Although electrical propulsion systems offer the advantage over chemical systems of much higher exhaust velocity or specific impulse, there is a penalty to be paid for this performance. The total mass of a chemical propulsion system is made up of the mass of the propellant or propellants and their storage tanks, the engine itself and the control system. Electric propulsion systems have, in addition to these components, a power source and a power controller. The mass of these components partially offsets the mass saving made by being able to fulfil the mission velocity change requirements using a reduced propellant mass.

The mass of the power supply scales in proportion to the required power:

		Power supply mass
		Power supply specific mass (mass per unit power)
		Power required for propulsion

The power required to obtain a desired thrust is given by:

		Power required for desired thrust
		Exhaust gas velocity
		Thrust

The efficiency with which the thruster converts input power into thrust power is:

Conversion efficiency

It follows that the power supply mass is given by:

showing that the power supply mass scales with the square of the exhaust velocity.

The required propellant mass scales inversely with the exhaust velocity so there exists, for any given required velocity change, some optimum exhaust velocity that minimises the sum of the propellant and power supply masses.

The equation for power supply mass shows the importance of achieving low power supply specific mass and high thruster conversion efficiency to reap the maximum benefit from the use of electric propulsion.

	Electric versus Chemical Propulsion
	Methods of Electric Propulsion