Lockheed NF-104A

A number of standard production F-104 Starfighters were obtained (including F-104D two-seat versions)[3] and used by the ARPS to simulate the low lift/high drag glide approach path profiles of the X-15 and the projected X-20 Dyna-Soar program. These maneuvers were commenced at 12,000 ft (3,700 m) where the F-104 engine was throttled back to 80% power; and with the flaps, speedbrakes and landing gear extended, the aircraft was established in a 30° dive with a pull-out for the landing flare starting at 1,500 feet (500 m) above the ground. These glide approaches gave little room for error.

JF-104 during RCS testing.

It was realized that normal aircraft control surfaces had little or no effect in the thin air of the upper stratosphere and that any aircraft operating at extremely high altitudes would need to be equipped with a reaction control system (RCS). A modified version of the Bell X-1 was used for initial RCS tests, but was grounded after technical problems. It was replaced in 1959 with a NASA-modified Lockheed F-104A (55-2961), which carried RCS systems on its wing tips and in the fuselage nose. This aircraft (designated JF-104) achieved a maximum altitude of 83,000 feet (25,300 m) during the test program. Pilots who flew this aircraft included Neil Armstrong, who gained valuable experience in using the RCS. Pilots complained that the instrument displays were difficult to read and were not accurate enough for the critical zoom climb profiles required to reach high altitudes.[4]

Lockheed was awarded a contract by the United States Air Force to modify three F-104A aircraft for the dedicated role of aerospace trainer (AST) in 1962. The airframes were taken out of storage at AMARG and transported to the company factory for modification.

The wingspan of the NF-104A was increased by the addition of wingtip extensions. This modification was needed to house the RCS roll control thrusters and decreased the wing loading.

The vertical fin and rudder were replaced by the larger area versions from the two-seat F-104 and were structurally modified to allow installation of the rocket engine.

The fiberglass nose radome was replaced with an aluminum skin and housed the pitch and yaw RCS thrusters.

The air intakes originally designed by Ben Rich were of the same fixed geometry as the F-104A, but included extensions to the inlet cones for optimum jet engine operation at higher Mach numbers. Internal fuselage differences included provision for rocket propellant oxidizer tanks, removal of the M61 Vulcan cannon, radar equipment and unnecessary avionics. A nitrogen tank was installed for cabin pressurization purposes. This was required, as there would be no bleed air available from the engine after its normal and expected cutoff in the climb phase.

In addition to the standard General Electric J79 jet engine, a Rocketdyne AR2-3 rocket engine was fitted at the base of the vertical fin. This engine burned a mixture of JP-4 jet fuel and 90% hydrogen peroxide oxidizer solution. The NF-104 carried enough oxidizer for approximately 100 seconds of rocket engine operation. The thrust level could be adjusted to maximum or approximately half power by the pilot using an additional throttle lever on the left side of the cockpit.

The reaction control system consisted of eight pitch/yaw thrusters (four for each axis) and four roll thrusters. They used the same kind of hydrogen peroxide fuel as the main rocket engine from a dedicated 155 lb (70 kg) fuel tank and were controlled by the pilot using a handle mounted in the instrument panel. The pitch/yaw thrusters were rated at 113 lbf (500 N) thrust each and the roll thrusters were rated at 43 lbf (190 N) thrust.

Chuck Yeager in the cockpit of an NF-104A, 4 December 1963

The NF-104A was able to reach great altitudes through a combination of zoom climbing (building up a high speed in a shallow dive at high altitude, and then climbing steeply, converting speed and momentum into altitude) and use of the rocket engine (to reach higher level speeds and to maintain climb rate for as long as possible after entering the zoom climb). A typical mission involved a level acceleration at 35,000 feet (10,700 m) to Mach 1.9 where the rocket engine would be ignited, and on reaching Mach 2.1 the aircraft would be pitched up to a climb angle of 50-70° by carefully applying a load equal to 3.5 g. The J79 afterburner would start to be throttled down at approximately 70,000 feet (21,300 m) followed shortly after by manual fuel cutoff of the main jet engine itself around 85,000 feet (25,900 m) to prevent fast-rising engine temperatures from damaging the turbine stages of the jet engine. After continuing over the top of its ballistic arc the NF-104 would descend back into denser air where the main engine could be restarted using the windmill restart technique for recovery to a landing.[5]

The first NF-104A (USAF 56-0756) was accepted by the USAF on 1 October 1963. It quickly established a new unofficial altitude record of 118,860 feet (36,230 m) and surpassed this on 6 December 1963 by achieving an altitude of 120,800 feet (36,800 m). The aircraft was damaged in flight June 1963 when a rocket oxidizer vessel exploded. It suffered an inflight rocket motor explosion in June 1971. Although the pilot was able to land safely, the damaged aircraft was retired and this marked the end of the NF-104 project. This aircraft was transferred to the National Museum of the United States Air Force. It is on display at Nevada County Air Park, Grass Valley, California. wearing the markings of "56-0751".

NF-104A Tail Number 760 at the U.S. Air Force Test Pilot School.

The second NF-104A (USAF 56-0760) was accepted by the USAF on 26 October 1963. After retirement, this aircraft was mounted on a pole outside the U.S. Air Force Test Pilot School at Edwards Air Force Base and can still be seen there today. The extended wingtips, RCS, metal nose cone, and other parts from 56-0760 were loaned to Darryl Greenamyer for his civilian aviation record attempts using a highly modified F-104. When he was forced to eject during a record flight, his aircraft was destroyed and the parts were never returned.[6]

The third NF-104A (USAF 56-0762) was delivered to the USAF on 1 November 1963, and was destroyed in a crash while being piloted by Chuck Yeager on 10 December 1963. This accident was depicted in the book Yeager: An Autobiography, and the book and film adaptation of The Right Stuff. The aircraft used for filming was a standard F-104G flying with its wingtip fuel tanks removed, but otherwise lacked any of the NF-104A's modifications, most visibly the angled-up rocket engine at the base of the vertical stabilizer.[7]

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• Drendel, Lou (1976). F-104 Starfighter in Action. Aircraft No. 27. Carollton, Texas: Squadron/Signal Publications. ISBN 0-89747-026-5. OCLC 2524513.
• Kinzey, Bert (1991). F-104 Starfighter: In Detail & Scale (1st ed.). Blue Ridge Summit, PA: Tab Books. ISBN 1-85310-626-7. OCLC 22665285.
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