Lockheed YF-12A

Last revised December 26, 2007

The Lockheed YF-12A Mach 3 interceptor of the mid-1960s had its origin in the top-secret A-12 spyplane which had been designed by Lockheed at Central Intelligence Agency request as a successor to the U-2 spyplane.

The basic U-2 concept dated back to 1954, but as early as 1957 the CIA had concluded that the high-flying U-2 might soon be vulnerable to missiles and would have to be replaced with something more advanced if reconnaissance missions over Soviet territory were to continue unmolested. The CIA wanted an aircraft with a maximum cruising speed of over Mach 3 which could operate at altitudes exceeding 80,000 feet, which should render the aircraft immune from interception by any known threat. The Agency gave the project the code name Project Gusto.

After some preliminary studies, both Convair and Lockheed were given funds to carry out detailed design studies. Even before this study had gotten underway, the Lockheed Skunk Works team (headed by the famous engineer Clarence R. "Kelly" Johnson) had already been trying to interest the government in a U-2 replacement, and had been experimenting with advanced propulsion concepts involving a turbojet powered by a liquid hydrogen/liquid oxygen mixture, but these ideas were abandoned as being impractical at an early stage in the process.

The design had begun with a project named "Archangel I", so named because it was intended to fly higher and faster than the U-2, which had been nicknamed "Angel". The project became known under the project designation of A-11, and in June 1959 the A-11 was declared the provisional winner over Convair's FISH proposal. However, the design had a high wing and engines mounted under the wings and was not considered sufficiently stealthy and the CIA directed that Lockheed redesign the aircraft to reduce its radar cross section, even at the expense of a slightly reduced performance. The design that finally emerged was designated A-12 by the company. The origin of the designation was apparently the fact that the design was the twelfth in the series that had begun with Archangel I. This designation was not consistent with either official USAF designataions or with the usual Lockheed Skunk Works designations, which began with CL, which stood for "California Lockheed".

On August 29, 1959, the Lockheed A-12 was officially declared the winner of the contest and was awarded a limited development contract for design, wind-tunnel testing, and the construction of a mockup. To provide an additional cover, Project GUSTO was closed and the project came to be known under the deliberately deceptive codename OXCART.

On September 3, 1959, the CIA authorized Lockheed to proceed with antiradar studies, aerodynamic structural tests, and engineering designs, and on 30 January 1960 the Agency gave the green light to produce 12 aircraft. Serial numbers were 60-6924/6935 . Later, three more single-seaters (60-6937/6939) and nine two-seaters (60-6940/60-6948) were ordered. The aircraft were to be tested at a secret test site in Nevada know as Groom Lake or "Area 51", which had originally been developed for the U-2.

The technical challenges faced were enormous. The aircraft was to operate at sustained speeds that were nearly three times greater than that of existing aircraft, and was to be able to cruise at altitudes above 80,000 feet.

The Pratt and Whitney Division of United Aircraft Corporation was chosen to develop the propulsion system. The J58 engine was based on a 1956 project known as Suntan, which was designed to produce a Mach 2.7 hydrogen-fueled aircraft. This project was cancelled in 1959. It was also based on the Pratt & Whitney JT9 single-spool high-pressure ration turbojet rated at 26,000 lb.s.t. in afterburning, which had been designed for a US Navy attack aircraft which had been cancelled. The design team came up with a unique idea of a variable-cycle engine (later known as a bleed-bypass engine) in which air was extracted from the fourth stage of the nine-stage single-spool axial-flow compressor and fed through a series of bypass ducts and reintroduced into the turbine exhaust near the front of the afterburner. This reduced the exhaust gas temperature and produced almost as much thrust per pound of air as the main flow that had passed through the main part of the engine.

To minimize stalling of the front stages of the rotor blades at low engine speeds, moveable inlet guide vanes were incorporated. They changed from axial to a cambered position in response to the main fuel control.

When operating at high cruising speeds, the turbine inlet temperatures reached over 2000 degrees Fahrenheit, which required the development of a new type of fuel, known as JP7. It had a much higher ignition temperature than JP4. It required the use of a new type of chemical ignition system based on a fluid known as tri-ethyl borane.

The A-12 was to be built almost entirely of heat-resistant titanium alloys, chosen because of their relatively light weight and great strength. However, titanium is relatively costly and is a very hard metal which is very difficult to mill and machine. Consequently, its use as an important aircraft component required the development of entirely new technologies. The Titanium Metals Corporation was chosen as the primary contractor for the metal. Quality control is especially difficult during titanium alloy manufacture--even the most minute of impurities can render the batch into a useless brittle metal which shatters under the weakest of stresses. Of the early deliveries from Titanium Metals Corporation, some 80 percent had to be rejected as being flawed, and it was not until 1961 that quality became consistently satisfactory.

Since the airframe would undergo aerodynamic heating during sustained supersonic cruise, significant thermal expansion was expected. In order to accommodate for this thermal expansion, a set of chordwise corrugations were added to the central portion of the wing, and expansion gaps were provided between some panels.

When operating at high cruising speeds, the turbine inlet temperatures reached over 2000 degrees Fahrenheit, which required the development of a new low-volatility type of fuel, known as JP7. It had a much higher ignition temperature than JP4. It required the use of a new type of chemical ignition system based on a fluid known as tri-ethyl borane. During protracted operations above Mach 3 it was estimated that the fuel tanks would heat up to about 350 degrees Celsius, and the tanks themselves had to be rendered inert with nitrogen gas to prevent fuel explosions in the tank because of the heat. Special lubricating oil had to be developed for operation at 600 degrees F., and contained a diluent in order to remain fluid at operation below 40 degrees.

The wing was of short-span modified delta planform, supplemented by long fuselage chines which extended forward all the way to the nose. These chines had the beneficial effect of contributing to yaw damping at high speeds and also provided additional space for fuel and equipment. The blending of the fuselage, chines, and wings also helped to reduce the A-12's radar signature over that of other aircraft.

The engines were housed in long nacelles that were mounted at approximately mid-span on each wing. The nacelles were provided with a complex inlet and ejector system, with the inlets contributing as much as 60 percent of the total engine thrust at Mach 3. During takeoff, suck-in doors mounted on the side of the engine nacelles open up to admit additional air to the engines. The engines contained large conical-shaped inlet spikes which could be pushed forward or backwards to adjust for aircraft speed. During takeoff and low-speed flight, the inlet spikes were fully forward, but as speed increased, the inlet spikes were pulled to the rear a maximum of 26 inches so that the shockwave could be maintained at the inlet throat.

A vertical tail was mounted above the rear of each engine nacelle. The vertical tails were all-moving, and were canted inwards. Fixed fins were mounted underneath each nacelle, and a fin was mounted underneath the rear fuselage which could be folded aside for takeoffs and landings. The landing gear was fairly conventional, with the three-wheeled main members retracting inwards into bays in the fuselage and the nosewheel retracting forward into a bay in the forward fuselage.

A small Skunk Works engineering team worked with engineers from Pratt & Whitney and were able to construct the first prototype in only 22 months. In December 1961, the first A-12 (article number 121, USAF serial number 60-6924) was trucked from Palmdale, California to the secret base at Groom Lake, Nevada, so chosen that it would be well-distant from prying eyes. However, the J58 engine was still not ready, and it was decided that a pair of Pratt & Whitney J75 engines would be used for the initial test flights.

Finally, on 26 April 1962, aircraft 121 was ready. On that day, Lockheed test pilot Louis Schalk took it for an unofficial, unannounced, maiden flight lasting some 40 minutes. The first official flight took place on April 30, 1962, piloted again by Louis Schalk. The aircraft broke the sound barrier on its second official flight, 4 May 1962, reaching Mach 1.1.

Aircraft No. 122 arrived at Groom Lake on 26 June, and spent three months in radar testing before engine installations and final assembly. Aircraft No. 123 arrived in August and flew in October. Aircraft No. 124, a two-seated version intended for use in training OXCART project pilots, was delivered in November. It was to be powered by the J58 engines, but delivery delays and a desire to begin pilot training prompted a decision to install the smaller J75's. The trainer flew initially in January 1963. The fifth aircraft, No. 125, arrived at the area on 17 December.

On October 27, 1962 a U-2, flown by a Strategic Air Force pilot on a SAC-directed mission, was shot down by a surface-to-air missile while flying over Cuba in search of Soviet missile sites. This raised the dismaying possibility that continued manned, high-altitude surveillance of Cuba might become too dangerous, and the OXCART program suddenly assumed greater significance and its achievement of operational status became one of the highest national priorities.

By the end of 1962, speeds of Mach 2.16 and altitudes of 60,000 feet had been achieved, still with the lower-powered J75 turbojets, although some flights did take place with just one J58 installed. However, by the end of January 1963, ten J58 engines were available, and the first flight with two of them installed occurred on 15 January. Thenceforth all A-12 aircraft were fitted with their intended propulsion system.

In the meantime, an entirely new mission had appeared for the OXCART. In September of 1959, the USAF had cancelled its contracts for the North American F-108 Rapier, a Mach 3-capable aircraft that had been proposed as the USAF's next-generation interceptor to replace the F-106. The reason given for the cancellation was that the F-108 was simply too expensive for the USAF, now that the primary Soviet threat to the US mainland was its battery of intercontinental range ballistic missiles rather than its fleet of long-range bombers. Nevertheless, Lockheed thought that the Air Force might still be interested in a less-costly F-106 replacement should the Soviet bomber fleet ever again be perceived as a significant threat. Lockheed suggested to the Air Force that the A-12 design would make a good platform for a Mach 3+ interceptor, and might make a good replacement for the F-106 at a more reasonable cost than the abortive F-108 Rapier. Sufficient interest was expressed by the Air Force that in October of 1960 they gave Lockheed permission to modify three A-12 Airframes (the 7th, 8th, and 9th) to interceptor configuration. The designation AF-12 was initially assigned to this project. Serial numbers were 60-6934/6936. One of the unsaid reasons for the Defense Department approval of the interceptor project was perhaps that it might make a good cover for the real CIA-supported intelligence-gathering nature of the project. In September of 1962, these three aircraft were secretly assigned the designation YF-12A in the new Defense Department tri-service scheme.

The YF-12A was quite similar in overall configuration to the A-12 from which it was derived. It differed from the A-12 primarily in having a second crewman in a position immediately behind the pilot This second crewman was added to operate the extremely powerful and capable Hughes AN/ASG-18 pulse Doppler fire control radar, which had originally been developed for the F-108 Rapier. The AN/ASG-18 was installed in the extreme nose of the aircraft, with the forward chines being cut back to accommodate the 40-inch radome. The ASG-18 radar supposedly had a search range as great as 500 miles. Infrared sensors were installed in the forward edges of the cut-back chines.

The YF-12A also differed from the A-12 in having armament. This armament consisted of four Hughes AIM-47A Falcon air-to-air missiles housed internally in chine bays that had previously been used to carry the reconnaissance equipment. The AIM-47A had originally been known as the GAR-9 and (like the ASG-18 radar) had originally been intended for the F-108 Rapier. When fired, the Falcon missiles were explosively ejected from their bays, and their rocket motors were fired. Powered by a storable-propellant liquid-fuelled rocket, the AIM-47A had a maximum speed of Mach 6 and an interception range of 115 miles. It had a launch weight of about 800 pounds. The missile relied on semiactive radar homing for midcourse guidance to the immediate vicinity of the target, homing in on reflections off the target resulting from transmissions from the huge ASG-18 radar. However, it used terminal infrared homing for the final run in to the target. The AIM-47 could carry a 250-kiloton nuclear warhead.

Early wind-tunnel testing indicated that there would be directional stability problems resulting from the revised nose and cockpit configuration, and a large folding fin was mounted under the aft fuselage and two shorter fixed fins were mounted underneath each nacelle.

The first YF-12A (60-6934) took off on its initial flight on August 7, 1963, piloted by James D. Eastham. It was equipped with a streamlined camera pod mounted underneath each engine nacelle for photographing AIM-47 missile launches. Three YF-12As were built. Serials were 60-6934/6936.

While on a routine INS test flight with A-12 number 123 on May 24, 1963, CIA detachment pilot Kenneth Collins entered some clouds. Water vapor froze in the pitot tube, giving an erroneous airspeed reading. With the airspeed indicator giving the wrong reading, the aircraft stalled. Collins ejected safely from the aircraft after it entered an inverted flat spin. The aircraft crashed 14 miles south of Wendover, Utah. The wreckage was recovered in two days, and people at the scene were identified and requested to sign secrecy agreements. A cover story for the press described the accident as occurring to a F-105, and is still listed in this way on official records.

Although this particular loss was ultimately traced to a problem that was easily corrected, it nevertheless precipitated a policy problem within the Agency. With the growing number of A-12s flying out in the western desert, the CIA felt that there was a danger that the OXCART project's cover could be blown at any moment. Although the program had gone through development, construction, and a year of flight testing without attracting any public attention, the Department of Defense was experiencing increasing difficulty in concealing its participation in the OXCART program because of the delays and cost overruns that had increased the rate of expenditures to such an extent that they might eventually get large enough to attract unwanted attention from congressional budget oversight committees. There was also a realization that the technological data would be extremely valuable in connection with feasibility studies for the SST. Finally, there was a growing awareness in the higher reaches of the aircraft industry that something new and remarkable was going on. Several commercial airline crews had reported sighting unidentified aircraft in flight. The magazine Aviation Week indicated to its readers that it was vaguely aware that there was some rather unusual project going on at the Skunk Works at Burbank.

Soon after President Lyndon Johnson took office following the assassination of President John Kennedy on November 22, 1963, he was briefed on the OXCART project and directed that some sort of cover announcement be prepared for the spring of 1964. On February 29, 1964, President Johnson announced that "The United States has successfully developed an advanced experimental jet aircraft, the A-11, which has been tested in sustained flight at more than 2,000 miles per hour and at altitudes in excess of 70,000 feet. The performance of the A-11 far exceeds that of any other aircraft in the world today. The development of this aircraft has been made possible by major advances in aircraft technology of great significance for both military and commercial applications. Several A-11 aircraft are now being flight tested at Edwards Air Force Base in California. The existence of this program is being disclosed today to permit the orderly exploitation of this advanced technology in our military and commercial program."

It was later speculated that the use of "A-11" rather than "A-12" in the Presidential announcement was a slip of the tongue on Lyndon's part. However, it now seems that this was a deliberate act of disinformation, since the "A-11" referred to by President Johnson had been the original design designation for the aircraft first proposed by Lockheed during the Project Gusto competition, prior to the adoption of stealth technology for the design. "Kelly" Johnson worked on the draft speech to be given by President Johnson and proposed that he use the designation "A-11", the non-stealthy version. Subsequently, it became the design designation for the YF-12A interceptor version. The parallel OXCART project remained black. Although there was at the time considerable speculation about a Central Intelligence Agency role in the A-11 development, this was never acknowledged by the government until the declassification of OXCART began in 1983. Only after this time did the CIA's A-12 become public knowledge.

The President also said that "the A-11 aircraft now at Edwards Air Force Base are undergoing extensive tests to determine their capabilities as long-range interceptors." This statement was untrue, since at the time the President spoke, there were no A-11's actually at Edwards and there never had been. Project officials were aware that some sort of public announcement was about to be made, but they had not been told exactly when. Caught by surprise, they hastily arranged to fly two of the Air Force YF-12A's to Edwards to support the President's statement. Thenceforth, while the OXCART continued its secret career out at the Groom Lake site, the A-11 performed in its YF-12A guise at Edwards Air Force Base under a considerable glare of publicity.

In 1964, Kelly Johnson was awarded the prestigous Collier trophy for his achievements in the YF-12A project, becoming the first person in history ever to be honored by this recognition twice.

On April 16, 1964, the first airborne AIM-47 missile separation test was conducted. Unfortunately, the missile's nose-down position was not correct, and had the missile's rocket motor actually fired, the aircraft would probably have shot itself down. On March 18, a YF-12A successfully engaged a Q-2C target drone at 40,000 feet while the interceptor was flying at Mach 2.2 at an altitude of 65,000 feet. The first powered launch was undertaken on March 18, 1956. Six out of seven AIM-47 tests resulted in hits, including One fired from an altitude of 75,000 feet and a speed of Mach 3.2 against a target approaching head-on at 1500 feet.

The three YF-12As served initially with the 4786th Test Squadron at Edwards AFB. The USAF was sufficiently impressed with the performance of the YF-12A that on May 14, 1965 they ordered a total of 93 definitive F-12B aircraft into production and Congress voted $90 million toward the project. However, Secretary of Defense Robert McNamara saw no need for the F-12B and refused to free up the money, the reason being given that the expanding war in Southeast Asia was consuming all available funds in the USAF budget. McNamara chose instead to purse the cheaper "F-106X" interceptor. The YF-12As served primarily in various and sundry operational evaluation projects throughout the remainder of the 1960s.

Even though there was to be no production of the YF-12A, the three prototypes carried out an extensive program of flight research with the Air Force. On May 1, 1965, the following world records were set by YF-12A 60-6936:

In December of 1969, YF-12A 60-6935 was handed over to NASA at the Dryden Flight Research Facility. 60-6936 was delivered to Dryden a year later.

While serving with NASA, YF-12A 60-6936 was lost on June 24, 1971 when USAF pilot LtCol Ronald J. Layton and WSO Maj. William A. Curtis were forced to eject after the plane caught fire due to a fuel line fatigue failure during approach to Edwards. Both crewmembers survived.

In November of 1979, its research program being completed, YF-12A 60-6935 was donated to the USAF Museum at Wright Patterson AFB in Dayton, Ohio. It is now permanently on display at the museum. This is the only YF-12A that survives.

The prototype YF-12A (60-6934) was briefly placed in storage following the end of its flight test program. In 1969, it was taken out of storage and cut up and used for parts in the construction of SR-71C 64-17981, a two seat trainer.

There was actually a FOURTH YF-12, sometimes referred to as a YF-12C. Its serial number is 60-6937. This aircraft was really the number two SR-71A serial number 64-17951 that had been retained by Lockheed until 1969 and used as part of the SR-71 test force at Edwards AFB. In that year, Lockheed turned the aircraft over to the USAF. The Air Force then changed its designation to YF-12C and assigned it a new serial number (60-6937) prior its delivery to NASA in July of 1971. This was apparently done so that no-one would know that NASA was using the plane. NASA returned the plane to the Air Force in December of 1978. It was later put on outdoor display at Edwards AFB. The plane is now with the Pima Air Museum of Tucson, Arizona in its Air Force markings.

The A-12s continued on to become operational spy planes and carried out numerous reconnaissance missions, the details of which are still highly classified even today. The OXCART document (assuming it to be genuine) gives a few of the details. The OXCART fleet was taken out of service and placed in storage in the late 1960s.

In February 1963, Lockheed undertook redesign of the basic A-12 with additional fuel tankage, broader forward nose chines, and the provision for inflight refueling and a seat for a second crewman. This eventually emerged as the SR-71.

Specification of Lockheed YF-12A:

Engines: Two Pratt and Whitney J-58 (JT11D-20B) turbojets, each rated at 32,500 lb.s.t. with afterburning. Performance (estimated): Maximum cruise speed: 2110 mph at altitude (Mach 3.2) Maximum operational ceiling: 85,000 feet Maximum unrefuelled range: 2500 miles Dimensions: Length: 101 feet 7 inches, Wingspan: 55 feet 7 inches. Height: 18 feet 6 inches. Wing Area: 1795 square feet Weights: 60,730 pounds empty, 127,000 pounds maximum takeoff. Armament: Four Hughes AIM-47A air to air missiles which are explosively ejected downwards from paired tandem missile bays.


  1. Lockheed Aircraft Since 1913, Rene J. Francillon, Naval Institute Press, 1987.

  2. The American Fighter, Enzo Angelucci and Peter Bowers, Orion Books, 1987.

  3. The Illustrated Encyclopedia of Aircraft Armament, Bill Gunston, Orion Books, 1988.

  4. Lockheed Blackbirds, Anthony M. Thorborough and Peter E. Davies, Motorbooks International, 1988.

  5. The OXCART Story, Thomas P. McIninch, available from Skunk Works Digest.

  6. Lockheed A-12/YF-12/SR-71, Paul F. Crickmore, Wings of Fame Vol 8, 1997.

  7. E-mail from Paul Suhler