With the abandonment of the North American XF-108 "Rapier" high-speed interceptor program for the United States Air Force (USAF) came an opportunity for Lockheed to sell its A-12 design as a possible replacement. The A-12 was developed as a high-altitude reconnaissance aircraft for the Central Intelligence Agency so much work on the product had already been completed when cancellation came to the XF-108. Lockheed was able to interest USAF authorities in a modified version of their high performance plane and this gave rise to the "YF-12A" designation which encompassed three converted A-12s pulled from the A-12 production line (aircraft numbers seven, eight and nine of fifteen).
Unlike the A-12, the YF-12A relied on a crew of two (a pilot and a fire control officer) seated in tandem. Additionally it carried the Hughes AN/ASG-18 Look-Down/Shoot-Down (LDSD) Fire Control Radar (FCR) in its nose and tied to an armament display of 3 x Hughes AIM-47A "Falcon" Air-to-Air Missiles (AAMs). These missiles were held in three internal bays (a forth was reserved for the fire control equipment). The radar fit was the same unit developed for the failed XF-108 product.
The A-12 was an excellent starting point for a Cold War interceptor as it was one of the earlier attempts by the Americans to feature inherent stealth features in its construction make up. A chined body and inward-canted vertical tail fins, along with Radar Absorbent Materials (RAMs), were all features of the airframe - qualities that would later be reproduced in the classic SR-71 spyplane. Radar Cross Section (RCS) reduction was also implemented where possible into the A-12's form. The chines at the nose of the A-12 were removed due to the unique radar installation.
Additionally the A-12 was a high-speed, high-altitude performer which the YF-12A would benefit from immensely. The interceptor form carried 2 x Pratt & Whitney J58/JTD11D-20A afterburning turbojet engines which outputted 20,500 lb thrust on dry and 31,500 lb thrust with rehear. Maximum speed reached Mach 3.35 (2,275 miles per hour) with a range out to 3,000 miles and a service ceiling up to 90,000 feet. Rate-of-climb was measured at 11,820 feet-per-minute.
Its general configuration followed the A-12: a long neck holding the cockpit well-ahead of the body was used. The engines were held outboard of the fuselage in individual tubular nacelles breaking up the flow of the wing mainplanes. Two tail fins were featured, one per nacelle dorsal line. This same configuration would be repeated in the upcoming SR-71 product by Lockheed as well.
YF-12A designated the three converted A-12 models which served in the pre-production role. Its in-service designator was to become F-12B but this was never used as the program was halted before production-quality units could begin construction. YF-12C was a fictitious designation applied to SR-71 testing handled by NASA in an effort to keep the SR-71 program under wraps from public eyes.
First flight of the YF-12A was had on August 7th, 1963 and the product was announced to the public in February of the following year to help keep the A-12 secret. Ninety-three F-12B production units were ordered by the USAF in May of 1965 but the growing American military commitment to the Vietnam War (1955-1975) and mounting budgetary needs elsewhere forced the F-12B procurement to the backburner. As time wore on, the need for this dedicated interceptor ebbed away until there was no viable requirement standing. Thusly the YF-12A was cancelled in January of 1968.
The YF-12As that were completed survived a time longer as research-collecting aircraft for both the USAF and NASA where it ended its days. While one of the three examples was lost to an accident in July of 1971, the remaining two examples have been preserved - one at the National Museum of the USAF in Dayton, Ohio and the other making up a portion of the SR-71C display at the Hill Aerospace Museum in Utah.
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