Both XB-70s used six single-shaft General Electric YJ93-GE-3 turbojets, each developing 28,800lb thrust at sea level or 19,000lb in military power. Their combined power apparently made the XB-70 the loudest aircraft of its time.
General Electric developed the highly sophisticated YJ93 engine for a planned supersonic interceptor, the F-108, and a bomber, the XB-70, designed to fly at speeds of 2,000 mph and altitudes of approximately 70,000 feet.
The US Air Force (USAF) canceled the F-108 program and ordered only two XB-70s.
Both XB-70s were powered by six single-shaft General Electric YJ93-GE-3 turbojets, each producing 28,800lb thrust at sea level or 19,000lb in military power. Peter E. Davies explains in his book North American XB-70 Valkyrie that their combined power reportedly made the XB-70 the loudest aircraft of its era. All six variable-thrust afterburners were necessary for takeoff, and accelerating half a million pounds of XB-70 required an extensive runway. Ground running demanded a clear area of 750ft behind the engine, and ear protection was mandatory. Large sound abatement mufflers could be attached to the exhausts with adapter rings during ground tests.
The engine was designed for continuous afterburner operation and pioneered the use of air-cooled titanium turbine blades that permitted higher operating temperatures than conventional steel blades, although resistance to foreign object damage was decreased.
The engine measured 20 ft in length and more than 4.3ft in width, offering a relatively favorable 6:1 thrust-to-weight ratio while burning JP-6 fuel (designed for the J93), although the afterburner performance tended to be unstable.
In an unusual development for that period, the engine was prepared for the aircraft’s first flight after extensive testing for over 5,000 hours, including 600 hours in Mach 2+ conditions, during which engineers discovered that fuel economy for the engine was superior at higher speeds than it would have been at slower speeds. The J93 was designed for a maximum altitude of 95,000ft and Mach 3.2, with optimal performance at Mach 3 and 65,000ft. Ground-testing was conducted aboard NB-58A Hustler 55-0662, though the flight-test program for that combination was canceled, and the Hustler, as a TB-58A, became the chase aircraft for the XB-70 program instead.
Ten feet of each engine was encased by a corrugated alloy sandwich heat shield with gold-plated inner surfaces to reflect heat. North American Aviation (NAA) employed a similar engine-bay shielding process for its A-5 Vigilante. Engine management occurred through electrical signals from the throttles to the John Oster engine control system for each engine, where a mechanical system took over. There was a wide range of advanced secondary systems. The four hydraulic systems operated at an unusually high pressure of 4,000psi and used a specially developed lightweight stainless steel and Oronite 70, a new type of fluid. Such high pressure was essential to operate the control surfaces under the extreme aerodynamic forces experienced at Mach 3. The system powered 44 motors and 85 actuators, as well as numerous valves and pumps, all constructed to tolerate temperatures of up to 630°F.
North American XB-70 Valkyrie is published by Osprey Publishing and is available to order here.
Photo by LIFE Magazine and U.S. Air Force
