Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: mike2niner4 on 27/04/2009 17:50:06
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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
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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.
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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.
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Thanks [;D] [;D] I've always wondered that...
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I remember that in the eraly days orbiting satellites squirted out jets of hydrogen peroxide in order to make minor adjustments in position.
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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.
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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.
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... 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.
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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 (http://howthingsfly.si.edu/propulsion/rocket-propulsion)
Quote from that link 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.
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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 (http://howthingsfly.si.edu/propulsion/rocket-propulsion)
Not really, sorry.
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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?
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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.
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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.
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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?:)
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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.
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We have them already.
Up above :)
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We have them already.
Up above :)
So com'on, show some video. Some facts then.
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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' :)
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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' :)
Why you reply with 'we'?
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:
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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 (http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion)- 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 (https://www.google.co.uk/search?q=newtons+3rd+law+demonstration&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:unofficial&client=firefox-a)
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
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Sorry but needs simple facts, not words only.
So, just put in a vacuum chamber that mini rocket:
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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 .. 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.
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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
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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 .. 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.
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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.
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Thanks imatfaal, I got a chance to lean something here.
Not sure about this, but aho...
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.
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?
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.
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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
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waytogo - you seem to be struggling with this concept.
Consider two identical cubes, each with an open face, floating in the "vacuum" of space. In the open face are two identical astronauts of identical mass, with their feet attached to the cube. The cubes are side by side so that the open faces are together and the astronauts are face to face.
They push against eachother. The two cubes fly apart from the original position at the same speed.
One of the astronauts then turns around and picks up a bowling ball that was in his cube (don't worry, the other cube had one as well :). he holds it in front of him and since there is no gravity and they are both moving at the same speed in the same direction, the bowling ball remains in front. He pushes on the bowling ball and while the bowling ball flies away, the cube also increases it's velocity away from the direction of the ball.
You can see where this is going. If you then pushed out some sort of gas, then it would push the vessel a bit further. Because gas molecules are generally smaller than your average rocket, you need to push lots out, really fast. This is essentially what rockets burning fuel in a vacuum do.
I think.
Also, consider the recoil of a gun. If you could somehow rig up a gun so that it could fire in the vacuum of space, there would still be a significant recoil from the bullet being pushed out at such high speed.
I personally don't have the means to conduct such experiments to prove such points myself, but I'm happy to go along with mainstream science on this one and just do the thought experiments on basic principles.
waytogo, I hope that helps you understand it a bit better, and everyone else, please correct me if i've over-simplified it.
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I think of it as imbalance Imatfaal, and so also an expression of entropy :) although in the fast lane :)
To me it has to do with 'symmetries' too in some ways as the situation clearly is non-symmetric, if we define it to a 'system' . But then again, in real life it's also a 'open-ended' system but using way2gos ideas of a tank having a vacuum we get a nice system. Anyone wanna buy one? And a rocket.
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Sorry but needs simple facts, not words only.
So, just put in a vacuum chamber that mini rocket:
It's curious how there is not a simple educational stuff on school labs to prove that... don't you?
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Sorry but needs simple facts, not words only.
So, just put in a vacuum chamber that mini rocket:
Do you really doubt that a rocket could work in the void, I mean, seriosly? If the answer is yes, you need some holidays in a quiet place for some time...
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Do you really doubt that a rocket could work in the void, I mean, seriosly?
I would to see a scientific stuff (demonstration) about that (however, I'm not the only one). I wonder why there are not any stuff in schools lab to study the behaviours of that physics at all relative to Vacuum Environment concerning action/reaction relative to gas engines?
If the answer is yes, you need some holidays in a quiet place for some time...
Do I Have to Answer That?
Thanks [;D] [;D] I've always wondered that...
Me too. ;)
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Being a rocket scientist (not) I too had some trouble with the concept of how a rocket could work in space, until I was shown the light (and kicked up the bum).
Whether your rocket is in space or in Earh's atmosphere is no different (I think). The thrust produced by the rocket pushes both ways. In Earth's atmosphere, the rocket would be subject to friction, but in space, it would not. So as I understand it, less thrust would be needed in space than in the atmosphere.
So in Earth's atmosphere the thrust would produce (O represents the point of thrust)
<<<<O>>>>
But due to friction would result in
<<<O>>>>>
While in space it would result in
<<<<O>>>>
ie, equal thrust both ways with no friction. Both the gaseous exhast and the rocket move appart at the same force. The only difference is that the point of thrust moves with the rocket and you can see the rocket move, but not the gaseous exhast.
I think that's about right, though my explanation probably leaves much to be desired.
If I am wrong, bugger it.
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Do you really doubt that a rocket could work in the void, I mean, seriosly?
I would to see a scientific stuff (demonstration) about that (however, I'm not the only one).
This would have a meaning if you had never heard of a real rocket going in the void, but you have...
Or you think that some rare atom around could make the difference?
About an experiment, they have already told you: stay in a chair with wheels and little friction with pavement, then throw away something heavy enough: you will go in the opposite direction ("action-reaction" law or "third principle of dynamics"). Where do air enter into the effect?
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If the answer is yes, you need some holidays in a quiet place for some time...
Do I Have to Answer That?
It would help me understand if you are serious or if you are just kidding us.