To get Rocket Fuel, you will first need to craft an Empty Canister and carry some in your inventory. Then, you will have to find some Crude Oil. Use a Portable Oil Extractor to get enough oil to fill up the canister. Make a Refinery to refine the oil into usable rocket fuel. Alternatively, other forge liquids are also usable, such as Buildcraft.
How To Use Rocket Fuel
In order to launch the Rocket, you must insert a charged Battery into the Fuel Loader, or attach it to a Coal Generator and the Rocket Launch Pad. If done correctly, the Fuel will start loading into the Spaceship. After the Fuel has fully loaded into the Spaceship, the player can launch out of the atmosphere.
Warning: highly flammable!
If you have a liquid block of Fuel in your game world (currently this is possible only in Creative mode or with aid of other mods), be careful: if there is any fire near it, it can cause an explosion. (Feature added in Galacticraft version 126.96.36.1995)