for the CBARS competition (courtesy of General Atomics) 

On February 1, 2016, the US Navy shelved the UCLASS program in favor of the Carrier-Based Aerial Refueling System (CBARS) for an unmanned aerial refueling tanker the size of the Super Hornet and with minimal ISR capabilities, deferring the strike capability and some communications roles long envisioned for the UCLASS to a future variant of the CBARS, and in July of that year, the CBARS was officially christened the MQ-25A Stingray. The three companies from southern California which had proposed designs for the UCLASS program now adapted those proposals for the MQ-25 competition after an RFP for the MQ-25 program was issued in October 2017. While Northrop Grumman offered an X-47B derivative with reduced stealth and aerial refueling capability, General Atomics envisaged a slightly enlarged Sea Avenger iteration for aerial refueling while Lockheed Martin unveiled a revised Sea Ghost design with greatly reduced stealth. On October 25, Northrop Grumman withdrew from the contest after saying that it would be not able to properly manage development of its X-47B derived design if it aimed to win the MQ-25 contract.

The Boeing MQ-25 Stingray, which won the CBARS competition in August 2018.

On August 30, 2018, the US Navy declared Boeing the winner of the MQ-25 Stingray competition. Back in 2014, Boeing had built in secret a wing-body-tail unmanned aircraft that incorporated lessons learned from the Phantom Ray and its other UAV programs when the UCLASS requirements were drastically revised, but it was not until December 2017 that this aircraft was unveiled as the Boeing submission for the MQ-25 contest. On September 19, 2019, the MQ-25 Stingray made its first flight, and 2021, it became the first UAV in history to refuel manned aircraft when it conducted refueling tests of an F/A-18F, F-35C, and E-2. Although the MQ-25 is mainly designed for aerial refueling, in April 2024 Boeing unveiled a strike/ISR variant of the MQ-25 armed with two AGM-158C LRASM stealthy cruise missiles below two underwing hardpoints, indicating that the Navy is keenly contemplating plans to give the Stingray the strike and ISR capabilities envisioned for the canceled UCLASS program. In this way, if the proposal for a strike/ISR variant of the MQ-25 does get built, it will not only fulfill the operational roles which the Sea Avenger, Sea Ghost, and Northrop Grumman's operational derivative of the X-47B were designed when first conceived but also function as a true successor to the A-6 Intruder in terms of providing the Navy with a purpose-built attack aircraft after the cancellation of the A-12 Avenger II program in 1991, the abortive A/F-117X and A/F-X programs of the mid-1990s, and the shelving of the UCLASS program in 2016.

For more on the General Atomics, Lockheed Martin, and Northrop Grumman UCLASS and CBARS proposals, see the following links:

https://www.ga-asi.com/ga-asi-introduces-sea-avenger-uas-for-uclass-carrier-operations

https://www.flightglobal.com/lockheed-unveils-unmanned-surveillance-and-strike-aircraft/109320.article

https://www.secretprojects.co.uk/threads/us-navy%E2%80%99s-uclass-cbars-mq-xx-mq-25-stingray-program.16346/

https://www.popularmechanics.com/military/aviation/a19600045/lockheed-martin-unveils-mq-25-stingray-tanker-drone-design-for-the-navy/

https://www.businessinsider.com/us-navy-carrier-based-drone-race-mq-25-stingray-2018-4

https://www.flightglobal.com/northrop-grumman-pulls-out-of-mq-25-competition/125865.article

Long-range carrier-based escort fighters from southern California that never were

The earliest years of the Cold War in the late 1940s saw a proliferation of requirements by the US Navy's Bureau of Aeronautics for different types of carrier-based fighter planes, namely day fighters, all-weather fighters, and interceptors, which produced the F9F, F3D, F3H, and F4D. However, mostly lost in talk with respect to US Navy jet fighter development in the 1946-1950 period is the fact that while the US Air Force tinkered with a penetration fighter requirement that led to the XF-88, XF-90, and YF-93, the BuAer had its own requirement for a long-range escort fighter, and although a number of companies from the Los Angeles area came up with extraordinary designs for such an aircraft, the Navy's long-range escort fighter specification of the late 1940s ended up being dropped before any design submission could be selected for full-scale development.

On December 30, 1947, the Navy's Bureau of Aeronautics issued the OS-112 requirement for a four-seat jet fighter with a top speed of 535 mph (861 km/h) at 40,000 ft (12,192 meters), a operational combat radius of 1,381 miles (2,224 km), a service ceiling of 45,000 ft (13,716 meters), and a climb rate of 5,000 ft/min (25.4 m/s). Although the USSR did not yet have a nuclear weapon and was seen by US intelligence as a long way off from testing one, the appearance of the Soviet Union's MiG-15 meant that the Navy may have felt it necessary to plan for operating an escort fighter to provide protection of the forthcoming North American AJ Savage strategic nuclear-armed bomber if that aircraft conducted nuclear strike missions deep inside the Soviet heartland from the high seas. A July 1948 deadline for companies to submit bids for the OS-112 spelled out of the Navy, and six companies worked out designs for a carrier-based escort fighter: Boeing, Curtiss-Wright, Douglas, Lockheed, McDonnell, and Vought.

Three view drawing of the Douglas Model 1163 naval escort fighter (dated July 30, 1948)

The Douglas company was well-positioned to undertake design studies for an escort fighter to fulfill the parameters of the OS-112 requirement buoyed by its experience with design and development of the F3D Skyknight all-weather fighter. The Santa Monica and El Segundo branches of Douglas worked out two independent naval escort fighter studies, with the Santa Monica division's proposal bearing the company designation Model 1163 and the El Segundo branch's design being called D-585. The Model 1163, conceived in July 1948, was a very large escort fighter design which measured 65 feet 3.5 in (19.90 meters) long and had wings with a moderately swept leading edge and straight trailing edge spanning 90 feet (27.43 meters) when unfolded and 35 feet (10.67 meters) when folded. It was powered by three 4,200 lb (18.7 kN) thrust Westinghouse J46 turbojets mounted below the fuselage aft of the rear wing spar and fed by air intakes on the sides of the forward fuselage beneath the cockpit canopy, and each of the engines would be enclosed in stainless steel casings separating them from each other and the rest of the aircraft. The Model 1163 would have the wing panels folded fore and aft of each other over the fuselage in order to easily accommodate itself aboard an aircraft carrier, and the nose section and a portion of the vertical stabilizer could also fold back. It had a top speed of 578 mph (930 km/h), a range of 2,595 miles (4,809 km), a service ceiling of 46,800 feet (14,265 meters), a maximum take-off weight of 52,000 lb (23,587 kg), and a climb rate of 5,170 ft/min (26.26 m/s). Armament was to consist of four 20 mm machine guns, two in the nose and two in a power-operated tail turret, and the tricycle landing gear had a narrow track and twin nosewheels. The Douglas company had opted to use turbojets rather than turboprops for the Model 1163 because it deemed a turbojet-powered escort fighter to have greater all-round climb performance in contrast to a turboprop design requiring an auxiliary turbojet to attain the climb rate specified in the OS-112 specification. Very little is known regarding the El Segundo division's D-585 escort fighter project, other than the fact that it was conceived in April 1948, and because no drawings or data exist in the BuAer section of the National Archives, the D-585 probably was not submitted for the OS-112 specification.

Drawing of the Lockheed L-180-2 from the project documents (dated July 1948)

Even before Douglas unveiled the Model 1163 proposal, Lockheed envisaged five distinct designs for combat aircraft in July 1948 under the designation L-180. The L-180 iteration submitted for the OS-112 requirement was the L-180-2, was a backswept wing aircraft with a length of 55 feet (16.76 meters) and wings spanning 81 feet 6 in (24.84 meters) when unfolded and 33 feet (10.06 meters) when folded. The L-180-2 was powered by two Allison T40 turboprops driving eight-blade counter-rotating propellers 13 feet (3.96 meters) in diameter, and to accommodate itself aboard the Oriskany-class aircraft carriers, it had the outboard wing sections fold to the rear. Armament consisted of four 20 mm machine guns, two in the nose and two in a tail turret, and upward- and downward-facing APS-29 radars were fitted aft of the fuel tanks behind the cockpit. Aware that the Navy would judge the L-180-2 design too slow and heavy to operate from small carriers if it were evaluated, Lockheed saw its design for a carrier-based version of the XF-90, the L-180-3, as a lighter alternative to the L-180-2 although the range of the L-180-3 would fall well short of the combat radius specified in the OS-112 parameters. Although outside the scope of this post, it should be noted that Lockheed saw the L-180-5 carrier-based strategic jet bomber design as having the potential to work in tandem with the L-180-3 when necessary to achieve the 1,381 mile combat radius on select combat missions.

All the design submissions to OS-112, including the Douglas 1163 and Lockheed L-180-2, were very interesting escort jet fighter projects, but in the end the US Navy discarded the long-range escort fighter concept before it could evaluate any of designs to decide which proposal should be ordered because the first successful nuclear weapons test by the USSR in September 1949 and deployment of the Tupolev Tu-4 strategic bomber (the Soviet copy of the B-29 Superfortress) made the Navy's need for an all-new carrier-based interceptor to tackle the emerging threat of nuclear-armed Soviet strategic bombers more urgent. Even if either the L-180-2 or Model 1163 had been selected by the Navy to enter full-scale development, they would have not been fast enough to take on Soviet jet fighters even while offering protection for the AJ Savage or the future A-3 Skywarrior if those aircraft conducted nuclear strike missions over the USSR.

References:  

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