This generates a hot gas mixture of air and combustion products which escapes at the back of the jet engine with very high velocity, thus generating thrust. Jet engines depend on the combustion of fuel with air, where the air is obtained from the atmosphere. A jet engine is an air-breathing engine. It can only operate if enough air is available in the surroundings. The vacuum of space cannot provide the air.
Rocket engines are therefore necessary for operation in the outer atmosphere and space.
A rocket engine carries both fuel and oxidizer and is completely independent of the atmosphere. There are several different ways of generating thrust with a rocket engine. The most common way, which resembles air-breathing engines, is to expand a hot gas thermodynamically and to accelerate it to supersonic speeds. A Rocket Engine And Nozzle For supersonic gas speeds to be attained, acceleration needs to be done using a convergent-divergent nozzle . The hot gas, however, can be obtained through a number of different ways.