However, the volume of liquid hydrogen necessary to lift the rocket off the ground would require the rocket to be so large that atmospheric drag would be unacceptably large. To keep rockets small enough for practical applications, designers often select liquid fuels that provide more energy per unit volume than per unit weight. One example is the fuel RP-1, refined kerosene, which was used in the first, or takeoff, stage of the Saturn 5 moon rocket. The United States space shuttles use a blend of solids in their two lateral boosters, liquid fuels in their main engines, and hypergolic liquids in their attitude and maneuvering thrusters. A number of propellant systems are being developed.
Hybrid rockets use a solid fuel and a liquid oxidizer.
Electricity, provided by solar cells or nuclear reactors, can heat rocket fuel without combustion. In an arc-jet rocket, an electric arc heats the propellant. Plasma-jet systems use a magnetic field to expel fuel from the rocket. Magnetohydrodynamic rockets are still more sophisticated. Ion rockets convert the fuel into electrically charged particles that are then expelled by an electric field.