Why Rocketry Doesn't Work in the Vacuum
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Sooner Boomer
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Not all rocket motors have nozzles. The "whistle rocket" for 4th of July is one example.
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The point is that the expanding gases in a rocket motor are not expanding freely into a vacuum. The nozzle - or whatever you want to call the opening on one end of the motor- causes an unbalanced pressure inside the combustion chamber which provides thrust.
-- Roger
maskedwarrior
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Thank you Jadebox and Mushtang, your replies are very kind. And Zebedee, I'm not sure to whom you are referring as 'trolls' but please rest assured...
I believe we can power rockets in space! I might not understand the explanations but it doubtless goes on!! At the very least there's far, far too much day-to-day evidence there are satellites above our heads to start implying there's an elaborate conspiracy that began 70 years ago and has been upheld unwaveringly ever since! So in the spirit of forsaking all forms of bigotry, dismissiveness and plain paranoia, let me reassure you I am a pure-intentioned human being. That is all.
And 'Yeah but...' is what the Student asks the Teacher, surely? Are school kids trolls? Forgive me if I find your reasoning faulty on that one!
And one more thing, I'm not issuing a challenge here by playing devil's advocate or by asking questions and demanding answers. I think to barge in here and do that is the height of rudeness! That you're sparing me any time at all I am more than grateful for, thank you!!
I will play devil's advocate a little stronger here to help explain in a really clear way where I'm failing to understand. But once again, I'm not a moon hoax crony!!! I'm just a curious layman who wants to test your statements and learn!!
So, just to quest a bit further: what seems to allow speculation to endure in certain quarters, like Clues Forum et al, might actually lead on from your answers....
In answer to this, the principle of 'equal and opposite reaction' is often evoked on certain threads. Here's the sort of thing they say. Doubtless it's very easily dismantled, but I'll present the case.
The principle of 'equal and opposite reaction' surely means that at the point of 'recoil' a division of energy occurs, and this is a very definite and important line. Two energetic forces mutually oppose each other at this line, each with their own momentum, each going its separate way. Complete opposites. Meaning, at the exact point and at the exact moment I let loose the bowling ball it goes its way, and me and my skateboard go mine.
A note about the 'Nozzle': Surely, since the nozzle is there to prevent Free Expansion, as gas leaves the rocket nozzle into space it will Free Expand and do no more work? So no gains from the nozzle end?! The moment of gas recoil can be the only inertial force under Newton's Third Law.
The fact we're talking about recoil happening inside the rocket's combustion chamber before leaving through the nozzle (which by definition is a restrictive structure) to me implies the fulcrum of the 'equal and opposite reaction' is happening 'inside' the rocket.... But surely, after the point of recoil, any impediment of the gas's momentum (ie. by the restrictive nozzle) will be absorbed by the rocket chassis itself, and all benefits (ie avoiding Free Expansion) will cancel themselves out in the F1 = -F2 equation??
If, after the point of recoil, the rocket absorbs any of the gas's momentum this will make a 'closed system' and will immediately cancel out.
And I would argue that this will work on an inverse matching scale. ie. ANY amount of impediment to gas momentum by the rocket chassis, big or small, will likewise cancel out. On my skateboard, if I throw the bowling ball but it brushes my trailing foot as I travel on my way, the contradicting momentum will negatively impact my progress. If I brush the ball lightly it will slightly effect it. If I brush the ball heavily I may make no progress at all.
Likewise, if I blank off the barrel of the shotgun, the recoil will be immediately absorbed and cancelled out upon the round and gasses hitting the obstruction in the barrel mouth. The gun will be a 'closed system' and the momentum in both directions will be cancelled out(and the gun barrel will probably explode!). If I restrict the bullet's progress with a nozzle the same could be said, to an inversely proportional degree.
So, if the rocket nozzle is indeed required to stop the exhaust gas Free Expanding in space this, by definition, must be restricting the gasses progress. But any work done to stop the gas Expanding made by the rocket itself would absorb the gas's momentum into the chasis and be immediately subtracted from the system, under Newton's Third Law.
Only the total momentum of the gas at the point of recoil, minus all subtractions due to impediment by the rocket chassis causing a 'closed system', could be said to produce force and movement under Newton's 3rd law. And since all work done to stop gas Free Expanding is subtractable from the equation F1 = -F2..... then aren't we left with free expanding gas in a vacuum with a force equal to zero???
That's the gist of the counterargument. How would you respond to that?? Thanks very much. Tony
I believe we can power rockets in space! I might not understand the explanations but it doubtless goes on!! At the very least there's far, far too much day-to-day evidence there are satellites above our heads to start implying there's an elaborate conspiracy that began 70 years ago and has been upheld unwaveringly ever since! So in the spirit of forsaking all forms of bigotry, dismissiveness and plain paranoia, let me reassure you I am a pure-intentioned human being. That is all.
And 'Yeah but...' is what the Student asks the Teacher, surely? Are school kids trolls? Forgive me if I find your reasoning faulty on that one!
And one more thing, I'm not issuing a challenge here by playing devil's advocate or by asking questions and demanding answers. I think to barge in here and do that is the height of rudeness! That you're sparing me any time at all I am more than grateful for, thank you!!
I will play devil's advocate a little stronger here to help explain in a really clear way where I'm failing to understand. But once again, I'm not a moon hoax crony!!! I'm just a curious layman who wants to test your statements and learn!!
So, just to quest a bit further: what seems to allow speculation to endure in certain quarters, like Clues Forum et al, might actually lead on from your answers....
In answer to this, the principle of 'equal and opposite reaction' is often evoked on certain threads. Here's the sort of thing they say. Doubtless it's very easily dismantled, but I'll present the case.
The principle of 'equal and opposite reaction' surely means that at the point of 'recoil' a division of energy occurs, and this is a very definite and important line. Two energetic forces mutually oppose each other at this line, each with their own momentum, each going its separate way. Complete opposites. Meaning, at the exact point and at the exact moment I let loose the bowling ball it goes its way, and me and my skateboard go mine.
A note about the 'Nozzle': Surely, since the nozzle is there to prevent Free Expansion, as gas leaves the rocket nozzle into space it will Free Expand and do no more work? So no gains from the nozzle end?! The moment of gas recoil can be the only inertial force under Newton's Third Law.
The fact we're talking about recoil happening inside the rocket's combustion chamber before leaving through the nozzle (which by definition is a restrictive structure) to me implies the fulcrum of the 'equal and opposite reaction' is happening 'inside' the rocket.... But surely, after the point of recoil, any impediment of the gas's momentum (ie. by the restrictive nozzle) will be absorbed by the rocket chassis itself, and all benefits (ie avoiding Free Expansion) will cancel themselves out in the F1 = -F2 equation??
If, after the point of recoil, the rocket absorbs any of the gas's momentum this will make a 'closed system' and will immediately cancel out.
And I would argue that this will work on an inverse matching scale. ie. ANY amount of impediment to gas momentum by the rocket chassis, big or small, will likewise cancel out. On my skateboard, if I throw the bowling ball but it brushes my trailing foot as I travel on my way, the contradicting momentum will negatively impact my progress. If I brush the ball lightly it will slightly effect it. If I brush the ball heavily I may make no progress at all.
Likewise, if I blank off the barrel of the shotgun, the recoil will be immediately absorbed and cancelled out upon the round and gasses hitting the obstruction in the barrel mouth. The gun will be a 'closed system' and the momentum in both directions will be cancelled out(and the gun barrel will probably explode!). If I restrict the bullet's progress with a nozzle the same could be said, to an inversely proportional degree.
So, if the rocket nozzle is indeed required to stop the exhaust gas Free Expanding in space this, by definition, must be restricting the gasses progress. But any work done to stop the gas Expanding made by the rocket itself would absorb the gas's momentum into the chasis and be immediately subtracted from the system, under Newton's Third Law.
Only the total momentum of the gas at the point of recoil, minus all subtractions due to impediment by the rocket chassis causing a 'closed system', could be said to produce force and movement under Newton's 3rd law. And since all work done to stop gas Free Expanding is subtractable from the equation F1 = -F2..... then aren't we left with free expanding gas in a vacuum with a force equal to zero???
That's the gist of the counterargument. How would you respond to that?? Thanks very much. Tony
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Rocket engine combustion happens inside the chamber, which has one side open. It's not a closed chamber. The combustion sends gasses in all directions which put a force on anything they hit. Some go left, or right, or forwards, or backwards, etc. The left and right forces do cancel out because they're pushing the chamber in opposite directions, but those particles are still bouncing around in the chamber and will eventually assist in thrust. The combustion gasses that go forwards push on the rocket but they have no opposite force to cancel them out because the back of the chamber is open. So the recoil happens in the forwards direction.
Nope, all the momentum isn't absorbed, it's not a closed system. The amount of momentum leaving the open end of the rocket is equal to the momentum absorbed by the rocket. The particles leaving the back have very little mass but a HUGE velocity. Each particle adds a tiny velocity to the HUGE mass (by comparison) of the rocket. There is so much mass leaving the back of the rocket a enormous velocities that the rocket gains momentum.
Your confusion is thinking that the impediment to gas momentum in a rocket is cancelled out. It's not.
The rocket nozzle is used to direct the exhaust gasses in the proper direction, and pick up maximum momentum from these gasses. It's not "required to stop the exhaust gas Free Expanding in space". Once the gasses leave the engine bell they're no longer helpful to the rocket and they can expand into space. Or not, it doesn't matter.
It's not a closed system, and the sum of all forces on the rocket are not equal to zero.
However, as I wrote that previous sentence I started to get a deeper understanding of why you think F1 = -F2, what you mean by "closed system" and therefore a rocket can't move. If you want the "closed system" to include the rocket and all gasses then it's true that the opposite forces cancel out and the system as a whole doesn't move. Some of the system goes one way, some of the system goes another way, but the average of all the masses stays in the same place.
If your bowling ball hits your leg, and therefore you can't throw it, you won't move - just like you said. If you tie a rope to the bowling ball and your leg, then throw the bowling ball, you will move in the opposite direction for a little while. When the rope pulls tight the rope will stop both you and the bowling ball. Still the same effect, it's just a larger system. With no rope you don't get stopped by the bowling ball. Is this still a "closed system". If you consider the ball's momentum along with yours then it can be. Once you throw the ball, without the rope, and you're moving away, nothing that happens to that ball will change the fact that you're moving. So the ball can hit a wall, explode, be pulled away by something (Free Expansion), whatever, and you will still have gained momentum from it and will be moving.
maskedwarrior
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Oh I see. Hmmm perhaps I've been assuming some things are a 'given' when perhaps they aren't... leading to some confusion.
It all seems to hang on what Free Expansion actually is! I know Free Expansion is a law of gas dynamics. It was not formulated by Newton (who didn't work with gas, apparently) but by later scientists who formulated this and other energy equations about gas, which are now accepted as principles of science.
https://en.wikipedia.org/wiki/Free_expansion
The principal of Free Expansion, from what I gather, essentially causes gas to act in a specific way - a way distinct from solid objects like a bowling ball or a bullet from a gun. The properties of gas depend on pressure to such an extent that it behaves in a very special way in a zero pressure environment. It loses its physical properties in a pretty fundamental way (Illustrated, perhaps, by the fact all gas energy equations use Pressure in their calculations). To quote Wiki on this: 'During free expansion, no work is done by the gas.'
But in reality what is this effect?? And is it a significant factor in space propulsion at all?
It is suggested on Clues Forum, by a contributor known as Botheus, that Free Expansion stops gas doing any Work in space - because space is a vacuum. And the properties of gas under Free Expansion in a vacuum preclude gas being propelled from the rocket with any Force.
Here's my interpretation of the argument: Gas under Free Expansion in a vacuum disperses to try and equalize the pressure differential between the rocket and the void beyond. And it does this pretty darned quickly. The rocket can't 'push' the atoms with any force great enough to counteract this tendency of gas to Free Expand. The gas molecules cannot be moved by the rocket's propulsion and thus cannot create an equal and opposite reaction to make the rocket move forward. The gas is moved, not by the rocket, but under its own impetus in an attempt to equalize the pressure difference.
So, goes the argument, since the rocket does not propel the gas there is no exchange of mass, and Newton's Third Law doesn't apply. Hence the rocket can't move!
I've heard the 'nozzle' argument used to explain why this doesn't happen. Hence my argument above re. nozzles, as I can see a hole in this reasoning. Please note also, when I was referring to 'closed system' and 'energy being cancelled out' in my previous post I was trying to point out my perceived flaw in this argument. I didn't mean to suggest that ALL energy was cancelled out of the equation, just whatever momentum was absorbed by the rocket to try and counteract Free Expansion. ie. IN THEORY: If gas was leaving the rocket with a Force of 0, any interaction with the rocket bell housing to counteract this Free Expansion effect would be absorbed by the chassis and would need to be deducted from the 'third law equation, in order to make it balance, and thus you'd still have a force of 0 in your calculations. The rocket can't provide its own 'atmosphere' into which to propel the gas - immune from free expansion - as this is akin to creating a self peddling bicycle or similar.
However maybe there's been some confusion (from me and perhaps between various parties) about just what free expansion is and what significance it plays in a real world scenario. So, what are the physical properties of Free Expansion??
The sole problems I have wrapping my head round this are as follows:
1. How fast do gas molecules in a vacuum physically move under the effect of Free Expansion?
2. How fast do gas molecules leave a rocket, under the impetus of the combustion explosion in a vacuum??
3. If the rocket can push gas molecules faster than free expansion moves them, what impact does free expansion have on the force exerted by the rocket in a vacuum??
4. If Free Expansion moves gas molecules faster than the rocket can propel them would this mean the rocket can't 'push against itself' and can't actually move forward??
A clear explanation of this physical phenomenon is proving hard to come by! And this is what's baking my noodle, at the moment!!
Do rockets need to overcome Free Expansion in space? If not, why? If so, how??
Thank you so much for your patience here. Any thoughts or links would be extremely well received.
Tony
It all seems to hang on what Free Expansion actually is! I know Free Expansion is a law of gas dynamics. It was not formulated by Newton (who didn't work with gas, apparently) but by later scientists who formulated this and other energy equations about gas, which are now accepted as principles of science.
https://en.wikipedia.org/wiki/Free_expansion
The principal of Free Expansion, from what I gather, essentially causes gas to act in a specific way - a way distinct from solid objects like a bowling ball or a bullet from a gun. The properties of gas depend on pressure to such an extent that it behaves in a very special way in a zero pressure environment. It loses its physical properties in a pretty fundamental way (Illustrated, perhaps, by the fact all gas energy equations use Pressure in their calculations). To quote Wiki on this: 'During free expansion, no work is done by the gas.'
But in reality what is this effect?? And is it a significant factor in space propulsion at all?
It is suggested on Clues Forum, by a contributor known as Botheus, that Free Expansion stops gas doing any Work in space - because space is a vacuum. And the properties of gas under Free Expansion in a vacuum preclude gas being propelled from the rocket with any Force.
Here's my interpretation of the argument: Gas under Free Expansion in a vacuum disperses to try and equalize the pressure differential between the rocket and the void beyond. And it does this pretty darned quickly. The rocket can't 'push' the atoms with any force great enough to counteract this tendency of gas to Free Expand. The gas molecules cannot be moved by the rocket's propulsion and thus cannot create an equal and opposite reaction to make the rocket move forward. The gas is moved, not by the rocket, but under its own impetus in an attempt to equalize the pressure difference.
So, goes the argument, since the rocket does not propel the gas there is no exchange of mass, and Newton's Third Law doesn't apply. Hence the rocket can't move!
I've heard the 'nozzle' argument used to explain why this doesn't happen. Hence my argument above re. nozzles, as I can see a hole in this reasoning. Please note also, when I was referring to 'closed system' and 'energy being cancelled out' in my previous post I was trying to point out my perceived flaw in this argument. I didn't mean to suggest that ALL energy was cancelled out of the equation, just whatever momentum was absorbed by the rocket to try and counteract Free Expansion. ie. IN THEORY: If gas was leaving the rocket with a Force of 0, any interaction with the rocket bell housing to counteract this Free Expansion effect would be absorbed by the chassis and would need to be deducted from the 'third law equation, in order to make it balance, and thus you'd still have a force of 0 in your calculations. The rocket can't provide its own 'atmosphere' into which to propel the gas - immune from free expansion - as this is akin to creating a self peddling bicycle or similar.
However maybe there's been some confusion (from me and perhaps between various parties) about just what free expansion is and what significance it plays in a real world scenario. So, what are the physical properties of Free Expansion??
The sole problems I have wrapping my head round this are as follows:
1. How fast do gas molecules in a vacuum physically move under the effect of Free Expansion?
2. How fast do gas molecules leave a rocket, under the impetus of the combustion explosion in a vacuum??
3. If the rocket can push gas molecules faster than free expansion moves them, what impact does free expansion have on the force exerted by the rocket in a vacuum??
4. If Free Expansion moves gas molecules faster than the rocket can propel them would this mean the rocket can't 'push against itself' and can't actually move forward??
A clear explanation of this physical phenomenon is proving hard to come by! And this is what's baking my noodle, at the moment!!
Do rockets need to overcome Free Expansion in space? If not, why? If so, how??
Thank you so much for your patience here. Any thoughts or links would be extremely well received.
Tony
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I actually tried to get into the forum mentioned to discuss this, never got a reply that allowed me in (so I don't think it really IS an open discussion on that forum, as opposed to this one and presumably most forums.)
A lot of the arguments made on that forum do indeed "seem" logical. I had two semesters of college physics at a fairly decent school (USAFA) and some engineering courses, and to be honest I was a bit bamboozled too.
couple simple things that may help straighten it out.
Get a garden hose and a big bucket of water (a pool or lake would be better.)
Put a nozzle on the hose and turn the water on full and feel the force the exiting water generates while you are holding the hose above your bucket/pool. The article would tell you that force is generated (or accentuated) because of the air around you. Seems logical at first. Now take that hose and stick it in the bucket or lake. Now your exiting water has a much denser substance to "push" against. Thus by the web site's logic, the force should be MUCH greater. It's not. Not only is it no greater, it is LESS. Reason is the velocity of the exiting water decreases due to drag. I really do encourage you to actually try this, I did.
The try to make the same argument for a jet ski, but they are still wrong. A jet ski goes faster when the jet directed posteriorly into the air rather than the denser water, and for exactly the same reason.
The free expansion of a gas argument is also misapplied. All the rules about free expansion of a gas are indeed true, in a CLOSED SYSTEM. For all practical purposes, space is the ultimate UNCLOSED system.
If a guy in space has wrench attached to the a cord, and he heaves it, he will travel in one direction and the wrench the other, until the cord stops both. If it is not elastic, both spaceman and wrench will then stop in space. That's essentially a closed system. The center of gravity of the combined spaceman and wrench will not change. If there is no cord, the spaceman keeps going in one direction, the wrench in the other. The center of gravity of the two combined STILL doesn't change, but both parties keep going in opposite directions. That's an OPEN system.
Rockets in space are the same principle. Pressure in motor and nozzle vector high velocity mass in one direction. That gas will expand in ALL directions (I guess you could call if "free" expansion) but it STILL has a NET velocity in one direction (yes, the cloud is expanding, but the CENTER of gravity of the cloud IS indeeed moving away from the nozzle--- there is no force in space to simply make it stop.) Newton's law states that if that cloud (which still has mass, even if it's gas) is going one way, there has to be an equally and accountable mass x velocity product (in this case our spacecraft) going the other way.
Hope this helps, and perhaps helps to know that other people like me (who should probably know better) struggled a bit with the web site's articles at first.
A lot of the arguments made on that forum do indeed "seem" logical. I had two semesters of college physics at a fairly decent school (USAFA) and some engineering courses, and to be honest I was a bit bamboozled too.
couple simple things that may help straighten it out.
Get a garden hose and a big bucket of water (a pool or lake would be better.)
Put a nozzle on the hose and turn the water on full and feel the force the exiting water generates while you are holding the hose above your bucket/pool. The article would tell you that force is generated (or accentuated) because of the air around you. Seems logical at first. Now take that hose and stick it in the bucket or lake. Now your exiting water has a much denser substance to "push" against. Thus by the web site's logic, the force should be MUCH greater. It's not. Not only is it no greater, it is LESS. Reason is the velocity of the exiting water decreases due to drag. I really do encourage you to actually try this, I did.
The try to make the same argument for a jet ski, but they are still wrong. A jet ski goes faster when the jet directed posteriorly into the air rather than the denser water, and for exactly the same reason.
The free expansion of a gas argument is also misapplied. All the rules about free expansion of a gas are indeed true, in a CLOSED SYSTEM. For all practical purposes, space is the ultimate UNCLOSED system.
If a guy in space has wrench attached to the a cord, and he heaves it, he will travel in one direction and the wrench the other, until the cord stops both. If it is not elastic, both spaceman and wrench will then stop in space. That's essentially a closed system. The center of gravity of the combined spaceman and wrench will not change. If there is no cord, the spaceman keeps going in one direction, the wrench in the other. The center of gravity of the two combined STILL doesn't change, but both parties keep going in opposite directions. That's an OPEN system.
Rockets in space are the same principle. Pressure in motor and nozzle vector high velocity mass in one direction. That gas will expand in ALL directions (I guess you could call if "free" expansion) but it STILL has a NET velocity in one direction (yes, the cloud is expanding, but the CENTER of gravity of the cloud IS indeeed moving away from the nozzle--- there is no force in space to simply make it stop.) Newton's law states that if that cloud (which still has mass, even if it's gas) is going one way, there has to be an equally and accountable mass x velocity product (in this case our spacecraft) going the other way.
Hope this helps, and perhaps helps to know that other people like me (who should probably know better) struggled a bit with the web site's articles at first.
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An analogy (not perfect, but ...) ...
Free expansion ... open a can of an uncarbonated beverage like Minute Maid Lemonade. Click.
Rocket Motor ... shake up a can of a carbonated soda like Diet Coke then open it. Click ... WHOOOOOOOOSH
Free expansion ... open a can of an uncarbonated beverage like Minute Maid Lemonade. Click.
Rocket Motor ... shake up a can of a carbonated soda like Diet Coke then open it. Click ... WHOOOOOOOOSH
ThirstyBarbarian
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Yes, the confusion is over what Free Expansion is. Free expansion is a description of a specific idealized condition that is useful for illustrating the properties of gases. People get hung up on the sentence you quoted above, "During free expansion, no work is done by the gas." That sentence is meant to describe the specific idealized condition of free expansion under which all the free expansion formulas will work. It is not meant to say that the expansion of gases into a vacuum cannot be harnessed for work. It is meant to say that if the expansion of gasses into a vacuum are harnessed for work, then the conditions for free expansion are not met, and the equations for free expansion do not apply.
The whole point of the design of a rocket motor is to harness the expansion of gases for as much work as possible, so free expansion does not apply.
In your post, you talk about an "effect of free expansion" and a "principle of free expansion." There is no such thing. Free expansion is a specific theoretical condition, not a principle or effect that applies any time gases expand into a vacuum.
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Let's imagine that there is a condition called Disappearing Bowling Ball (DBB) where any bowling ball that isn't touched by human hands immediately ceases to exist. This is similar to what you have in your mind as Free Expansion - that gasses from a rocket engine disperses so quickly in the ultimate vacuum of space that they are pulled away from the rocket and are of no use, and that no work is done by the gasses during this expansion and therefor cannot do anything to help the rocket move. So let's say that DBB is the same and that as soon as you let go of it the ball is gone, so quickly that from that point in time you're unable to use it.
I put you on your skateboard. You throw the ball in such a way that you'd expect the skateboard to roll in the opposite direction (assuming it doesn't hit your leg or any other issues). While you're swinging the ball the skate board starts to move, then you throw the ball away. Right before it disappears you've given it some Velocity in one direction and yourself Velocity in the opposite. As soon as you let go DBB takes it away from the system and yet you're still rolling backwards.
This happens because the momentum is taken before you let go, and once you let go of that ball nothing that happens to it (DBB or just rolling away) will change what you do.
Rockets gain momentum (quite a bit of velocity) from the exhaust gasses before they're released into the vacuum of space. Once they are released you can assume that Free Expansion whisks them away and no further work can be taken from them if you want, but that doesn't mean that it wasn't possible to use them to move the rocket.
I put you on your skateboard. You throw the ball in such a way that you'd expect the skateboard to roll in the opposite direction (assuming it doesn't hit your leg or any other issues). While you're swinging the ball the skate board starts to move, then you throw the ball away. Right before it disappears you've given it some Velocity in one direction and yourself Velocity in the opposite. As soon as you let go DBB takes it away from the system and yet you're still rolling backwards.
This happens because the momentum is taken before you let go, and once you let go of that ball nothing that happens to it (DBB or just rolling away) will change what you do.
Rockets gain momentum (quite a bit of velocity) from the exhaust gasses before they're released into the vacuum of space. Once they are released you can assume that Free Expansion whisks them away and no further work can be taken from them if you want, but that doesn't mean that it wasn't possible to use them to move the rocket.
ThirstyBarbarian
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We shouldn't even use the term Free Expansion unless we are talking about the specific scenario of a Free Expansion experiment or theoretical scenario. The term Free Expansion does not refer to just any old situation where gas is expanding in a vacuum. It is a specific case in which gas is expanding from one volume into a larger volume, and no energy is allowed to enter or leave the chamber. (It is in fact impossible in the real world to create a Free Expansion scenario. No chamber is perfectly insulated. Energy will always flow into or out of the chamber.)
A lot of our technology is based on extracting power from the expansion of gases --- internal combustion engines, gas turbines, and rocket engines. They all exploit the fact that gases under pressure will expand to fill a larger volume at lower pressure, and you can harness the movement of that gas to do work --- you can pull energy out of the system. In a free expansion experiment, you would not be allowed to pull energy out of the system, but in reality we do it all the time.
Imagine an internal combustion engine with the tailpipe in a vacuum. It would still function pretty much the same way it would normally with atmospheric pressure, right? The fact that the exhaust expands into a vacuum does not mean it can't do work.
Imagine a gas turbine with a vacuum on the exhaust side. It still works pretty much the same as it does at sea level.
Same thing for a rocket motor. Just because there is vacuum on the exhaust side does not mean it suddenly stops working. It works pretty much the same as it does in the atmosphere.
A lot of our technology is based on extracting power from the expansion of gases --- internal combustion engines, gas turbines, and rocket engines. They all exploit the fact that gases under pressure will expand to fill a larger volume at lower pressure, and you can harness the movement of that gas to do work --- you can pull energy out of the system. In a free expansion experiment, you would not be allowed to pull energy out of the system, but in reality we do it all the time.
Imagine an internal combustion engine with the tailpipe in a vacuum. It would still function pretty much the same way it would normally with atmospheric pressure, right? The fact that the exhaust expands into a vacuum does not mean it can't do work.
Imagine a gas turbine with a vacuum on the exhaust side. It still works pretty much the same as it does at sea level.
Same thing for a rocket motor. Just because there is vacuum on the exhaust side does not mean it suddenly stops working. It works pretty much the same as it does in the atmosphere.
Thirsty is exactly right. Free expansion is a specific condition used in textbooks to create homework problems and text questions that are simple and straightforward to solve without unnecessary complicating factors. In real life, free expansion doesn't happen. Even the gasses that are expelled from a rocket engine (with a bell or without) are still capable of doing work. Even though they are a gas and not a solid (assuming that they didn't freeze into crystals) they still have mass and they still have an imparted velocity through space. If they were to, at some future time, impact another object (gas molecule, asteroid, space station, or whatever) they would transfer that momentum onto whatever object that they impact. Even in space, even in a vacuum, the conservation of energy and the conservation of momentum is maintained.
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