Offline BRrollin

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Rocket Propulsion
« on: April 28, 2020, 09:02:03 PM »
A previous post regarding space travel diverged into a discussion on rocket propulsion. In an effort to gather the conversation, I’m starting a new topic on it.

Below show some links which summarize the current theory on rocket propulsion, namely how thrust is obtained as a consequence of CONSERVATION OF MOMENTUM.

The analysis follows directly from first principles, and stands as a  rebuttal against the claim that this propulsion cannot occur in a vacuum.

http://hyperphysics.phy-astr.gsu.edu/hbase/rocket.html

https://www.grc.nasa.gov/WWW/K-12/rocket/conmo.html

I’d like to note that a discourse on thermodynamics is irrelevant here, as this analysis holds for ANY type of rocket system - whether it burns a fuel or some other mechanism.

This analysis could easily extend to solar sail systems.
“This just shows that you don't even understand the basic principle of UA...A projectile that goes up and then down again to an observer on Earth is not accelerating, it is the observer on Earth who accelerates.”

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Offline somerled

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Re: Rocket Propulsion
« Reply #1 on: May 14, 2020, 03:12:51 PM »
No rebuttal there.
 
Any change in momentum requires application of a force - always . Defined by the equation ,  F = ma as stated by the laws of physics .

The Principle of Conservation of Momentum applied to rockets stems from theoretical physics , Neorems theory applied to Newton's laws , which allows calculation of velocities from change of momentum and other variables , but the starting point is always that F must be greater than zero , no force = no change in momentum .

Your first link generalizes this but omits to point out the fact that expansion of hot gas into a vacuum produces no work or force and ignores the fact that thrust is reactive force that requires pressure .

Nasa ignores the laws of science and this neat math trickery which leads to such statements as " rockets work better in a vacuum",
which anyone carrying out an actual experiment in vacuum chamber can see is wrong.

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Offline JSS

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Re: Rocket Propulsion
« Reply #2 on: May 14, 2020, 03:23:06 PM »
Nasa ignores the laws of science and this neat math trickery which leads to such statements as " rockets work better in a vacuum",
which anyone carrying out an actual experiment in vacuum chamber can see is wrong.

The idea that "rockets work better in a vacuum" seems perfectly sound to me.  Lets do a thought experiment.

1. Rocket in a vacuum, the exhaust can come out with nothing trying to push it back in = Maximum thrust.
2. Rocket in atmosphere, the pressure is trying to push the exhaust back in so that has to be overcome = Less thrust.
3. Rocket at the bottom of the ocean, the pressure strong enough to force water INTO the engine = Zero thrust.

It seems pretty logical to me. Rockets move because the inside pressure is greater than the outside pressure. Stuff comes out, rocket goes forward. If stuff can't come out as fast, less movement.

If you want an actual demonstration, watch what happens when opening a can of Coke at the bottom of the ocean.


Offline BRrollin

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Re: Rocket Propulsion
« Reply #3 on: May 14, 2020, 04:23:19 PM »
No rebuttal there.
 
Any change in momentum requires application of a force - always . Defined by the equation ,  F = ma as stated by the laws of physics .

The Principle of Conservation of Momentum applied to rockets stems from theoretical physics , Neorems theory applied to Newton's laws , which allows calculation of velocities from change of momentum and other variables , but the starting point is always that F must be greater than zero , no force = no change in momentum .

Your first link generalizes this but omits to point out the fact that expansion of hot gas into a vacuum produces no work or force and ignores the fact that thrust is reactive force that requires pressure .

Nasa ignores the laws of science and this neat math trickery which leads to such statements as " rockets work better in a vacuum",
which anyone carrying out an actual experiment in vacuum chamber can see is wrong.

You’re sort of right. Except in order to get an acceleration you need a NET force, not just a force. Two forces can act on an object and cancel each other out. Won’t get any acceleration in that case.

Conservation of momentum derives from theory by Newton’s third law. When you have a third law force pair, and a next external force of zero, then momentum is conserved in that system.

It does not mean that the internal forces are zero! Heck, that’s the third law to begin with!

Also, the expansion of gas is a thermodynamic issue, but the momentum transfer that provides the gas with a velocity is a Newtonian issue.

Lastly, thrust is defined in theory as the mass loss rate times the exhaust velocity. There is no pressure involved here.
“This just shows that you don't even understand the basic principle of UA...A projectile that goes up and then down again to an observer on Earth is not accelerating, it is the observer on Earth who accelerates.”

- Parsifal


“I hang out with sane people.”

- totallackey

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Offline stack

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Re: Rocket Propulsion
« Reply #4 on: May 14, 2020, 09:38:24 PM »
No rebuttal there.
 
Any change in momentum requires application of a force - always . Defined by the equation ,  F = ma as stated by the laws of physics .

The Principle of Conservation of Momentum applied to rockets stems from theoretical physics , Neorems theory applied to Newton's laws , which allows calculation of velocities from change of momentum and other variables , but the starting point is always that F must be greater than zero , no force = no change in momentum .

Your first link generalizes this but omits to point out the fact that expansion of hot gas into a vacuum produces no work or force and ignores the fact that thrust is reactive force that requires pressure .

I'm not sure why you're still hung up on the Joule's 'gas does no work in a vacuum' catchphrase. It's already been shown to you that that particular aspect of thermodynamics does not apply to rocket propulsion. You're misapplying the law over and over again.

Nasa ignores the laws of science and this neat math trickery which leads to such statements as " rockets work better in a vacuum",
which anyone carrying out an actual experiment in vacuum chamber can see is wrong.

It would not be just NASA, it would be every space agency/company and missile manufacturer/launcher performing said trickery. But math trickery isn't even required, just logic. Rockets go faster in a vacuum for two very simple reasons:

- There's no atmospheric resistance, which is obviously massive down here nearer to terra firma
- Most likely when a rocket or missile reaches the vacuum of space its weight to thrust ration is much less due to burned off fuel and even staging
Not much is known about the celestial bodies and their distances.

Re: Rocket Propulsion
« Reply #5 on: May 15, 2020, 11:44:21 PM »


Rockets move because the inside pressure is greater than the outside pressure. Stuff comes out, rocket goes forward. If stuff can't come out as fast, less movement.


Curious about this statement. I thought the force of the thrust is what pushes it forward, not the fact that it has greater pressure on the inside? 

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Offline RonJ

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Re: Rocket Propulsion
« Reply #6 on: May 16, 2020, 12:15:33 AM »
If you look at the nozzle end of a operational rocket you can see plenty of very fast moving gasses coming out.  That gas has a mass and a very high speed.  Rocket fuel is very heavy. That mass is converted to gas (with the same mass).  It starts out at a speed of zero in the combustion chamber and exits at a very high speed into the vacuum.  That means by pure logic that there's a very high acceleration rate.  So you have a mass plus acceleration and the formula F=MA.  Since every action has an opposite reaction the force of the accelerating gas is pushing against the rocket and there's a nice momentum transfer taking place between the exiting gasses and the rocket itself.  That momentum transfer results in a force vector in the direction that the rocket is going and the rocket accelerates in the opposite direction of the accelerating gas using the same formula F=MA.  Remember that Force and Acceleration are vector quantities.  When the rocket is surrounded by a vacuum there's less of a impeding force vector on the nose of the rocket and no air for the accelerating gas to move aside so a rocket in a vacuum is more efficient.  It's just basic physics if you really stop to think about it. 
« Last Edit: May 16, 2020, 12:17:28 AM by RonJ »
For FE no explanation is possible, for RE no explanation is necessary.

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Offline JSS

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Re: Rocket Propulsion
« Reply #7 on: May 16, 2020, 12:18:46 AM »


Rockets move because the inside pressure is greater than the outside pressure. Stuff comes out, rocket goes forward. If stuff can't come out as fast, less movement.


Curious about this statement. I thought the force of the thrust is what pushes it forward, not the fact that it has greater pressure on the inside?

I'm no rocket scientist so feel free to read up on it yourself, and I'm being as absolutely as simple as possible.

Pressure is what causes the gas to be expelled.

The gas being expelled pushes the rocket forward.

So anything that affects pressure will affect the thrust in the end.

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Offline RonJ

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Re: Rocket Propulsion
« Reply #8 on: May 16, 2020, 02:23:24 AM »
Pressure is also required in the combustion chamber to expel the burnt fuel out thru the rocket engine nozzle.  The nozzle is the part where the speed increase (acceleration) occurs of the expelled burnt fuel.  Now, before the hot combustion products even leave the rocket engine itself there is lots of accelerating mass. You have an accelerated mass expelled out the rocket engine with a momentum in one direction, so it is required to have a force that provides momentum in the opposite direction.  All of these forces are entirely self contained within the rocket engine itself and the rocket could be in any kind of environment, vacuum or otherwise.  The accelerating mass of the fuel is going out in one direction and that momentum is transferred to the rocket in the opposite direction so the net momentum is zero.  Basic rocket science.  The rocket has a thrust in the direction of travel so will accelerate in that direction.  Any ambient conditions outside the rocket engine itself is entirely irrelevant.  Everything happens inside the rocket engine itself.  There's not a vacuum inside the rocket engine, but a very high pressure.  An accelerating force is against the inside of the rocket engine directly opposite the nozzle.  The rocket is much more massive than the portion of the fuel that's being burned at the time so the burned fuel is pushing against the rocket as it's exiting thru the nozzle.  Momentum in one vector direction equals the momentum in the opposite vector direction.  I've gone over the theory in several different ways, maybe it will be understood now.
For FE no explanation is possible, for RE no explanation is necessary.

Re: Rocket Propulsion
« Reply #9 on: May 26, 2020, 12:31:00 PM »
If you are standing on a skateboard, and you throw a bowling ball, what would happen? You would slide backwards in the opposite direction of the bowling ball. It's been established that force = mass*acceleration, so that is what happens with a rocket. It sends mass in one direction at high speed, and as a result, it accumulates a high net force in the other direction. That is also how a rocket can function in a vacuum. Rockets bring their own oxygen, and when they fire their thrusters they are sending mass out at very high velocities. As a result, they are exerting a powerful force on themselves in the other direction
Everything is possible. It's just that some things are really f*cking hard.

Re: Rocket Propulsion
« Reply #10 on: June 01, 2020, 11:07:46 PM »
Force does indeed = mass * acceleration. That is the principle of momentum and is the subject of Newtons 2nd law.  However what you describe about opposing forces is Newtons 3rd law.  For every action force there is an equal but opposite reaction force.  The rocket for example expels exhaust gases in one direction which pushes the rocket in the opposite direction.  Same as a jet engine.