# The Naked Scientists Forum

### Author Topic: How do rockets in space propel themselves?  (Read 13606 times)

#### mike2niner4

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##### How do rockets in space propel themselves?
« on: 27/04/2009 17:50:06 »
Hey,

I'm wondering how rockets propel themselves in space with the little burner things they have as if its a vacuum, what does it have to push against?

Thanks, Mike

#### lightarrow

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##### How do rockets in space propel themselves?
« Reply #1 on: 27/04/2009 18:17:37 »
Hey,

I'm wondering how rockets propel themselves in space with the little burner things they have as if its a vacuum, what does it have to push against?

Thanks, Mike
It push against the gas it expels, the principle is "reaction" or "momentum conservation". If you throw away a body of mass m and velocity v, you acquire a momentum m*v. So, if your mass is M, your velocity V will be: V = m*v/M.

#### lyner

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##### How do rockets in space propel themselves?
« Reply #2 on: 27/04/2009 18:35:34 »
An aloternative (but equivalent) answer is that Newton's Third Law of Motion, which says that for every force (action) there is an equal and opposite force (reaction).
The force needed to squirt the gases out of the nozzle produces a force back into the nozzle.

#### mike2niner4

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##### How do rockets in space propel themselves?
« Reply #3 on: 27/04/2009 18:39:05 »
Thanks  I've always wondered that...

#### blakestyger

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##### How do rockets in space propel themselves?
« Reply #4 on: 27/04/2009 22:32:50 »
I remember that in the eraly days orbiting satellites squirted out jets of hydrogen peroxide in order to make minor adjustments in position.

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #5 on: 09/09/2012 22:44:56 »
Hey,

I'm wondering how rockets propel themselves in space with the little burner things they have as if its a vacuum, what does it have to push against?

Thanks, Mike

Hi there

I really still not have a clue. And I'm serious.

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #6 on: 09/09/2012 23:36:22 »
An aloternative (but equivalent) answer is that Newton's Third Law of Motion, which says that for every force (action) there is an equal and opposite force (reaction).
The force needed to squirt the gases out of the nozzle produces a force back into the nozzle.

Hi,

(green text) Right, BUT for me that does not explain nothing in a vacuum: immagine you are an astronaut in space without any thing you can touch near you; well, about action-reaction theory: you can move yourself such as arms, legs, how much you want, but its very probably you will stay in that start position in space since someone will pick up you.

PS - This if the space is really what they have explained since now, but I have some reserve on that.
« Last Edit: 09/09/2012 23:42:59 by waytogo »

#### RD

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##### Re: How do rockets in space propel themselves?
« Reply #7 on: 09/09/2012 23:50:50 »
... that does not explain nothing in a vacuum: immagine you are an astronaut in space without any thing you can touch near you; well, about action-reaction theory: you can move yourself such as arms, legs, how much you want, but its very probably you will stay in that start position ...

Vacuum isn't a special case : there would still be a reaction force, e.g. a counter-rotation in the astronaut's torso while their limb was moving. When the limb was moved back to the previous position the torso would also return to it's previous position, so they would return to the start position/posture.
« Last Edit: 10/09/2012 00:04:50 by RD »

#### lean bean

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##### Re: How do rockets in space propel themselves?
« Reply #8 on: 11/09/2012 16:58:49 »
Hey,

I'm wondering how rockets propel themselves in space with the little burner things they have as if its a vacuum, what does it have to push against?

Thanks, Mike

Hi there

I really still not have a clue. And I'm serious.

Does the ballon picture on this link help http://howthingsfly.si.edu/propulsion/rocket-propulsion
Quote
When we think of rockets (or jet engines) we rarely think of balloons. Instead, we think of the big rockets that carry satellites, supplies, or people into space. However, balloons and rockets are very similar. The only significant difference is the way the pressurized gas is produced. With rockets, the gas is produced by burning propellants that can be solid or liquid in form or a combination of the two.
In my younger days I used to think thrust came out the back. In the picture thrust is the pressure arrow pointing to the balloon's front.
Compare both balloons, the untied balloon has a thrust (pressure) going forwards but the equal pressure going backwards is escaping through the open nozzel, so the pressure inside the ballon is unbalanced along the a line from front to end.the pressure on the front takes the balloon forwards. The pressure in other directions is balanced.
« Last Edit: 11/09/2012 17:08:28 by lean bean »

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #9 on: 11/09/2012 23:19:02 »
Hey,

I'm wondering how rockets propel themselves in space with the little burner things they have as if its a vacuum, what does it have to push against?

Thanks, Mike

Hi there

I really still not have a clue. And I'm serious.

Does the ballon picture on this link help http://howthingsfly.si.edu/propulsion/rocket-propulsion

Not really, sorry.

#### yor_on

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##### Re: How do rockets in space propel themselves?
« Reply #10 on: 12/09/2012 08:31:15 »
It is the same principle as with a garden hose. Or in your shower, the more water you apply, turning up the water pressure, the harder that nozzle will want to move the opposite way. It's a principle, or 'law'. But yeah, I see your point, I think? Try to think of it as molecules or atoms bouncing around inside that chamber, as they get 'heated' they have more energy and becomes agitated. Then they find a way out (the hole) and the 'principle of action and reaction' steps in as they 'transform/organize' themselves into one direction. But, I think this one is for Damocles actually :)

How do they transmit the force to the walls of that chamber?
Specifically the opposite wall, if you think of it as molecules?
« Last Edit: 12/09/2012 08:34:00 by yor_on »

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #11 on: 12/09/2012 08:39:51 »
It is the same principle as with a garden hose. Or in your shower, the more water you apply, turning up the water pressure, the harder that nozzle will want to move the opposite way. It's a principle, or 'law'.

... well officially there is not weight, pressure, friction in space. So your example does not fit the deal.
« Last Edit: 12/09/2012 08:41:24 by waytogo »

#### yor_on

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##### Re: How do rockets in space propel themselves?
« Reply #12 on: 12/09/2012 08:43:38 »
It's not about the outside, it's purely about a 'imbalance' as I think of it. Imbalance can exist in all sort of environments, or densities. To me the principle is similar to the way hot water mix with cold to give us tepid, all about equalizing imbalances. In space that imbalance will be fixed when the chamber contain about the same as outside, a vacuum.

#### yor_on

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##### Re: How do rockets in space propel themselves?
« Reply #13 on: 12/09/2012 09:04:17 »
You might think of it as half the chamber having less pressure than the part closest to the wall 'pushing' the rocket forward, possibly. A pressured sphere will, approximately, have a equal pressure in all directions. But if you open a hole the side where the hole is will have less pressure and inside the chamber you now have a situation in where the wall furthest away will be the one most 'agitated' whilst the one with the hole will have a weaker 'force' acting as the distance between molecules widen due to that they leave through the hole, possibly :) and those molecules furthest away will act more forcefully on the opposite wall as they have a higher energy/density per cubic centimeter. So it is a imbalance inside the chamber as well as the imbalance between inside and what's outside.

But let's see what JP and Damocles, to mention a few, have to say about it?
I could be bicycling in the great green younder, although I think it is right, well, sort of?:)

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #14 on: 12/09/2012 09:15:47 »
Hi, The facts are the only way to show something.

So, if you are still conviced that its all explained, just put a little rocket in a vacuum chamber and let us to see your scientific stuff.

#### yor_on

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##### Re: How do rockets in space propel themselves?
« Reply #15 on: 12/09/2012 09:26:18 »
Up above :)

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #16 on: 12/09/2012 09:30:25 »
Up above :)

So com'on, show some video. Some facts then.

#### yor_on

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##### Re: How do rockets in space propel themselves?
« Reply #17 on: 12/09/2012 10:09:04 »
If you're asking if we have rockets, real ones? In space, no less? I'm sure you can google up a u-tube video to find it confirmed? And they all use the same 'principle', as far as I know, what Newton called 'action and reaction' :)

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #18 on: 12/09/2012 10:35:03 »
If you're asking if we have rockets, real ones? In space, no less? I'm sure you can google up a u-tube video to find it confirmed? And they all use the same 'principle', as far as I know, what Newton called 'action and reaction' :)

I'm not meaning the real ones obviously and Saturn 5 does not fit in an vacuum chamber...

However, that's a mini rocket example:

#### imatfaal

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##### Re: How do rockets in space propel themselves?
« Reply #19 on: 12/09/2012 11:23:02 »
OK Here is a video of the space shuttle doing a back flip whilst being filmed from the ISS.

Rockets work in space.  They work because of the reality of an idea that Newton formalised - that for each action there is an equal and opposite reaction.  If you throw stuff out of the back of the rocket - then the rocket moves forward

You could look at these demonstrations of Newton's 3rd law

If you don't understand this let us know and we will try to explain a bit more.  Yoron probably uses "we" through a sense of shared humanity and because like me he paid for them through his taxes - it might be a tiny remote connection, but it is one I am proud of

#### waytogo

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##### Re: How do rockets in space propel themselves?
« Reply #20 on: 12/09/2012 15:53:46 »
Sorry but needs simple facts, not words only.

So, just put in a vacuum chamber that mini rocket:

#### lean bean

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##### Re: How do rockets in space propel themselves?
« Reply #21 on: 12/09/2012 16:40:30 »
If you're asking if we have rockets, real ones? In space, no less? I'm sure you can google up a u-tube video to find it confirmed? And they all use the same 'principle', as far as I know, what Newton called 'action and reaction' :)
I always said the moon landing was an hoax :)  :) :)

Waytogo
Quote
.. well officially there is not weight, pressure, friction in space. So your example does not fit the deal.

Just checking, are you thinking you need  something outside the chamber for the nozzle exhaust to push against ? Like an atmosphere? If so, it's not like that.

----------------

Quote
Newton’s Third Law of Motion is often given as an explanation for the generation of thrust. But it explains the effect of thrust, not the cause of thrust. Pressure and shear stress are the only two ways nature exerts an aerodynamic force on an object. Pressure is the basic source of thrust produced by a propeller, jet engine, or rocket engine.
http://howthingsfly.si.edu/propulsion/engines

« Last Edit: 12/09/2012 16:52:22 by lean bean »

#### imatfaal

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##### Re: How do rockets in space propel themselves?
« Reply #22 on: 12/09/2012 17:41:06 »
If you're asking if we have rockets, real ones? In space, no less? I'm sure you can google up a u-tube video to find it confirmed? And they all use the same 'principle', as far as I know, what Newton called 'action and reaction' :)
I always said the moon landing was an hoax :)  :) :)

Waytogo
Quote
.. well officially there is not weight, pressure, friction in space. So your example does not fit the deal.

Just checking, are you thinking you need  something outside the chamber for the nozzle exhaust to push against ? Like an atmosphere? If so, it's not like that.

----------------

Quote
Newton’s Third Law of Motion is often given as an explanation for the generation of thrust. But it explains the effect of thrust, not the cause of thrust. Pressure and shear stress are the only two ways nature exerts an aerodynamic force on an object. Pressure is the basic source of thrust produced by a propeller, jet engine, or rocket engine.
http://howthingsfly.si.edu/propulsion/engines

I can see what they are getting at - but I think they are desperately trying to be controversial.  You get thrust by throwing stuff backward - how you do that is pressure in a rocket engine (as they correctly explain very nicely), it could be coulomb force in an ion thruster (which kinda contradicts their "only two ways"), or by throwing baseballs from the back of the ship.  In case of a rocket - the rocket pushes the exhaust backwards - the exhaust pushes the rocket forwards.

#### lean bean

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##### Re: How do rockets in space propel themselves?
« Reply #23 on: 12/09/2012 20:30:56 »
Thanks imatfaal, I got a chance to lean something here.
it could be coulomb force in an ion thruster

A electromagnetic field is produced by some apparatus, when ions react with the field they are channelled electromagnetically to the rear and out of the ship, there is resistance in the field to the work being done (channelling the ions backwards)  the field pushes against the field generator and that against its supports  and the ship moves forwards…I think.

Quote
throwing baseballs from the back of the ship.
You push against the ball, the opposite force is transmitted to the floor by your feet, the ship goes forwards?

Quote
In case of a rocket - the rocket pushes the exhaust backwards - the exhaust pushes the rocket forwards.
Couldn't this be ...
The explosion in the chamber creates equal pressures in all directions to the chamber walls, but since the rear (nozzle) is open, the pressure going forward is not countered or balanced and so there is a net pressure going to the forward wall and great enough to overcome the rocket's inertia.
I think.
« Last Edit: 12/09/2012 20:49:24 by lean bean »

#### imatfaal

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##### Re: How do rockets in space propel themselves?
« Reply #24 on: 12/09/2012 22:57:42 »
yes, yes, and yes almost (I think and hope)  :-)  Newton's 3rd law is what allows us to say that the force which allows momentum to be imparted to the ion/ball/hot exhaust gases must be opposed by an equal force which in turn will accelerate the ship in the opposite direction.  ie it explains why a contained pressure imbalance causes both the gas to accelerate in one direction and the spaceship in the other.

it's very much a chicken-egg situation - but I think of it in these terms; why should hot gas in a chamber cause the the rocket to move 1.  The scenario causes gas to be expelled at high velocity out of the back of the ship by some means that is not that important - that force must have an equal and an opposite which acts on the ship  2.  The hot gas is molecules/atoms hitting the sides of the combustion container  - each time they hit the forward wall rather that side wall they bounce out with the exhaust and the bounce is unbalanced. The reaction force to the unbalanced bounces is forward.   1 & 2 are basically the same explanation - the second is basically a simplistic view of pressure the first a similar view of 3rd law; but they just take different perspectives.  I prefer the first because it is easily transferred to the ion-thruster and baseball thruster argument - but I can also see the benefits of the other, but I think these are only apparent in more complex views of the application

#### The Naked Scientists Forum

##### Re: How do rockets in space propel themselves?
« Reply #24 on: 12/09/2012 22:57:42 »