Spacecraft Propulsion System
SPacecraft propulsion systems generally consist of several different parts. There are several competing priorities for the design of the system they generally are:
1) The compounding cost of 'shipping' we need a to have a reasonably efficient propulsion system. The spacecraft masses get unyieldingly large if the engines don't deliver a lot of thrust, for a long time, (thrust * time = Impulse) for a little propellant (eg Specific impulse (ISP) is high.) We similarly need light tanks, valves and other assorted components.
2) The engines need to be controllable. That is to say the ability to control the duration or thrust level of the engine to handle changing conditions. This means the propulsion system really can't be a simple solid rocket motor.
3) Simple storage tanks. Cryogenics are probably to difficult to store for a reasonably long period in changing conditions, and many compressed gases would have a prohibitively high tank weight.
The resulting field of system type selection is narrowed to either Hybrid or liquid engines with single or multiple liquids.
Hybrids have some advantages and some disadvantages. The disadvantages seem to be the fuel (generally) exposure to the space environment, the difficulty in throttling (leading to mixture ratio variation), and the reusability. A liquid engine, if it is restartable (mandatory) is easily reused by refilling the propellant tank(s) and go again. This also gives tha ability to stage off empty tanks which adds up to a significant mass savings in multi-event missions.
Chemical Liquid engines are our current choice for a propulsion system. The rest of the system now consists of