# How do rockets in space works?

#### Brett13

Nasa uses chemical rockets in space right? On earth I could understand how they work, the one reaction produces an equal and opposite reaction. So on earth the rocket would be pushing on air to propel itself. But theres no air in space... Edit:::: I added an s to "works" in the title, my bad

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Staff Emeritus
Rockets on earth don't push against the air either. It's quite simple - exhaust goes backwards, so to conserve momentum the rocket goes forward.

#### Brett13

Even then though wouldnt the exhaust have to push on something to propel the rocket? its 2:48 in the morning maybe im just tired haha...

#### Curl

It's not "pushing air" to propel itself, that's wrong.

Rockets just eject mass in one direction and the rocket head is pushed in the opposite direction. The center of mass of rocket + fuel cannot move since there is no net force on the system, so the rocket head must move in order to conserve momentum.

Has nothing to do with pushing air, you're thinking of a propeller.

#### Brett13

Oooooh so the ejection of the exhaust(the mass) is what propels it. OK thank you very much

#### Curl

yes you have to lose mass. It is a very inefficient way to do it but nobody has figured out a better way.

its like standing on ice and throwing bananas... you'll start sliding a little but with much effort

#### jtbell

Mentor
its like standing on ice and throwing bananas... you'll start sliding a little but with much effort
If you could throw bananas with a speed equal to the exhaust speed of a typical rocket engine, you would do very well!

#### AJ Bentley

I believe grapes would be more efficient? :rofl:

#### arildno

Homework Helper
Gold Member
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Oooooh so the ejection of the exhaust(the mass) is what propels it. OK thank you very much
It is perfectly acceptable to try to understand this in terms of Newton's 3.law of motion, as long as you do that PROPERLY:

You have then, at all times, TWO "objects":
Object 1: Ship+remaining fuel
Object 2: The fuel that is being exhausted.

Object 1 acts upon Object 2 with a force so that Object 2 is ejected.
According to Newton's 3.law, then, Object 2 acts upon Object 1 with a reaction force of equal magnitude, causing THAT to accelerate as well.

When we look, however, on the TOTAL system ship+fuel (ejected or still inside ship), there will never be any dislocation of its center of mass, under the assumption of no external forces acting upon it.

At the initial take-off, the Earth WILL act upon that total system's center of mass, by additional mechanism to, say, gravity:
Namely to push exhausted fuel hitting it.

The dislocation of the total system's center of mass must not, however, be confused by the dislocation of the SHIP's center of mass...(the latter one arguable the most important one!)

#### AJ Bentley

There's another way to look at it, that is equally valid.

If you think about the rocket's combustion chamber, it has a front wall, side walls and no back wall.

The expanding gasses inside the chamber press against the walls.
The side walls all cancel out - there's as much pressure top as bottom and left as right. So the rocket isn't pushed sideways.
But the front wall has pressure against it that isn't balanced by the pressure against the back wall (because the IS no back wall!).
That pressure against the front of the combustion chamber pushes the rocket forward.

This isn't a different explanation - it's really the same one from a different point of view.
(You can often look at things in different ways in Physics)

#### arildno

Homework Helper
Gold Member
Dearly Missed
There's another way to look at it, that is equally valid.

If you think about the rocket's combustion chamber, it has a front wall, side walls and no back wall.

The expanding gasses inside the chamber press against the walls.
The side walls all cancel out - there's as much pressure top as bottom and left as right. So the rocket isn't pushed sideways.
But the front wall has pressure against it that isn't balanced by the pressure against the back wall (because the IS no back wall!).
That pressure against the front of the combustion chamber pushes the rocket forward.

This isn't a different explanation - it's really the same one from a different point of view.
(You can often look at things in different ways in Physics)
Quite so.

In an action/reaction-couple, what counts as the "action" and what is the "reaction" is largely arbitrary.

Of course, by having a chamber that can be tilted sideways, then the rocket will be pushed in a slightly different direction, along the direction indicated by the open end and the "front"

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