Does Rocketry Work beyond Earth's atmosphere?

[hoi.polloi]

Here is a post from the ab Irato blog in response to Simon's:

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]

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]

Here is a post from the ab Irato blog in response to Simon's:

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

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]

... 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.

... 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]

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]

... 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.

... 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?

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"?

[Heiwa]

...
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 .

[simonshack]

*

THE ROCKET NOZZLE QUESTION



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:

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 will happens at, say, 100km of altitude (the so-called Kàrmàn line where air density is 2.2million times(!!!) thinner than at sea level) as a rocket approaches the 'edge of space' - while still combating 90% gravity pull? Are rockets still airworthy at such altitudes - and beyond? Will they keep escaping gravity and keep ascending in the skies - in spite of the rapidly waning atmosphere?

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

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

Excerpt from above webpage: "The opposite situation, in which the atmospheric pressure is lower than the exit pressure, is called underexpanded. In this case, the flow continues to expand outward after it has exited the nozzle. This behavior also reduces efficiency because that external expansion does not exert any force on the nozzle wall. This energy can therefore not be converted into thrust and is lost." (my bolds)

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



...

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

<---- Rocket exhaust expands at "360°" (in all directions)


In other words, wouldn't the rocket's exhausts eventually expand so much as to simply nebulize in all directions (much like a gardenhose's fully "expanded" sprinkler that will hardly impart any noticeable 'recoil jolt' to your arm) and thus ceasing to provide the necessary thrust/force to counter the pull of gravity?

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


Links for reference:
"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]

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]

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.

[hoi.polloi]

If an explosion worked — on the outside of the ship — what would prevent the atoms from the explosion, which did hit the ship, from transferring momentum to the ship?

I am not saying it's easy to make this happen, but then we are talking about propulsion from explosions rather than ejection.

[brianv]

http://tvtropes.org/pmwiki/pmwiki.php/M ... onsInSpace

I have to say I'm in the Boethius camp, always have been on this one. Space is defined as a 'near perfect vacuum', a void. One atom of hydrogen per cubic meter is being generous. There is simply nothing to push against to cause forward motion. The reason rockets work on earth is because the ground pushes them up. The harder you push against the ground the harder is pushes back. For every action there is an equal and opposite reaction. And as a side note, EV at the earth's surface is approximately 25,000mph, so the hypothetical rocket must generate that much thrust on take off to leave the earth's gravitational pull -- as I see it.

When lux was sitting in his car and he pushed the steering wheel or dash, did the car move? The harder he pushed the faster it went? He may have stood outside the car and pushed the dash, as we all have after a breakdown. The energy being transferred from the ground through our legs. No such thing in space.

If a nuclear weapon is exploded in a vacuum-i. e., in space-the complexion of weapon effects changes drastically:

First, in the absence of an atmosphere, blast disappears completely.

http://history.nasa.gov/conghand/nuclear.htm

YahooAnswers

http://answers.yahoo.com/question/index ... 515AAxxx2n

Rockets 'push against' the rocket exhaust to move in a vacuum.

Good grief "Bicycle pedals itself".

PS I'm also greatly puzzled as to how pressurised tin can's can exist in space. My senses recreates the 'balloon in thin-air" scenario, where it becomes stretched and disintegrates, but that's for another day!

[simonshack]

If an explosion worked — on the outside of the ship — what would prevent the atoms from the explosion, which did hit the ship, from transferring momentum to the ship?

I am not saying it's easy to make this happen, but then we are talking about propulsion from explosions rather than ejection.

Hoi...

The NASA kooks have thought about this before - you're underestimating the extent of their fervid insane imagination!

Project Orion (nuclear propulsion)

"Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft (nuclear pulse propulsion). Early versions of this vehicle were proposed to have taken off from the ground with significant associated nuclear fallout; later versions were presented for use only in space."

"Supporters of Project Orion felt that it had potential for cheap interplanetary travel, but it lost political approval over concerns with fallout from its propulsion."

http://en.wikipedia.org/wiki/Project_Or ... pulsion%29

[Boethius]

...
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 .

Heiwa, are you saying we can burn fuel in the vacuum of space without the benefit of an atmosphere or gravitational field like we have on the earth or the sun? Even if we bring along our own liquid oxygen I don't see how we can burn anything in the vacuum of space except inside of a closed container. Having an open nozzle would violate the integrity of our closed container.

[Boethius]

http://tvtropes.org/pmwiki/pmwiki.php/M ... onsInSpace

I have to say I'm in the Boethius camp, always have been on this one. Space is defined as a 'near perfect vacuum', a void. One atom of hydrogen per cubic meter is being generous. There is simply nothing to push against to cause forward motion. The reason rockets work on earth is because the ground pushes them up. The harder you push against the ground the harder is pushes back. For every action there is an equal and opposite reaction. And as a side note, EV at the earth's surface is approximately 25,000mph, so the hypothetical rocket must generate that much thrust on take off to leave the earth's gravitational pull -- as I see it.

When lux was sitting in his car and he pushed the steering wheel or dash, did the car move? The harder he pushed the faster it went? He may have stood outside the car and pushed the dash, as we all have after a breakdown. The energy being transferred from the ground through our legs. No such thing in space.

If a nuclear weapon is exploded in a vacuum-i. e., in space-the complexion of weapon effects changes drastically:

First, in the absence of an atmosphere, blast disappears completely.

http://history.nasa.gov/conghand/nuclear.htm

YahooAnswers

http://answers.yahoo.com/question/index ... 515AAxxx2n

Rockets 'push against' the rocket exhaust to move in a vacuum.

Good grief "Bicycle pedals itself".

PS I'm also greatly puzzled as to how pressurised tin can's can exist in space. My senses recreates the 'balloon in thin-air" scenario, where it becomes stretched and disintegrates, but that's for another day!

Yes, brianv, none of the explanations and analogies about how rockets work make sense to me. I am still waiting for someone to set me straight.

I have also seen the "rocket pushes out a little gas then pushes on that gas to propel itself" argument. These theories started popping up around 2011, claiming to be better explanations than Newton's 3rd law or Newton's 2nd law. It's funny to me that after 40 years and thousands of "successful" space flights someone could come up with a new theory. Don't we have enough experimental data to tell for sure? Unless, of course, none of the rockets actually work in space.