JP-7

JP-7 (Jet Propellant 7, MIL-DTL-38219) is a jet fuel developed by the U.S. Air Force for use in supersonic aircraft because of its high flash point and thermal stability. It is the fuel used in the Pratt & Whitney J58 engines, used in the Lockheed SR-71 Blackbird. The air compression 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 the match will be extinguished (although this is common in any low-volatility fuel including kerosene and diesel).

The Boeing X-51 Waverider uses JP-7 fuel to run the Pratt & Whitney SJY61 scramjet engine, with fuel capacity of some 270 lb (120 kg). As with the SR-71, the X-51A design also uses fuel to absorb heat from the engine.

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 cesium containing compound known as A-50, which aided in disguising the radar signature of the exhaust plume. The Blackbirds used approximately 36,000 - 44,000 pounds of fuel per hour of flight.

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 has low vapor pressure and high thermal oxidation stability. It must operate across a wide range of temperatures, from near freezing at high altitude, to high temperature of airframe and engine parts being cooled. Its volatility must be low to make it flash-resistant 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 initiate combustion and allow afterburner operation 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."

JP-7 Aerial Refueling Requirements

The SR-71 required in-flight refueling to replenish the fuel expended on take-off and during long duration missions. To accomplish this, specialized KC-135Q tankers were required to refuel the SR-71. The "Q-model" tanker had a modified "high-speed boom," which would allow refueling of the Blackbird at nearly the tanker's maximum airspeed, with minimum flutter. The Q-model also had a separate fuel system, which kept the tanker's fuel (either JP-4 or later JP-8) isolated from the JP-7 needed by the SR-71. During its operational life, the SR-71 had dedicated KC-135Qs and aircrews sitting on alert, ready to launch within minutes to tank up a Blackbird. Some of these alert birds were located on the east coast (Pease AFB, Loring AFB, Plattsburgh AFB etc.); on the west coast; at Hickham AFB, Hawaii; Mildenhall, England; and Guam. Several of the original KC-135Q aircraft are still serving on active-duty in the U. S. Air Force inventory as the KC-135T; a new designation given after the aircraft have been upgraded/reengined to the KC-135R standard.

See also

• JP-1
• JP-4
• JP-5
• JP-6
• JP-8
• JPTS
• Jet fuel
• Aviation fuel
• KC-135Q

References

Notes

Bibliography

• Rich, Ben R. and Leo Janos. Skunk Works: A Personal Memoir of My Years at Lockheed. New York: Little, Brown and Company, 1994. ISBN 0-316-74330-5.