For this chain-of-thought reasoning and answer, what was the question?
The relevant sentence in the passage is: 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.
A: JP - 7
Q: 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.

Based on this passage, what kind of fuel does the sr-71 use?