On May 1, 1960, Soviet air defense missiles downed a U-2 flown by Francis Gary Powers, and, with 24 missions flown, overflights of the Soviet Union were halted. America’s first purpose-built spyplane had, until that day, avoided such a fate by flying extremely high, but the U-2 was slow, and U.S. officials had always known its days were numbered. If the airplane’s follow-on was to be less vulnerable, and assuming the high-altitude requirement persisted, as it had to, then only one area of performance remained: speed.
Under a project code-named Gusto, the Lockheed Skunk Works, headed by the legendary Kelly Johnson, and the Convair division of General Dynamics completed designs for a successor to the U-2 in the summer of 1959. Convair proposed a manned, ramjet-powered parasite reconnaissance airplane that would be launched from a special version of the B-58B Super Hustler bomber flying at Mach 2.2 above 35,000 feet. The parasite would reach Mach 4.2 and 90,000 feet, according to Convair. The project, named Fish, was terminated because the Air Force canceled the B-58B and ramjet technology was unproven. Convair later came back with the ground-launched Kingfish, based on its F-106 Delta Dart interceptor and the B-58, but it too was doomed.
That left the Skunk Works entry, which had evolved from A-1 (for “Archangel”) to A-11 and then A-12, which in turn evolved into the Air Force’s SR-71. There is disagreement even today at the Skunk Works on whether the CIA’s version was the A-11 or the A-12. Garfield J. Thomas, vice president of Reconnaissance Systems at what is now Lockheed Martin, says it was the A-11. Albert T. “Bud” Wheelon, who was head of the CIA’s Directorate of Science and Technology in the early 1960s, agrees. Engineers at Pratt & Whitney who designed its engine claim it was really the A-12. They’re all correct. Two Johnsons—Kelly in private and Lyndon Baines in public—called it the A-11. And the basic airplane was the A-11. It became the A-12 when its metal vertical tail, engine inlets, and the forward edges of its nacelles were changed to a kind of composite plastic to foil enemy radar. One thing is certain: Both aircraft were in a program code-named Oxcart. In January 1960, the Skunk Works got an order for 12.
Oxcart was to cruise at 90,000 feet and Mach 3.2—faster than a rifle bullet. At that speed, the friction of the air would create tremendous heat, but airframe heating in itself was not insurmountable. North American’s X-15 almost routinely exceeded Mach 3 and in October 1967 would set a world record of Mach 6.7. But the X-15 flew supersonically for mere minutes, not for the many hours required of a long-range reconnaissance aircraft. The F-106, the nation’s premier operational interceptor at the time and for years to come, could fly at Mach 2 for only 15 minutes at a time.
The heat that would be created by flying at Mach 3+ for long periods bedeviled engineers and required unparalleled inventiveness. As Kelly Johnson said, “Everything on the aircraft, from rivets and fluids, including materials and power-plants, had to be invented from scratch.” Long before the A-12 flew, its designers calculated that their creation’s leading edges would get hotter than a soldering iron, and without adequate cooling in the cockpit, it would literally get hot enough to bake a cake.
In early 1991, with the cold war over and much of the SR-71 declassified, the Graduate Aeronautical Laboratories at the California Institute of Technology presented a course entitled “Ael07, Case Studies in Engineering: The SR-71 Blackbird.” The course was based on a detailed three-inch-thick engineering summary, and virtually every section of the book, whether the subject was grease, hydraulic fluid, engines, oil, electrical plugs, or tires, shared one word: heat.
To cope with both altitude and heat, a fuel called JP-7 was developed with an exceptionally high flash point: A lighted cigarette tossed into a pail of JP-7 would go out. This tolerance for high temperatures allowed it to be the airplane’s primary heat sink. And there was a lot of it: The fuselage and inner wings formed fuel tanks with a capacity of 40 tons, and they were unlined, since no plastic liner could survive.
B.F. Goodrich came up with main landing gear tires that were compounded with aluminum powder to produce a silvery rubber that reflected heat from the airframe. And they were filled with nitrogen, not air, because oxygen would only incite combustion. The tires, which had a 22-ply rating, cost $2,300 apiece and were good for between 15 and 20 landings, depending on the pilot’s touch. The wheel wells into which they retracted were shielded and surrounded by tanks of fuel that performed double duty as a massive cooling system. The heat sink worked by routing the fuel to a pump that raised its pressure to 45 pounds per square inch and sent it to several valves and heat exchangers, where it cooled subsystems before being returned to the tanks to be consumed in the engines. The JP-7 circulated throughout the fuselage and wings the way blood does in animals.
A hydraulic system was invented to operate between -65 and 650 degrees Fahrenheit. After responding to an ad in a technical journal touting a fluid that would work in temperatures as high as 900 degrees, Johnson received a large canvas bag filled with white powder. He called the manufacturer and was told that it would turn to liquid when it got hot enough. The problem was solved by Penn State’s Petroleum Refining Laboratory, however, which came up with a super-refined petroleum-based oil that met the requirement.
The glass through which the all-important cameras peered had to be free of optical distortion even though the inside temperature was 150 degrees and the outside 550 degrees. (Pilots have reported that the windshield of an SR-71 gets so hot at cruise that they can’t touch their gloved hand to it for more than a couple of seconds.) That one was solved by the Corning Glass Works and Perkin-Elmer, a lens manufacturer, in three years and at a cost of $2 million. They fused quartz glass windows to the metal frame using high-frequency sound waves, which had never been done before.