electrothermal propulsion[i¦lek·trō′thər·məl prə′pəl·shən]
Vehicular propulsion that involves electrical heating to raise the energy level of the propellant. In contrast, chemical rockets use the chemical energy of one or more propellants to heat and accelerate the decomposition products (monopropellants) or combustion products (bipropellants) for thrusting purposes. In both instances, the high-energy propellant gases are exhausted through a nozzle where they are accelerated to a high velocity, and thrust is produced by reaction. By decoupling the heating or energy addition process from the restraints of propellant chemistry considerations, electrothermal devices can be operated on a wide variety of materials, many of which would not otherwise be considered to be propellants. Water and space station liquid-waste streams are two examples of such propellants being considered for electrothermal propulsion purposes. See Rocket propulsion
Practical electrothermal thrusters come in two forms, resistojets and arcjets. In resistojets, which are now flying in a station keeping role on many communications satellites, the electrical energy is first deposited in a heater or resistive element and then transferred to the propellant. The need to first heat a material limits the maximum operating temperature and the maximum enthalpy of the propellant. As a consequence, the essential simplicity of the device is balanced by well-defined limitations on exhaust velocity or specific impulse. Arcjets circumvent this limitation by using the propellant as the heater element. An electric arc discharge passes directly through the propellant. High temperatures and specific-impulse values can be achieved but only at the price of design complexity. See Specific impulse
One further and fundamental distinction is that electrical thrusters are power-limited whereas chemical thrusters are energy-limited. By definition, electrical propulsion systems must have an associated power supply for operation. Solar panels or nuclear power supplies can supply well-defined power to the thruster for essentially unlimited time. Consequently, although the power level is well constrained, the total energy available is virtually boundless. Chemical systems are exactly opposite. In this case, the total energy available for propulsion is well defined by the propellant volume. However, the rate at which this propellant is used, the rate of energy usage per unit time, or the power can be exceedingly high. Chemical thrusters are ideal for escaping the Earth's gravity well; electrical thrusters are ideal for moving payloads in low-acceleration conditions removed from gravity wells, that is, for the more ambitious missions far removed from low Earth orbit.