# Thread: How does rocket engine make ship go forward ?

1. *Source : Rocket engine - Wikipedia, the free encyclopedia

The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound at sea level air are not uncommon.
About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle (see diagram). As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other.

*Source : What Is a Rocket? | NASA

Why Does a Rocket Work?

In space, an engine has nothing to push against. So how do rockets move there? Rockets work by a scientific rule called Newton's third law of motion. English scientist Sir Isaac Newton listed three Laws of Motion. He did this more than 300 years ago. His third law says that for every action, there is an equal and opposite reaction. The rocket pushes on its exhoust. The exhaust pushes the rocket, too. The rocket pushes the exhaust backward. The exhaust makes the rocket move forward.

> I have a question here : ship moving from left to right :

-'The rocket pushes on its exhaust',
but how does the first amount, and further outcoming amounts, of exhaust stay in place to have
the rest of the outcoming exhaust pushing against it ?
The exhaust gas expands, but there is nothing 'on the left' to expand against, only 'on the right' were the ship/rocket is.

(the bold comes from copy/paste, sorry)

2.

3. The exhaust gas doesn't need anything on the left to "expand against".
The action/ reaction takes place at the "interface" of exhaust gases inside the bell of the engine and the surface of that bell itself.
The diagram attached to that Wiki article shows it quite clearly:

(Except this is moving right to left, so transpose where pertinent).

4. Originally Posted by Noa Drake
(the bold comes from copy/paste, sorry)
Best way to get rid of unwanted formatting is to highlight the formatted text, then click on the icon at the upper left which says "Remove Format" when you hover your mouse over it.

5. Originally Posted by Noa Drake
*Source : Rocket engine - Wikipedia, the free encyclopedia

The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound at sea level air are not uncommon.
About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle (see diagram). As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other.

*Source : What Is a Rocket? | NASA

Why Does a Rocket Work?

In space, an engine has nothing to push against. So how do rockets move there? Rockets work by a scientific rule called Newton's third law of motion. English scientist Sir Isaac Newton listed three Laws of Motion. He did this more than 300 years ago. His third law says that for every action, there is an equal and opposite reaction. The rocket pushes on its exhoust. The exhaust pushes the rocket, too. The rocket pushes the exhaust backward. The exhaust makes the rocket move forward.

> I have a question here : ship moving from left to right :

-'The rocket pushes on its exhaust',
but how does the first amount, and further outcoming amounts, of exhaust stay in place to have
the rest of the outcoming exhaust pushing against it ?
The exhaust gas expands, but there is nothing 'on the left' to expand against, only 'on the right' were the ship/rocket is.

(the bold comes from copy/paste, sorry)
If you fire a gun, you experience a recoil, don't you? Well, a rocket simply uses the recoil from "firing" its exhaust. So if you understand why a gun recoils, you should have no trouble understanding a rocket (or a jet engine - the principle is the same.).

If you do not understand why a gun recoils, we can discuss.

6. It is like Dywyddyr says.
Think of burning the fuel inside of a rocket engine as a controlled explosion.
Think of it just as the explosion without anyting around it.
The explosion creates a pressure wave.
Now imagine if you put a flat steel sheet at one side of that explosion.
The pressure would push it.
A basic rocket is a tube with one end open and a controlled explosion inside.
At the open end of the tube the pressure is very low, at the closed end the pressure is very high. The explosion pushes hard against the closed end but at the open end there is nothing to push against.
A fancier rocket engine has a nozzle that lets the continuing expansion of the hot gases push a bit more against a bit more area.

7. Originally Posted by Dywyddyr
The exhaust gas doesn't need anything on the left to "expand against".
The action/ reaction takes place at the "interface" of exhaust gases inside the bell of the engine and the surface of that bell itself.
The diagram attached to that Wiki article shows it quite clearly:....
Beautiful my good man, beautiful!

8. Dear Noa,

I'm confused as to what the purpose of this thread is. You seem to know a great deal about how a rocket engine works so what is this thread about? What do you wish to learn? Thanks.

9. Originally Posted by Dywyddyr
The exhaust gas doesn't need anything on the left to "expand against".
The action/ reaction takes place at the "interface" of exhaust gases inside the bell of the engine and the surface of that bell itself.
The diagram attached to that Wiki article shows it quite clearly:

(Except this is moving right to left, so transpose where pertinent).
Ok, i am trying to understand, looking at the image here : why doesn't the hot gas exit to the right and keeps on going right ?
Leaving the rocket more stationary ?

10. It does keep going right, but it doesn't leave the rocket stationary. Are you familiar with the concept of conservation of momentum? If you send the exhaust gas at high velocity to the right, something else has to go speeding off to the left.

11. And then we introduce more reactants into the chamber to generate a further explosion, the whole typically happening continuously.

I am also puzzled with where you are heading with this.

12. Originally Posted by Noa Drake
Originally Posted by Dywyddyr
The exhaust gas doesn't need anything on the left to "expand against".
The action/ reaction takes place at the "interface" of exhaust gases inside the bell of the engine and the surface of that bell itself.
The diagram attached to that Wiki article shows it quite clearly:

(Except this is moving right to left, so transpose where pertinent).
Ok, i am trying to understand, looking at the image here : why doesn't the hot gas exit to the right and keeps on going right ?
Leaving the rocket more stationary ?
The rocket in the pictures has a mass and carries propellant of mass . The propellant is burned and gas is ejected at the far end of the rocket. The speed of the ejected gas is . The increase in the speed of the rocket is:

See the term increase? What does it mean, Noa?

13. Stand on a skateboard holding a bag of bricks.
Throw a brick. Throw another brick. Throw another brick.

14. Originally Posted by pzkpfw
Stand on a skateboard holding a bag of bricks.
Throw a brick. Throw another brick. Throw another brick.
And before long you end up moving VERY fast.
Because you have to leg it before the cops arrest you for breaking windows.

15. Originally Posted by John Galt
And then we introduce more reactants into the chamber to generate a further explosion, the whole typically happening continuously.

I am also puzzled with where you are heading with this.
That's why I asked. However my question was ignored.

16. Originally Posted by pzkpfw
Stand on a skateboard holding a bag of bricks.
Throw a brick. Throw another brick. Throw another brick.
With a little more time I'll expand on this:
The 2nd brick isn't pushing on the 1st brick. It's the throwing of the 2nd brick that gives me some more impulse.
The 3rd brick isn't pushing on the 2nd brick. It's the throwing of the 3rd brick that gives me some more impulse.

It's not pushing the brick against air that gives me that impulse either - it's me pushing against the brick itself.

Same with the rocket engine.

The exhaust emitted at 2 seconds isn't pushing on the exhaust emitted before, it's the emission itself that provides impulse.
The exhaust emitted at 3 seconds isn't pushing on the exhaust emitted before, it's the emission itself that provides impulse.

The exhaust doesn't have to push against anything at all. The exhaust itself has mass, just like the brick. And throwing it out the back of the rocket gives the rocket an "equal and opposite" reaction.

(i.e. throwing a brick basically means pushing on it. That's the kind of push meant by the quote in post #1.)

17. How hard can it be to understand this?!
It's not rocket science!
...oh, wait. It is.

18. Consider this analogy.

19. @ jr monroe. I think you are skating on thin ice with that comparison.

20. Originally Posted by John Galt
@ jr monroe. I think you are skating on thin ice with that comparison.
OH, a pun, and nicely done.

21. Originally Posted by dan hunter
Originally Posted by John Galt
@ jr monroe. I think you are skating on thin ice with that comparison.
OH, a pun, and nicely done.
Aha, a rhyme, and just in time.

22.

23. I was not aiming at anything in particular, just didn't catch the details of how it worked. Thank you.

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