The Naked Scientists Forum

Author Topic: How does rocket thrust work in the vacuum of space?  (Read 31708 times)

Glenn Romaniuk

• First timers
• Posts: 3
How does rocket thrust work in the vacuum of space?
« on: 27/02/2010 18:13:11 »
If rockets launched at sea level use thrust produced by pushing against our gases in our atmospher how can a rocket work in the vacuum of space. How is thrust produced in that environment?

Thanks,
Glenn

[MOD EDIT - PLEASE PHRASE YOUR POST TITLES AS QUESTIONS, WHICH IS THE FORUM POLICY. THANKS. CHRIS.]
« Last Edit: 27/02/2010 21:52:15 by chris »

Vern

• Neilep Level Member
• Posts: 2072
Re: How does rocket thrust work in the vacuum of space?
« Reply #1 on: 27/02/2010 19:09:58 »
Rockets don't get thrust by pushing against the air. Rockets get thrust by pushing against the rocket on one side and pushing against nothing on the other side of an explosion.

Geezer

• Neilep Level Member
• Posts: 8328
• "Vive la résistance!"
Re: How does rocket thrust work in the vacuum of space?
« Reply #2 on: 27/02/2010 19:18:13 »
The rocket ejects a mass of burned fuel to produce a force. The force produced is proportional to the amount the mass of the combustion products (the exhaust) was accelerated. This follows from F=ma (Force = mass x acceleration)

So, the more you accelerate the exhaust, the more force is applied to the rocket.

Imagine you are floating in space. If you take off a shoe and throw it away from you really fast, you will move in the opposite direction from your shoe. You can even do this without going into space if you are standing on a very low friction surface like ice.

Robro

• Full Member
• Posts: 69
Re: How does rocket thrust work in the vacuum of space?
« Reply #3 on: 27/02/2010 20:36:22 »
Also, the shape of the rocket exhaust nozzle is made or 'tuned' in such a way that it captures the expanding gasses escaping the combustion chamber of the rocket engine as efficiently as possible.
« Last Edit: 28/02/2010 03:26:06 by Robro »

LeeE

• Neilep Level Member
• Posts: 3382
How does rocket thrust work in the vacuum of space?
« Reply #4 on: 28/02/2010 12:24:40 »
If rockets launched at sea level use thrust produced by pushing against our gases in our atmospher...

Rockets (and jet engines) work because of Newton's third law, not because they push against something.

yor_on

• Naked Science Forum GOD!
• Posts: 11731
• Thanked: 1 times
• (Ah, yes:) *a table is always good to hide under*
How does rocket thrust work in the vacuum of space?
« Reply #5 on: 03/03/2010 08:44:45 »
I like to look at it this way.

An explosion is a little like a ball growing in a empty space, it generally can be said to grow in all directions symmetrically (depending on material, geometry etc). Introduce a hinder in all directions, leaving one 'open' to that space outside, like the walls of that rocket engine. Now you've stymied the symmetry of growth for that explosion in all directions except one. The open direction will let it out unhindered but in all other directions the force will 'push' on the walls of its confinement before being reflected one way or another. Thats why it's so important to think of how that explosion will reflect, and how to build the best explosion chamber. The ultimate, as I see it, must be to let it 'push' just once, in the direction you want to move, and then leave, out the chambers hole.

To get it to leave several 'pushes' seems very hard to do as it (the 'force' of the explosion) then will have to be angeled in several directions, not contradicting the velocity you want, and then somehow still succeed to 'push' once again, or more, in the correct direction.
« Last Edit: 03/03/2010 08:47:45 by yor_on »

Robro

• Full Member
• Posts: 69
How does rocket thrust work in the vacuum of space?
« Reply #6 on: 03/03/2010 18:40:52 »
If rockets launched at sea level use thrust produced by pushing against our gases in our atmospher...

Rockets (and jet engines) work because of Newton's third law, not because they push against something.
Yes, the engine doesn't push on anything, it's the controlled expansion of the oxidized fuel that pushes the engine.
« Last Edit: 04/03/2010 08:03:31 by Robro »

Geezer

• Neilep Level Member
• Posts: 8328
• "Vive la résistance!"
How does rocket thrust work in the vacuum of space?
« Reply #7 on: 03/03/2010 19:56:47 »
There is no "explosion". The fuel oxidizes (burns) at a controlled rate.

yor_on

• Naked Science Forum GOD!
• Posts: 11731
• Thanked: 1 times
• (Ah, yes:) *a table is always good to hide under*
How does rocket thrust work in the vacuum of space?
« Reply #8 on: 04/03/2010 07:06:11 »
Gezeer, I agree in that you can see it a lot of ways as a matter of definition :)
You can also say that it 'explodes' at a controlled rate, or 'expands', or applies a 'pressure' or...

But Nasa have chosen to call it 'explosions' " In a  rocket engine , fuel and a source of oxygen, called an oxidizer, are mixed and exploded in a combustion chamber. The combustion produces hot exhaust which is passed through a nozzle to accelerate the flow and produce thrust. For a rocket, the accelerated gas, or working fluid,  is the hot exhaust produced during combustion."

You're quite right in that it's hard to see any 'push' in the example of the guy throwing a shoe. Maybe you can say that he pushes against the shoe which pushes against him, as we are discussing this from a Newtonian perspective (action & reaction).

Then, you might see it as the walls of your rockets engine chamber(s) are 'pushing' back against the force of the 'explosion/very fast burn/pressure, pushing at its walls with an equal force, and as the walls won't bend to it the whole rocket are induced to move.

A sort of wall induced action&reaction :)

Robro

• Full Member
• Posts: 69
How does rocket thrust work in the vacuum of space?
« Reply #9 on: 04/03/2010 08:09:23 »
OK, Geezer, corrections made. I will try to be more accurate :)

Geezer

• Neilep Level Member
• Posts: 8328
• "Vive la résistance!"
How does rocket thrust work in the vacuum of space?
« Reply #10 on: 04/03/2010 20:33:34 »
People often refer to "explosions" in the combustion chambers of gasoline internal combustion engines producing power. They don't.

When things are working properly, the fuel/air mixture burns in a controlled manner to produce increasing pressure. When things are not working properly, the fuel/air mixture explodes and produces very little useful power while it damages the engine.

Sometimes semantics are important.

fontwell

• Jr. Member
• Posts: 39
How does rocket thrust work in the vacuum of space?
« Reply #11 on: 15/03/2010 11:55:19 »
Ages ago I heard someone define an explosion as "where the flame front moves faster than the speed of sound" but I've never come across it since. Periodically I wonder if this does actually ever happen and even if so, does this definition help anyone? Also, hard to see how you could apply this definition to a rocket in space.

The Naked Scientists Forum

How does rocket thrust work in the vacuum of space?
« Reply #11 on: 15/03/2010 11:55:19 »