Resolution of the difficult flight conditions which the A-12 was to encounter, being at that period relatively unexplored, proved to be a complex matter. Sustained flight above Mach 3 at high altitudes - demanded extended trials in the supersonic envelope (initially conducted with the J-75) as well as the study of new fuels and lubricants. Moreover, due to the influence temperatures would have upon the function of the engine, every component would have to be manufactured from specialized alloys (stainless steel, Hasteloy X, titanium) which to up that time had only been used for such smaller components as turbine-blades. The trials being carried out at the period with the experimental "X" planes were of little help, as these were mainly rocket propelled and their flights of short duration.
At the outset variable geometry jet-pipes were to be an integral part of the engine. It was decided, jointly with Lockheed, that these would be incorporated in the compression chamber, so as to keep dimensions within acceptable limits. Pratt & Whitney was accountable for the nozzle and post-combustion chamber. On top of this Pratt & Whitney was busy designing the control mechanisms because the engineers at Lockheed had absolutely no experience of material requirements for gears, bearings and welding able to withstand such temperatures. Pratt & Whitney had hardly any more experience, but were compelled to acquire it so as to see the program through.
The J-58 went through numerous modifications which, by 1960, included changes to the bypass ducts, increasing the compressor stages from 8 to 9, so augmenting the rather low compression rate, and adopting an afterburner - intended for continuous operation throughout a mission. The J-58-P4 thus conceived for the A-12, had few common parts with the initial J-58-P2 studied for the US Navy. Only the overall dimensions, the aerodynamics of the compressor blades and the turbine unit were retained for the time-being. Some time later even these would also be modified.
The result of the tests carried out in conjunction with Lockheed evidenced, from the start, that at the intended higher Mach numbers the new engine would be unable to cope with the volume of air coming through the air-intakes. This would result in compressor stalling with accompanying loss of efficiency and thrust at high speeds. Pratt & Whitney therefore modified their JT-11 by installing a series of fixed flow-vanes downstream of the 4th compressor stage, which directed the surplus airflow along six longitudinal jet pipes running along the engine casing. The surplus was then carried straight to the afterburner chamber serving to cool the burners, whilst enriching the mixture; so enabling higher combustion temperatures or increased thrust.
This principle, innovative at that time, figured largely in the future development of other engines called "bypass jets. The J-58 cannot be properly referred to as being bypass jet, an unknown principle at that time, as that did not derive from flow-vanes with an irregular cooling affect, so serving to relieve the compressor at certain stages of flight, which was obtained by a bypass flow constantly shifting air from within the compressor towards the turbine outlet. Nevertheless the chosen formula allowed an effective limitation of the phenomenon of unstarts and compressor fall-off due to variations in inlet temperature, on top reducing specific consumption from 10 to 15%. This is the system which Ben R Rich (Kelly Johnson's successor as head of the "Skunk Works") qualified as being a "bypass jet engine by air withdrawal".
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