pounds overweight, and the driver was ordered to unload the excess. The air force had to go to the governor to get the load released. The Sun Tan group thought about painting on a second axle but quickly realized this would be too obvious. When the new trailer was built, it had two axles, the second purely for cover.
A third incident occurred during construction of a liquid hydrogen plant near Pratt and Whitney's Florida test facility. Its cover was as a 'fertilizer plant,' but word soon spread that the facility produced hydrogen. A local civil defense official became alarmed that a hydrogen bomb was being built in the area. It took a delegation of security officials to convince him to keep quiet.
Use of liquid hydrogen affected every part of the CL-400. It boiled at negative 423 degrees F, yet, at Mach 2.5, the plane's skin would reach 746 degrees F. The liquid hydrogen would have to be protected from this heat.
The fuel lines, which would have to pass through the hot wing structure before reaching the engines, had a vacuum-jacketed insulation. Tests of the insulation were done at Fort Robertson using five ovens. Heat tests were also run on the engines, booster pumps, valves, controls, and other components. [104]
While the Skunk Works was designing the CL-400, Pratt and Whitney was conducting tests on the hydrogen- fueled engine. The initial work, code-named 'Shamrock,' was to convert a J57 engine to burn hydrogen. The modifications worked very well; the engine could be throttled down until the fan blades were spinning slowly enough to be counted. The throttle could then be smoothly opened to full power.
The success of the modified J57 encouraged development of the Model 304 engine that would power the CL- 400. On a normal jet engine, fuel is sprayed directly into the combustion chamber. With the Model 304 engine, the liquid hydrogen first passed through a heat exchanger. This contained nearly five miles of stainless steel tubing. The liquid hydrogen was heated by the exhaust, going from negative-423 degrees F to 1,340 degrees F and changing from a liquid to a hot gas. The hydrogen gas was fed through a turbine, which spun the compressor fans and liquid hydrogen pump via a reduction gear. Some of the hydrogen was sprayed out the burners and ignited. The rest was sent to an afterburner.
The first runs of the 304 engine began on September 11, 1957. In all, twenty-five and a half hours of operation with liquid hydrogen were completed during the next year. Despite failures with the turbines, heat exchanger, and bearing, the development was seen as progressing satisfactorily. [105]
The CL-400 would never get to test its wings, however. By October 1957, the Sun Tan project had effectively ended. The problem was the plane's short range. The end came when Johnson was visited by Assistant Air Force Secretary James H. Douglas Jr. and Lt. Gen. Clarence A. Irvine. They asked how much 'stretch' was in the CL-400. Johnson told them only 3 percent.
The plane was a flying thermos bottle. The only space was the cockpit, and fuel could not be carried in the hot wing structure. Douglas and Irvine asked Pratt and Whitney how much improvement could be made in the 304's fuel efficiency. The answer was only 5 or 6 percent over five years.[106]
To increase the CL-400's range, its size would have to be increased considerably. The Skunk Works looked at planes as long as a football field.
This made the plane even less practical, and Johnson urged that Sun Tan be canceled. The air force was also short of money for several higher-priority projects, and there were doubts Eisenhower would approve overflights.
With this, the project ended. The prototype CL-400s were canceled in October 1957, although the engine tests continued through 1958. The formal cancellation was made in February 1959. In all, between $100 and $250 million had been spent. Not until 1973 was the Sun Tan project revealed.[107]
Sun Tan was only one thread in a number of post-U-2 ideas. After the failure of the Dirty Bird U-2s, Johnson studied a large flying-wing design.
The span of the swept-back wings was larger than that of the U-2. It was powered by two jet engines fed from a nose intake. Fins were located near the wing tips. In overall shape, it resembled the World War II Go 229 German fighter. The design was capable of very high altitudes, but still at relatively low speeds.
It was not until the fall of 1957 that the emerging high-speed reconnaissance aircraft program began to coalesce. Bissell arranged for a study of how a plane's speed, altitude, and radar cross section affected its probability of being shot down. The study found that supersonic speeds greatly reduced the chances of radar detection. The aircraft would need a top speed of Mach 3, to fly at altitudes over 80,000 feet, and to incorporate radar-absorbing material.[108]
To achieve such speeds was a nearly impossible task. At this time, there had been only one manned Mach 3 flight. On September 27, 1956, the X-2 rocket-powered research aircraft reached Mach 3.196, equivalent to 2,094 mph. The plane went out of control, killing the pilot, Capt. Milburn Apt.[109]
Even this had been a brief, rocket-powered sprint. The reconnaissance aircraft would need to maintain these speeds for a prolonged time, while being subjected to more severe airframe heating than on Sun Tan.
To put in perspective what was required, the plane would have a
If these speeds could be reached, however, it would vastly complicate the problem facing Soviet air defenses. A U-2 flying directly toward an SA-2 SAM site would be detected about ten minutes before reaching it and would be in range for about five minutes. A Mach 3 aircraft would have a warning time of less than two minutes. Only
Speed would greatly reduce the reaction time of air defenses. Use of radar-absorbing material would further reduce the range at which the plane could be detected.
An airplane with these capabilities would be very expensive — far more than the U-2 had been. A clear assessment of the plane's feasibility was needed. (Sun Tan had, by this time, proven to be a 'wide-body dog.') Bissell put together a panel to provide this assessment. The chairman was Dr. Land, and the panel included two aerodynamic experts and a physicist.
The assistant secretaries of the air force and navy for research and development also attended some of the six meetings.[112]
The navy, Convair, and Lockheed were made aware of the general requirements and submitted designs. (As yet, no money or contracts had been issued.) The navy submitted a design for a ramjet-powered aircraft with rubber inflatable wings. It would be carried to high altitude by a huge balloon. The aircraft would then be boosted by a rocket to a speed at which the ramjets could start. The navy proposal proved to be totally impractical. It was determined that the balloon would have to be a mile in diameter and the aircraft's wing area one-seventh of an acre.
Convair proposed a ramjet-powered Mach 4 aircraft that would be launched from a B-58. This proposal, although far more practical than the navy concept, also had shortcomings. The B-58 could not reach supersonic speed with the aircraft attached. Moreover, it was thought the aircraft's ramjet would suffer 'blowouts' during maneuvers. The total flight time for the Marquardt ramjet was less than seven hours, but Convair engineers continued to refine the design.
Lockheed and Johnson were studying a wide range of concepts for what was initially called the 'U-3' project. Many were based on the Sun Tan airframe, but using kerosene fuel. Different size aircraft were looked at, with both two and four engines. Johnson also looked at exotic concepts. These included towing the U-3 to altitude behind a U-2; using a booster stage; carrying the U-3 to altitude under a balloon; aircraft with jet, rocket, and ramjet engines; designs that used coal slurries or boron fuel; vertically launched aircraft; and a design with inflatable wings
