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)
I read these to mean:
1. No work is done at the time
free expansion takes over.
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.
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).
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.
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.
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?