# Why do people think that rockets can thrust in the vacuum of space? [duplicate]

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.

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.

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:

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.

• I’ve already explained that the force is provided by pressure gradient force and no force is needed to be provided by the rocket to satisfy the rule of conservation of momentum. Mar 13, 2019 at 16:55
• @Jsphappy if the gas inside the nozzle is pushed back into the nozzle, where does it go, or why does it not? Mar 13, 2019 at 16:57
• The air can only go back into the nozzle if there’s resistance. Get a clear pipe with one end closed. Put the pipe horizontally and level. Fill the pipe with some water. See it flow to the end. When the water hits the end, it starts to flow back Mar 13, 2019 at 18:15
• @Jsphappy, from the other comment thread it seems you are quite skeptical about a fact which is commonly accepted by the scientific community (that rockets work). That's fair, and I am all for discussing why the scientific community agrees on this, but to do that we need some common ground to start from. Is there any parts of commonly accepted physics you do not accept as empirically well-tested facts? Mar 13, 2019 at 20:54
• @Jsphappy I've edited my answer to make it more clear what is going on. This is quite simple to analyze through momentum conservation - I don't understand what pressure gradient force has to do with the situation? Mar 13, 2019 at 21:05