# How jet engine works in vacuum

Its not about availability of oxygen for combustion of gases. I want to know since there is no air or friction providing things in vaccum, how the force applied by a jet engine get can make thrust to move the vechile

• If you use the usual definition "jet" engines don't work in vacuum because they take their working mass from the surrounding medium. Rockets work in vacuum. Commented Mar 18, 2014 at 13:00
• Possible duplicate of How can thrust be generated in space which is a vacuum?? Commented Mar 18, 2014 at 13:02

Think about the recoil of a gun. The gun shoots some mass (the projectile) with very high momentum into negative direction. Because of the balance of momentum the gun gets a momentum into positive direction.

The jet engine does the same with highly accelerated gas which has also mass and therefore also a high momentum at high speed.

• thanks since your answer. I know this fact as it is the basic principle but your answer changes my wrong way of thinking... Commented Mar 18, 2014 at 11:07

Every action has an equal and opposite reaction.

So to propel ourselves "forwards" we must propel something else backwards.

In ground vehicles we push against the ground. Since the ground is practically an immovable object this is very efficient.

In airplanes we propel outselves forward by propelling air (and a small ammount of combustion products) backwards. Generaly we reffer to planes where all the rotating components are inside a duct and push air out of an opening as "jets". Since reaction mass is basically free and energy is at a premium there has been a trend towards engines that move more air at lower velocity.

In space we don't have any air. So we can't use it as oxidiser and more importantly we can't use it as reaction mass. So we have to carry all our reaction mass with us. Here propellant mass is usually the limiting factor, so we try to maximise the exhaust velocity.