Military Aviation

Serpentine Six Pack: A quick look at the General Electric YJ93, the engine that made the XB-70 the loudest aircraft of its time

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 designed the highly-advanced YJ93 engine to power a planned supersonic interceptor, the F-108, and a bomber, the XB-70, at speeds of 2,000 mph and at altitudes of about 70,000 feet.

The US Air Force (USAF) cancelled the F-108 program and ordered only two XB-70s.

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. As explained by Peter E. Davies in his book North American XB-70 Valkyrie, their combined power apparently made the XB-70 the loudest aircraft of its time. Takeoff required all six variable-thrust afterburners, and accelerating half a million pounds of XB-70 took a long 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 for ground tests.

Designed for continuous afterburner operation, the engine pioneered the use of air-cooled titanium turbine blades that allowed higher operating temperatures than conventional steel blades, although resistance to foreign object damage was reduced.

The 20ft-long engine was more than 4.3ft wide, and it offered a comparatively favorable 6:1 thrust-to-weight ratio burning JP-6 fuel (designed for the J93), although the afterburner performance tended to be unstable.

Unusually for that time, the engine was ready for the aircraft’s first flight after extensive testing for more than 5,000 hours, including 600 hours in Mach 2+ conditions, where it was found that fuel economy for the engine was better than it would have been at slower speeds. The J93 was designed for a maximum altitude of 95.000ft and Mach 3.2, with optimum performance at Mach 3 and 65,000ft. It was ground-tested aboard NB-58A Hustler 55-0662, although 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.

A corrugated alloy sandwich heat-shield, with its inner surfaces gold-plated to reflect the heat, encased ten feet of each engine. North American Aviation (NAA) used a similar engine-bay shielding process for its A-5 Vigilante. Engine management was by 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, operating at an unusually high pressure of 4,000psi, used a specially developed lightweight stainless steel and Oronite 70, a new type of fluid. Such high pressure was needed 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 of which were built 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 credit: LIFE Magazine and U.S. Air Force

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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