Does Rocketry Work beyond Earth's atmosphere?

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Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

lux wrote:
Boethius wrote:
In physics work is done only when energy leaves the boundary of a system.
This statement differs greatly from this definition of work:
In physics, a force is said to do work when it acts on a body so that there is a displacement of the point of application, however small, in the direction of the force. Thus a force does work when it results in movement.
Take a charged battery as an example. It is possible for the battery to exert a force without energy leaving the battery?
As per the equation Work = Force x Distance a battery does no work, generates no force until it's energy is passed to some object outside of the battery.

Perhaps it was unclear that I was referring to a closed system in my previous post. A closed system is one where no energy is exchanged with any object outside of itself. A ship in space represents a closed system as it does not interact with space and space does not interact with it nor does a ship in space interact with any field or force. If a ship cannot transfer energy to some object or entity outside of itself, how does it use its energy, potential or kinetic, to do work?

hoi.polloi
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Re: Why Rocketry Doesn't Work in the Vacuum

Boethius wrote:So, looking at two models of rocket thrust
1. If we open the exhaust to let out gas free expansion takes over and no work is done. (NASA model)
2. If we create an imbalance inside the ship's combustion chamber we are in a closed system and no work is done. (Postulated alternative model)

1. No work is done at the time free expansion takes over.
and
2. Imbalance creates vibration that - when settled - results in no work being done.

However, does that still leave open the possibility for:

The front part of the chamber to warp in the opening of a vibration, followed by the free expansion 'take over'?

---

Also, to address the crux of lux's grievance with Boethius' writings, let me try to rephrase the 'problem':

If a balloon is somehow situated in space and its end were opened, it would collapse instantly, all its helium or air being released without any real struggle to do so, and this would theoretically result in no momentum for the balloon. However, if the collapse didn't go perfectly and some part of the balloon had to squirm and squiggle to let the last bit of air out of itself, isn't it likely that the balloon's sudden deformity would cause it to spin, dash or tumble? Nothing is holding the balloon in place so it is free to go nuts. Are we assuming — even given a flawless, precision opening of its end — this would result in a perfect collapse, revealing no imbalance in the rubber skin, in the air itself, etc.?

Similarly, a perfect jet engine that could somehow survive the journey into a pressureless environment, and somehow maintain use of the liquid fuel combustion machine, should be going through some kind of rapid transformation as its fuel were exiting in whatever form.

lux seems to be asking if it is the rapid and violent transformation itself which is propelling the rocket through the air, rather than a build-up of pressure beneath and/or behind the rocket. So, is it?

We are not concerned about the philosophical or scientific idea of whether the subject is using perfect definitions of potential energy, kinetic energy or work. We are concerned with its ability to move from some designated relative point from Earth to a different controlled distant point — point A to point B, so to speak — on a course that its pilot(s) intend(s).

Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

lux wrote:In my quest for the elusive straight answer I will try rephrasing the question:

The title of the thread is: Why Rocketry Doesn't Work in the Vacuum

Now, in view of the fact that ...
1) A vacuum is an absence of atmosphere.
and
2) Rockets do fly when in an atmosphere

My question then becomes: (Assuming the rocket is supplied with oxygen so its fuel can burn) When you remove the atmosphere and thus create a vacuum what are you removing as far as the rocket's ability to move is concerned?
Hello lux, I hope the following answers are straightforward enough for you. Here is what happens to a NASA style rocket powered by liquid propellant when it turns on it's engines in the vacuum of space:

1) The gasses inside the rocket do not to produce a force as they exit
In the vacuum of space the accelerated gasses inside the ship's nozzle are no longer pushed out of the ship. They enter the vacuum via the principle of free expansion (without performing work) and thus do work neither against the rocket itself nor the vacuum of space.
=>No work done by gasses inside of the ship

2) The gasses that leave the rocket do not produce a force once they have exited
In the vacuum of space the gasses that exit the rocket do not accelerate, they move at a constant velocity forever. If an object (including a molecule of gas) travels without accelerating it cannot produce a force. Hence the rocket produces no force.
=>No work done by gasses outside of the ship

I do not see where the force comes from that moves a NASA space rocket.

You have been asking me, I believe, to explain how rockets work in the atmosphere and I continue to say that I do not feel it's worthwhile to investigate the subtleties and nuances of atmospheric flight. The question at hand is,
do rockets produce thrust in space via NASA's theories and equations or is it all part of the same great space hoax?

Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

hoi.polloi wrote:
Boethius wrote:So, looking at two models of rocket thrust
1. If we open the exhaust to let out gas free expansion takes over and no work is done. (NASA model)
2. If we create an imbalance inside the ship's combustion chamber we are in a closed system and no work is done. (Postulated alternative model)

1. No work is done at the time free expansion takes over.
and
Yes, hoi. Also remember that the only work that can possibly be done on the ship is due to the expansion of the gasses as there is no gravity or other force available to accelerate it. If the gas in the ship is not allowed to do any work (prohibited by free expansion) the ship has no source of thrust.
hoi.polloi wrote: 2. Imbalance creates vibration that - when settled - results in no work being done.

However, does that still leave open the possibility for:

The front part of the chamber to warp in the opening of a vibration, followed by the free expansion 'take over'?
The issue is that the ship is a closed system much like a charged battery. There is nowhere for the energy to go, no way for it to do any work until it is connected to something external. The ship is perfectly insulated by space.

Yes, the ship can deform, bend, crinkle, etc... but this will not generate work. Much in the same way that while you cannot lift yourself off the floor by the scruff of your neck, because you and the floor are a closed system under influence of gravity and friction. Still, you can pull out your teeth, poke out your eyes, etc... (you can twist and deform your body but you cannot generate force enough to leave the floor).

Note that you can jump off the floor when you add potential energy by flexing your knees. This amounts to opening the system (interacting with gravity to create more force).
hoi.polloi wrote: Also, to address the crux of lux's grievance with Boethius' writings, let me try to rephrase the 'problem':

If a balloon is somehow situated in space and its end were opened, it would collapse instantly, all its helium or air being released without any real struggle to do so, and this would theoretically result in no momentum for the balloon. However, if the collapse didn't go perfectly and some part of the balloon had to squirm and squiggle to let the last bit of air out of itself, isn't it likely that the balloon's sudden deformity would cause it to spin, dash or tumble? Nothing is holding the balloon in place so it is free to go nuts. Are we assuming — even given a flawless, precision opening of its end — this would result in a perfect collapse, revealing no imbalance in the rubber skin, in the air itself, etc.?
From the experiments I have read about when gas exits via free expansion it does not generate heat, that is the gas molecules do not "fight to get out", they don't bump into each other or push each other out of the way. It is an orderly exit which makes sense if no work is done. Therefore the balloon should collapse without motion.

hoi.polloi wrote: Similarly, a perfect jet engine that could somehow survive the journey into a pressureless environment, and somehow maintain use of the liquid fuel combustion machine, should be going through some kind of rapid transformation as its fuel were exiting in whatever form.

lux seems to be asking if it is the rapid and violent transformation itself which is propelling the rocket through the air, rather than a build-up of pressure beneath and/or behind the rocket. So, is it?
Discussing the mechanics of rocket flight in an atmosphere is a potential quagmire that I see only having value after the question of space flight is more or less concluded one way or the other. The problem is atmospheric flight is experimentally verifiable, well-studied, provable and extremely complicated. Once a discussion starts down this avenue it would be very easy for someone to drag it out forever. Rather than have an army of fluid mechanics/aerodynamic experts come crashing the party, given that these disciplines do not apply to space, lets keep our eyes on space and what actually does/can go on up there.
hoi.polloi wrote: We are not concerned about the philosophical or scientific idea of whether the subject is using perfect definitions of potential energy, kinetic energy or work. We are concerned with its ability to move from some designated relative point from Earth to a different controlled distant point — point A to point B, so to speak — on a course that its pilot(s) intend(s).
Well, I would say the first question is, if a ship is motionless in space, not under the influence of gravity and having no momentum, will NASA rockets allow the ship to move?

If we give the ship some initial momentum we are increasing the problem's complexity. If the rocket doesn't move from a dead stop then what is it, just a big cannonball with lights and sparkles?

lux
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Re: Why Rocketry Doesn't Work in the Vacuum

Boethius wrote:
You have been asking me, I believe, to explain how rockets work in the atmosphere and I continue to say that I do not feel it's worthwhile to investigate the subtleties and nuances of atmospheric flight. [/b]
I asked you for the difference between atmospheric flight and flight in a vacuum. Since we know that rockets fly in an atmosphere and since you are introducing the idea that they won't fly in a vacuum, I am asking you what the specific difference is between these 2 conditions such that the rocket won't fly in a vacuum.

Further, I am not asking for any "subtleties and nuances" of anything. Just a simple straight answer to the question asked.

Since you haven't given a straight answer to any of the questions I have asked you so far I presume this will be the case here so I will answer my own question:

In the case of a rocket flying through the atmosphere with its own supply of oxygen for burning fuel the only atmospheric factor affecting the rocket is air friction slowing the forward speed of the rocket.

The atmosphere provides no other force and is in no way assisting the forward motion of the rocket.

Therefore when this rocket enters a vacuum the only possible result is the removal of air friction thus speeding up the flight of the rocket.
Last edited by lux on Tue May 28, 2013 3:04 pm, edited 2 times in total.

hoi.polloi
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Re: Why Rocketry Doesn't Work in the Vacuum

Here is a post from the ab Irato blog in response to Simon's:
larry wrote:I have been considering joining CF to respond to this thread. The claim that the “free expansion of gas into a vacuum” proves the impossibility of rocket thrust in space is bogus. This model would be valid if the rocket engine was enclosed by a box which is attached to the ass end of the rocket. Obviously wouldn’t work.

In fact, a rocket would work better in no atmosphere. Imagine yourself on roller skates firing a shot gun. What would happen? Everyone would guess (I believe) that you would move in a direction opposite to the shot gun slug flying away. It’s hard to imagine how air pressure or resistance acting on the slug would have any effect on you. What’s happening is that momentum is conserved. Momentum is mass times velocity. When you and the slug were at rest, lets call the total momentum for you and slug both zero. (m1 x v1) + (m2 x v1) = 0 . After the shot gun is fired, the momentum for the slug is m1 x v2, where v2 is the new speed of the slug. For the total momentum of the system to be the same, m2 x v3 = – (m1 x v2), or v3 = – (m1 x v2) / m2, where v3 is the new velocity for the shooter. If you were falling in space, with no atmosphere, v2, the speed of the slug would be greater with no air resistance impeding it, so v3 would also be greater in the opposite direction.

BTW, I don’t believe that there is anything man made functioning in space. As alluded to on CF, the radiation problems are immense. Not to mention that the speed required to stay in orbit is many times greater than the fastest speeding bullet, so hitting a spec of an asteroid, which might be coming from an opposing direction at a comparable speed would be highly problematic for the mylar protection. Another problem I haven’t seen mentioned is that if the atoms that are extant within low earth orbit range are ionized, what would that do to the mylar. Or perhaps the radiation would cause the protective mylar to itself become ionized and then simply dustify into atoms, like the WTC.
I like Simon’s perpetual motion machine. But it wouldn’t work.
Conservation of energy is the “reason”, but I’ll spare you the details.
Boethius, my initial thinking on this comment is that 'larry' is considering the slug to be a valuable ballast, in opposition to being subject to the rule of free expansion. However, the slug is not gas, and you are specifically referring to NASA's gas-based propulsion as being worthless. Since you've already addressed this problem by saying ballast could work, but not gas, does it deserve a response? I am curious what you make of the idea of a rocket that uses a gun firing dense BBs for propulsion.

Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

lux wrote:
Boethius wrote:
You have been asking me, I believe, to explain how rockets work in the atmosphere and I continue to say that I do not feel it's worthwhile to investigate the subtleties and nuances of atmospheric flight. [/b]
I asked you for the difference between atmospheric flight and flight in a vacuum. Since we know that rockets fly in an atmosphere and since you are introducing the idea that they won't fly in a vacuum, I am asking you what the specific difference is between these 2 conditions such that the rocket won't fly in a vacuum.

Further, I am not asking for any "subtleties and nuances" of anything. Just a simple straight answer to the question asked.

Since you haven't given a straight answer to any of the question I have asked you so far I presume this will be the case here so I will answer my own question:

In the case of a rocket flying through the atmosphere with its own supply of oxygen for burning fuel the only atmospheric factor affecting the rocket is air friction slowing the forward speed of the rocket.

The atmosphere provides no other force and is in no way assisting the forward motion of the rocket.

Therefore when this rocket enters a vacuum the only possible result is the removal of air friction thus speeding up the flight of the rocket.

As I posited in my response to you, my research indicates the gasses of a rocket cease to do work in the vacuum of space. The gasses will not produce force while either inside or outside of the ship.

Your example above relates to a rocket that starts out in the atmosphere and ends up in the vacuum of space. What I feel will happen is that once the rocket enters space its engines will stop functioning. It becomes a great big rock, subject to gravity and momentum. It will rise based on its momentum until the force of gravity yanks it back towards earth. I agree that its momentum will not be dampened by air resistance while in the vacuum.

Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

hoi.polloi wrote:Here is a post from the ab Irato blog in response to Simon's:
larry wrote:I have been considering joining CF to respond to this thread. The claim that the “free expansion of gas into a vacuum” proves the impossibility of rocket thrust in space is bogus. This model would be valid if the rocket engine was enclosed by a box which is attached to the ass end of the rocket. Obviously wouldn’t work.

In fact, a rocket would work better in no atmosphere. Imagine yourself on roller skates firing a shot gun. What would happen? Everyone would guess (I believe) that you would move in a direction opposite to the shot gun slug flying away. It’s hard to imagine how air pressure or resistance acting on the slug would have any effect on you. What’s happening is that momentum is conserved. Momentum is mass times velocity. When you and the slug were at rest, lets call the total momentum for you and slug both zero. (m1 x v1) + (m2 x v1) = 0 . After the shot gun is fired, the momentum for the slug is m1 x v2, where v2 is the new speed of the slug. For the total momentum of the system to be the same, m2 x v3 = – (m1 x v2), or v3 = – (m1 x v2) / m2, where v3 is the new velocity for the shooter. If you were falling in space, with no atmosphere, v2, the speed of the slug would be greater with no air resistance impeding it, so v3 would also be greater in the opposite direction.

BTW, I don’t believe that there is anything man made functioning in space. As alluded to on CF, the radiation problems are immense. Not to mention that the speed required to stay in orbit is many times greater than the fastest speeding bullet, so hitting a spec of an asteroid, which might be coming from an opposing direction at a comparable speed would be highly problematic for the mylar protection. Another problem I haven’t seen mentioned is that if the atoms that are extant within low earth orbit range are ionized, what would that do to the mylar. Or perhaps the radiation would cause the protective mylar to itself become ionized and then simply dustify into atoms, like the WTC.
I like Simon’s perpetual motion machine. But it wouldn’t work.
Conservation of energy is the “reason”, but I’ll spare you the details.
Boethius, my initial thinking on this comment is that 'larry' is considering the slug to be a valuable ballast, in opposition to being subject to the rule of free expansion. However, the slug is not gas, and you are specifically referring to NASA's gas-based propulsion as being worthless. Since you've already addressed this problem by saying ballast could work, but not gas, does it deserve a response? I am curious what you make of the idea of a rocket that uses a gun firing dense BBs for propulsion.
Hello hoi,

this post brings up an important difference between how gasses and solids function in space. Gasses, from what I can see, do absolutely no work in space (or any vacuum). Solids, however, continue to produce forces during collisions, etc..

Since I cannot find any evidence of gasses doing work against or within the vacuum of space (or any vacuum) I cannot explain how a gas based rocket would function in space.

I agree that a "shotgun in space" would work fine as long as it fired a slug, because it requires work to push the slug out of the gun as such there will be conservation of momentum, the slug goes one way, the shotgun the other.

Notice that the poster states
larry wrote:Momentum is mass times velocity
whereas the NASA rocket equation states force is mass times velocity.
He's disagreeing with me and he hasn't even bothered to check the NASA equation he is tacitly defending. It is interesting how people who doubt certain NASA claims (Apollo moon landings, Satellites, etc...) will not challenge them on others (the Rocket Thrust equation)

lux
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Re: Why Rocketry Doesn't Work in the Vacuum

Boethius wrote: ... my research indicates the gasses of a rocket cease to do work in the vacuum of space.
But this definition of work states that "a force does work when it results in movement."

The gasses of the rocket engine in space being propelled outward are moving, therefore work is being done.
Boethius wrote:... I cannot explain how a gas based rocket would function in space.
But, isn't the premise of this thread your contention that a gas based rocket would not function in space at all?

hoi.polloi
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Re: Why Rocketry Doesn't Work in the Vacuum

It seems that Boethius's main argument is that:

When a chunk of exploding gas is set right against the body of a rocket, the gas will freely push against the rocket with an infinitesimally light touch versus its completely powerful push against the rest of the gas - because that push against itself is free (instead of costing any effort as it would on Earth - in an atmosphere) compared to the push against the sturdy ship. Therefore, the explosion touches the ship, but does no work.

It seems lux's main counter-argument is that:

When a chunk of exploding gas is set right against the body of a rocket, the gas will push against itself - temporarily ignoring free expansion - because the force of the explosion is so fantastic - and therefore the gas will push against itself to motivate the ship, before dissipating into the void. The atoms bouncing off the ship would do some work by transferring their momentum to the ship, thereby effectively making a thrust.

Which is correct?

Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

lux wrote:
Boethius wrote: ... my research indicates the gasses of a rocket cease to do work in the vacuum of space.
But this definition of work states that "a force does work when it results in movement."

The gasses of the rocket engine in space being propelled outward are moving, therefore work is being done.
The gasses of a rocket are not propelled outwards into the vacuum of space. The enter space by means of free expansion, without doing any work without the application of any force. This is not some fantasy of mine. NASA admits free expansion exists although they keep it buried deep in the sand, as far away from their "rocket theory" as possible.
Boethius wrote:... I cannot explain how a gas based rocket would function in space.
lux wrote: But, isn't the premise of this thread your contention that a gas based rocket would not function in space at all?
Quite so. If I had been presented with a logical explanation of how a gas-based rocket functions in space I would not have started this thread.

While looking for an answer to how rockets work in space, I found:

1. Internet boards making "Newton's 3rd Law is why" claims with the rationale that even an idiot knows this is true. After some research I found Newton's 3rd is inapplicable because gas is not propelled from a rocket into a vacuum due to an effect known as free expansion.

2. The NASA rocket thrust equation based on Newton's 2nd Law which I have shown to be in error as well because force is not equal to mass x velocity (for starters).

3. People applying the properties of solids in a vacuum to gas in a vacuum. Solids do work in a vacuum, when they collide with other solids for example. Solids can have work done on them; you can throw a rock in space. Gas in a vacuum, on the other hand, seems to be incapable of either acting on an object or having an object act on it. For example, you can punch someone in a vacuum but can you "blow them over"?

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Re: Why Rocketry Doesn't Work in the Vacuum

Boethius wrote: ...
There are 4 major ideas on presented on the Internet, including NASA web sites, as to how rockets generate thrust in space
1. Newton’s 3rd Law : for every force there is an equal and opposite
2. Newtons’s 2nd Law : Force = Mass x Acceleration
3. Conservation of Momentum
4. The use of a specialized nozzle to accelerate the gas inside the ship, concentrate and aim the gas jet.

...

The problem with applying Newton’s 3rd is that the rocket’s propellant does not generate force in a vacuum according to the laws of physics and chemistry. If the force of the propellant is 0 then Newton’s 3rd states that
Force on Rocket=-Force of Gas.
If Force of Gas = 0 the rocket does not move.
Evidently a rocket engine produces a force, when its onboard liquid or solid fuel burns to produce a gas/smoke that is ejected through the rocket engine nozzle. It happens in vaccuum, in a gas and in a liquid. Problem is just to find a rocket engine nozzle and fuel that can produce that force to, e.g. stop Apollo 11 to crash on the Moon - http://heiwaco.tripod.com/moontravel.htm#EV5 .

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Re: Why Rocketry Doesn't Work in the Vacuum

*

Lux,

I may be starting to wrap my head around the notion of free expansion - as introduced by Boethius. Of course, I'm no rocket scientist and am only doing my best here to grasp some basic rocket-propulsion principles, so please bear with me as I think aloud - hopefully making some progress along the way. I've now read a fair amount of literature concerning rocket nozzles - and specifically as what concerns their primary design dilemma for operating at various altitudes/air densities. Let me just summarize the little I've read and learned so far - in simple language and graphics:

THE ROCKET NOZZLE QUESTION

Let's look at this fundamental aspect of rocket nozzle design:

"The optimal size of a rocket engine nozzle to be used within the atmosphere is when the exit pressure equals ambient (atmospheric) pressure, which decreases with altitude. For rockets travelling from the Earth to orbit, a simple nozzle design is only optimal at one altitude, losing efficiency and wasting fuel at other altitudes."
: http://en.wikipedia.org/wiki/Rocket_engine_nozzle

Right away, I'd say this sounds awfully problematic... in other words: at only one given altitude" X", a rocket (with a fixed rocket nozzle) performs at maximum efficiency. At ALL other altitudes, the rocket loses efficiency and wastes fuel. (Additionally, it appears that the airflow around a vehicle travelling at Mach speeds also reduces its surrounding external pressure - thus further reducing the thrust of any given jet/or rocket). So what happens between, say 60 and 100km of altitude as a rocket approaches the 'edge of space' (the so-called Kàrmàn line where air density is 2.2million times thinner than at sea level) - while still combating 90% gravity pull? Are rockets still airworthy there? Only NASA knows, I guess.

But let's go to Aerospaceweb and see what they tell us about...

"Nozzle Overexpansion & Underexpansion"
http://www.aerospaceweb.org/question/pr ... 0220.shtml

You may wish to read the above-linked article. Here's the basic problem illustrated by their 3-step diagram:

Now, wouldn't this be the logical progression of the above phenomena?

In other words, wouldn't the rocket plume eventually expand so much as to simply nebulize in all directions, thus ceasing to provide the necessary thrust/force to counter the pull of gravity? (This, of course, unless you believe that beyond a 'certain altitude', gravity ceases to be a force - and the spaceship gets 'flung' by its sheer momentum into 'free-fall' orbit...)

Lastly, you may ask, what type of rocket nozzle is used on modern spacecraft? Amazingly, it seems that the old De Laval design (1888 !) is still very much the (fixed)rocket nozzle widely used today... so much for technical innovation, NASA!
"Very nearly all modern rocket engines that employ hot gas combustion use de Laval nozzles." http://en.wikipedia.org/wiki/De_Laval_nozzle

"Rocket Nozzle Design: Optimizing Expansion for Maximum Thrust" http://www.braeunig.us/space/sup1.htm
"ROCKET PROPULSION": http://www.braeunig.us/space/propuls.htm
"Rocket engine nozzle" http://en.wikipedia.org/wiki/Rocket_engine_nozzle
"Rocket engine" http://en.wikipedia.org/wiki/Rocket_engine
.

Boethius
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Re: Why Rocketry Doesn't Work in the Vacuum

hoi.polloi wrote:It seems that Boethius's main argument is that:

When a chunk of exploding gas is set right against the body of a rocket, the gas will freely push against the rocket with an infinitesimally light touch versus its completely powerful push against the rest of the gas - because that push against itself is free (instead of costing any effort as it would on Earth - in an atmosphere) compared to the push against the sturdy ship. Therefore, the explosion touches the ship, but does no work.

It seems lux's main counter-argument is that:

When a chunk of exploding gas is set right against the body of a rocket, the gas will push against itself - temporarily ignoring free expansion - because the force of the explosion is so fantastic - and therefore the gas will push against itself to motivate the ship, before dissipating into the void. The atoms bouncing off the ship would do some work by transferring their momentum to the ship, thereby effectively making a thrust.

Which is correct?
Hello, hoi, those are not my claims at all.

Based on your comments I believe this is a more accurate representation of what I theorize to be true:

1. When a rocket's combustion chamber is filled with accelerated gas opening the nozzle to expel the gasses into the vacuum of space does not generate a force against the ship. This is due to the principle of free expansion.

2. No amount of combustion or pressure inside the space ship can move the ship until that combustive force or pressure is exchanged with some object, entity, or field outside of the ship (a space ship is a closed system).

3. Based on 1 and 2 there is no way to move the ship by releasing gas and no way to move the ship by keeping the gas inside. A space ship cannot generate force with a gas based propulsion system. Space rockets are the stuff of fantasies not science or physics.

lux
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Re: Why Rocketry Doesn't Work in the Vacuum

I see what you're saying, Simon, about the nozzle operating at lower atmospheric psi and it does seem like a good point (providing, of course, that it's an accurate representation of real world performance).

But, I would think that for rockets designed to fly in space a modification of the nozzle design would minimize that effect perhaps. I don't really know -- I'm just guessing here as we all are.

My point about rocket thrust is that the burning of fuel inside the rocket produces pressures in all directions. All of these pressures are countered by pressures in opposite directions except the forward pressures which are not countered due to the escaping gasses to the rear. The resulting force vector then will be one in a forward direction and the rocket moves in that direction. The "free expansion" bit at the tail would have no effect on this model. As I think hoi worded it, there would be an imbalance of forces and forward movement would result.