Turbine Fuel Low Volatility JP-7, commonly known as JP-7 (referred to as Jet Propellant 7 prior to MIL-DTL-38219 ) is a specialized type of jet fuel developed in 1955 for the Central Intelligence Agency (CIA) for use in its reconnaissance aircraft, the Lockheed A-12, and subsequently for aircraft with similar high speed performance, the Lockheed YF-12 and Lockheed SR-71. It was also used for the higher speed Boeing X-51 Waverider.

Usage

JP-7 was developed for the Pratt & Whitney J58 (JT11D-20) turbojet engine, which was used primarily in the Lockheed SR-71 Blackbird which retired in 1999. The SR-71 design speed was Mach 3.2 at which very high skin temperatures occurred due to aerodynamic heating. A new jet fuel with a high flash point and high thermal stability was developed as the fuel had to be used as a heat sink for the severe high temperature environment in the aircraft. The Boeing X-51 Waverider also used JP-7 fuel in its Pratt & Whitney SJY61 scramjet engine, with fuel capacity of some 270 lbs.

Composition

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 36000 - 44000 lb 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 (<3%) 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 (TEB) 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 (usually 16) for restarts, 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.

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