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I can't seem to find a specific answer to this anywhere.

I understand that in a rocket there is a chemical reaction that causes gas particles to leave the rocket at high velocity. By Newton's third law, and the conservation of momentum, this caused the rocket to be propelled.

What is missing is a physical explaination of what exactly causes this force on the rocket, as most answers annoyingly miss this final bit!

People often use a skateboard-bowling ball analogy. However, in this analogy the person throws the ball and the ball provides an equal and opposite force on the person as it is thrown. The problem is, rocket doesn't 'throw' the exhaust out as it directly doesn't accelerate the particles - this is a result of a reaction. So what exactly causes the force on the rocket itself? Is it the case that in the explosion some gas particles collide with the rocket base, and the nossle is designed as to maximise collisions that will provide an upward force?

I have seen people say this is wrong, or suggest it is right. Wikipedia says:

'About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber, and the rest comes from the pressures acting against the inside of the nozzle'

I want to be able to understand this in terms of particle collisions. I know there is a force due to the physical laws but people don't seem interested in the mechanics of the force itself.

Back to the analogy: if someone where to throw a bowling ball over your skateboard, you wouldn't move, just as particles leaving a rocket, without collision of any kind, wouldn't cause the rocket to move (I know that's not possible, but hopefully makes my point). Hope this question makes sense.

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Assume that you want the rocket to move to the left.

In the combustion chamber there is a chemical reaction which leads to the increase in the kinetic energy of the atoms/molecules and these molecules move in all directions.
As momentum is conserved then the increase in momentum of the molecules moving to the right is balanced by the increase in momentum of molecules moving to the left.
The molecules moving to the right go out of the combustion chamber through a nozzle where the speed of the gases is greatly increased. Molecules rebound off the inside of the nozzle due the force on them due to the walls of the nozzle and in turn the molecules exert a force on the nozzle in the forward direction which contributes a significant amount of the thrust of the rocket.

The molecules moving to the left hit the left hand side of the combustion chamber and rebound.
So the left hand side of the combustion chamber has exerted a force to the right on the molecules to make them rebound and by Newton's third law the molecules which were moving to the left have exerted a force to the left on the combustion chamber and also the rocket which is connected to the combustion chamber.

This is slightly different from the analogy of throwing a bowling ball in that it is obvious as to where the interaction between the thrower and the bowling ball occurs because that must be due to the contact forces between the bowling ball and the thrower.
Note that throwing the bowling ball in the direction of motion will results in the thrower having a momentum backwards and hence result in a backward force on the skateboard.
However if at the front of the skateboard there was a vertical plank which the forward moving bowling ball hit and rebounded from then the forward force on the skateboard due to the bowling ball rebounding would be greater than the backward force on the skateboard due to the thrower throwing the bowling ball. So there will be a net force on the skateboard in the forward direction.

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  • $\begingroup$ In an actual chemical rocket part of the propulsive force does not originate in the propulsion chamber but in the fuel pump or the pressurized fuel tank. Moreover, the nozzle plays an extremely important role, actually, THE most important role in producing propulsive force. That is where the random direction of molecules is being directed in mostly one direction and where the expansion converts thermal energy near the thermodynamic optimum. See en.wikipedia.org/wiki/Rocket_engine_nozzle for a few pointers where the really important physics happens from a thermodynamic point of view. $\endgroup$
    – CuriousOne
    Mar 31, 2016 at 21:46
  • $\begingroup$ @CuriousOne As always thank you for your very informative comment. There does not seem to be clarity as to the relative contributions to the thrust by the combustion chamber and by the nozzle. One source seems to indicate a 50:50 split. There is a very comprehensive discussion of rocket propulsion here. braeunig.us/space/propuls.htm $\endgroup$
    – Farcher
    Mar 31, 2016 at 23:00
  • $\begingroup$ If you don't have absolute clarity about the distribution of forces in a rocket, your rocket will typically disintegrate at launch. All of this can be measured and has been measured plenty on real engines. It's just not something physicists are required to know and are prepared to know... unless they have practical experience with the design of rocket engines. $\endgroup$
    – CuriousOne
    Mar 31, 2016 at 23:21
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Even a chemical reaction would maintain conservation of momentum. So, the blocked side (rocket) moves forward, and the open side (exhaust gas) moves backward. The chemical reaction is needed to produce the gas. The speed/amount of exhaust alone is enough to compute overall forward thrust of rocket. If you open the mouth of an inflated balloon, it moves while the air comes out of it, in opposite direction. Same way rocket moves, with a difference that chemical reaction keeps producing hot gas with high pressure/speed.

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There is a recurring flaw in nearly every explanation of how a rocket is propelled. Newton’s Third Law states that “for every action there is an equal and opposite reaction”. However, it is not good enough to simply say that this rocket driving force is “the reaction to another force”. You still have to identify and explain what that force is!

In the case of a rocket, the force which drives the rocket forward and upwards is the imbalance of pressure forces within the combustion chamber.

Lets imagine a cubic container, lets say a biscuit tin, with 6 sides, each 1 foot (I’ll use imperial units since they are much easier to visualise) long and with a lid on the bottom which is held but can be released by a catch. Lets put a valve on one side through which air can be pumped. The box is suspended free and is not incontact or close to the ground.

Now put a pressure of 1 psig (pound per square inch guage) in the box. (for understanding, atmospheric pressure is about 14.7 psia or 0psig (a = absolute, g= gauge) and a typical car tyre is about 35psig. So there is a differential pressure between the inside of the box and the outside of 1 pound per square inch. That means there is a total force acting on each side, the top and the bottom, of 144 (1x12x12) pounds of force. (if you are, say 65 kgs, or 144 pounds weight, you are exerting that force onto the ground so it is kinda like you standing on the box)

While the lid is on, all the forces are in balance. Each side force balances the opposite side and the force on the top balances the force on the lid. Now release the catch on the lid. Immediately, the force on the lid no longer pulls on the box and as a result the force on the opposite side, the top, is 144 pounds of force. THAT is the driving force for a rocket!! To maintain the thrust, you need to maintain the pressure in the chamber, so if air is continually added to the box, then it will continue to experience an imbalance of forces and will continue to move/accelerate.

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