In early 1964, President Lyndon B. Johnson announced the existence of an “advanced experimental aircraft” 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 plane he was referring to was the Central Intelligence Agency’s A-12 OXCART reconnaissance airplane, but what Johnson announced was the experimental fighter version of the same airframe, the YF-12.
I’d always assumed that the YF-12 was just a cover story to explain the operations of the CIA’s A-12s, which, to be fair, it was. But it was also a genuine program to give the US Air Force a Mach 3+ fighter. Under the name Project KEDLOCK, the USAF examined the possibility of using the A-12/YF-12 as an interceptor to protect continental North America against Soviet bombers.
Details on the project seem relatively scarce, at least compared to the A-12 and its Air Force successor, the SR-71 Blackbird. The YF-12 was a two-seat variant of the CIA’s A-12 design, equipped with an ASG-18 radar and AIM-47 missile combination that had been designed for the USAF’s previous attempt at a high-speed, long-range interceptor, the XF-108. Three prototypes were built, and they flew a testing regime that culminated in six successful missile engagements. Despite Congressional appropriation of money to start the production process, the Secretary of Defense wanted a cheaper plane, and the program was cancelled in 1968. The YF-12s ended up having their greatest impact as NASA test aircraft, flying a range of high speed, high altitude tests from 1969 to 1979. (The full NASA history of those tests can be found here.)
A lot of sources present the cancellation of the YF-12 as a tragedy that robbed the US Air Force of an exceptional airplane. But would the YF-12 have made a great interceptor? Unlike most what-if planes, the airframe had already flew, and the performance of the A-12 and the SR-71 bear out pretty much all the claims. The ASG-18/AIM-47 radar/missile combination was already in development, and much of it ended up the F-14’s very successful AWG-9/AIM-54 system, so the electronics and weapon seem relatively mature too.
Would it have been the right plane for the job? That’s a much bigger question. The F-12 would have had all sorts of features that were undesirable for its mission. Sealing off the fuel tanks of an A-12/SR-71 was almost impossible (because of the heating and cooling that came with flying at Mach 3) and regular practice was to take-off without full tanks and fill them with in-flight refueling. Because the plane used a special fuel, JP-7, which meant that it also needed special tanker aircraft (the KC-135Q). Like most contemporary fighter aircraft, the F-12 was being designed without an internal gun – unlike aircraft like the F-106 Delta Dart and the F-4 Phantom, it would have been pretty much impossible to carry one in a pod or to use one at the plane’s optimum speed. Those sorts of challenges would have been particularly inconvenient, since US Air Force interceptors would spend quite a lot of 1980s escorting subsonic Soviet bombers along the edges of the US Air Defense Identification Zone.
Ostensibly, the YF-12 was in competition with several other potential interceptors, including an advanced version of the F-106 and an air defense variant of the swing-wing F-111 bomber. In reality, all these designs were looking at the end of an era. The F-106 was the last American fighter to be designed exclusively to intercept Soviet bombers. Its replacements, the F-4, F-15, and F-16, were all general-purpose fighters or multi-role aircraft. The interception mission wasn’t significant enough to justify a distinct design, and flexibility was worth more than specialization. The F-12 would have been the US Air Force’s fastest fighter by a vast margin, but even it couldn’t outrun the obsolescence of the concept on which it was based.