JP-9 ? My understanding is that this synthetic fuel was used primarily in cruise missiles but has been replaced with JP-10
We currently run JP-8 because that's what is readily available.
JP-4 was more volatile as it contained a high percentage of Avgas. It might cause any jet engine to run a little stronger due to increased BTU output but the fuel controllers are set for fuels with different specific gravity so it is probably a moot point.
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Definition : JP-4, or JP4 (for "Jet Propellant") was a jet fuel, specified in 1951 by the U.S. government (MIL-J-5624E). It was a 50-50 kerosene-gasoline blend. It has lower flash point than JP-1, but was preferred because of its better availability. It was the primary U.S. Air Force jet fuel between 1951 and 1995. Its NATO code is F-40. It is also known as avtag.
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JP-8 definition : JP-8, or JP8 (for "Jet Propellant") is a jet fuel, specified in 1990 by the U.S. government. It is kerosene-based. It is a replacement for the JP-4 fuel; the U.S. Air Force replaced JP-4 with JP-8 completely by the fall of 1996, in order to use less flammable, less hazardous fuel for better safety and combat survivability. U.S. Navy uses a similar formula to JP-8, JP-5. JP-8 is projected to remain in use at least until 2025. It was first introduced at NATO bases in 1978. Its NATO code is F-34. It is specified by MIL-DTL-83133 and British Defence Standard 91-87.
The Navy went to JP-5 because it was less flammable. It is primarily used on carriers.