<P> Turbine Fuel, Low Volatility, JP - 7, commonly known as JP - 7, was referred to as Jet Propellant 7, to MIL - DTL - 38219 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 . </P> <P> The Boeing X-51 Waverider also uses JP - 7 fuel to run the Pratt & Whitney SJY61 scramjet engine, with fuel capacity of some 270 pounds (120 kg). As with the SR - 71, the X-51A design super-cools this fuel (cooled by extended subsonic flight in the stratosphere; prior to acceleration to supersonic speeds); then, when in supersonic flight, the fuel is heated by its circulation through heat exchangers which transfer to it the heat load of the interior spaces of the airframe . The fuel is then pumped through rotating mechanical parts of the engines and auxiliary mechanical equipment, providing both lubrication and cooling . Finally, at a temperature of nearly 550 ° F (290 ° C), it is pumped into the fuel nozzles of the engines . </P>

What kind of fuel does the sr-71 use