Aviation Ancestry - November 2012
From “Gusto” to “Oxcart”- SR-71 Part Three
By Scott Schwartz
The radar signature of a full-scale A-12 model is tested at a certain secret base in Nevada. Production A-12’s were painted black- mainly to reflect heat and cool down the aircraft. (Photo courtesy of the U.S. Government)With 20 months in which to get an A-12 airborne for the first time, the Skunk Works team had several challenges ahead of them. The first of these was the fact that techniques used in building a conventional airplane could not be used on the A-12; like using aluminum in its construction, for example. The friction caused by Mach 3 flight would generate enormous temperatures. Aluminum begins to weaken at 300 degrees Fahrenheit; the “coolest” part of the A-12 at Mach 3 would be the windshield at 600 degrees Fahrenheit. The temperature at the engine nacelles would be 1,200 degrees. Obviously, something stronger was needed.
Stainless steel was briefly considered, but, of course, that would have resulted in an extremely heavy aircraft. One engineer suggested the use of titanium, which was fifty percent lighter but just as strong as stainless steel. But, the use of titanium posed its own challenges.
For one thing, the only American company producing titanium at the time, produced sheets of it, with quality that was inconsistent. Plus, the drill bits that worked fine on aluminum came apart when used on titanium.
Lockheed engineer Ben Rich proposed a solution to the problem. He suggested the use of a softer grade of Titanium – albeit a grade that lost its strength at a mere 550 degrees Fahrenheit. To compensate for this weakness Rich suggested painting the entire airplane black, which would cause more heat to radiate away from the aircraft than would be absorbed.
A closer look at the J-58 engine. (Scott Schwartz)Since weight was a big concern, it should come as no surprise that Kelly Johnson balked at this idea. In fact, he flat-out rejected the idea at first, in the belief that the added weight (approximately 100 lbs.) would negate any benefits. That is, until Johnson and Rich met again at the next planning meeting. Apparently Johnson had done his own research, because he immediately approached Rich and admitted that he’d been wrong about the paint.
Of course, there was still the problem of obtaining enough titanium – since the supplier that Lockheed was using could supply only small quantities of it. After searching the world over, the CIA settled on buying titanium from a country that was a leader in exporting titanium at the time. Through the use of A Pratt & Whitney JP-58 engine; this engine was originally designed for a never-produced Navy fighter/attack aircraft. Lockheed modified the engine for use in the A-12. This example rests at the March Field Air Museum. (Scott Schwartz)dummy corporations and intermediaries, the Agency was able to buy Titanium from the Soviet Union!
Well, settling on titanium for the airframe structure solved only one of the problems posed by the high temperatures that the aircraft would be exposed to. Most of the aircraft’s other components would have to be tougher than average, as well. Through the years, the Skunk Works had become adept at scavenging parts from other Lockheed airplane designs – namely engines, avionics, and flight control systems. That would not work in this case.
Wiring would have to be gold- plated in order to withstand the heat (indeed, wind tunnel testing demonstrated that the airframe would actually stretch three inches at Mach 3). Hydraulic lines would have to be made of stainless steel, for the same reason. Not to mention the fact that the Skunk Works would have to make its own titanium rivets.
Because the fuel to be carried by the A-12 (13,000 gallons of it, by the way) would reach a scorching 350 degrees Fahrenheit during supersonic flight, the Shell Oil Company had to develop a fuel (known as JP-7) with a flash-point high enough to keep it from igniting at that temperature. In fact, a lit match thrown into a puddle of spilled JP-7 (of which there were many, since the aircraft leaked fuel constantly, when parked on the ground) would not ignite it. JP-7 also contained additives that reduced the visible shock waves that emanated from the engine’s tail pipes-making the aircraft less visible from the ground. As it turned out, the JP-7 would do more than feed the A-12’s engines. It also acted as a coolant, absorbing the heat that was generated inside the aircraft through the incorporation of heat-exchangers. And, if this wasn’t enough, the Skunk Works team developed a special valve that was able to sense variations in temperature. The valve routed the hottest fuel to the engines while keeping the cooler fuel available for cooling the retracted landing gear and the avionics.
Lubricating oil for the engines was just another in the series of challenges posed by the hostile environment in which the A-12 was expected to operate.
Initially, Kelly Johnson had located someone in Texas, who claimed to have created a type of oil that could tolerate operating temperatures in the neighborhood of 900 degrees Fahrenheit. Johnson, of course, was ecstatic. That was, until he received a sample of the “oil.” The sample arrived in the form of a canvass bag that was filled with powder crystals – crystals that did not become an oily lubricant until they were heated to 900 degrees Fahrenheit. Since heating a powder to nearly 1,000 degrees before it could be put into the aircraft was….impractical, Lockheed referred the problem to the petroleum research department at Penn State University. Penn State was eventually able to develop an oil that stayed viscous at the various operating temperatures to be encountered in the operation of this remarkable aircraft.
Virtually, the only “off-the-shelf” (sort of) parts to be used on the A-12 would be its engines. Originally intended for a mach 2 Navy fighter that was never put into production, the Pratt & Whitney J-58 had previously been subjected to roughly 700 hours of testing, before funding for the project had been stopped. A scaled down (80 percent) version of the JT9 engine that was used in the XB-70, the J-58 – in its “stock” form – generated 26,000 pounds of thrust. But, as we’ll see, the engine would hardly be an “off-the-shelf” item, once Lockheed engineers were through modifying it.
To be continued.
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