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A force is required to change the momentum of the rocket exhaust which consists mainly gas moving out of the rocket chamber. People think that it is the rocket that pushes the gas out. If this were true, yes the gas would push back. However, the gas moves because of pressure gradient force and not because of the rocket pushes it out. Therefore, there would be no push on a rocket.

https://en.m.wikipedia.org/wiki/Pressure-gradient_force

When the gas moves out at high velocity, the atmosphere provides resistance which creates back pressure of flow causing thrust. In space, there is no such resistance.

Jsphappy
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1 Answers1

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Fuel contains potential energy. When burned/consumed, that energy is translated into momentum in all directions. If a rocket directs the momentum of the gas resulting from burning fuel in one direction, the rocket itself must gain momentum in the opposite direction since momentum is conserved. The atmosphere does not really play a role in this process (other than providing resistance against the rocket).

Here is a picture illustrating the situation: Rocket

At $t_1$ the rocket is hanging still (total momentum $p(t_1) = 0$) in vacuum. At $t_2$ the fuel has been ignited and expands (explodes) in all directions. Since it can only expand freely out of the bottom of the rocket, it gets a net momentum $p_2$ out of the bottom. By conservation of momentum $$0 = p(t_1) = p(t_2) = p_1 + p_2$$ the rocket must gain $p_1 = -p_2$ such that the total momentum is still zero. The physical force $F = dp_1/dt$ acting on the rocket is the gas pushing against the walls containing it. The two side walls cancel, but the force acting on the top wall is not counteracted by anything.

I do not see how pressure gradient force is related to the question. This is also how a rocket functions inside the atmosphere.

Codename 47
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