NASA Dryden FRC, 1972.
Beginning in 1955, the USAF began seeking proposals under the Long-Range Interceptor-Experimental (LRI-X) program to replace the F-102 and F-106. The specifications called for included a ceiling above FL60, a speed in excess of Mach 1.7, a range of 1,000 miles, a crew of two, at least two engines, and an integrated fire control system allowing interception of targets 60nmi from the plane. Northrop, Lockheed and North American all put forward designs, but none had advanced very far when the program was terminated in 1956. LRI-X was reinstated in 1957, with North American’s design, which shared the engine and escape capsule from the company’s B-70 design, was designated the F-108. North American got as far as construction of a full-scale mockup of the XF-108, given the nickname ‘Rapier’, before the program was cancelled again in 1959. NA continued refining the design elements however, which eventually saw success in the A-5 Vigilante bomber for the Navy.
Other work for the F-108 also continued, with Hughes working on its AN/ASG-18 radar, the first Pulse Doppler radar unit in the US, a paired infra-red search and track system (IRST), and the GAR-9 (later AIM-47) missile. This work was leveraged by Lockheed, which was proposing an armed interceptor variant of the A-12 Cygnus it was developing for the CIA. The AF-12, as it was then known, added a second crew member to monitor and operate the fire control system, the radar and IRST for the AN/ASG-18, and three AIM-47 missiles. Adding the radar and IRST involved cutting back the chines along the nose, which required adding stationary fins under the engine nacelles and a folding fin under the belly.
The first flight of the YF-12 was on 8/7/1963, with LBJ revealing the existence of the program, called KEDLOCK, to the public the next year to disguise the parallel A-12 test program.
The USAF test program continued until 1967, resulting in an altitude record of 80,257.86 feet and three separate speed records on May 1st, 1965: 2,070.102 mph on a 15/25 straight course, 1,643.042 mph on a 500km closed course, and 1,688.891 mph on a 1,000 km closed course. Afterwards, -936 would wear three Blackbird silhouettes under the cockpit.
Six out of seven tests of the AIM-47 were successful, with the last being launched from 74,000 feet and striking a JQB-47E drone flying at 500 feet. removing a 4-foot section of the tail. It was during one of those flights that Kelly Johnson flew in the RSO seat of a YF-12, his second Oxart/Blackbird flight.
Lockheed further refined the F-12 in preparation for production, building a mockup of the forward section showing a partial chine on the radome, along with the IRST integrated into the chine on the fuselage.
These changes removed the need for the folding fin, though the fixed fins under the nacelles remained. The USAF tendered an order of 93 (some sources say 96) F-12Bs for the Air Defense Command. Despite these successes, the YF-12 program was canceled in 1967 by Robert McNamara in favor of Convair’s F-106X project, which never materialized, and also due to increasing costs of the Vietnam War. Hughes continued work on the radar and AIM-47, which later were upgraded into the AN/AWG-9 and AIM-54, intended for the F-111B and used in the F-14.
The YF-12s were handed over to NASA for testing, which included an improved computer control system for the engine bypass system, which virtually eliminated unstarts (the ejection of the supersonic shockwave from the inlet, which resulted in wild changes in aircraft attitude) on the A-12 and SR-71 series, and the Cooperative Airframe-Propulsion Control System (CAPS), which reduced slight deflections in the aircraft’s direction, which at Mach 3 could result in deviations of thousands of feet. Another test required painting a white cross on the underside of 60-6936, which allowed ground cameras to track the plane with high precision.
Another experiment was Coldwall (sometimes parsed as “Cold Wall”), which involved mounting a large cylinder under the fuselage, chilled with liquid nitrogen and covered by an insulating jacket.
At altitude and Mach 3, the jacket would be blown away by primacord, exposing the cylinder to a high heat and pressure environment. Data was then collected about temperature and air pressure changes, which could be compared to theoretical and wind tunnel data, all of which greatly improved knowledge of fluid dynamics.
Two of the three YF-12s were lost in accidents, with 60-6934 falling victim to overheating in 1966. The crew were able to escape, but the internal damage was judged to be too severe to be repaired. The aft fuselage was later mated to a ground test unit to form the SR-71C trainer after the second SR-71B was lost. 60-6936 was destroyed in 1971 after a fire broke out on the left engine on approach to Edwards. As before, the crew escaped, but the plane was written-off. A leak in the left engine nearly destroyed 60-6935 as well, after ground crew noted a contrail of escaping JP-7 behind the plane. The pilot was able to land the plane, and the crew scrambled out, jumping on to the roof of the base commander’s car, and in the process the pilot punched holes into the chine with the spurs (used to lock into the ejection seat) on the back of his boots. The holes were patched and the fuel line replaced, and the plane later placed into storage for a time.
60-6935 was flown to the USAF Museum in Dayton on November 7th, 1979, and is now on display.