Your dismissal of air or ground making a total of three forces and not a pair, is fundamentally wrong. Anything fired out the exhaust of a rocket will be met with resistance from any kind of matter that's in that location, whether it's solid or fluid. The amount of resistance will be much higher in the case of solid concrete, for instance, than water, or air. But regardless of that, whatever the rocket emits is effectively pushing against what's just outside, and all matter takes some effort to move. Therefore your three things are just two things. The opposing force is highest for solids, and decreases for fluids.Penelope » March 29th, 2018, 5:07 am wrote:Patrix, your belief that Newton's 3d doesn't come into play in vacuum is because you interpret Newton's 3d to mean that rockets travel by pushing against either air or ground. This is expressly denied by Newton's 3d because it deals only with relationships between pairs. Air or ground makes 3.by patrix » March 28th, 2018, 1:02 pm
I have no problem with Newton's laws of motion. . . perfectly fine and valid. they never come into play in the rockets in vacuum scenario
You may avow Newton's 3d or that third entities like air or ground determine the outcome of paired forces transacted by paired material objects-- namely the rocket and its exhaust one instant before it separates from the rocket. You can't have both; you have to choose. Please review my discussion last post concerning force vectors. Just as numbers are necessary for math, force vectors are necessary to analyze forces.
Further, regardless of the misinterpretation & misapplication of the Free Expansion experiment, I'm sure that you are aware that vacuum is not a force, and therefore cannot stop or slow speeding bits of exhaust. However this is actually irrelevant because the transactional force between the exhaust & rocket will have already taken place, an instant prior to separation. This means that the force (resulting in motion) will already have been apportioned between them, so that nothing which happens to one can now affect the other.
This makes sense when you light a firework rocket, which accelerates from the ground very strongly, but then slows down. But for some reason, every 'space' launch I've ever seen on TV has the rocket lifting incredibly slowly from the launchpad, before apparently getting to tens of thousands of mph once out of sight. The only logical explanation I have for this slowness is that we are not watching a real thing.
Rocket thrust decreasing as the density of the outside medium decreases makes complete sense to me, and a natural extrapolation of the concept makes no surprise of tending toward zero thrust if the local material is a vacuum.
As regards the 'vacuum of space' though, I'm by no means wed to the idea that a vacuum is what 'space' consists of - as I'm certain man has never been there to find out.
By all means stick to theoretical arguments, but much of the history of space travel suggests that about 100km is pretty much the limit for any man-made technology to date. The US Air Force nudged up to that height with the experimental rocket planes , and encountered great difficulty staying there. If it was possible to go further, I very much doubt that the military, who were owning the effort then, would suddenly lose all interest and hand it over to a newly created civilian body, i.e. NASA. As a rule, when military are in posession of some technology that would allow military use, they get right on and use it, they don't give it away to a bunch of civilians.