JP-7 was developed for the Pratt & Whitney J58turbojet engine, which was used primarily in the now retired Lockheed SR-71 Blackbird. During flight, the SR-71 could attain speeds in excess of Mach 3+, which was the most efficient cruising speed for the J58 engines. However, very high skin temperatures are generated at this speed due to friction with the air. A new jet fuel was needed that was not affected by the heat, so JP-7 jet fuel, with a high flash point and high thermal stability, was developed for this purpose. The Boeing X-51 Waverider also uses JP-7 fuel in its Pratt & Whitney SJY61 scramjet engine, with fuel capacity of some. As with the SR-71, the X-51A design super-cools this fuel ; then, when in supersonic flight, the fuel is heated by its circulation through heat exchangers which transfer to it the heat load of the interior spaces of the airframe. The fuel is then pumped through rotating mechanical parts of the engines and auxiliary mechanical equipment, providing both lubrication and cooling. Finally, at a temperature of nearly, it is pumped into the fuel nozzles of the engines.
JP-7 is a compound mixture composed primarily of hydrocarbons; including alkanes, cycloalkanes, alkylbenzenes, indanes/tetralins, and naphthalenes; with addition of fluorocarbons to increase its lubricant properties, an oxidizing agent to make it burn more efficiently, and a caesium-containing compound known as A-50, which is to aid in disguising the radar and infrared signatures of the exhaust plume. The SR-71 Blackbirds used approximately of fuel per hour of flight. JP-7 is unusual in that it is not a conventional distillate fuel, but is created from special blending stocks in order to have very low concentration of highly volatile components like benzene or toluene, and almost no sulfur, oxygen, and nitrogen impurities. It has a low vapor pressure, and high thermal oxidation stability. The fuel must operate across a wide range of temperatures: from near freezing at high altitude, to the high temperatures of the airframe and engine parts that are being cooled by it at high speed. Its volatility must be low enough to make it flash-resistant at these high temperatures. The very low volatility, and relative unwillingness of JP-7 to be ignited, required triethylborane to be injected into the engine in order to initiate combustion, and allow afterburner operation in flight. The SR-71 had a limited capacity for TEB, and therefore had a limited number of available 'shots' of TEB for re-starts, and those had to be managed carefully on long-duration flights with multiple stages of relatively low-altitude air refueling and normal high-altitude cruise flight.
