JP-7 Fuel

Turbine Fuel, Low Volatility, JP-7, commonly known as JP-7, was referred to as Jet Propellant 7, to MIL-DTL-38219[2] is a specific jet fuel that was developed for the United States Air Force (USAF), for use in its supersonic military aircraft that required a jet fuel with a high flash point, and high thermal stability. JP-7 is the fuel that was developed, which was required by the Pratt & Whitney J58 (JT11D-20) turbojet engines, 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 was developed for this purpose.

 

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 36,000–44,000 pounds (16,000–20,000 kg) 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 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.

 

"The operating envelope of the JT11D-20 engine requires special fuel. The fuel is not only the source of energy but is also used in the engine hydraulic system. During high Mach flight, the fuel is also a heat sink for the various aircraft and engine accessories which would otherwise overheat at the high temperatures encountered. This requires a fuel having high thermal stability so that it will not break down and deposit coke and varnishes in the fuel system passages. A high luminometer number[nb 1] (brightness of flame index) is required to minimize transfer of heat to the burner parts. Other items are also significant, such as the amount of sulfur impurities tolerated. Advanced fuels, JP-7 (PWA 535) and PWA 523E, were developed to meet the above requirements."

SR-71A Flight Manual, Section I, page 4

Properties

Melting point: −30 °C (−22 °F)

 

Boiling point (at 1 standard atmosphere (100 kPa)): 282–288 °C (540–550 °F)

Density (at 15 °C (59 °F)): 779–806 kg/m³

 

Vapor pressure (at 300 °F (149 °C)): 155 millimetres of mercury (3.00 psi) (20.7 kPa)

 

Flashpoint: 60 °C (140 °F)

 

Net heat of combustion: min. 43.5 megajoules per kilogram (5.48 kWh/lb)