How does propulsion work?

What are the types of rocket propulsion?

There are several types of rocket propulsion systems:

Type Uses Advantages Disadvantages
Solid fuel chemical propulsion main booster simple, reliable, few moving parts, lots of thrust

not restartable

Liquid fuel chemical propulsion main booster, small control restartable, controllable, lots of thrust


Cold-gas chemical propulsion small control restartable, controllable low thrust
Ion in space booster restartable, controllable, high specific impulse complex

The solid motor is used mainly as a booster for launch vehicles. Solid motors are almost never used in space because they are not controllable. The boosters are lit and then they fire until all the propellant has burned. Their main benefits are simplicity, a shelf life which can extend to years as in the case of missiles, and high reliability.

Liquid motors come in many shapes and sizes: Most of them are controllable (can be throttled up and down), restartable, are often used as control and maneuvering thrusters. Liquid thrusters can be broken into three main types: monopropellant, bipropellant, and cryogenic thrusters. Monopropellants only use one propellant such as hydrazine. Bipropellants use a fuel and an oxidizer such as RP-1 and H2O2. Cryogenic systems use liquefied gases such as LiH and LOX (liquid hydrogen and liquid oxygen). Cryogenic means super-cooled. You would have to super-cool hydrogen and oxygen to make them liquids. With each step from monopropellant to bipropellant to cryogenic the thruster complexity goes up but the performance also goes up.

Cold-gas motors have controllability similar to liquids but are the simpler and lighter. They are basically a high pressure tank with switches which flip between the open and shut state. They function a little like spray paint, with the contents under pressure inside, and when the valve is opened, they stream out.

Ion engines are vastly different from chemical (solid, liquid) engines in that they are low thrust engines which can run for extended periods of time. The length of use of chemical engines is usually from seconds to days while the length of use of ion engines can be anywhere from days to months.

What is a propellant?
How are rockets designed?
Why does DS1 have two different types of propulsion?
How does DS1's engine work?
What is the difference between an ion engine and a conventional one?

What is thrust?
What would happen if DS1 ran out of fuel?
How do things burn?

How is rocket propulsion different from jet propulsion?
What is a monopropellant?
What are some rocket propellants?
Why are cryogens hard to handle?
How does density affect tank volume?