JP-7

JP-7 (Jet Propellant 7, MIL-T-38219) is a jet fuel developed by the U.S. Air Force for use in supersonic aircraft because of its high flashpoint and thermal stability. It is the fuel used in the Pratt & Whitney J58 engines, used in the Lockheed SR-71 Blackbird. The air friction of Mach 3+ cruising flight generates very high skin temperatures, therefore this special fuel was needed. The story told by Ben Rich in his book Skunk Works is that a lit match can be dropped in a bucket of JP-7 and the fuel will not ignite and match will be extinguished (although this is common in any low-volatility fuel including kerosene and diesel).

JP-7 is a 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 better, and a caesium containing compound known as A-50, which aided in disguising the radar signature of the exhaust plume.

JP-7 is unusual in that it is not a 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 is specified by MIL-DTL-38219. It has low vapor pressure and high thermal oxidation stability. It has to operate within a wide range of temperatures, from near freezing of high altitudes to high temperature of airframe and engine parts being cooled. Its volatility must be low to make it flash-resistant even in 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 light it up, and to light up the afterburner in flight.

Properties

Melting Point −30 °C
Boiling Point (1 atm) 282–288 °C
Density (at 15 °C) 779–806 kg/m³
Vapor Pressure (at 300 °F (149 °C)) 155 mmHg (20.7 kPa)
Flashpoint 60 °C
Net heat of combustion min. 43.5 MJ/kg

From the SR-71 Flight Manual:

"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 (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."