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The heart of the SR-71 "Blackbird" : the mighty J-58 engine

by Philippe Ricco
translation : Graham Warrener
Marc Binazzi


1.5 : Projects and variants

An initial batch of 50 motors was delivered to Lockheed in 1963 for fitting in the A-12 and the YF-12s. A second batch of one hundred examples was dispatched in 1964 to equip the SR-71s.

Pratt & Whitney had great plans for their JT-11 super engine. They saw the prototype and the those as JT-58s destined for the Blackbirds unfolding into a production series. Adaptations of this motor were proposed for all the design studies of the period for hypersonic programs, much treasured in the 1950s - 1960s, such as the SST or the Convair BJ-58. This latter four-engine design, also known as the B-58C, was contemplated as a supersonic transport flying at Mach 2.5 and cruising at more than 70,000 ft (21,300 m), powered by four J-58s (without after-burning) of 23, 000 lbs thrust (10.400 kgp) mounted in two wing-tip nacelles.

A military version of this Convair model 58-9 was also announced under the name of model 62. It would have had the capability to transport 52 troops. Twin-engine interceptor variants called B-58D (for Air Defense Command) and B-58S (for the Tactical Air Force) were also proposed fitted with J-58s capable of propelling a load of more than 30,000 lbs (13.600 kg). It is likely that the Pentagon encouraged this profusion of projects using these engines so as not to draw attention to the A-12.
Much later development of a double-flux version of the J-58 using compressors of greater size was contemplated.

At the same time Pratt & Whitney proposed a nuclear version of the J-58 using an indirect propulsion cycle, fuelled by a solid-fuel reactor cooled by a twin line system? The USAF intended being equipped with a guided missile capable of low-altitude penetration. The program, named CAMAL was cancelled in 1960, but it had demanded of Convair, involved with the project - in competition with Pratt & Whitney and General Electric, construction of two subsonic NX-2 aircraft to experiment with the mode of propulsion.

These studies mainly permitted a comparison of the two classes of nuclear flux engines, those envisaged by General-Electric (the simplest but dirtier system as the radioactive material was sent directly into the combustion chambers) or those using the indirect cycle proposed by Pratt & Whitney which utilized liquid sodium as a high-temperature conductor between the nuclear reactor and the combustion chambers of the J-58 propulsion unit.

The planes would fly in 1965 under the designation of WS-125A/L, but had been registered in 1961 as a back-up for the WS-110A "Weapon System" which gave birth to the North American XB-70 Valkyrie. Certain Pratt & Whitney nuclear reactors were adopted a little later by the commission for nuclear energy in its "System for Nuclear Auxiliary Power" (SNAP) However, despite some trial installations of a nuclear reactor aboard a giant B-36 bomber (alias the X-6) no atomic propulsion unit would ever be air-tested. Not even the General Electric system, design of which had progressed further than that of Pratt & Whitney's system.

The description of another project derived from the J-58, namely the SNECMA M-35 is of equal interest. In 1959 the French SNECMA company signed a partnership agreement with Pratt & Whitney which offered enviable prospects. Not least because, at the start of the nineteen-sixties, France decided to conquer the challenge of supersonic-transport, SNECMA thus had to design a propulsion system for the future aircraft. That led to negotiations concerning the possibility of manufacturing under license the JT-11B3, a civil variant which was being proposed at the then stage of J-58 development. The Franco-British agreement of 1961, brought an end to this project, with the decision to equip the future "Concorde" with a turbojet of British origin. However, one sometimes finds reference to the M-35 engine among the many Mach-3 aircraft projects fashionable in France at that time.

Aérostories 2002                                                        > next page

1 : Genesis

> 1.1 Development
> 1.2 Evolutions
> 1.3 Ground and flight tests
> 1.4 Improvements
> 1.5 Projects and variants
> 1.6 Note about designations

2: Technical description (soon in N°13)

> 2.1 Compressor
> 2.2 Combustion chambers
> 2.3 Turbine
> 2.4 Afterburning
> 2.5 Bypass system
> 2.6 Accessories and equipments
> 2.7 Regulation system
> 2.8 Lubrication
> 2.9 Special propellants
> 2.10 Performances

3: The J-58 in use (soon in N°14)

> 3.1 Introduction
> 3.2 The Pratt & Whitney turbo-ramjet
> 3.3 Inlet
> 3.4 Exhaust
> 3.5 In flight
> 3.6 Unstarts
> 3.7 Automatic controls
> 3.8 Technology updates
> 3.9 Ground starting

> More infos about  SR-71

The Convair B-58C bomber project was to be equipped with 4 J-58 engines without after burner.
© Convair

In its civilian version, the Convair 62 was to be equipped with the same engines.
© Convair

This strange experimental aircraft, the Convair NX-2 was to be built (two of them) to test the General Electric and Pratt & Whitney nuclear engines. Effectively, an atomic version of the J-58 was seriously considered.
© Convair

The SNECMA Company considered building the M35, which was the equivalent of the JT11B built under license for the supersonic transport airplane project (future Concorde). It is without doubts the reason that Pratt & Whitney introduced this J-58-P2 at the 1963 Le Bourget air show, whereas the "Blackbird" the only airplane equipped with this engine was still totally unknown. Its existence was indeed not revealed until February 29, 1964.
© coll. Bodemer