Re: How does a rocket propulsion system work in a vacuum (ie. space)?

A rocket, unlike an automobile engine or jet engine, typically operates in a vacuum. It therefore must bring along an oxidizer to replace the atmospheric oxygen used by more common engines. Typically NASA uses liquid oxygen for the oxidizer in liquid fueled rockets. Hydrogen or a hydrocarbon is the fuel of choice for most of these engines. For solid fuel rockets, the chemicals are chosen, mixed, and packed in such a way as to incorporate the oxidizer within the solid fuel itself. Thermite has a similar chemical reaction.

Once the fuel and oxidizer are brought together and ignited in the combustion chamber, the hot gases generated are exhausted at very high speed through the rocket nozzle. These gases produce the thrust that pushes the rocket forward.

In terms of vectors, the high speed gases exiting the rocket engine produce a force in the negative z direction. The force is proportional to the kinetic energy which in turn is related to the square of the velocity of the gas. There are no or little effects of gravity and drag even in low Earth orbit, so those vectors can be ignored. Newton's various Laws of Motion can then applied, and it can easily shown that the rocket experiences an equal and opposite force acting on the rocket through the engine to drive the rocket forward.

To emphasize the effect of the hot gas stream on the rocket, consider alternative rocket engines that do not require any chemical reaction to drive them forward. One that is under active consideration for a manned trip to Mars is a nuclear rocket. In this design, a bed of nuclear fuel is used to heat hydrogen or another gas to a high temperature. The design of the chamber and the nozzle forces the heated gas out of the nozzle at high speed. The force imparted is derived from the kinetic energy of the exiting gas as with the chemical rocket, but here there is no chemical reaction. As you might imagine, the higher the velocity of the exiting gas, the larger the kinetic energy and hence the higher the force.

Ion engines are another alternative engine that can produce thrust without a chemical reaction. Typically a high density fluid is ionized in a chamber. The ionized fluid is attracted to a grid in front of the nozzle with an opposite charge. The movement of the fluid from one side of the engine and out the nozzle produces the thrust. Heavy fluids such as mercury are preferred since they have a high mass and can impart a larger kinetic energy