The North American Aviation XB-70 Valkyrie was the prototype of the B-70 nuclear-armed, deep-penetration strategic bomber for the Strategic Air Command of the U.S. Air Force.
In the 1950s, the North American Aviation company designed the Valkyrie bomber as a large, six-engine aircraft capable of reaching Mach 3+ while flying at 70,000 feet (21,000 m), which velocity and altitude capabilities would allow the evasion of interceptor aircraft, then the only effective weapon against bomber aircraft.
In 1961, improved, high-altitude surface-to-air missiles (SAMs), the U.S. Air Force’s doctrinal change to low-level penetration bombing, the large development costs of the B-70 program, and the introduction of intercontinental ballistic missiles (ICBMs) to the U.S. nuclear arsenal, led to the cancellation of the B-70 program. As such, two prototype aircraft were built, and designated XB-70A; these aircraft were used for supersonic test-flights during 1964–69. In 1966, one prototype crashed after colliding in midair with a smaller jet aircraft; the remaining Valkyrie bomber is in the National Museum of the United States Air Force, in Ohio.
Design:
The Valkyrie was designed to be a high-altitude Mach 3 bomber with six engines. Harrison Storms shaped the aircraft with a canard surface and a delta wing, which was built largely of stainless steel, sandwiched honeycomb panels, and titanium. The XB-70 was designed to use supersonic technologies developed for the Mach 3 Navaho, as well as a modified form of the SM-64 Navaho’s all-inertial guidance system.
The XB-70 used compression lift, which was generated from a prominent wedge at the center of the engine inlets that created a shock wave below the aircraft. The wing included inboard camber to more effectively use the higher pressure field behind the strong shock wave (the airflow at the XB-70 wing’s leading edge was subsonic). The compression lift increased the lift by five percent.
Unique among aircraft of its size, the outer portions of the wings were hinged, and could be pivoted downward by up to 65 degrees. This increased the aircraft’s directional stability at supersonic speeds, shifted the center of lift to a more favorable position at high speeds, and strengthened the compression lift effect. With the wingtips drooped downwards, the compression lift shock wave would be further trapped under the wings.
The XB-70 was equipped with six General Electric YJ93-GE-3 turbojet engines, designed to use JP-6 jet fuel. The engine was stated to be in the “30,000-pound class”, but actually produced 28,000 lbf (124.6 kN) with afterburner and 19,900 lbf (88 kN) without afterburner. The Valkyrie used fuel for cooling; it was pumped through heat exchangers before reaching the engines.
To reduce the likelihood of autoignition, nitrogen was injected into the JP-6 during refueling, and the “fuel pressurization and inerting system” vaporized a 700 lb (320 kg) supply of liquid nitrogen to fill the fuel tank vent space and maintain tank pressure.
The U.S. Air Force followed these developments closely, and in 1955 issued General Operational Requirement No. 38 for a new bomber with the payload and intercontinental range of the B-52 and the Mach 2 top speed of the Convair B-58 Hustler. The new bomber was expected to enter service in 1963.
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