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. Although electric rockets are efficient, their thrust is low. Nuclear propulsion, on the other hand, offers both high thrust and efficiency. In the simplest nuclear propulsion system, a graphite core reactor heats liquid hydrogen to a rapidly expanding gas that produces the thrust. Various exotic propulsion systems have been proposed for rockets of the future.