NORTH AMERICAN XB-70 VALKYRIE
THE NORTH AMERICAN AVIATION XB-70 VALKYRIE WAS THE PROTOTYPE VERSION OF THE PLANNED B-70 NUCLEAR-ARMED, DEEP-PENETRATION STRATEGIC BOMBER FOR THE UNITED STATES AIR FORCE STRATEGIC AIR COMMAND.
Designed in the late 1950s by North American Aviation (NAA), the six-engined Valkyrie was capable of cruising for thousands of miles at Mach 3+ while flying at 70,000 feet (21,000 m). At these speeds, it was expected that the B-70 would be practically immune to interceptor aircraft, the only effective weapon against bomber aircraft at the time. The bomber would spend only a brief time over a particular radar station, flying out of its range before the controllers could position their fighters in a suitable location for an interception. High speed also made the aircraft difficult to see on radar displays and its high-altitude and high-speed capacity could not be matched by any contemporaneous Soviet interceptor or fighter aircraft.
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 SM-64 Navaho,
as well as a modified form of the Navaho’s inertial guidance system. The XB-70 used compression lift, which arose from a shock wave generated by the sharp leading edge of the central engine intake splitter plate below the wing. At Mach 3 cruising speed, the shock wave attached along the wing leading edge, preventing the high pressure behind the shock front from leaking up over the wing. The compression lift provided five percent of the total lift.The wing included inboard camber to use the higher-pressure field more effectively behind the strong shock wave. Unique among aircraft of its size, the outer portions of the wings were hinged and could be pivoted downward by up to 65 degrees, acting almost as a type of variable-geometry wingtip device. This increased the aircraft’s directional stability at supersonic speeds, shifted the center of pressure to a more favourable 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.
Like several other delta-wing aircraft designed to cruise at extremely high speeds, the Valkyrie included a streamlined visor which could be lowered for the pilots to see the ground during the nose-high take-off and landing. In the B-70 design, the visor moved down into the main nose and the outer window panels moved with it to become more vertical, at 24 degrees slope. With the nose raised into its high-speed position, the outer windows were almost horizontal. A system that blew air at 600 °F (316 °C) from the engines was used for both defogging and rain removal.The lower forward section included a radar bay, and production machines were to be equipped with a refuelling receptacle on the upper surface of the nose.
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 produced 28,000 lbf (120 kN) with afterburner and 19,900 lbf (89 kN) without afterburner. The Valkyrie used fuel for cooling, which was pumped through heat exchangers before reaching the engines. To reduce the likelihood of autoignition, nitrogen was injected into the JP-6 during refuelling, whilst the ‘fuel pressurisation system’ vapourised a 700 pounds (320 kg) supply of liquid nitrogen to fill the fuel tank vent space and maintain tank pressure.
The XB-70’s maiden flight was on 21 September 1964. In the first flight test, between Palmdale and Edwards AFB, one engine had to be shut down shortly after take-off, whilst an undercarriage malfunction warning meant that the flight was flown with the undercarriage lowered as a precaution, limiting speed to 390 mph, about half that planned. During landing, the rear wheels of the port side main gear locked, the tyres ruptured and a fire started.