As I discussed in my previous post, the Convair XFY and Lockheed XFV were touted by their respective companies as magic-bullet solutions to overcome the speed limitations of helicopters by combining the vertical lift of a helicopter with the combat performance of a fighter plane. However, these aircraft were handicapped by the pilot having difficulty landing the plane (in other words, figuring out how close he was approaching the ground), and the Convair and Lockheed "Pogos" also would have been no match for Soviet fighter planes even if they had gone into production due to the turboprop powerplant around which were designed. However, US aircraft manufacturers in southern California did not completely give up on the tail-sitter concept, instead merely choosing to design tail-sitting VTOL fighters powered by a jet engine rather than a propeller engine. While the Ryan Aeronautical Company of San Diego is best known for taking the lead in jet-powered tail-sitting VTOL aircraft design, the very Convair and Lockheed that had created the turboprop-powered "Pogo" aircraft also undertook design studies for tail-sitters with jet propulsion. Therefore, this post will focus on jet-powered designs for tail-sitting VTOL fighters from southern California.
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| Artist's impression of the Ryan Model 38 VTOL jet fighter project |
In January 1947, the Ryan Aeronautical Company of San Diego began design studies for a single-seat VTOL fighter, the Model 38, in response to a Pentagon requirement for a prototype jet-powered, single-seat VTOL fighter. The first Model 38 design, the Model 38-1, was to use either one Rolls-Royce Nene or Allison J33 turbojet (both of which had a thrust of 5,000 lb [22.2 kN]) and had a gross weight of 7,700 lb (3,492 kg). Because the weight of the proposed aircraft exceeded the available thrust of the turbojet, Ryan's chief designer, Benjamin Salmon, proposed to equip the Model 38-1 with four rocket-assisted take-off (RATO) boosters that would be used in take-off mode until the aircraft reached sufficient altitude in order to conduct a transition to forward flight. By February, two additional Model 38 proposals were envisaged, the Model 38-2 with an X-shaped tail and tip-mounted jets, and the Model 38-3 with a tail empennage featuring four fins. In April 1947, the US Navy awarded Ryan a contract for further development, with the aim being to construct two prototypes. A test rig was built by Ryan to use an Allison J33 for VTOL testing in late 1948, and Ryan carried out wind tunnel tests of various configurations for the Model 38 for the next few years. By September 1951, a somewhat larger Model 38 design was envisaged with delta wings, a weight of 17,500 lb (7,937 kg) and one 21,000 lb (93.4 kN) thrust General Electric J53 turbojet. The proposed production Model 38 would be armed with four 20 mm cannons and air-to-surface missiles. Unusual features for vertical take-off and landing included a vectorable exhaust outlet for providing low-speed pitch and yaw control, plus ducts feeding engine compressor air to outlets in the wings for low-speed roll control. The aircraft would have flown at subsonic speeds at an altitude of 50,000 feet (15,240 meters). By late 1951, however, the Navy chose not to fund full-scale development of this proposal, largely due to concerns about cost and performance, not to mention that the J53 had earlier begun test runs. Ryan then sought to win back Navy support in 1953 by conceiving the Model 38R with a T-tail delta wing planform and one Pratt & Whitney J67 turbojet, but this design did not progress beyond the drawing board (Miller 2001).
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| Left: Ryan X-13 Vertijet prepared to hook itself to a flatbed launch/transport trailer after a test flight, 1957 Center: Second X-13 Vertijet (serial number 54-1620) on display at the National USAF Museum in Dayton, Ohio. Right: First X-13 Vertijet (serial number 54-1619) on display at the Gillespie Field Annex of the San Diego Air and Space Museum in San Diego, California. |
Although the Navy lost interest in Ryan's VTOL jet fighter design, the US Air Force was very interested in Ryan's proposal, so Ryan itself proposed the Model 69 VTOL technology demonstrator with one non-afterburning 10,000 lb (44.4 kN) thrust Rolls-Royce Avon turbojet, a high-mounted delta wing spanning 21 feet (6.4 meters), small wingtip stabilizers, and a large vertical stabilizer. In 1953, the US Air Force awarded Ryan a contract to build two experimental aircraft (serial numbers 54-1619/1620) designated X-13, and the aircraft was officially christened Vertijet. Assembly of the first X-13 began in early 1954 and was completed by the fall of 1955. The Vertijet made its first flight on December 10, 1955 with Ryan test pilot Pete Girard at the controls; this was a brief horizontal flight to prove airworthiness, with an improvised, fixed tricycle landing gear. As flight tests progressed, the tricycle undercarriage was replaced by the tail-mounted Roller Skate apparatus with dampened wheels, and on May 28, 1956, the first X-13 vertical take-off and landing was conducted with the Roller Skate. The second X-13 (54-1620) was completed in early 1956, differing from the first aircraft in featuring a modified cockpit with improved visibility for the pilot when the Vertijet was in an upright position. It was shipped to Edwards Air Force Base on a trailer in the spring of 1956 and fitted with fixed tricycle landing gear, making its first flight on May 28 in horizontal takeoff and landing mode. After initial flawless flights in horizontal mode, the aircraft was fitted with necessary attachment gear that would allow it to operate from a special 'pogo rig', prompting the addition of an attachment hook below the X-13's nose that would engage with a small trapeze when the aircraft was at rest. A trailer system featuring a vertical flatbed was built to serve as a launch and recovery platform for the second X-13, and on November 28 the aircraft made its first transition from level flight to hover at 6,000 feet (1,828 meters), before returning to level flight. On April 11, 1957, a vertical take-off from the trailer was carried out followed by a transition to horizontal flight and a vertical return to the pogo rig. By July, the second X-13 was ferried to Pentagon via the Panama Canal aboard the USS Young America to conduct demonstration flights for the military top brass, with Pete Girard as well as Ryan test pilots William "Lou" Everett and William Immenschuh taking part in those flights. After several short demonstration flights at Andrews Air Force Base in Maryland, Girard himself carried out a demonstration flight on July 30 before 3,000 Pentagon officials and media reporters lasting 7 minutes, aiming to prove to the Air Force the potential combat applications of the X-13. The flight unexpectedly ended in mishap just as the X-13 was to land in front of the Pentagon, as Girard noted that the fuel meter was low, so he decided to land the plane in a rose garden. The X-13 program was terminated in early 1958 and the two aircraft subsequently were given to museums, with the first aircraft now on display at the Gillespie Field Annex of the San Diego Air and Space Museum in San Diego, and the second aircraft displayed at the National Museum of the US Air Force at Wright-Patterson Air Force Base in Dayton, Ohio.
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| 3-view drawing of the Ryan Model 115C tail-sitting supersonic VTOL jet fighter |
Although the Model 38 and X-13 dominated much of Ryan's focus on VTOL jet fighter design, in early 1954 the company was asked by the US Air Force to study the notion of a tail-sitting supersonic VTOL jet fighter. Drawing upon experience in designing the Model 38 and X-13, Ryan proposed the Model 84 hook-suspended jet fighter, of which 14 different single- and twin-engine configurations were worked out. I was initially proposed to power the Model 84 with one Pratt & Whitney J75 turbojet, but Ryan later felt that two side-by-side General Electric J79 turbojets were essential to give the Model 84 a better combat radius. A modified version of the Model 84 was also proposed, the Model 84F-7, which had two 20,000 lb (89 kN) thrust General Electric X-301 afterburning turbofans, a top speed of Mach 2.5, and armament comprising 20 mm cannons and air-to-air missiles or one free-fall nuclear weapon. The Model 84 design resembled the Convair F-102 Delta Dagger and Dassault Mirage IIIA in having engine inlets on the sides of the forward fuselage, a delta wing, and a single vertical stabilizer. A hook would be fitted below the forward fuselage, and the pressurized cockpit was fitted with a swiveling ejector. Ryan submitted the Model 84 design to the Wright Air Development Center, and after this proposal was well-received, it began work in 1955 on an improved supersonic VTOL tail-sitting fighter design, the Model 112, which had two General Electric J79s. The Model 112 design study continued into 1956, and Ryan offered two versions of the Model 112 to the US Navy, the Model 113 and Model 114. Meanwhile, the Air Force decided to continue funding Ryan's supersonic VTOL tail-sitter studies, and thus Ryan proposed the Model 115, which was similar to the Model 112 but had a longer fuselage to accommodate a larger bay housing a single tactical nuclear weapon or four air-to-air missiles. The final version, the Model 115C, envisaged in 1957, had a slightly stretched fuselage, greater operating range, circular inlets for the J79s, and greater range, and the airframe would be made from stainless steel. Alternative methods of boosting take-off performance were considered, including water injection and use of exotic fuels, and the Model 115C also was to used a tricycle landing gear. Despite promising lower operating costs, none of the Ryan designs for supersonic VTOL tail-sitter designs progressed to the hardware phase.
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| Top: Convair Configuration IVa Center: North American tail-sitting supersonic VTOL design study Bottom (clockwise from top): Lockheed CL-295-1, CL-295-3, CL-295-4, and CL-295-2. The CL-295-2 was also offered to the Navy as the CL-349-17. |
Even before testing of the Convair XFY and Lockheed XFV began, Convair and Lockheed investigated design studies for supersonic VTOL tail-sitting jet fighters, which like the turboprop-powered "Pogos" hewed to the design philosophies for the fighter jets built by Convair and Lockheed. Under a six-month USAF contract to study a lightweight supersonic tail-sitter issued in December 1953, Convair devised several proposals for supersonic VTOL tail-sitters, all of which shared a delta wing. One of the first supersonic VTOL tail-sitter design studies to emerge had a fuselage containing a jet engine fed by air flowing through a nose intake and unguided rockets carried in retracted forward-located packs. The aircraft stood on four dampened struts in wing nacelles and two vertical stabilizers, and the cockpit was housed in a large pod on the vertical stabilizer, with the pilot lying in a prone position. A revised design had the cockpit at the front of the aircraft, surrounded by the engine inlet. The Convair Configuration IVa had a fairly typical cockpit canopy design, an air inlet for the turbofan below the cockpit, and a ventral pod housing one M61 Vulcan 20 mm cannon and the landing strut. Wind tunnel tests of this design at NACA Langley in 1954 showed this proposal to be aerodynamically sound, but concerns about directional stability prompted Convair to devise a new Configuration IV design with delta-shaped canards on the nose and the ventral fin eliminated. In the meantime, Lockheed in 1954 investigated several designs for tail-sitting VTOL supersonic jet fighter under the company designation CL-295. The first two proposals, the CL-295-1 and CL-295-3, were based on the F-104 Starfighter and featured a retractable hook below the nose similar to that of the Ryan supersonic VTOL tail-sitters, as well as an exhaust flow control system, reaction jets, and a secondary stabilizing horizontal stabilizer for VTOL. The CL-295-1 was powered by one Wright TJC32C4 turbojet whereas the CL-295-3 used one General Electric X-84 turbofan and had a slightly shorter fuselage and wingspan in addition to being lighter, and both proposals would be armed with one 20 mm M61 Vulcan cannon and reach a speed of Mach 2. The next CL-295 design, the CL-295-4, was powered by two General Electric X-84 turbofans and stood on two vertical stabilizers and two wingtip nacelles when standing upright for VTOL, while featuring a canards on the nose. Successful test runs of the General Electric J79 turbojet prompted Lockheed to propose a J79-powered derivative of the CL-295-4 with twin dorsal vertical stabilizers, the CL-295-2. A version of the CL-295-2 was also offered to the US Navy as the CL-349-17 to meet the parameters of the TS-140 specification for a VTOL jet fighter. The CL-295-68 was similar to the F-104 in having fuel tanks at the wingtips but had rear-mounted backswept wings, a cruciform tail empennage similar of that of the earlier XFV, and one Wright J67 turbojet with low-speed vane control in the exhaust flow, fed by air flowing through a ventral engine intake. The final design for the CL-295, the CL-295-77, had nose canards as in the CL-295-2/4 and CL-349-17 but had rear-mounted backswept wings with two General Electric X-84 turbofans at the wingtips. Armament for the CL-295-77 comprised Sidewinder air-to-air missiles, and the CL-295-77 used vanes in the exhaust flow and compressed air nozzles in the wingtips for low-speed vertical control, with normal control surfaces for horizontal flight. North American is also known to have worked on a design for tail-sitting VTOL supersonic fighter, which stood upright on three vertical stabilizers and had three jet engines in the rear fuselage for vertical takeoff and four more in pairs in wingtip-mounted nacelles for forward flight, but this proposal is known only from drawings and no technical data is available at the moment.
By the late 1950s, the US Air Force and US Navy had come to the conclusion that the tail-sitter idea was conceptually a dead end when it came to operational practicality, and after hearing the new of flight tests of the Rolls-Royce Thrust-Measuring Rig (nicknamed the Flying Bedstead) realized that the best way for jet fighter to achieve vertical take-off and landing was to use swiveling jet engines, lift jets, and lift fans. In other word, by allowing a jet fighter to rise vertically above the ground through means of downward air, lift fans, swiveling jet engines, and separate jets could give the pilot visibility during the process of landing his/her plane vertically. Thanks to the British, the US armed forces and the aircraft industry in southern California could now look in this new approach to vertical take-off and landing for jet fighters.
References:
Bradley, R., 2013. Convair Advanced Designs II: Secret Fighters, Attack Aircraft, and Unique Concepts 1929-1973. Manchester, UK: Crécy Publishing.
Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.
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