The two most important characteristics of a rocket engine are its thrust and its specific impulse. Thrust is a measure of how much force the rocket can exert. In order to take off from Earth's surface, a rocket's thrust must be greater than the force of gravity upon it. In engineering terms, the thrust-to-weight ratio must be greater than one.
Specific impulse can be thought of as a rocket's "gas mileage". It measures the amount of speed obtainable from a unit mass of propellant. It is related to the speed of the rocket's exhaust.
Chemical rockets are useful for their extremely high thrust. For example, a single engine of the Saturn-V rocket has 6,800,000 N of thrust at 300 s specific impulse. Currently, chemical rockets are the only propulsion technology that has been used to reach Earth orbit. No, as yet conceived of, electric thruster has a thrust-to-weight ratio greater than one. However, electric thrusters have much greater specific impulse than conventional rockets, and can therefore ultimately propel a spacecraft to much greater speeds using much less propellant mass once the craft has reached Earth orbit.
The table below compares the thrust and specific impulse of several different propulsion systems.
|Specific Impulse (s)||Propellant|
|PPS-1350 Hall Thruster
|NSTAR Ion Engine
(Deep Space 1)
|NEXT Ion Engine
|VASIMR® VX-200||200||5||5000||Ar (Optional:
D, N, Xe)