That New Black Magic

In the early years of the cold war, enter Kelly Johnson and an clean sheet of paper—long enough to accommodate an 80-foot wingspan.

Clarence L. “Kelly” Johnson of Lockheed’s secret “Skunk Works” facility, chief designer of the U-2. (USAF)
Air & Space Magazine

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"Put a little patch of paper up there to show what it looks like," Johnson answered. "Then you'll have to redraw it because the blueprint machine won't handle anything wider than 42 inches."

Altitude would be the U-2's best defense, but altitude also constituted its single most difficult engineering challenge. Its engine, rated at more than 10,000 pounds of thrust at sea level, would produce only about 700 pounds of thrust at altitude. Hydraulic systems were heavy, so Johnson eliminated the customary hydraulic boost for the controls. To save time and cost, they used the bucket seat and the control yoke from a P-38. Some of the pilots recruited to fly the first U-2s disliked the yoke (which they associated with transport aircraft), but it took the strength of both arms to fly the airplane, and the additional leverage of the yoke was necessary.

Johnson saved even more weight by designing the airplane for load factors of only 2.5 Gs, a fraction of that for normal combat aircraft. Instead of using a wing spar that passed through the fuselage, the wings, which also carried almost the entire fuel supply, were simply bolted on. This would turn out to be an ingenious solution, for the airplane would spend little time in turbulent air. (At a CIA symposium on the U-2 in September, gleeful officials reported that a recent structural evaluation indicated the current airframe is good for over a hundred years' more service.)

And in his pursuit of weight reduction Johnson also eliminated landing gear. He wanted the CL-282 to take off from a wheeled dolly and land on its belly, which would act like a skid. But reality, in the person of flight test engineer Ernest Joiner, intruded. Joiner told his boss that the flight test program would quickly fall apart if the airplane had to have its belly repaired after every landing.

It was therefore decided to install a dual-wheel main landing gear and tailwheel in the U-2's fuselage and use flexible struts, or pogos, as they came to be called, to prop up the wingtips during takeoff, after which they would fall away. But that created another problem. The U-2's all-important payload, a very large, heavy camera, was to be carried in a so-called Q-bay behind the cockpit. The engine would be right behind the bay. The only place to put the forward landing gear was therefore between the engine's air intake ducts, which formed a pair of pants whose legs straddled the Q-bay. This was not the ideal place to put the forward landing gear, Baldwin says. It was too far forward and made landing somewhat tricky. But the engineers were stuck with it.

The engine was a modified version of the reliable Pratt & Whitney J-57, which powered F-100s and B-52s. But here, too, there were birthing problems. The P-31 version of the engine, specially adapted to high-altitude flying, was dedicated to another program for the Air Force. The P-37 version, the first U-2 engine, was not designed for altitudes above 65,000 feet. It therefore tended to flame out repeatedly above 57,000 feet. Pilots dreaded the prospect of having to descend to 35,000 feet, where the MiGs and missiles waited, to restart their windmilling engines.

Then there was the oil problem. Because the atmospheric pressure at altitude was so low, oil leaked through the J-57's seals and got into the U-2's air conditioning and de-fogging systems. Engineers calculated that during an operational mission, 64 quarts of oil--the maximum capacity of the system--might be lost. The de-fogging ducts sprayed the windshield with hot air from the engine's compressor, and during long flights a gradually thickening coat of oil would form on the glass. This was solved by providing pilots with long sticks with diaper cloth attached to the ends so they could wipe the windshields clean. Someone even got the idea of welding a small metal box on the de-fog line and stuffing it with Kotex to absorb the oil. But the hot air was under so much pressure that it bent the box out of shape, says Lockheed's Bob Murphy, who was involved in many aspects of the U-2 and SR-71 programs. The problem ended only when the P-31 engine, which required less oil and was optimized for high altitude, replaced the model 37 in 1956.

Another worry was jet fuel: The standard JP-4 and JP-5 would boil away at high altitude. General James Doolittle, an executive with Shell Oil, had been a technical advisor to the reconnaissance community, and he persuaded Shell to develop a new jet fuel, designated JP-7, that had very low volatility. Production of JP-7 required most of the stocks of the petroleum products the company used to manufacture insect sprays, and although few Americans knew why, there was a nationwide shortage of bug spray in 1955.

In the near vacuum of the upper atmosphere, pilots also required special protection so that their body fluids would not bubble and boil. To this end, the David Clark Company of Worcester, Massachusetts, devised a partial-pressure suit, which was the first of its kind for keeping pilots alive in near-space conditions. This even led to the first specialized food and water provisions. Pilots could push a tube through a little hole in the face mask and suck on sweetened water or cheese- and bacon-flavored food mixtures squeezed from soft containers.

Undoubtedly the most daunting problem faced by U-2 pilots was the infamous "coffin corner," the terribly tiny margin between Mach-shock and stall buffet. At altitudes above 65,000 feet, the first U-2s had an interval of only six knots (7 mph) indicated airspeed between the onset of Mach buffet and a stall. In other words, the difference between the U-2's slowest flying speed and its fastest was only six knots. The margin was so narrow that Lockheed test pilots reported that in a bank, a U-2's inner wing could be stalled while its outer one was buffeting wildly from excess speed.

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