I have finally discovered a mystery that's been bother me for the longest time.

I'm a civil engineer and not a mechanical engineer so engines are not part of what I would think about often.

My question is this : Jet engines propel air through the engine to create thrust. This requires air in the surroundings in order for it to work. So if you put a jet in space, it will just stay there because there's no air for the jet to suck.

Now what about rocket engines which propel shuttles in space? There's no air so what does the sprout of spray push against?

Simply put, the rocket engine is enclosed and the combusted fuel has no other place to go then out the engine through the back. Using Newton's 3rd law, the conservation of momentum means the engine has to move in the opposite direction. The exhaust rushing out as you would see on tv plays no part in the propulsion. It is the internal reaction that creates this force.

And you know how we associate roaring engines with the power of them? It is partially right. The roar comes from the sudden release and change in pressure when the gas escapes the engine. Like a balloon popping. It's not that the engine is working like mad and the internal machinery is clunking away to provide a macho sound for us.

Makes me think about farts. If we want our farts to come out silently, fart into an inflated balloon. The higher pressure in the balloon will ensure a slow and smooth transition of the stuff without much hoohah. But of course you have to make sure you fart with sufficient strength or the air from the balloon will rush into your backside and you'll prob feel terrible.