The Pratt & Whitney J58 (company designation JT11D-20) is an American jet engine that powered the Lockheed A-12, and subsequently the YF-12 and the SR-71 aircraft. It was an afterburning turbojet engine with a unique compressor bleed to the afterburner that gave increased thrust at high speeds. Because of the wide speed range of the aircraft, the engine needed two modes of operation to take it from stationary on the ground to at altitude. It was a conventional afterburning turbojet for take-off and acceleration to Mach 2 and then used permanent compressor bleed to the afterburner above Mach 2. The way the engine worked at cruise led it to be described as "acting like a turboramjet". It has also been described as a turboramjet based on incorrect statements describing the turbomachinery as being completely bypassed. The engine performance that met the mission requirements for the CIA and USAF over many years was later enhanced slightly for NASA experimental work (carrying external payloads on the top of the aircraft), which required more thrust to deal with higher aircraft drag. Development Origins The J58, company designation JT11, had its origins in the larger JT9 (J91) engine. It was a 3/4 scale JT9 with a mass flow of , down from . The JT11 was proposed to the US Navy under their designation J58. It was also proposed for various Navy and Air Force aircraft, e.g. Convair F-106, North American F-108, Convair B-58C, Vought XF8U-3 Crusader III, and North American A3J Vigilante, but none of these applications followed. The J58 began development for the US Navy to power the planned Martin P6M jet flying boat. The P6M started out using Allison J71-A-4 engines and then switched to the Pratt & Whitney J75, due to J58 development delays. Upon cancellation of the P6M, it was selected for the Convair Kingfish and for the Lockheed A-12, YF-12A and SR-71. Other sources link its origin to the USAF's requirement for a powerplant for the WS-110A, the future XB-70 Valkyrie. Re-design for Mach 3.2 Analytical calculations of the performance of the original J58 showed that at Mach 2.5 the "exhaust pressure was equal to the inlet pressure, the compressor was deep in surge, and there was no cool air to the afterburner liner that would therefore melt". The first problem was caused by too-high a compressor delivery temperature, which did not allow enough energy to be added