Question

why not use gasoline instead of methane as rocket fuel

Answer 1

this is bcoz methane is more efficient.......

Answer 2

As others have pointed out, gasoline*has* been used as rocket fuel, but it has some disadvantages. Most of the time we use RP-1, which is*similar* to kerosene, diesel or Jet fuel, but is not quite the same.RP-1 is specially refined to have very few short chained hydrocarbons, so is less volatile than kerosene/diesel/Jet-A and *much* less volatile than gasoline. In addition very few long chain hydrocarbons are allowed, in fact it’s quite centered on 12 carbon chains, and almost all of those chains are highly branched. All that makes handling easier (it is actually considerably less toxic than gasoline/kerosene/diesel/Jet-A), as well as reduces the amount of ash buildup in turbopumps and injectors. It also has considerably more resistance to thermal and shear breakdown,and so is less prone to producing gunk to clog pipes, valves, pumps, injectors and whatnot.There are also very low sulfur and other contaminant levels.On the flip side, there are no real advantages for gasoline or kerosene/diesel/Jet-A, except for cost. All of those, including RP-1, have quite similar energy densities, in both volume and mass terms.OTOH, the cost of RP-1 is not really that high, especially compared to the cost of the rocket. About 20 years ago I was involved in a project that budgeted RP-1 atabout 25 cents per pound and LOX at 5 cents per pound (you need about 2.5lbs of oxygen per pound of RP-1). Those prices have doubtless changed some over time, but it’s not going to be a big part of the budget. A Falcon-9 probably has on the order of 300,000lbs of RP-1 on board, at $.25/lb, that’s only $75K, which is barely going to be a rounding errorin the budget.Liquefied methane (natural gas/LNG) also has similar energy density in mass terms, but is poorer in volume terms, but it also has the advantage of low cost, high stability and clean burning, although you do have to build somewhat larger tanks.