Why do rocket engines work in the vacuum of space?

[Sephiroth2088]

I have a feeling someone is going to be like "Duhhh" on this one. But I got to thinking.

I recall reading somewhere that the guy that came up with the rocket engine for use in space got an F on his paper because "I think" the professor said that there would be nothing to push back on the rocket in space.

So I got to thinking, I know Einstein said for every action there is an equal and opposite reaction... well.. for example I push a wall, it's pushing back on me with equal force.

However a rocket in space pushes on well.. a vaccuum, a vacuum can push back?

In the same way I know that if I was surrounded by air and waved my arms around in a gravityless room with air inside I would move. But if I was in a space suit in space and did this I feel that I wouldn't move, because their is nothing to push back.

 

[cesiumfrog]

A rocket pushes on it's fuel

 

[olgranpappy]

...I know Einstein said for every action there is an equal and opposite reaction.

No. No. No. Try Newton.

 

[rcgldr]

A rocket expels part of it's own mass (burnt fuel) at a very high rate of speed. The spent fuel goes one direction, the rocket goes the other direction.

 

[chronon]

In 1920 the New York Times stated that rockets would not work beyond the atmosphere (they published a correction in 1969)

 

[FredGarvin]

However a rocket in space pushes on well.. a vaccuum, a vacuum can push back?

A rocket engine (or any other reaction engine) only "pushes" on the fluid stream inside it. It carries both fuel and oxidizer (for a bi-propellant system) on board so it can operate in an atmosphere like space.

The ignition of the fuel creates an acceleration in the fluid stream which causes the exhaust stream to exit at a much higher velocity. Due to Newton's 3rd law, the reaction of accelerating that fluid stream to a higher velocity creates a reaction force in the opposite direction, i.e. thrust.

 

[mgb_phys]

Perhaps a simpler picture is to imagine the exploding fuel pushing on the top of the fuel tank as it jets out of the bottom.

 

[russ_watters]

Ask and ye shall receive: http://www.braeunig.us/space/propuls.htm

(top of the combusion chamber, though, not the fuel tank...but I know what you meant)

 

[FredGarvin]

Perhaps a simpler picture is to imagine the exploding fuel pushing on the top of the fuel tank as it jets out of the bottom.

That certainly is a more simple picture. Although, any burner designer would cringe at you using the word "exploding." There is a desired and controlled combustion process that happens but let's not get picky.

 

[DaveC426913]

In the same way I know that if I was surrounded by air and waved my arms around in a gravityless room with air inside I would move. But if I was in a space suit in space and did this I feel that I wouldn't move, because their is nothing to push back.

You're right you wouldn't move. What you need to do to move in space is reach into the duffel of tennis balls you cleverly brought with you and start throwing them away from you one at a time as hard as you can**. That will get you moving. And by logical extension of thrown tennis balls to thrown atoms of gas, you can see why a rocket works quite well.


**BTW, you don't need vacuum to use this technique. It will work nicely sitting on an office chair or on an ice rink too (at least, in principle).

 

[Danger]

What you need to do to move in space is reach into the duffel of tennis balls you cleverly brought with you and start throwing them away from you one at a time as hard as you can**.

If you're as lazy as I am, however, it's a lot easier to temporarily disconnect your oxygen supply hose and direct it rearward. (Note that the term temporary is very important to the success of the manoeuvre.)

 

[mgb_phys]

Although, any burner designer would cringe at you using the word "exploding."

It's the memory of my mechanics professor, with a very strong Austrian accent - after calculating the burn rate needed to get a Satrun V off the ground ( something like 20t/s) he says "Zis is not burning - zis is exploding"
Not chemically correct but a good description of 20t of rocket fuel a second going bang!