Lockheed's VTOL flat-rising jet fighter designs of the late '50s and early '60s

In the late 1950s, the US Air Force and Navy abandoned the tail-sitter concept as unfeasible 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 in VTOL mode 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, D-188, and D-188A VTOL combat jets, the latter whose design philosophy influenced West Germany's EWR VJ 101C prototype VTOL jet fighter. Bell, however wasn't the only US company to undertake design studies for flat-rising VTOL combat jets. Lockheed decided to work out flat-rising VTOL jet fighter designs of its own after becoming aware of the impracticality of the tail-sitter idea, and the resulting panoply 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. 

Last VTOL tailsitter day fighter designs from southern California

In early 1954 the Ryan Aeronautical 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 Vertijet, it 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. One Model 84 iteration, the Model 84F-7, 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.

 

3-view drawing of the Ryan Model 115C tail-sitting supersonic VTOL jet fighter

In 1955 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.

Top: Convair Configuration IVa
Bottom: North American design study for a supersonic tailsitter day fighter

Even before testing of the Convair XFY, Convair investigated design studies for supersonic VTOL tail-sitting jet fighters, which like the turboprop-powered XFV hewed to the delta wing design philosophies for the F-102 Delta Dagger. 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. 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. 

Selected Lockheed CL-295 design studies, clockwise from top to bottom: 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.

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 in response to the Navy's 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.

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.

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

Convair's CX-4 concepts

In 1962-1963, pursuant to the CX-4 requirement issued by the US Air Force in 1962 for for a heavy-lift transport to replace the C-133, the Convair San Diego division of General Dynamics worked out a series of advanced concepts for a heavy-lift strategic airlifter. Convair's San Diego division had some previous experience with jet airlifter design, having proposed the Model 63 design for the SOR-182 competition won by the Lockheed C-141 Starlifter, and it therefore decided that participation in the CX-4 program was worth a try.

Top: Six-engine concept by Convair San Diego for CX-4
Bottom: Convair's preferred concept for CX-4 with four STF-200-D2s

Convair San Diego's resulting design studies for the CX-4 requirement investigated three turboprop and six turbofan options. One set of designs focused on a heavy-lift transport with six Pratt & Whitney T57 turboprops fitted with variable camber propellers, and the turboprop-powered iteration measured 195 feet 6 in (59.6 meters) long with a wingspan of 210 feet (64 meters), a gross weight of 560,000 lb (254,200 kg), a payload capacity of 165,000 lb (74,842 kg). However, the propellers extended over the whole span and offered very little ground clearance. The initial baseline jet-powered design for CX-4 was powered by six Pratt & Whitney TF33 turbofans and had a low-mounted horizontal stabilizer, and alternate engine options for this iteration included six Pratt & Whitney STF-200-D2s or eight turbofans housed in four paired nacelles. The preferred CX-4 design from Convair San Diego was powered by four STF-200-D2s and measured 204 feet 6 in (62.3 meters) long with a wingspan of 230 feet (71.3 meters), a gross weight of 546,000 lb (248,200 kg), and a T-tail configuration. The high- and low-mounted CX-4 designs featured a side-hinged nose positioned ahead of the cockpit and a tail ramp to allow both drive-through loading and air-dropping, and the four-engine iteration had the flight deck raised in a bulge over the cargo area.    

In addition to land-based studies, Convair looked at an amphibious CX-4 transport proposal to address the possibility that a destination airbase might not be available and not at every brushfire war location. It was powered by four STF-200-D2 turbofans and measured 204 feet 6 in (62.3 meters) long with a wingspan of 234 feet (71.3 meters) and a payload capacity of 165,000 lb (74,842 kg). The amphibious CX-4 iteration featured a pair of clamshell doors, which were designed to load cargo at a land base and deliver it to a seadrome, and the aft fuselage had a side-loading door.

In November 1963 the CX-4 program was renamed CX-HLC (Heavy Logistics Carrier), which changed its name to CX-HLS (Heavy Logistics System) in January 1964. None of Convair's designs would ever see the light of day, and Convair itself neglected to take part in the CX-HLS competition.

References:

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