Why are petrol and diesel not used as rocket fuel like kerosene ? Which properties make them unfit for use in rocket fuel ?
I'll try to find a source for this, but it is my understanding that in the early days of rocketry (read: 1950's, in this context), engineers tried to use jet fuel (and probably other fuels), but the way the fuel was produced led to large variations in thrust and specific impulse from test to test.
Basically, they'd fuel up the rocket engine (on a test stand), light it up and measure the thrust. Then they'd do it again, and they would get a substantially different thrust. This led to a specification called RP-1 (Rocket Propellant 1) which dictates that any fuel carrying the designation of RP-1 must have a certain level of various impurities (i.e. sulfur, etc.) and no more. This specification is tighter than specifications for jet fuel and other hydrocarbon fuels.
EDIT: The wikipedia page on RP-1  gives a more detailed description of what I said, although they attribute the problems more directly to problems with cooling the engine.
Rockets work differently to internal combustion engines. It is easier to cool an engine than it is to cool a rocket, due to the scale and usage of the exothermic reactions. rockets use specific propellants that are tested for their purity. Car engines can have fuel that have some impurities because it is ignited intermittently and is used to transform chemical energy into kinetic energy. Most Rockets ignite their fuel constantly and the resulting release of force from the chemical energy pushes the rocket away from the aperture of the rocket, even in vacuums.
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