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A-12 OXCART Blackbird - Design

At the high speed the A-12 would reach, most of the aircraft’s skin would be subjected to temperatures between 500 and 600 degrees F., and over 1,000 degrees F. at some spots near the engines. Lockheed chose to build over 90 percent of the A-12’s airframe out of a titanium alloy. It was almost as strong as stainless steel but weighed half as much and could handle the intense heat. Titanium proved to be very difficult to work with, however. Its extreme hardness caused problems in machining and shaping the material.

Lockheed decided to gothe route of a titanium aircraft rather than the stainless steel honeycomb concept of the B -70, because of the better strength / weight ratio possible and the more straightforward construction that could be used. The Skunk Works did not have the capability to build a stainless steel honeycomb aircraft with the incredible amount of precise tooling required; nor did Lockheed foresee, either, the incredible amount of tooling that it would take to make it out of B-120 titanium. This material was not in use by any other project.

To operate at design speeds and temperatures, the A-12 required fuel, lubricants, hydraulic fluids, and sealants that had not been invented yet. The fuel tanks, holding almost 11,000 gallons, made up the largest proportion of the aircraft and would heat up to about 350 degrees F. At that temperature the most advanced fuel blends then in use would boil off or blow up. Instead, a special fuel, called JP-7, with a low vapor pressure and high flash point had to be developed; a lighted match would not ignite it. A liquid chemical that exploded on contact with air would start the engines. Through the use of heat exchangers and smart valves, the fuel would also act as an internal coolant.

No sealant for the fuel tanks was ever developed that was simultaneously impervious to chemical effects caused by the fuel, and elastic enough to expand and contract as the tanks heated and cooled and were subjected to large pressure changes. Consequently the A-12’s tanks leaked, a quirk that was not detected until the first aircraft was delivered to the test site and filled with fuel, setting off a reaction that broke down the sealants. A “leak rate” of between five and 60 drops per minute, depending on the source, was considered acceptable. When the A-12 was about to go off on a test or mission, it would receive only enough fuel to get airborne. It would then rendezvous with a KC-135, top off its tanks, and immediately climb to operating altitude, which caused the metal to expand and the leaks to stop.

The J58 turbojet engines that would enable the A-12 to fly so high and fast were the most persistent problem. Designed in 1956 for a Navy aviation project that was canceled, the engines had to undergo major modifications to turn them into the most powerful air-breathing propulsion devices ever made. Just one J58 had to produce as much power as all four of the Queen Mary’s huge turbines — 160,000 horsepower or over 32,000 pounds of thrust. To crank it up, two Buick (later, Chevrolet) racecar engines on a special cart were used. The unmuffled, big block engines put out over 600 horsepower and made a deafening roar. The J58s themselves put out an almost incredible din. Recalling his visit to the test site to watch a midnight takeoff, DCI Richard Helms wrote that “[t]he blast of flame that sent the black, insect-shaped projectile hurtling across the tarmac made me duck instinctively. It was if the Devil himself were blasting his way straight from Hell."

By 1966 the A-12 aircraft radar cross-section had been reduced considerably. The Soviets’ TALL KING radar would be able to identify and track the A-12 despite its small, nonpersistent radar return. Though the aircraft could be detected by SOVBLOC radar, defensive countermeasures equipments had been developed, tested and installed to considerably reduce the risk to the aircraft in a hostile environment. Due to the phase-down no further new efforts to reduce radar cross-section were under consideration. Development of a second generation countermeasures equipment had been completed and was being tested. Results to date were very encouraging. Upon completion of testing it would be introduced to the operational fleet.

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