A genuine question about fuel engine and in vacum trust in general.

[SpacePsilo]

Hullo, so I have a question, in vaccum, there is almost nothing to allow the expelled gaz to have an opposite force, so why don't the gaz continue to infinity.

Because for jet engine, you have engine --> <- atmosphere. So in vaccum it should be engine --> void...

I know it's not like this since ships have trusts, but I wonder what allow the force to have an opposit in vaccum.

If someone has a link for a simple explaination or can give it I would greatly appreciate it.

[KerikBalm]

I don't even understand the question....

You're asking how a rocket engine can create thrust in a vacuum?

It pushes against the exhaust... If I'm in space, and holding a 100 kilo weight... it has inertia, if I push against it, it starts moving 1 way, and I move another way.

Rockets are just like this, instead of pushing huge chunks of mass all at once, they do it continuosly with the combusting fuel pushing against the nozzle and combustion chambers

[SpacePsilo]

Yes that is what I was wondering, so, for an engine similar to the Ion, would it be plausible to do it inverted?

So if you have a spaceship on top of it you put the engine exaust facing the way you want to go, then you put a metal plate in front, so the engine push the plate that is linked to ship and generate trust? If the gaz you expell is faster than the trust it generate would your ship move in the direction the plate is pushed or the opposite way?

[magnemoe]

Simpler. think recoil, you notice recoil not only on guns but on stuff like water hoses with decent pressure. Rocket works just like this, non of this pressures against the atmosphere, only propellers, turbofans and rotors do.

[Vanamonde]

Hullo, so I have a question, in vaccum, there is almost nothing to allow the expelled gaz to have an opposite force, so why don't the gaz continue to infinity.

That pretty much IS what the exhaust does. It is ejected and disperses and just becomes part of the gases and dust of space.

If I understand what you're asking, the atmosphere behind the jet is not something the engine is pushing against for propulsion. As the gases heat and expand within the engine, they press on the walls of the combustion chamber, bounce off, and are expelled out the back. That is where the propulsive force on jets and rockets comes from: expanding gases pushing on the inside of the engine.

[sgt_flyer]

'basically' rocket nozzles are specifically shaped to maximize the usage of gas expansion - as the gas expands, they press onto the nozzle's walls - but because the nozzle walls are at an angle, they 'deflect the gases, providing net thrust (without a nozzle, a gas would expand as a sphere shape - with a nozzle, you're directing all the expansion to one side)

- this is also why we need different nozzle lengths while in atmo / vacuum - atmospheric pressures prevent full gas expansion - so if your nozzle is too long in atmosphere, the gases won't 'stick' to the walls of the nozzle, limiting the amount of force they can impart on the nozzle (and you also have a penalty in dry weight because of the too long nozzle )

[FlyingPete]

According to Newton, rockets don't move- if we define the rocket as the structure and all the fuel it contains. In a vacuum, we expel the exhaust gases in one direction. The force exerted on the rocket by the expanding gases is what gives it its thrust. However, the center of mass of the system as a whole doesn't move from its original position. What you need for the rocket to be efficient is to direct all the exhaust gas in a single direction- any expansion of the gases sideways doesn't contribute to thrust.

Jet engines (and propellers) work differently- they take in their reaction mass from the atmosphere and accelerate it- in this case the acceleration of the atmospheric air provides the thrust.

[Leszek]

The easiest way to see it, is like a boat. Imagine a small rowboat. You are standing on the back. You jump out to dive. The boat will move forward. This is because you have pushed off the boat in your dive. Now imagine if you were in a spacesuit and there was no atmosphere and you jumped off the back of the boat. The boat will still move forward. The atmosphere has nothing to do with it and it should be plain as day.

In a rocket the exhaust is pushing off the engine, not the atmosphere.

[Bill Phil]

The exhaust is pushing against the rocket engine. Same for jet engines. In fact, just about any​ reaction engine works like this.

[Dkmdlb]

So if you have a spaceship on top of it you put the engine exaust facing the way you want to go, then you put a metal plate in front, so the engine push the plate that is linked to ship and generate trust?

Like a troll drive?

[magnemoe]

Like a troll drive?

Pretty much, however my troll drive would be to accelerate trolls up too high speed and live on the youtube feed then they hitting the concrete slab.

[meve12]

The exhaust is pushing against the rocket engine. Same for jet engines. In fact, just about any​ reaction engine works like this.

Make this any engine at all. The tires of a car pushes the asphalt backwards...

[Bill Phil]

Make this any engine at all. The tires of a car pushes the asphalt backwards...

The wheels are only driven by the engine... The engine drives the mechanism that propels the vehicle, not directly propelling the vehicle.