So I just read this and it didn't make any sense

It is not necessary for the rocket exhaust to push against anything EXCEPT THE SHIP ITSELF. You see, when the combustion of fuel takes place inside the rocket (think of a long vertical cylinder with the 'bottom' open) the exhaust gasses produced expand quickly in all directions. The molecules slam into anything in their path exerting a small force each. Thus some molecules push against the 'right' side and some push against the 'left' side, and all these sideways forces cancel each other out. Some molecules slam against the 'top' of the cylinder, but since there is no bottom of the cylinder, there is no force to cancel this out! Therefore the net force will be in the 'up' direction.

By the very definition; there would be no movement still. Also engines require oxygen to burn which doesn't exist in a non existent environment. up, left, right, are not directions.

  • 4
    $\begingroup$ Possible duplicate of What is a rocket engine thrusting against in space? $\endgroup$ Mar 1, 2019 at 10:16
  • $\begingroup$ In space, we bring oxygen with us for burning. Also, there are rocket engine designs that don't require oxygen. As for how it works, ever sat in an office chair with a fire extinguisher and propelled yourself around the room? It's like that. Conservation of momentum says if I start out not moving and then shoot a lot of matter (like air or exhaust) out in one direction, I have to move in the other direction to make the total momentum add to zero still. It's why guns have recoil, fire hoses are unsafe, and rockets fly through space $\endgroup$
    – Jim
    Mar 1, 2019 at 14:28
  • $\begingroup$ Re, "engines require oxygen to burn." They carry it with them. cdn.britannica.com/s:500x350/94/95394-004-8CA5FFE8.jpg $\endgroup$ Mar 1, 2019 at 14:32
  • $\begingroup$ ...Except, it's not always oxygen as such. Rockets depend on powerful, exothermic chemical reactions, and one of the fuel components often will be pure oxygen or an oxygen bearing substance (e.g., perchlorate); but some fuel systems use no oxygen at all: Hydrazine is used as fuel for maneuvering thrusters. It contains no oxygen atoms at all, but it exothermicly decomposes when it meets a catalyst in the rocket nozzle. You can instantly turn a hydrazine-fueled thruster on or off just by opening or closing a valve. $\endgroup$ Mar 1, 2019 at 14:43

3 Answers 3


Sure there would be movement. The rocket moves in the opposite direction as where the exhaust nozzle is pointed to.

Imagine that instead of gas particles, there are little balls bouncing around. Each time one of them hits the wall closer to the top of the rocket, it will transfer a bit of its momentum to the rocket. This means that the rocket then moves a bit faster in the forward direction than before the collision. In the other direction there is no wall that the balls can bounce into. If a ball is traveling towards the end of the rocket, it will just leave the system without a collision. This means the momentum of the rocket stays constant.

From a larger perspective, you can employ the conservation of momentum. The momentum of any system is constant, unless there is an external force acting on it. Consider a rocket that hasn't fired yet and is floating somewhere in space. We can say that its momentum is zero in this moment. Now turn on the rocket engine - the (heavy) rocket accelerates (slowly) forward, while the (light) exhaust gases move (very quickly) backwards. Yet the net momentum is still zero.

If we are only interested in the movement of the rocket, we can just ignore the exhaust gases after they have left the rocket. If they bounce into anything or not doesn't change the fact that they have already supplied their momentum to the rocket.

Regarding the comment about oxygen in space: rockets carry both fuel and oxidizer in liquid form and don't need any oxygen from the air. That differentiates them from jet engines, which do use oxygen from the air and only carry fuel.


Why would there be no direction?

There is such a thing as left up and down if we're operating in a frame of reference relative to the craft. Then those axis's would be well defined. If the engines would require oxygen this would be part of the fuel which would be brought with them.

The molecules in the engine is not tied in any way but are independent of the craft, thus when they crash into the "top" of the engine, they create a forward thrust.


I'm not sure you're looking for this but this is what I've got. Picture yourself standing on a skate and throwing tennis ball in one direction and surely you'll begin to move in the other direction, and as you empty the bag of balls, you'll move even faster. Fundamentally the Newton's third law of motion settles this argument, but if that's not enough, here's a molecular explanation that may help but a bit counterintuitive. As the rocket shoots out molecules of gas, there's something called inertia which is a seemingly opposing force to a motion. The cumulative inertia of thousands and millions of gas molecules being energetically accelerated is the resultant force exerted on the rocket. To picture this better, imagine explosions happening in its thousands between every individual molecule of gas and the rocket pushes both in the opposite direction.


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