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Aeronautics: The Supersonic Cobra

6 minute read

The Air Force officers who christened their controversial new research plane the Valkyrie were probably confused. Valkyries were the screaming maidens of Nordic mythology who selected the warriors who were to fall in battle, and conducted them to Valhalla. But when the long-delayed XB-70A was finally rolled out of its hangar at North American Aviation last week, no one else knew what to call it either. Some said it looked like a hooded cobra; to others it was a praying mantis, a flying anteater, a banana split towing an orange crate. To Brigadier General Fred J. Ascani, Air Force chief of the project, the shining white monster was a beautiful flying bird. “It looks like it’s doing Mach 3 just sitting on the ground,” he said fondly.

Strange Design. According to present Defense Department plans, the XB-70A will never be used as a bomber, and only two prototypes will be built. But from its graceful curving nose to its folding wingtips the plane is radically new; into its strange, almost frightening design, $1.3 billion of engineering and scientific imagination have already been poured. Heavier and longer than any other airplane, it is designed to cruise at three times the speed of sound (roughly 2,000 m.p.h.) for an “intercontinental” distance. The engineering innovations that were tried in its construction may well affect every high-speed airplane built in the foreseeable future.

One of the aerodynamic advances was the use of “compression lift.” Every supersonic airplane generates shock waves in the air around it, and at three times the speed of sound those waves are extremely powerful. The engines and air intakes of the XB-70A are placed under its delta wing, and their bulk encourages a shock wave to form just where the upward force of its compressed air can be caught and used as free lift. Speedboats do something similar when they climb out of the water and plane along on their bottoms.

Compression lift will be a help, but the Valkyrie needs much more of a boost. It could not hope to succeed without unprecedented strength combined with lightness and heat resistance. At 70,000 ft., where it will fly, the temperature is far below zero, but at Mach 3, air friction will heat parts of the structure to 650° F., which is well above the softening point of aluminum and magnesium. To avoid such dangers, some parts of the ship were made of titanium, that new miracle metal, lavishly used in the delta-wing All. But more than half of the XB-70A’s structural weight is “stainless steel honeycomb sandwich.” This sophisticated material, which is comparable to the light cellular wing bones of large birds, is made by brazing thin steel skins to hexagonal steel cells. It is extremely light, but almost as strong as if it were made of solid metal.

The steel sandwich is also an excellent insulator, keeping the searing heat outside from reaching the plane’s vulnerable innards. The rear fuselage, though, is heated from inside by the engines and from outside by the racing air. In flight it will reach 1,000° F., so it is made of high-strength tool steel, which does not lose its strength even when white-hot.

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.

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