Everything about Jp-7 totally explained
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 won't ignite. The match will be extinguished.
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 isn't 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 isn't 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 won't 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."
Further Information
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