Southern California's lost long-range interceptors, part 2: Lockheed YF-12

The cancellation of the F-108 Rapier in September 1959 was a setback to efforts by the US Air Force to field an advanced long-range interceptor to take on advanced strategic bombers being developed by the USSR, including the Myasishchev M-50 and Tupolev Tu-22. However, the quest by the aircraft industry of southern California to provide the Air Defense Command with a Mach 3 interceptor did not totally die out with the F-108's termination. Not too long after the F-108 program was axed, a revolutionary aircraft of similar speed to the F-108 would come to form the basis for another US effort to acquire a Mach 3 long-range interceptor, this time from Lockheed.

Desktop model of the YF-12 at the Planes of Fame Museum, Chino (photographed by me in 2018)

In 1960, while undertaking development of the A-12 (Archangel-12) spy plane, Lockheed proposed a long-range interceptor variant of the A-12 for the US Air Force under the internal designation AF-12. The AF-12 design was to have provisions for a second crewmember (radar operator), missile bays for folding fin versions of the GAR-9 (later AIM-47) long-range air-to-air missile, and a Hughes ASG-18 fire control search-and-track radar. Pleased with the AF-12 proposal, in October 1960 the US Air Force signed a contract whereby the seventh through ninth A-12s on order (serial numbers 60-6934/6936) would be completed as AF-12s. The CIA assigned the codename KEDLOCK to the AF-12 program, and a mock-up was inspected by Air Force officials in May 1961, after which the AF-12 design was tweaked the following month to include three ventral fins (two below the engine nacelles and a third larger fin below the extreme rear of the fuselage). To accommodate the Hughes ASG-18 fire-control radar, the chines were cut back insofar that they extended to the front of the cockpit, and the nose was now made of plastic composite material that was transparent to radar and resistant to thermal loads expected from Mach 3+ flight. The AF-12 had a length of 101 feet 8 in (30.97 meters), a wingspan of 55 feet 7 in (16.95 meters), a height of 18 feet 6 in (5.64 meters), a wing area of 1,795 square feet (167 square meters), an empty weight of 60,730 lb (27,604 kg), a gross weight of 124,000 lb (56,200 kg), and a maximum takeoff weight of 140,000 lb (63,504 kg).

Select photos of the Lockheed YF-12A in flight 

Construction of the first AF-12 began in August 1962 at the Lockheed factory in Burbank, California, where other A-12s were being manufactured. After the Defense Department introduced the Tri-Service aircraft designation system on September 18, 1962, the AF-12 was officially designated YF-12 as a matter of convenience given its A-12 heritage. The first YF-12A was completed in the first half of 1963 and transported to Area 51 in Groom Lake, Nevada, where it made it first flight on August 7; the second YF-12A aircraft flew for the first time on November 23, 1963, and the third prototype followed suit on March 13, 1964. The YF-12A's existence was publicly disclosed on February 29, 1964 when President Lyndon B. Johnson referred to it as "A-11" at his first nationally televised news conference, and the aircraft was publicly unveiled at Edwards Air Force Base on September 30. For security reasons, LBJ may have chosen to referred to the YF-12A as "A-11" because the Soviets had breached Lockheed and CIA security, even though the A-11 was a fundamentally different design than the YF-12A (Lowther 2021, pp. 99-100). In the meantime, the AIM-47 Falcon was ejected from a YF-12A for the first time on April 16, 1964, and on March 18, 1965 the YF-12A scored its first kill when an AIM-47 hit an aerial  target 36.2 miles (58.26 km) away. On May 1, 1965, the first and third YF-12As set speed and altitude records of 2,070 miles per hour (3,331.505 km/h) and 80,258 feet (24,462.60 meters) respectively, and four months later, on September 28, the first YF-12A fired an AIM-47 at a speech of Mach 3.2 at an altitude of 75,000 feet (22,860 meters), intercepting a JQB-47E target drone 32.2 miles (51.8 km) away at 500 feet (150 meters). In all, 13 missile firings were conducted from the YF-12A up to 1966.

Forward fuselage mock-up of the Lockheed F-12B, the proposed production F-12

The USAF's Air Defense Command was quite impressed by test flights of the YF-12A, and May 14, 1965 it placed a production order for 93 F-12Bs. The F-12B was similar to the YF-12A but differed in utilizing the nose chines of the A-12 and SR-71 but also eliminating the central large stabilizing fin below the rear fuselage, not to mention that the underside of the forward fuselage was flattened and the tail of the rear fuselage extended as in the SR-71. Instead of using ejectors to toss the AIM-47s from the internal weapons bays, the F-12B would have used deployable trapeze launch rails that could deploy the missiles in a substantially nose-down altitude. Plans called for spreading out the F-12B fleet to different Air Force bases, with three squadrons of 16 F-12Bs each sent to bases in the northeast US, and three squadrons of 16 aircraft deployed on the west coast of the US. However, US Secretary of Defense Robert McNamara did not support development of the F-12B, and due to Vietnam War cost and a lower priority placed by updated intelligence analysts upon defense of the continental US, the F-12B program was cancelled in January 1968. Not too long before the F-12B's cancellation, Lockheed had also looked into a fighter-bomber version of the F-12, unofficially called FB-12 by the company, which resembled the SR-71 and would carried either four AGM-69 nuclear-armed land attack missiles, two AGM-69s and two AIM-7 Sparrow air-to-air missiles, four AIM-7s, three AIM-7s and one M61 Vulcan machine gun, four AIM-47s, or three AIM-47s and one M61. However, the FB-12 went no further than the drawing board because the FB-111 and F-15 would soon be developed to fulfill the FB-12's intended air-to-air and air-to-ground roles. In the meantime, the first YF-12A (serial number 60-6934) suffered a landing mishap that caused the forward fuselage to be damaged beyond repair, leaving the remainder of the first YF-12A to be mated with a static test SR-71 airframe to create the SR-71C two-seat trainer. The second and third YF-12A later were transferred to NASA in 1969 for scientific missions ranging from sonic boom tests to high-altitude air sampling; serial number 60-6936 was lost in a crash near Edwards Air Force Base on June 24, 1971 after an in-flight fire, but both pilots ejected safely, while the sole remaining YF-12A continued serving NASA until November 17, 1979, when it was flown to the US Air Force Museum at Wright-Patterson Air Force Base in Dayton, Ohio. (NASA acquired SR-71 serial number 61-7951 in 1971 for aerodynamic research purposes after the crash of the third YF-12A and assigned it the bogus designation YF-12C and the fair serial number 60-6937 due to the secrecy surrounding the SR-71, operating this aircraft until December 1978, but that is another story.)

Although the F-108 Rapier was canceled without ever reaching the hardware phase and the F-12 almost made it into production when the Defense Department canceled F-12 production plans after deciding that defense of the continental US wasn't too important, some legacies of the F-108 and F-12 programs would find their way into future jet fighters. For example, the AIM-47 Falcon designed to be carried by the F-108 and F-12 would form the basis of the AIM-54 Phoenix, the chief armament of the US Navy's F-14 Tomcat, while the ASG-18 radar was later developed into the more advanced AWG-9 and APG-71 all-weather radars that equipped the F-14. Today, the F-15 Eagle and F-22 Raptor utilize the long-range air-to-air combat role envisaged for both the F-108 and F-12 in tandem with the air superiority role for which they are primarily designed.

References:

Jenkins, D.R., and Landis, T.R., 2008. Experimental & Prototype U.S. Air Force Jet Fighters. North Branch, MN: Specialty Press.

Landis, T.R., and Jenkins, D.R., 2005. Lockheed Blackbirds (Warbird Tech Series, Volume 10), Revised edition. Minneapolis, MN: Specialty Press. 

Lowther, S., 2021. Lockheed SR-71 Blackbird - Origins and Evolution. Horncastle, UK: Tempest Books.

Southern California's lost long-range interceptors, part 1: North American F-108 Rapier

For many US aircraft manufacturers, the 1950s would see quantum leaps in combat aircraft technology when it came to supersonic speed, punctuated by the USAF's introduction of its first supersonic jet fighter, the North American F-100 Super Sabre, but also the Douglas D-558-2 Skyrocket becoming the first plane to reach Mach 2 in November 1953. The Convair F-102 Delta Dagger and F-106 Delta Dart, first flown in 1953 and 1956 respectively, constituted quantum leaps in the development of US supersonic fighter planes because they gave America the first-ever purpose-built supersonic interceptors to take down Soviet long-range bombers intruding into North American airspace. With development of the North American XB-70 Valkyrie Mach 3 supersonic bomber underway, however, the US Air Force began shopping for an all-new long-range interceptor capable of traveling at Mach 3 and countering the USSR's upcoming supersonic bombers like the Myasishchev M-50, Tsybin RSR, and Tupolev Tu-22. In response, the aircraft industry in southern California unveiled a spree of Mach 3 long-range interceptor designs, some conceived from scratch and others derived from existing aircraft designs. Due to the prolonged quest by the USAF for a Mach 3 interceptor, I am writing a two-post series on Mach 3 interceptor design in the Los Angeles basin; the first post will focus on the North American F-108 Rapier.

Top: Northrop N-144 interceptor design submission for the 1954 LRI competition
Bottom left: Desktop model of the North American submission to the 1954 LRI competition
Bottom right: An artist's conception of the North American NA-236, the winner of the LRI-X competition and the final step towards the F-108 Rapier  

In late April 1954, the USAF announced the Long-Range Interceptor (LRI) program for an advanced long-range interceptor with an operating altitude of 60,000 feet (18,000 meters) at Mach 1.7 (1,122 mph (1,806 km/h) with a maximum range of 1,151 miles (1,852 km), and the ability to detect enemy targets over a distance of 115 miles (185 km). The parameters for the LRI requirement were covered under the designation WS-202A, and ten companies undertook design studies for the LRI requirement, of which seven submitted bids. Northrop submitted a revised design for its earlier N-126 'Delta Scorpion' design for the WS-202A specification with a long, slim fuselage, turbojets below the wings, and low-mounted horizontal stabilizers, and it also worked out two additional designs, the N-144 (a scaled-up version of the N-126) and the lightweight N-149. North American's WS-202 submission had twin upper and lower vertical stabilizers, a cropped delta wing, a long, slim fuselage, and delta wing canards, and Lockheed's submission, the CL-288, looked like an F-104 Starfighter with mid-wing mounted turbojets. None of the seven designs offered for the LRI competition fully met the performance requirements in the LRI specification, but the N-144 was judged by Wright Field to come closest to meeting the parameters of the LRI requirement. As the submissions for the LRI requirement did not meet the required operational altitude, the evaluators at Wright Field asked for a relaxation of the original requirements, and on July 20, 1955, the US Air Force initiated the LRI-X (Long-Range Interceptor - Experimental) program, which like the earlier LRI program stipulated that a new-generation long-range interceptor should fly at 60,000 feet (18,000 meters) at a speed of Mach 1.7 (1,122 mph (1,806 km/h), but called for the new interceptor to utilize an integrated fire-control system to allow for interception of a bomber over a range of 69 miles (111 km), with the ability to make three kills. The GOR-114 requirement was initiated on October 6 to cover the LRI-X operational parameters, and by October 11, the USAF awarded study contracts to three companies (Lockheed, Northrop, and North American). The Lockheed CL-320 was similar to the CL-288 but was much larger and heavier, and it was powered by four General Electric J79 turbojets housed in two mid-wing nacelles, along with outrigger landing gear below the nacelles. North American proposed the NA-236, which resembled the 1954 North American interceptor proposal in having delta canards but differed in utilizing a delta wing, the canards atop the forward fuselage, and two side-by-side General Electric J93 turbojets. A large central vertical stabilizer was situated between the turbojets, and two smaller vertical stabilizers were mounted in mid-wing position along the wing's trailing edge. Northrop's LRI-X design, the N-167, had four General Electric J79 turbojets housed inside the fuselage, fed by air flowing through intakes in the wing roots, and two designs were investigated, the baseline N-167 with a tail empennage similar to that of the F-104 and wings with pronounced anhedral, and the N-167A design of April 1956 with the horizontal stabilizer on the rear side of the fuselage and of the same span as the wings. By January 1956, the NA-236 was declared the winner of the LRI-X contest, but budgetary constraints forced the Pentagon to cancel the LRI-X program on May 9. At the behest of the Air Defense Command, however, the Pentagon revived the long-range interceptor program on April 11, 1957, and in June of that year North American won a contract for the aircraft, which eventually was designated F-108.The Air Defense Command at the time was hoping to procure 480 F-108s.

Top: Model of the North American F-108 Rapier at the Lyon Air Museum, photographed by me in November 2021.
Bottom: Full-scale mockup of the F-108 at the North American plant in Inglewood, California, January 1959.

The initial XF-108 design (company designation NA-257) conceived in early May 1958 was similar to the NA-236 but had the trailing edge flaps split into two downward curving surfaces. It measured 84 feet 11 in (25.9 meters) long with a wingspan of 52 feet 11 in (16.1 meters), a wing area of 1,400 square feet (130.2 square meters), and a gross weight of 99,400 lb (45,088 kg), and it was powered by two General Electric J93s. Beginning in September, however, the F-108 design was revised whereby the canards were removed and larger underwing vertical stabilizers were provided to offer better handling and directional stability at low speeds and high angles of attack. Three months later, the wing design for the F-108 was changed to a cranked arrow shape with substantial tip extensions offering improved stability at high lift conditions, and large folding ventral stabilizers were fitted to the corners of the lower fuselage to address concerns about directional stability. In this form, the overall F-108 design was to be 89 feet 2.5 in (27.2 meters), with a wingspan of 57 feet 5 in (17.5 meters), a height of 22 feet 1 in (6.7 meters), a wing area of 1,865 square feet (173.4 square meters), an empty weight of 50,907 lb (23,098 kg) and a gross weight of 102,234 lb (46,373 kg). The F-108 was armed with three GAR-9 (later AIM-47) Falcon long-range air-to-air missiles, which had a range of 115 miles (185 km/h), and the top speed was to be Mach 3, the same speed as the company's own XB-70 Valkyrie supersonic strategic bomber. A full-scale mock-up of the F-108 was completed and inspected by US Air Force officials in January 1959, and the F-108 was officially christened Rapier in May. By this time, the first flight of the F-108 was scheduled for March 1961 with service entry planned for mid-1963, and the USAF order for 30 service test F-108s was reduced from 30 to 21 aircraft. However, the Pentagon was facing a tight budget stemming from the development of the Atlas and Titan ICBMs, the XB-70 Valkyrie supersonic bomber, the Polaris SLBM, and the Navy's fielding of its first generation of ballistic missile submarines, and given the rising cost of the F-108 program, on September 23, 1959, the USAF announced the cancellation of the Rapier. Several months before the F-108's cancellation, there was a proposal for a Mach 3 target drone for the testing the F-108's weapons systems, and in July 1959 the designation XQ-11 was requested for this vehicle, but the USAF Headquarters turned down the request, stating that a specific designation for the proposed F-108 target was unnecessary at an early stage of the F-108 program, and like the F-108, the XQ-11 did not proceed to the hardware phase.

References:

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

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

Jenkins, D.R., and Landis, T.R., 2008. Experimental & Prototype U.S. Air Force Jet Fighters. North Branch, MN: Specialty Press.

Experimental pusher fighters from the Los Angeles basin: Vultee XP-54 and Northrop XP-56

The annals of US fighter development in World War II before and after the Japanese attack on Pearl Harbor in December 1941 were defined by a proliferation of advanced designs for piston-engine fighter planes to either supplant or augment existing fighters in service with the US Army Air Force, namely the P-40 Warhawk, P-47 Thunderbolt, and P-51 Mustang. However, several fighter plane designs for the USAAF stood out relative to other designs in that utilizing radical design philosophies, including the pusher-engine layout, tailless flying wing layout, and swept wings. Among the radical USAAF piston-engine fighter projects conceived in the crucible of World War II, two designs were created on the drawing boards of the aircraft industry in the Los Angeles basin of southern California, the Vultee XP-54 and Northrop XP-56 pusher-engine fighters. 

Models of the Vultee XP-54, Curtiss XP-55, and Northrop XP-56, the winners of the R-40C competition. Photo taken by me at the Lyon Air Museum in November 2021. 

On November 27, 1939, the US Army Air Corps initiated Pursuit Specification XC-622 for a single-seat, single-engine fighter able to climb to 20,000 feet (6,096 meters) in 7 minutes and reach 425 miles per hour at altitudes of 15,000-20,000 feet (4,572-6,096 meters). The USAAC considered the 1,800 hp Pratt & Whitney H-3130 liquid-cooled H-block piston engine as having sufficient power to allow the fighter plane covered by XC-622 to meet the stated requirements. By February 20, 1940, the USAAC announced the Request for Data R-40C, which covered the requirements laid out in XC-622, and a total of six companies (Bell, Curtiss, McDonnell, Northrop, Republic, and Vultee) submitted bids; technical details of the proposals for the R-40C competition are discussed in Balzer (2008). Vultee's submission, the Model 70, was a twin-boom fighter with a length of 37 feet 6 inches (11.43 meters), a wingspan of 40 feet (12.2 meters), one Pratt & Whitney H-2600 H-block piston engine in pusher configuration, and armament comprising two .30-caliber and two .50-caliber machine guns plus two 20 mm cannons in the nose; three versions of the Model 70 were investigated, of which Versions 1 and 3 both had three-blade counter-rotating propellers but differed in gross weight (9.000 lb for Version 1, 9,055 lb for Version 3), while Version 2 was to use a single four-bladed propeller and weigh 8,788 lb (3,986 kg) when fully loaded (Buttler and Griffith 2015, p. 26). Northrop's proposal, designated N-2, was a tailless aircraft with drooping wingtips similar to those of the N-1M experimental flying wing and one ventral vertical fin , and it also featured a piston engine arranged in pusher configuration. Five variants were envisaged, with various powerplants investigated including the Allison V-1710, Pratt & Whitney R-1830 Twin Wasp, Pratt & Whitney R-2800 Double Wasp, and Pratt & Whitney H-2600. The first three N-2 designs (N-2, N-2A, and N-2B) were armed with two .50-caliber machine guns and two 20 mm cannons in the nose, while the N-2C and N-2D had two .30-caliber machine guns substituted for the .50-caliber guns (Buttler and Griffith 2015, p. 24). In late May 1940, the Vultee Model 70 and Northrop N-2B were selected by the USAAC for full-scale development and designated XP-54 and XP-56 respectively. (The Curtiss P-249C, the most unorthodox of the Curtiss submissions to R-40C, was designated XP-55 after being selected for full-scale development by the Army Air Corps over the orthodox CP-40 and P-248, but this aircraft) On September 26, one XP-56 prototype (serial number 41-786) was ordered, and a few months later, on January 8, 1941, a contract was signed for one XP-54 prototype with the serial number 41-1210). The US Army Air Force later ordered a second XP-54 prototype (serial number 42-108994) on November 28, 1941 (contract signed March 17, 1942), and a contract was issued for a second XP-56 prototype (serial number 42-38353) on February 13, 1942. The XP-54 was nicknamed "Swoose Goose" after a song about Alexander who was half-swan and half-goose, dubbed Alexander the Swoose. The XP-56 was nicknamed the Black Bullet, Silver Bullet, or Dumbo, but none of these names were ever officially applied to this aircraft.

Top: First XP-54 prototype (serial number 41-1210) at Muroc Army Air Field, early 1943.
Bottom: Second XP-54 (serial number 42-108994) in flight over the San Bernandino Mountains, early 1944.  

Even before the XP-54 prototype contract award, in December 1940 the powerplant for the XP-54 changed to one Lycoming XH-2470 H-block flat piston engine after Pratt & Whitney canceled the H-2600 program in October due to disappointing results of test runs of the H-2600. A full-scale mock-up of the XP-54 was inspected in May 1941, by which time the US Army Air Corps now wanted a high-altitude fighter with a pressurized cabin rather than a medium-altitude, high-speed fighter. In response, Vultee reworked the XP-54 design to incorporate a pressurized cockpit, a longer fuselage, larger wings, greater armor protection, and a turbo-supercharged engine, and the revised design of the XP-54, called the Model 84, was 53 feet 10 in (16.41 meters) long, with a wingspan of 54 feet 9 in (16.69 meters), a wing area of 456 square feet (42.4 square meters), an empty weight of 15,262 lb (6,923 kg), a gross weight of 18,233 lb (8,270 kg), and two 37 mm cannons and two .50-caliber machine guns in the nose. In September 1941 Vultee discussed with the US Army Air Force a proposed version of the XP-54 with one Wright R-2160 Tornado radial piston engine, and the designation XP-68 was assigned. The option was made to complete the second XP-54 prototype with the Tornado engine, but expected lengthy development of the Tornado engine led to the XP-68 project being cancelled on November 22, 1941. Meanwhile, the first XP-54 prototype was completed in late 1942 and made its first flight on January 15, 1943, with test pilot Frank Davis at the controls. Over 100 flights were carried out until October 28, when the first XP-54 was flown to Wright Field, Ohio, for service tests. Despite showing good handling characteristics, the XP-54 reached a top speed of only 380 mph (611 km/h), far below the guaranteed  top speed jotted out by the USAAF, and the Lycoming engine was hamstrung by serious technical woes, so the US Army Air Force decided not to order the XP-54 into production. The possibility of fitting the first XP-54 prototype with an Allison V-3420 liquid-cooled V-cylinder piston engine was considered, as was a proposal to fit the XP-54 with a turbojet, but both schemes were rejected as cost-prohibitive. The second XP-54 made its first flight on May 24, 1944, and ten flights were carried out until April 2, 1945, but the engine-turbosupercharger combination was pronounced unreliable and returned to Vultee to be replaced by another engine. (Many sources say that the second XP-54 flew only once, but this is not borne out by existing flight logs.) Oddly, photos of the second XP-54 in flight show the aircraft with the serial number 41-1211 on the vertical stabilizer, but this serial was actually allocated to a Vultee BT-13 Valiant trainer, and the second XP-54 most likely had 41-1211 applied to the vertical stabilizer in error, because it was ordered several months after the first XP-54, and there is a photo of one North American RF-100A Super Sabre erroneously marked with the serial number 53-2600 (which was actually used for a Northrop F-89 Scorpion). Neither of the XP-54s survive today, the first aircraft having been tested to destruction during static tests at Wright Field and the second XP-54 being scrapped after its last flight. 

Left: First Northrop XP-56 (serial number 41-786) during taxi tests in early September 1943
Right: Second Northrop XP-56 (serial number 42-38353) in flight

The other winner of the R-40C competition developed in the Los Angeles basin, the XP-56, was 27 feet 6 inches (8.38 meters) long and 11 feet (3.35 meters), with a wingspan of 42 feet 6 inches (12.96 m), a wing area of 306 square feet (28.44 square meters), an empty weight of 8,700 lb (3,955 kg), a gross weight of 11,350 lb (5,159 kg), and maximum takeoff weight of 12,145 lb (5,520 kg). The anticipated speed of the XP-56 was to be 465 miles per hour (749 km/h), and armament comprised four .50-caliber machine guns and two 20 mm cannons. After being told that Pratt & Whitney had canceled the H-2600 program, Northrop had the powerplant for the XP-56 design changed to one Pratt & Whitney R-2800 Double Wasp radial piston engine driving counter-rotating propellers. The XP-56 had to be constructed from magnesium alloy due to wartime shortages of aluminum, so Northrop engineer Vladimir Pavlecka utilized the new heliarc welding technique (company designation N-13) to manufacture magnesium in large sections. The first XP-56 was completed in March 1943 and underwent engine tests at before being shipped to Muroc Army Air Field in April for ground tests, but problems arose during taxi trials, including faulty wheel brakes, necessitating the installation of manual hydraulic brakes. The first flight flight of the XP-56 took place on September 6, 1943, piloted by John Myers, but yaw problems and nose heaviness were noticed during testing, so a dorsal vertical stabilizer was fitted in order to cure stability problems. A few additional flights were made at Muroc, but on October 8, the aircraft suffered tire blowout during a high-speed taxi run, causing the XP-56 to flip over and become wrecked. Myers himself survived with minor injuries, largely thanks to him wearing a polo player's helmet. After the crash of the first XP-56, Northrop decided to replace the cotton fabric tires of the first prototype with virgin rubber tires on the second XP-56 prototype, which was completed in January 1944. The second XP-56 first flew on March 23, with Northrop test pilot Harry Crosby at the controls, and a total of ten flights were carried out. Although the nose heaviness disappeared when the landing gear was retracted, stability still remained an issue, and the second XP-56 attained 320 miles per hour (515 km/h) instead of the projected 465 mph (749 km/h). On May 30, 1944, NACA was asked to use its wind tunnel at Moffett Field, California, to investigate the causes of the XP-56's low performance, but after complaints during the tenth flight of the second XP-56 about extreme tail heaviness on the ground, low power, and excessive fuel consumption, the USAAF decided not to conduct any more flights tests, and the XP-56 program was abandoned in late 1944. The second XP-56 prototype, miraculously, escaped the breaker's torch; on December 20, 1946, US Army shipped it to Freeman Field, Indiana, to be put into long-term storage. The second XP-56 became part of the National Air and Space Museum's collection in 1950 –1951 when the Smithsonian moved it to the Paul Garber Restoration Facility in Suitland, Maryland, where it sits today.

Never-realized derivatives of the XP-54: the Vultee Model 78 (left) and carrier-based Model 79 (right)

In an interesting footnote, in early January 1941 Vultee proposed a version of the XP-54 for the export market, the Model 78, which measured 48 feet (14.63 meters) in length and had a wingspan of 46 feet (14.02 meters), a gross weight of 8,500 lb (3,855 kg), and six .50-caliber machine guns in the nose, with power provided by one Allison V-1710 liquid-cooled V-cylinder piston engine (Balzer 2008, pp. 60-61). The same month, a carrier-based derivative of the XP-54, the Model 79, was envisaged for the Navy's SD-112-18 requirement, and two variants were devised, the 79A and 79C, which slightly differed in length, wingspan, and gross weight, not to mention that the Model 79A used counter-rotating propellers whereas the Model 79C had a single propeller (Buttler and Griffith 2015, p. 166). The Model 78 did not progress beyond the drawing board, and the Model 79 design lost out to the Grumman Model 51 (which became the F7F Tigercat) in the SD-112-18 contest. Northrop, for its part, looked at a derivative of the XP-56 powered by a turbojet (probably a Westinghouse J30 or General Electric J31), recognizing the onset of the jet age, but this proposal never materialized because Northrop focused its attention on P-61 production and development XB-35 flying wing bomber and XP-79B flying wing jet fighter (Chong 2016, p. 15).

References:

Balzer, G., 2008. American Secret Pusher Fighters of World War II: XP-54, XP-55, and XP-56. North Branch, MN: Specialty Press.

Buttler, T., and Griffith, A., 2015. American Secret Projects 1: Fighters, Bombers, and Attack Aircraft, 1937 to 1945. Manchester, UK: Crecy Publishing.

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

Southern California's foray into ducted fan aircraft: the story of the Doak Aircraft Company

One day at the Western Museum of Flight, I was touring a hangar of the museum housing a Harriet jump jet, gazing at memorabilia and desktop models of aircraft pertaining to the history of aerospace industry in southern California, when I happened to come upon a collection of artifacts and documents regarding vertical take-off aircraft built by the Doak Aircraft Company. One of those artifacts was a wind tunnel model marked with the label "Doak XF-1", and on the wall of the glass casing for Doak-related artifacts and memorabilia was an engineering drawing. I had remembered hearing the name Doak because it built one of several US Army VTOL research aircraft, but even though the Bell X-22 is the best-known tilt-duct fan aircraft, I didn't know until recently that the Doak company was based in Torrance, the very home of the Western Museum of Flight. Given that the location of the Western Museum of Flight was also the headquarters of Doak Aircraft, I thought it might now be a perfect idea to dedicate this post to telling the history of the Doak company and the VTOL aircraft designed and built by that manufacturer.

The Doak DRD-1 prototype primary trainer (civil registration NX25698)

The Doak Aircraft Company was founded in Hermosa Beach, on October 7, 1940, by Edmund R. Doak, Jr., a self-taught engineer and vice president of Douglas Aircraft Company. The company later moved to a new factory based in Torrance in 1941, undertaking the construction of wooden parts and products for aircraft under a contract signed with the furniture firm Kroehler in 1942. Every major US aircraft manufacturer was subcontracted to build subassemblies for military aircraft, including molded plywood fuselages for the North American AT-6 Texan and Vultee BT-13 Valiant trainers, and doors, hatches, and gun turrets for a wide variety of combat aircraft. Throughout the course of US involvement in World War II Doak reached a peak employment of 4,000 workers (Lobb 2006). Doak also built a primary trainer aircraft of its own, the DRD-1, of which a single example (civil registration NX25698) was built and flown in 1941. The DRD-1 was powered by one Continental R-760 radial piston engine and featured laminated plywood construction, with a length of 26 feet (7.92 meters) and a wingspan of 35 feet (10.67 meters); despite being submitted for evaluation by the US Army Air Corps, the DRD-1 was not ordered into production or given a military designation.   

 

Doak ducted-fan flight/wind tunnel test vehicles on display at the Western Museum of Flight in Torrance, California, from left to right: XF-1 wind tunnel model, Model 11 wind tunnel model, full-scale XF-1 tethered test vehicle. On the upper left is a three-view engineering drawing for what would later become the VZ-4. 

Doak's real aviation interest, however, lay in vertical take-off and landing aircraft. Since 1935, he had been experimenting with ducted fans and similar air-moving concepts, and Cold War tensions in the late 1940s and early 1950s raised worries among the US military top brass that the Soviet Union and its Warsaw Pact allies in Central and Eastern Europe might carry out air attacks on unprepared airbases in West Germany, Italy, or France with combat aircraft or theater ballistic missiles. These concerns, along with recognition of the built-in speed limitations of helicopters thanks to the aerodynamics properties of the rotor blades, prompted Edmund Doak to submit a VTOL aircraft design to the US Army's Army Transportation and Research and Engineering Command at Fort Eustis in Newport News, Virginia, in 1950. He believed that his design could take off and land in a small area, hover and loiter over an enemy target, and fly backwards like a helicopter without the usual noise and vibrations of a helicopter, while incorporating the forward flight mode, high speed, weapons load, and combat mission flexibility of a fighter aircraft. Since conventional combat planes were susceptible to Soviet attacks on unprepared airbases with concrete runways, the US Army found Doak's idea attractive, and on April 10, 1956, Doak received an Army contract to build a VTOL research aircraft to test the ducted fan method of vertical take-off and landing. As a first step in testing his ideas for ducted-fan aircraft, Doak built a plethora of ducted-fan vehicles for flight and wind tunnel testing from 1950 to 1957, including the Model 11, Model 13, and XF-1. 

The Doak VZ-4 (VZ=VTOL Research), known by the company designation Model 16, was an aircraft of all-metal construction, powered by a single Lycoming T53 turboprop engine buried in the fuselage that drove two wingtip-mounted tilting ducted fan propellers through a T-shaped box on the engine that transmitted power to the propellers via a 4 in (10.2 cm) aluminum tubular shaft and two smaller shafts. Each propeller had a diameter of 4 feet (1.22 meters) while the fan ducts each had a diameter of 5 feet (1.52 meters), swiveling upwards during vertical take-off and landing with a liftoff rotation speed of 4,800 revolutions per minute, and tilting forwards during forward flight. The wings and tail unit were of all-metal construction, and the cockpit had the pilot and observer seated in tandem, with the pilot using a standard stick and rudder to control the VZ-4 in flight. The VZ-4 was originally made of uncovered welded steel tubing, but later molded fiberglass was installed over the nose section and thin aluminum sheeting placed over the aft fuselage. To save time and money, some component were used during the construction of the VZ-4, including seats from a P-51 Mustang, landing gear from a Cessna 182, and the duct actuators from a Lockheed T-33 (trainer version of the P-80/F-80 Shooting Star jet fighter).

Top: The Doak VZ-4 performing one of its first hover flights, 1958. Note that the first two digits of the VZ-4's serial number are incorrectly reversed (the serial number on the vertical stabilizer should have read 56-6942, not 56-9642).
Bottom: Desktop models of the Doak Model 20 and Model 22 ducted-fan transport projects.

The VZ-4 (serial number 56-6942) was completed by the end of 1957. It carried out its first hover flight on February 25, 1958, and the transition from vertical to forward flight was conducted on May 5 of that year. Although initial flight tests were successful, the overall short take-off and landing performance was less satisfactory than was hoped for, as the VZ-4 itself tended to make a nose-up while transitioned to forward flight mode. After 18 hours of tethered hovering and 32 flight testing hours, in October 1958 the VZ-4 was transferred to Edwards Air Force Base for a new round of flight tests, and it underwent another 50 hours of test flights, attaining a maximum speed of 230 mph (370 km/h), a cruise speed of 175 mph (282 km/h), a range of 250 miles (403 km), a flight endurance of one hour, and a service ceiling of 12,000 feet (3,658 meters) with the turbines (Stevenson 2014, pp. 14-15). The US Army formally accepted the VZ-4 in September 1959, leading to it being called VZ-4DA (the code suffix DA denoting the Doak Aircraft Company), and the aircraft was subsequently passed on to the NASA Langley Research Center in Hampton, Virginia, on May 5, 1960 for additional testing. Even before the Army acquired the VZ-4, Doak proposed two ducted-fan transport designs in 1959, the Model 20 with two ducted-fan engines and a butterfly-shaped tail, and the larger Model 22 with four ducted fans, of which two were at the tips of the low-mounted wing near the crew cabin and the other two were situated at the tips of a high-mounted wing just ahead of the vertical stabilizer. By this time, however, pressure from Congress meant that the Army could only field helicopters and that only the Air Force could operate airplanes, and even though the VZ-4 was classified as a plane, the USAF showed little interest in developing an operation ducted-fan VTOL aircraft, so in late 1960, Doak Aircraft laid off 90 percent of its employees and went out of business in 1961. The VZ-4, meanwhile continued to serve with NASA until 1973, when it was retired and eventually donated to the US Army Transportation Museum in Fort Eustis, Virginia, where it remains on display today.

Top: Designs by Douglas for transport derivatives of the VZ-4 (1-ton payload design on left, 4-ton payload design on right)
Bottom left: Douglas D-828 four-engine tilt-duct transport project 
Bottom right: Douglas D-850 four-engine tilt-duct research aircraft (which lost out to the Bell X-22) 

After Doak Aircraft vanished into history's dustbin, Douglas acquired the patent rights and engineering files for Doak's VTOL designs, while hiring four former Doak engineers to work at its naval aircraft division based in El Segundo. In 1960 Douglas had proposed two transport derivatives of the Doak VZ-4 for submission to the US Army, one with a 2,000 lb (907 kg) payload and another with a payload of 8,000 lb (3,628 kg), but these designs were rejected by the Army. On January 27, 1961, the Department of Defense announced the Tri-Service Assault Transport Program for a large VTOL transport to be jointly operated by the Army, Air Force, and Navy, and Douglas responded in late 1961 with the D-828 ducted-fan transport, which was similar to the Doak Model 22 but had shoulder-mounted wings, with tilt-duct fans at the housing 8 feet (2.44 meter) diameter six-blade propellers, each powered by four General Electric T64 turboshafts. The wings spanned 48 feet 5 in (14.77 meters) when unfolded or 30 feet (9.15 meters) when folded, and the fuselage was 50 feet (15.25 meters) long. However, the D-828 was not proceeded with because US armed services disagreed on the best method of vertical for a large VTOL transport, with the Air Force and Army favoring the tiltwing and tiltrotor arrangement and the Navy preferring the tilt-duct system. (The D-829 tiltrotor proposal envisaged in parallel with the D-828 initially seemed acceptable to the Air Force and Navy, but was rejected by the Air Force along with a North American tiltwing proposal in favor of the Ling-Temco-Vought VHR-477 tiltwing design [which became the XC-142] and thus is not considered here as it was not a tilt-duct project.) After withdrawing from the Tri-Service VTOL transport program, in the spring of 1962 the Navy announced a competition for a tilt-duct technology demonstrator. In response, Douglas proposed the D-850, which featured the same tilt-duct engine layout and placement as the D-828 but had a cockpit similar to that of the Bell X-15 experimental tiltrotor and a shorter wingspan, while eliminating the mechanism for folding wings and tails. However, Bell's rival design to the D-850, known as the D-2127, won the Navy competition for the tilt-duct research aircraft context in 1964, eventually receiving the designation X-22 and making its first flight on March 17, 1966.

The Doak Aircraft Company remains an unsung aircraft manufacturer from southern California when it comes to the development of vertical take-off machines, being largely overshadowed by Hiller Aircraft of central California in terms of name recognition. However, Doak helped to put to the test the concept of a VTOL aircraft with tilting ducted fans by building the VZ-4 tilt-duct technology demonstrator, and even though the VZ-4 never led to an operational production machine of its design, not to mention that VTOL designs envisaged by the El Segundo division of Douglas derived from Doak's unbuilt tilt-duct transport projects either did not make the cut due to wrangling among the US armed forces over what form of VTOL was suitable for large VTOL transport or lost out to design proposals put out by other manufacturers, the volumes of aerodynamic data gathered by the VZ-4 helped influence development and testing of the Bell X-22 (as a matter of fact Bell has historically led the pack of American aircraft manufacturers in building and testing various types of VTOL aircraft besides helicopters, from vectored thrust jet aircraft to convertiplanes). 

References: 

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

Harding, S., 1997. U.S. Army Aircraft Since 1947. Atglen, PA: Schiffer Publishing.

Lobb, C., 2006. Torrance Airport. Charleston, SC: Arcadia Publishing. ISBN 978-0-7385-4662-9.

Stevenson, R., 2014. "Doak's One-Off." Aviation History (July 2014): 14–15.

Douglas A2D Skyshark: Heinemann's turboprop-powered flop

Much has been written in the US naval aviation literature about the Douglas AD/A-1 Skyraider piston-powered attack aircraft (nicknamed "Spad" by many airmen) and the prolific A4D/A-4 Skyhawk (better known as "Heinemann's Hot Rod"), both of which were designed by Ed Heinemann (1908-1991), chief designer at the El Segundo Division of Douglas, However, lost in talk regarding the design of naval warplanes by Heinemann in the early years of the Cold War is the fact, Heinemann himself undertook the first Douglas design for the US Navy with gas turbine power, the A2D Skyshark. Recently, I have seen desktop models of the Skyshark at a number of aviation museums in southern California, the Western Museum of Flight in Torrance and the Planes of Fame Museum in Chino. Unfortunately, for all of its good intentions in being a first step in bringing US naval attack aviation into the gas turbine age, the Skyshark turned out to be a disaster for Ed Heinemann due to technical problems with its engine. Therefore, I will discuss in detail Heinemann's failed effort at a turboprop-powered successor to the Skyshark.

Top: Douglas AD/A-1 Skyraider (BuNo 126997, photographed by me at the Planes of Fame Museum in Chino in April 2019), the aircraft that the A2D Skyshark would have replaced
Bottom: Three-view drawing of the Douglas D-557C design with one Westinghouse X25D2 turboprop

On June 25, 1945, the US Navy's Bureau of Aeronautics asked Douglas to design a turboprop-powered attack aircraft as a long-term successor to the new BT2D (later AD/A-1) Skyraider. A few months earlier, on January 25, Douglas had proposed the D-557 turboprop-powered attack aircraft, which was powered by two General Electric TG-100/T31 turboprops mounted side-by-side in the bottom of the fuselage below the pilot, both driving counter-rotating propellers. Armament would consisted of two 20 mm cannons and a 2,000 lb (907 kg) bomb or torpedo under the fuselage rack, plus 1,200 lb (544 kg) rockets or twelve 5 in (12.70 cm) rocket projectiles in wing racks. This proposal was unofficially called BT3D-1 by Douglas in anticipation of gas turbine propulsion rendering the Skyraider obsolete, and BT3D thus never became an official designation. By the second half of 1945, three subsequent designs for the D-557 were worked out. The D-557A was a tricycle design powered by two T31s in wing nacelles, with two 20 mm cannons and wing racks for carrying bombs, unguided rockets, or a torpedo, while the D-557B had two side-by-side T31s below the fuselage driving counter-rotating propellers and provisions for two 20 mm cannons, two 2,000 lb (907 kg) bombs mounted below the wing roots, and two 1,000 lb (454 kg) bombs mounted on mid-wing racks. The D-557C was similar to the D-557B in appearance but had one Westinghouse X25D2 turboprop driving counter-rotating propellers. By April 1946, mockups of all three proposals had been inspected by the Navy and were deemed satisfactory, but none of these projects progressed beyond the drawing board due to developmental problems with the T31 and X25D2.

Model of the Douglas A2D Skyshark (photographed by me at the Planes of Fame Museum in July 2021)

Although the initial D-557 proposals remained paper projects only, Douglas received a Letter of Intent from the US Navy on June 11, 1947, for a turboprop-powered attack plane capable of operaring from the Casablanca-class escort carriers with a combat radius of 690 miles (1,110 km) and a reduced bomb load. Heinemann himself went back to the drawing board to propose a derivative of his earlier D-557 designs powered by one Allison T40 turboprop driving counter-rotating propellers. The new design, measuring 50 feet (15.24 meters) long with a wingspan of 41 feet 2 in (12.55 meters), a height of 17 feet 1 in (5.21 meters), and a wing area of 400 square feet (37 square meters), had a slightly lower wing root thickness compared to the Skyraider and taller and larger vertical stabilizer with greater area, and armament comprised four 20 mm cannons and 5,500 lb (2,500 kg) of bombs, unguided rockets, or a torpedo mounted under 13 hardpoints. A mock-up was inspected in early September, and on September 25 a contract was signed for two prototypes designated XA2D-1 (BuNos 122988/122989). Because the T40 faced technical problems during development and test runs, the first A2D prototype did not make its first flight until March 26, 1950, with test pilot George Jansen at the controls. Although flight tests were marred by vibration problems with the T40, the outbreak of the Korean War prompted the Navy to place an order for 10 production A2D-1s (BuNos 125479/125488) on June 30, followed by another production order on August 18 for 81 more Skysharks (BuNos 127962/128042). Additional orders for 258 more production A2D-1s (BuNos 132793/133042, 134438/134445) were placed on February 10, 1951.

Left: First Douglas XA2D-1 Skyshark prototype (BuNo 122988)
Right: One of the ten completed production Douglas A2D-1s (BuNo 125480) in flight

Despite the Navy anticipating the success of the Skyshark, the A2D flight test program suffered a major setback on December 19, 1950, when the first XA2D-1 prototype crashed on its 15th flight during a landing attempt because of a failure of the gearbox of the Allison T40 turboprop, killing Navy test pilot Hugh Woods. Consequently, the second XA2D-1 prototype was fitted additional instrumentation and an automatic decoupler for the T40, making its first flight on April 3, 1952. Even so, the T40 continued to be bedeviled by gearbox problems, and the Navy was having doubts about the Skyshark overcoming all problems with the T40 turboprop, not to mention that carrier-based jet aircraft were starting to become commonplace. By mid-1952, the Navy project office in charge of the Skyshark recommended that the A2D program be cancelled, and later that year, all production orders for the A2D except for the first ten production aircraft under construction were cancelled. The second production A2D-1 (BuNo 125480) became the first production aircraft to fly, making its first flight on June 10, 1953 with George Jansen at the controls. Five more A2D-1s (BuNos 125479, 125481/125484) were completed and delivered to the Navy later in 1953, and the remaining production A2D-1s (BuNos 125485/125488) were built but not flown. During one A2D test flight in 1953, test C.G. "Doc" Livingston was pulling out of a dive when the T40 gearbox failed, caused oil from the engine to cover the airplane's windscreen and the propellers to fall off the A2D, but Livingston safely landed the plane. The second A2D-1 was lost in an accident near Lake Los Angeles on August 5, 1954 after suffering a gearbox failure, and the pilot bailed out of the plane. By this time, the new Douglas A4D Skyhawk had just begun flight tests, and the escort carriers that the Skyshark had been designed to operate from were phased out of service.

The only extant Skyshark (BuNo 125485) on display at the Gillespie Field Annex of the San Diego Air & Space Museum in San Diego

Following cancellation of the A2D program, the second Skyshark prototype was loaned to Allison for help in solving problems with the T40 turboprop in April 1953, and two production aircraft (BuNos 125481 and 125484) were intended to be flight ready and delivered to Allison; BuNo 125484 had originally been slated to be delivered to a naval storage facility in Litchfield Park, Arizona, but was instead flown to Edwards Air Force Base for use by Allison. After the completion of engine tests, the second XA2D-1 as well as the third and sixth production A2Ds were transferred to Navy facilities in Maryland and Rhode Island for various tests, including armament and barrier tests. The fate of these aircraft is unknown, but they may have been broken up for scrap. Meanwhile, the remaining A2D-1s at the Douglas factory in El Segundo were scrapped except for BuNo 125485, which was eventually used for ground radar calibration tests (minus its engine) at Los Angeles International Airport before being donated to the Ontario Air Museum in Ontario, California in the 1960s. It eventually languished in storage at Chino Airport near the Planes of Fame Museum in Chino, until 1993, when it was trucked to Idaho Falls Airport in Idaho Falls, Idaho, and restored to display status by Pacific Fighters. In the early 2010s, the aircraft was moved to the Gillespie Field Annex of the San Diego Air and Space Museum for static display, where it resides today.   

Despite offering better performance over the Skyraider, the A2D Skyraider turned out be an engineering disaster because the Allison T40 turboprop failed to live up to expectations in performance, much to the frustration of not only the US Navy but also Douglas and other US companies that had designed several naval aircraft around the T40. In any case, Ed Heinemann learned from the failures of the A2D program and designed a lightweight jet-powered attack plane as an alternative, which became the A4D Skyhawk and eventually supplanted the Skyraider along with the later Grumman A2F/A-6 Intruder all-weather attack aircraft. Although no longer in service with its country of origin, the Skyhawk still serves with the militaries of Argentina and Brazil.

References:

Allen, F., 1994. "Shark With No Teeth: The Story of the Douglas A2D Skyshark". Air Enthusiast 53 (Spring 1994): 69–75. 

Buttler, T., and Griffith, A., 2015. American Secret Projects 1: Fighters, Bombers, and Attack Aircraft, 1937 to 1945. Manchester, UK: Crecy Publishing.

Francillon, R., 1988. McDonnell Douglas Aircraft Since 1920: Volume I. Annapolis, MD: Naval Institute Press.

Markgraf, G., 1997. Douglas Skyshark, A2D Turbo-Prop Attack (Naval Fighters No. 43). Simi Valley, CA: Ginter Books.

Early vertical take-off and landing fighters from southern California, part 3: Lockheed VTOL flat riser projects

In the late 1950s the US Air Force and Navy had come to recognize the operational impracticalities of the tail-sitter concept and instead looked to the concept of lift jets and lift fans being pioneered in the United Kingdom by Rolls-Royce in the 1950s as the most suitable method of VTOL for a combat jet. With the lift jets (or lift fans) situated in the center fuselage, the pilot would easily land his plane on an airfield or any unpaved surfaces. In addition to lift jets/lift fans, the US aircraft industry looked at equipping VTOL jet fighters with jet engines housed in nacelles that could swivel upwards during vertical take-off and landing but tilt longitudinally in forward flight. The Bell Aircraft Corporation of Buffalo, New York, was the chief leader in developing flat-rising VTOL jet aircraft in the US during the late 1950s, as exemplified by the Model 65 experimental tilt-jet and the X-14 vectored thrust technology demonstrator, before turning to the design of the D-139 and D-188 VTOL combat jets, the latter whose design philosophy influenced West Germany's EWR VJ 101C prototype VTOL jet fighter. But Bell wasn't the only US aircraft manufacturer to undertake design studies for flat-rising VTOL combat jets. As will be emphasized in my third and final post in my three-part blogpost series on early American VTOL fighter designs, as interest in the tail-sitter concept gradually evaporated on part of the Air Force and Navy, Lockheed decided to envisage flat-rising VTOL jet fighter designs of its own after becoming aware of the impracticality of the tail-sitter idea; the resulting spree of proposals for flat-risers from Lockheed in the late 1950s would range from designs with lift jets/lift fans to aircraft with swiveling jet engines.

Top: Lockheed CL-346-1 (left) and CL-346-31 (right) designs
Bottom: Artist's conception of the CL-407 heavy fighter/light bomber

Around the time that design studies for the CL-295 and CL-349 tail-sitter projects concluded, Lockheed proposed a tilt-jet VTOL interceptor/tactical strike aircraft under the designation CL-346-1. This design retained the airframe of the F-104 Starfighter but differed in eliminating the fuselage engine and instead having two General Electric J79 turbojets in wingtip nacelles, and the horizontal stabilizer was situated on a ventral fin below the tail empennage. The J79s would tilt upwards for vertical take-off and when the CL-346-1 transitioned to forward flight, the engines would tilt horizontally so the aircraft could cruise to Mach 2 at 60,000 feet (18,288 meters). The CL-346-1 would have carried four Sidewinder air-to-air missiles below the wings in its role as an interceptor or a single nuclear bomb below the fuselage in tactical strike role. Another CL-346 design study, the CL-346-31, was an F-104 airframe with the entire propulsion housed in the inner wing section, with swiveling units and four additional lift jets in the fuselage, and the tail empennage had a tailwheel and long front undercarriage struts to provide good exhaust clearance during vertical take-off. A series of VTOL heavy fighter/light bomber designs were worked out by Lockheed under the designation CL-407, including the canard delta wing CL-407-37/40 with two jet engines in the rear fuselage that could swivel for VTOL and three lift jets situated behind the cockpit, and the canard delta wing CL-407-47-2 with two jet engines that could swivel backwards for VTOL and wingtips that could fold downwards. The CL-407 would have had a top speed of Mach 3 and an altitude of 70,000 feet (21,336 meters).

Three-view drawing of the CL-704

Beginning in 1958, Lockheed envisaged a VTOL derivative of the F-104G, the CL-521-1. which was similar to the F-104G but differed in having two large wingtip pods, each containing four Rolls-Royce RB.108 turbojets. The fuel in the wingtip pods was optionally transferred to the fuselage for use in the General Electric J79 and the engine pods could easily be removed, returning the aircraft to non-VTOL form and allowing the carriage of wingtip fuel tanks or missiles. The CL-521-1 would have a combat radius of 287 miles (460 km) and a top speed of Mach 1.4 when carrying the engine pods, although its range/payload could be increased if VTOL wasn't required, and armament comprised the 20 mm M61 Vulcan cannon and 4,000 lb (1,814 kg) of Sidewinder missiles, air-to-ground rocket, free-fall bombs, and tactical nuclear weapons. The CL-704 was a reconnaissance version of the CL-521-1 designed to conduct reconnaissance missions from NATO bases in Europe, with a secondary strike capability, and it was similar to the RF-104G in housing three KS-67A spy cameras. A total of 34 design variations of the CL-704 were probably worked out, even though documentation is lacking. 

CL-802-12 VTOL jet fighter design

From January 1962 to May 1963, Lockheed undertook design studies for a single-seat carrier-based multi-role VTOL combat jet, the CL-706, of which one proposal, the CL-706-13, was powered by two General Electric X-84 turbofans and ten 4,000 lb (17.79 kN) thrust Continental Model 365 lift jets, of which six were housed in the wingtip pods and four were situated in the forward fuselage section. Armament for the CL-706-13 consisted of a rotary 30 mm cannon, free-fall bombs, rockets, and air-to-surface missiles. A series of additional flat-rising VTOL supersonic jet fighter designs was devised under the company designation CL-802. The CL-802-12 was an F-104 Starfighter with the General Electric J79 turbojet moved further back and twelve 4,000 lb (17.79 kN) thrust Continental Model 365 lift jets, of which eight were housed in the wingtip pods and four were situated behind the cockpit, and it could be used for close air support, cargo transport, and light observation. The performance of the CL-802-12 would have been only Mach 1 or less, slower than other VTOL Starfighter design studies. The CL-802-7 was a two-seat Mach 2 fighter design with a single tail fin, two afterburning turbofans, and eight lift fans in the center fuselage section, and it would have carried four underwing munitions plus a variety of weapons in an internal weapons bay. On the other hand, the CL-802-14-3 proposal featured a two-seat Mach 2 VTOL jet fighter with two General Electric J79 turbojets in nacelles on the rear fuselage and ten Continental Model 365 lift jets (four behind the cockpit, four behind the wing's trailing edge, and two towards the tail). Armament for the CL-802-14-3 would have comprised a rotary cannon, underwing air-to-air or air-to-surface missiles, and a tactical nuclear weapon in the internal weapons bay.

Lockheed CL-757 VTOL test rig

To test the flight behavior of its VTOL jet fighter designs, Lockheed built an experimental VTOL test rig, the CL-757, which had an open cockpit for the pilot and observer and measured 23 feet (7 meters) long and 32 feet (9.75 meters) wide, with a gross weight of 7,920 lb (3,592 kg). Power came from six upright General Electric turbojets on each side of the craft, and those engines were supplied and fitted by US Air Force technicians. A total of 19 test flights of the CL-757 were carried out at Edwards Air Force Base in mid-1963, of which the initial ones were tethered. Although the Lockheed flat-rising VTOL jet fighter designs envisaged in the late 1950s and early 1960s looked good on paper and offered operational advantages over the earlier Lockheed tail-sitter designs in terms of landing characteristics, not a single one of Lockheed's flat-rising VTOL jet fighter projects progressed beyond the drawing board, largely because the US Air Force had lost interest in VTOL jet fighter technology and Lockheed itself was over-committed to other projects, including the A-12/SR-71 Blackbird and L-2000 supersonic airliner. Nonetheless, the VTOL propulsion systems utilized by Lockheed for its design studies for flat-rising VTOL combat jets would find their way into the EWR VJ 101C, Hawker Siddeley (later British Aerospace) Harrier, and Hawker Siddeley P.1154 combat aircraft.

References:

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

Rose, B., 2013. Vertical Take-off Fighter Aircraft. Hersham, UK: Ian Allan Publishing.