Ultimately, Oxcart was seen as needed and was continued under CIA control. Satellites would be restricted to coverage of the Soviet Union for the foreseeable future. It would also be many years before a satellite camera had the resolution of the U-2's B camera. If the USSR was off limits for the U-2, it could still provide coverage of Communist China, Cuba, Vietnam, or the Mideast. In a few years, however, these areas could no longer be overflown with impunity. The Chinese already had SA-2 SAMs, and other countries would have them by the early and mid-1960s. The Oxcart would soon be needed to conduct overflights of even Third-World countries.
Once the future of Oxcart was resolved, the initial development work continued. Temperature affected every aspect of the Oxcart's design. Even though the plane would be flying at the edge of space, friction would raise the skin temperature to over 500 degrees F. The coolest part of the engine, the inlet, reached 800 degrees F. The afterburner section would reach 3,200 degrees F.[122] The plane would have to be built of stainless steel or titanium.
Stainless steel honeycomb was being used in the Mach 3 XB-70, then under development, but Johnson rejected this when he saw the production problems it entailed. The honeycomb had to be produced in a clean room, under sterile conditions. The Skunk Works motto was "KISS" (Keep It Simple, Stupid). Stainless steel was too complicated and was likely to cause problems.
Johnson decided to use heat-treated B-120 titanium alloy. This was still a major step into the unknown. Although it had been used in aircraft before, nobody had ever tried to build an entire airframe out of the material. Even drilling a hole was a problem, due to titanium's extreme hardness. Drills would be worn out after only seventeen holes. A special West German drill was found that could drill 150 holes before needing resharpening.
Before beginning production, Johnson decided to build a sample of the wing structure and nose section. When the wing structure was put in the "hot box," to simulate the high temperatures, it literally wrinkled. The solution was to put corrugations in the wing skin. At high temperatures, the corrugations only deepened slightly. Johnson was jokingly accused of building a Mach 3 Ford Trimotor (which also had a corrugated skin). The nose segment was used to study requirements for cooling the pilot, camera, and systems.[123]
A continuing problem during development was the poor quality of the titanium. A full 80 percent was rejected; the material was so brittle that it would shatter like glass if dropped. This problem continued into 1961, until a group from CIA headquarters went to the Titanium Metals Corporation and briefed company officials about Oxcart. The supply soon became satisfactory.[124] Lockheed also established an extensive quality-control program.
There were times, Johnson later recalled, "when I thought we were doing nothing but making test samples."[125]
Sometimes the problems with titanium bordered on the bizarre. During heat tests, bolt heads would simply fall off after one or two runs. It was found that cadmium plating had flaked off the tools used to tighten the bolts. This was enough to "poison" the titanium, causing a spiderweb network of cracks to form. All cadmium-plated tools had to be thrown in a big vat that was boiling "like a witch's brew" to strip off the plating. It was also found that welds of wing panels done during the summer soon failed, while those made during the winter lasted indefinitely. Again, it was a chemical reaction. The parts were washed before welding, and in the summer, Burbank city water had chlorine added to reduce algae. Even an ordinary pencil was dangerous. A shop worker took a pencil and wrote some numbers on a piece of titanium; a week later, it was discovered the graphite had etched the metal.[126]
Not simply the airframe, but every part would have to withstand temperatures higher than ever before endured by an aircraft. Johnson said later,
"Everything on the aircraft, from rivets and fluids up through materials and power plants, had to be invented from scratch." All electrical connections were gold-plated, as gold retained its electrical conductivity better at high temperatures than copper or silver. The control cables were made of Elgiloy, a steel, chromium, and nickel alloy normally used in watch springs.[127] A hydraulic fluid was developed to withstand temperatures of 650 degrees F (150 degrees hotter than normal).[128]
Fuel was a difficult problem. During subsonic cruise, such as during refueling, temperatures would drop to negative-90 degrees F. At Mach 3, the fuel would be heated to 285 degrees F. It would then be pumped through the afterburner exit flaps, acting like hydraulic fluid to control their position.
This would raise its temperature to 600 degrees. The fuel would then be pumped into the J58 engine. Conventional fuel would boil and explode at such temperatures. The fuel developed was JP-7, also called LF-2A. It had a low vapor pressure; if a match was thrown into a pool of JP-7, the match would go out.[129]
The internal stress caused by such heat affected the quartz glass window for the camera. The heat had to be even throughout the window, or there would be optical distortion. This one problem took three years and $2 million to solve. The quartz window was fused to its metal frame using high-frequency sound waves.
The effect of these many problems was to delay the program and raise its cost.
Development of the J58 engines and their nacelles proved the most difficult problem. The J58 program was begun in late 1956 to power a navy attack plane with a dash speed of Mach 3. This speed would be maintained for only a few seconds. By late 1959, however, navy interest was fading, and it was decided to cancel the engine. The CIA requested the work be continued and the engine be modified for a continuous speed of Mach 3.2.
A contract was issued for three ground test and three flight test engines.[130]
With the many design changes needed to accommodate the extreme heat, virtually nothing remained of the original navy J58 engine when development was finished. To give one example, a standard ground test stand could not simulate the heat and altitude conditions required. Pratt and Whitney built a new test stand in which a J75 engine's exhaust was run through and around the J58. Speeds over Mach 3.6 and altitudes of 100,000 feet could be simulated.[131]
For all its power, the J58 engine alone was not enough to drive the A-ll to Mach 3 by brute force. The nacelles were the key that opened the way to those speeds. They were not simply a place to put the engines, but an integral part of the propulsion system. Up to 1,600 mph, air would come in through the intake and a ring of centerbody bleed vents to feed the engine.
As the A-ll approached Mach 3, the flow cycle would change. Air was now vented out the centerbody bleed vents. The effects were amazing — at Mach 3, a full 56 percent of the total thrust came from the intake. Another 27 percent came from the afterburner, while only 17 percent came from the J58 engine itself. In effect, the J58 was a flow inducer and the nacelles pushed the airplane.[132]
127
Richard P. Hallion,
131
William H. Brown, "J58/SR-71 Propulsion Integration or The Great Adventure into the Technical
132
Clarence L. Johnson, "Some Development Aspects of the YF-12A Interceptor Aircraft," AIAA Paper No. 69-757 (July 14–16, 1969): 7.