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Folding Tips. Like all delta-wing planes, the XB-70A tends to get nose-heavy at high speeds when the wing's center of lift shifts toward the stern. To counteract this tendency, two small wings called canards are set like large trim tabs, just aft of the cabin. But even the canards are not enough; as Mach 3 approaches, the tips of the delta wing will be folded downward. This will shift the center of lift forward and add directional stability. It also adds a hazardous complication to the plane's construction.
The 550,000-lb. XB-70A will be shoved along by six General Electric YJ-93 engines, each with 30,000 Ibs. of thrust. The engines' turbine blades are air-cooled to keep them from melting, and the afterburners, which are used continuously at high speed, run white-hot. The two boxlike air intakes, each one feeding three engines, are 80 ft. long and high enough for a man to walk erect in their gaping maws. They are rigged with movable walls, ports and bypass doors to keep the entering air at the right pressure and temperature. The engines are grouped close to the centerline of the plane so that if one of them fails, the loss of thrust will not cause a dangerous yaw.
The hydraulic system that works the giant control surfaces uses up 2,000 h.p., more than the output of both engines of a wartime B-25 bomber. If built conventionally, it would have been far too heavy; for the XB-70A, fluid pressure was raised to 4,000 Ibs. per sq. in. in unusually thin tubing. Such changes save weight, but they also increase the hazards of a system that has already proved a notorious source of aircraft trouble.
Protective Fuel. The fierce heat that will surround the XB-70A in flight was an overriding problem for its designers. Almost every part had to be heat-resistant. The tires, for example, are made by B.F. Goodrich out of material that stands twice the temperature that melts ordinary airplane tires. To dispel the heat that will fight its way toward the crew, North American's engineers decided to make the fuel carry it away. While the XB-70A is cruising at Mach 3, its fuel will circulate, cooling the interior, absorbing enough heat every minute to evaporate four gallons of water. Inside, if nothing goes wrong, an air-conditioning unit will be able to keep the cabin comfortable.
All through the plane are details that cause cold shudders as well as admiration. Titanium and stainless steel skins are "sculptured" chemically, sometimes to a thinness of .007 in. to save ounces of weight. Electric motors run at a temperature that would bake a cake. Such novel techniques—and thousands more that have been used in the XB70A—are interesting but highly experimental. They will call for elaborate and repeated testing before the dangerous cobra can attempt its first high-speed flight, scheduled for this summer.