Same goes for a ship in space. You can combust all the gasses you want. If you don't generate an external force you're not going anywhere. People say "the ship is pressing on the gasses" but the gasses don't exist outside the ship. Gas doesn't exist in the vacuum. So the ship is left pressing against itself. A space ship is like a car with an engine but no wheels.
Imagine a ship in space consisting of two parts A and B each with mass m. Thus the space ship A+B has mass 2 m. The space ship has a little internal mechanism, e.g. a spring, that can eject part A away from the space ship at velocity v leaving space ship B alone. When the mechanism is activated and applies a force on part A, space ship B gets velocity -v.
: rockets do not use springs to eject solids. They propel gasses.
Newton, you know.
Let's evaluate your Calling on Newton
1. Newton didn't consider gasses, only solids. The gas/chemistry pioneers like Joule, Thompson, Boyle and others generated results which disagree with rocketry. That's why NASA types keep going back to Newton in the 1600's instead of going forward to Joule in the 1800's.
2. Newton's 3rd Law, equal and opposite forces, action/reaction
, only applies if you have the necessary net external force
. An object can't move itself. Not understanding or ignoring this principle leads to a misapplication of Newton.
3. Newton's 2nd Law, Force = Mass x Acceleration
only applies if the ship is already accelerating
(changing velocity) when its mass is reduced.
4. You don't specify which Newton
you are going for, 2nd Law, 3rd Law, both?
5. NASA folk only seem to know Newton as far as NASA wants them to. For instance completely ignore the fact that gasses are not solids and that a different set of equations are used to evaluate the work done by and forces generated by gasses
Continuing on with the Strawman
You've described a rocket that uses a solid propellant and now, of course, you'll switch it to using gasses without any consideration of the different physical properties of a gas or the fact that gasses are undefined in space. Nearly 200 years of gas/chemistry research in the vacuum nullified by NASA without any reason given except, trust us, we're scientists
I can evidently replace my mechanical spring by a liquid that burns producing a hot gas that ejects part A. That's why my space ship works in vacuum.
The difference between gas and solid is ignored.
Let's just say...this may be your blind spot in the analysis...
You cannot eject
gasses into space. Why not?
Because free expansion
states that no work is done by a gas entering a vacuum.
Ejecting something requires work.
If the gasses do no work in space they cannot help move the ship once they enter space.
So the only force available to move the rocket are the accelerated gasses in ship itself which cannot move it because it is not a net external force
(proper application of Newton's 3rd Law).
Problem is that I have to get rid of part A every time I want to change velocity so my space ship gets smaller and smaller in the process.
This is another misapplication of Newton
Force = Mass x Acceleration
only works if you are already accelerating. If Acceleration = 0 then Force = 0 no matter how much Mass you release.
In space, when you release exhaust gas, that gas is just going to drift away. Yes, the gas will drift away very, very quickly, at many km/s but it's still just drifting off. It's not pushing the rocket because it imparts no force. Acceleration = 0, Force = 0.