The Real X-Jet
From Transformers to the X-Men, the Blackbird is still Hollywood's favorite futuristic jet. Here's the real story of its birth.
- By William E. Burrows
- AirSpaceMag.com, March 01, 1999
(Page 3 of 7)
Pratt & Whitney executive Arnold J. Gunderson crystallizes the engineering challenge by pointing out that while the J-58 had parts that were machined to thousandths of an inch, the whole engine got so hot at cruise that it expanded two and a half inches in width and grew six inches in length. (The airframe expanded and contracted as well, which was why fuel leaked from its wings.) That problem was solved, says Gunderson, by putting the gear box that drives the generators, hydraulic pumps, and other subsystems on the bottom of the engine, where it effectively floats, and hanging the engine itself from the top of the nacelle so it could expand and contract without stressing its parts.
The heat also made the A-12 and the SR-71 trickier to land than to launch. Descending too quickly and shock-cooling the airframe and engines would cause a sudden shrinking of parts that could be more stressful than the expansion that occurred during heat-up. Pilots had to avoid cooling the engine at a rate that could cause shrinkage and make the ends of the turbine blades rub against their seals, ruining the engine.
Mach 3.2 was the A-12’s (and then the SR-71’s) design point. The airplane could hit Mach 3.6 and, in theory, Mach 4 if the air was cold enough. But, as Gunderson explains, “Kelly designed the airplane to fly at Mach 3.2 at 80,000 feet. The airplane cruises there. It is very, very comfortable there because that’s the point at which it was designed to fly, where every system is snug and tight-fitting.... All of the enhancements and refinements we did over the many, many years of the program were done to reinforce that. Moving away created more problems than it was worth. Every time we flew faster and higher, we ran into areas where we had not so much experience. Things would go wrong. You’d swallow the [aero]spikes, you’d flame out.”
Unlike their engines, 93 percent of the airplanes’ structure and skin were made of titanium. The metal’s quality varies widely, and the first batch of titanium sponge (which looks like a giant soap pad) had to be bought by the CIA from the Soviet Union and smuggled past U.S. Customs. Ironically, the Soviets had most of the world’s supply of the metal and exported a product of exceptionally high quality. The plane’s landing gear was the largest titanium forging produced in the United States and the only titanium landing gear in use in any U.S. aircraft. Forging and cutting special metals to tolerances as tight as .005 inch, then welding them together, were exotic specialties that were virtually developed from scratch.
Welding brought with it new mysteries. Skunk Works engineers were at a loss to explain why A-12 wing panels that were spot-welded in the summer failed early, while those that were welded in the winter held together indefinitely. They traced the trouble to a characteristic of titanium: It is absolutely incompatible with chlorine. “We finally traced the problem to the Burbank water system, which had heavily chlorinated water in the summer to prevent algae growth, but not in winter,” Johnson related in a classified CIA journal article in 1982. “Changing to distilled water to wash the parts solved the problem.”
The A-12’s inlets, which include forward air bypass doors that automatically manipulate the air—and the famous spikes—were designed under Lockheed engineer (and later Skunk Works president) Ben Rich’s supervision and are the key to the airplane’s performance. The B-58 Hustler bomber used fixed spikes in front of its General Electric J-79s. Like his counterparts at Convair, Rich found that air hitting the unprotected front of a turbojet at high Mach numbers creates insurmountable pressure problems.
The spike’s purpose was to control the supersonic shock wave and, by working in combination with doors and openings to bleed away excess air, prevent supersonic airflow from entering the compressor intake. It accomplished this trick by moving back 26 inches into the throat of the inlet on a programmed schedule as the speed built up in a manner somewhat like the way the nozzle on a garden hose changes to adjust the water’s flow. At Mach 3.2, each inlet swallowed about 100,000 cubic feet of air a second.
When a spike failed to work perfectly, which happened repeatedly for many years, supersonic air pressure instantly built up and choked the compressors. They reacted without warning by violently spitting the shock wave back out. This event was the dreaded “unstart,” which, at Mach 2 or higher, would cause a thunderous bang that the crew heard and felt, while the airplane jerked so violently in the direction of the unstarted inlet that, according to Lockheed Martin’s Garfield Thomas, “it was like running into a brick wall.” He remembers one unstart yaw violent enough that the pilot “slammed his head so hard against the sill on the window [that it] cracked the helmet and knocked him semi-conscious.” A computer restarted the inlet, Thomas adds, and the pilot survived.