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.
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.
hoi.polloi wrote:On Earth, shooting something causes friction with the thing being shot. Least of all, air all around us. The shooter will be effected by the action of shooting. However, in a vacuum, there being no friction with anything, shooting something just wastes that thing and sends it soaring uselessly into the void.
But if that's true, then you're saying a gun (by a magic gunman and gun that can exist and fire in a perfect vacuum) would not be pushed back by the bullet, it would just eject the bullet without an effect on the gun or the arm of the gunman? Simply because of a lack of air pressure and friction and so on?
So there is no jet propulsion that would work because any explosive reaction that could even occur in space would be wasted in it completely.
hoi.polloi wrote:I think I see what you mean. To try to put this in unnecessarily simple layman's terms: because the vacuum is just complete void, it can freely take on just about an infinite amount of anything, at any rate, without actually anything significant happening. Hence, we hold on to the idea that mass exiting a craft into a vacuum would actually cause any motion in the craft only because we are used to such behavior in a non-vacuum.
I will try to present my findings with a minimum of math and formulae as these are often used to drawn us into traps, causing us to argue the minutiae of red herrings or chase ghosts. These ruses remind me of the joke about on which side of the barn roof the rooster’s egg will fall. How often do people forget that rooster’s don’t lay eggs?
"(Engineering / Aeronautics) a condition of an aircraft in flight in which a reduction in speed or an increase in the aircraft's angle of attack causes a sudden loss of lift resulting in a downward plunge."
simonshack wrote:I believe that the impossibility of propelling a rocket out of our planet's atmosphere was discovered at an early stage of space travel experiments. Any rocket reaching a certain, critical altitude (for which I surely won't pretend to provide/ specify any exact figure) simply stalls - due to the absence of air, and plummets back into the atmosphere.
Boethius wrote:Short answer: Yes a gun recoils in space. No, the analogy does not apply to rockets.
Longer version: Shooting a gun in space would happen theoretically as follows: pressurized gas accelerates the bullet through the barrel until the bullet leaves the muzzle. At that point the gas that was pushing the bullet escapes without doing any more work i.e. via free expansion. The energy of the bullet (its momentum) travels with the bullet and the gun recoils by principle of conservation of momentum.
The gun analogy does not apply to a NASA-type space rocket as their pressurized gas escapes without doing any work at all. A NASA rocket is a gun without a bullet.
lux wrote:So, a gun without a bullet (i.e., a blank cartridge) wouldn't recoil if fired in space? I don't think so.
The expanding gasses expand in all directions. They would meet little or no resistance at the muzzle but they would meet resistance in the opposite direction and the shooter would feel that as recoil.
I don't doubt that rockets in space wouldn't work as well as NASA claims but I don't see that they wouldn't move at all.
when I say "a gun without a bullet" I mean a gun with an empty chamber, a gun that expels only hot gas, basically a simplified model of a rocket.
In this case "free expansion" states that the gas from the gun enters the vacuum of space without doing any work, absorbed instantaneously and without recoil. Recoil in a gun only occurs when gasses do work on a projectile, to expel it and/or on the atmosphere outside/inside of the muzzle encountered as resistance.
The principle of free expansion comes from Joule and Thompson in the 1800's and has been experimentally verified.
Boethius wrote:The second term (Pressure Difference between inside the rocket and the vacuum of space) x Nozzle Area
violates the “free expansion” effect, part of the first law of thermodynamics by which pressurized gas moves into a vacuum without any work being done. It does not matter how highly pressured the gas is inside the rocket nor how fast it comes out. Because it is going into a vacuum the gas makes the trip “for free” and does not do any work, does not expend any energy and does not create any force or thrust.
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