Rockets in Space.

[stevepidge]

So I read the following and I want to know what you guys think?

Why doesn’t the propellant generate any force, it's expanding, right?
There is something known as “Free Expansion” or the “Joule-Thomson” effect, named after James Prescott Joule and J.J. Thompson two of the founders of the field of Physical Chemistry.
http://www.etomica.org/app/modules/site ... ound2.html
Free Expansion states that when a pressurized gas is exposed to a vacuum the gas expanding into the vacuum without any work being done. The gas is not “pulled” or “sucked” into the vacuum nor is it “pushed” out of the high-pressure container. In other words no work is done, no heat or energy is lost.
This result has been experimentally verified numerous times since its discovery in the 1850’s.
[for example a paper in the Journal of Physical Chemistry from 1902: http://pubs.acs.org/doi/pdf/10.1021/j150043a002]

As if Free Expansion wasn’t enough to invalidate the theory of rockets producing a force in a vacuum there is also a result from thermodynamics:
Work = Pressure x Change_in_Volume
that is easily found searching for “W=PV”
http://lsc.ucdavis.edu/~ahart/Alicia2B/Thermo.pdf
If the pressure of a system is 0 then the work done by the expanding gas into that system is 0. Gas expanding in a vacuum doing no work agrees with Free Expansion. This can also be understood as the gas meets no resistance as it exits into the vacuum and thus transfers neither heat nor energy to its surroundings. If the gas loses neither heat nor energy then it has done no work.

At this point we have a rocket with high-pressure gas generated from liquid fuel that can release the gas into a vacuum but has no way to produce a force while doing so. As soon as the nozzle is opened the gasses escape without doing any work. Therefore the 3rd Law is rendered useless.

Soo.. If a rocket fires into the vacuum of space what are the expanding gasses pushing off of? Would not the "thrust" meet zero resistance and do no work? Hence no propulsion?

[Aardwolf]

The gas isn't expanding into vacuum in all directions. In the direction of the spacecraft there is a material object to work against.

[stevepidge]

The gas isn't expanding into vacuum in all directions. In the direction of the spacecraft there is a material object to work against.

Are you saying there are rockets that have boosters which create expanding gases into some kinda self contained pressure chamber located in the front of the rocket? If so, would the amount of thrust be limited to the amount of gases the container can hold? What about Newton's law? Seems to me this is like saying a bike can peddle itself.

[Grey Cloud]

I understand Aardwolf to be saying that the gas tries to escape/expand in all directions but the rocket gets in the way so the gas effectively 'pushes' the rocket. Similar to when you inflate a balloon and then let it go. The balloon moves off in the opposite direction to the air streaming out of the blowy-uppy, tubey part thing. (That last bit is a technical term )

[stevepidge]

But the gasses out of the nozzle are pushing off of the earth and the atmosphere. How can the gasses do work in free space where there is no atmosphere or planetoid to push off of. If the gasses are expanding into a vacuum there is nothing to provide that push back in accordance with newtons 3rd law. Newton was not aware of the properties of vacuums. Vacuums as large as free space will absorb all the energy instantly and disperse the gasses without doing work.

[perpetual motion]

I've been trying to debate and inform people of this situation for years, but like
most of the posts on this forum, people look at you like you have lost your mind
or haven't went to college to be brain washed with their so called physics books
and brain washed professors.

[comingfrom]

Think of an astronaut on a space walk, and he's carrying a hammer. Now he throws the hammer into empty space. What do you think will happen to the astronaut's trajectory?
1. if he throws it slowly, or 2. if he throws it as hard as he can.

[stevepidge]

Think of an astronaut on a space walk, and he's carrying a hammer. Now he throws the hammer into empty space. What do you think will happen to the astronaut's trajectory?
1. if he throws it slowly, or 2. if he throws it as hard as he can.

Here let's visualize it more accurately. You are saying that rockets are using newtons 3rd law for propulsion. That means that the instant that there is uneven pressure, as in a nozzle is introduced to a compression chamber, that instantly FTL there is an equal and opposite reaction that propels the craft forward. If this is the case then things such as muzzle breaks should not reduce recoil in a firearm. The instant nature of the application of the third law would mean that newtons effect would occur before any gas could possible act on the system and you would always get recoil.

[stevepidge]

What I'm saying is that Newton didn't know about vacuums and therefore newtons law is dependent on interaction with another mass. Hence rockets can't work in a vacuum.

You can't say newtons 3rd law has an a priori effect, or a magical and instantaneous interaction as the result of an imbalance in a system. It is the expanding gasses pushing the atmosphere and the atmosphere in turn is pushing back on the rocket to propel it in an atmosphere.

Therefore Newtons law is only valid in an atmosphere or in a closed system like the earth.

[willendure]

Conservation of momentum.

The gas ejected out of the nozzle is moving at a very high speed, and has mass. You agree? Therefore it has momentum. But momentum of the gas/rocket system must be conserved, so opposite pf the momentum being imparted on the hot expanding gas must be imparted on the rocket, increasing its momentum along its direction of travel.

[willendure]

From Wikipedia:

https://en.wikipedia.org/wiki/Work_(thermodynamics)

"For a process in a closed system occurring slowly enough for accurate definition of the pressure on the inside of the system's wall that moves and transmits force to the surroundings, described as quasi-static,[19][26][27] pressure-volume work is represented by the following equation between differentials:

delta W = P dV"

A process in a _closed system_ _occurring slowly_ obeys W = PV. But this is not a closed system or a slow change. The rocket is open to space, and the chemical reaction is vigorous to put it mildly.

[comingfrom]

Hence rockets can't work in a vacuum.

But we know they do work, because we have sent rockets into space.

So something is wrong in your thinking.

All satellites have ionic propulsion motors for adjustments to orbit.
The motors spit out a very tiny amount of mass, but because it is at very high speed, it effects the trajectory of the craft.

The principal is the same as with the astronaut throwing a weight. He is going to experience a force in the opposite direction.

Backpack jets work too, enabling astronauts to maneuver around in space.

[stevepidge]

The gas has to do work to move a physical system. The gas cannot do work on the vacuum of space. The gasses do not expand into the vacuum of space they instantly return to the default energy state of the vacuum.

[stevepidge]

Hence rockets can't work in a vacuum.

But we know they do work, because we have sent rockets into space.

So something is wrong in your thinking.

All satellites have ionic propulsion motors for adjustments to orbit.
The motors spit out a very tiny amount of mass, but because it is at very high speed, it effects the trajectory of the craft.

The principal is the same as with the astronaut throwing a weight. He is going to experience a force in the opposite direction.

Backpack jets work too, enabling astronauts to maneuver around in space.

No you don't know, you have been told we did. If the 3rd law is instantaneous then there can be no muzzle breaks. I know we have muzzle breaks because I have first hand experience with them. Flying in space... Not so much. My thinking is rock solid and it is a logically drawn conclusion based on how NASA says the 3rd law of motion works. If it is not instantaneous then it must rely on an atmosphere and muzzle breaks can't exist... Yet I know first hand that they do.

[nicho247]

Hi Stevepidge,

Something along these lines was asked a few months ago, see link. I linked a general Nasa source. Essentially thrust is generated via a working fluid leaving a rocket. The faster it leaves the more thrust is generated. An atmosphere exerts a pressure onto this working fluid hindering its ability to leave a rocket. This pressure will always be some value, under water, in the air, and in space. The lower the pressure, the less the hindrance and consequentially greater thrust. For this reason, I bet rockets are more effective in space.

I think you are considering a vacuum as zero pressure and therefore we are dividing by zero or something, a false premise. Maybe I am missing something.

Nick