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.