What does a retro-rocket push against if space is a vacuum?

It's pushing equally against all the inside surfaces of the rocket itself. As the gases expand from the ignition, the nozzle out the back provides the path of least resistance, so the the pressure is relieved in that direction. The rocket is propelled then, because the pressure (push) remains higher on the forward side, than the rear.

The answers is that the vacuum itself isn't what we push against.

F=MA, so Force equal mass time acceleration, notice that there is no mention of surface to push against in the vacuum.

Here is a way to look at it...

Stand on roller skates and throw a bowling ball. You will move backwards.

Now, imagine the same thing in a vacuum. You will still move backwards.

If you changed this to a ping-pong ball, you would be applying a force to an object in one direction, and get an equal an opposite force pushing you backwards.

The retro-rockets propel gas, and this will result in a force.

I hope that I was helpful.

Rocket propulsion would not require a medium to push against. for each action there is an equivalent and opposite reaction. The action, in ths case, is the forcing of rocket exhaust out of the engine by utilizing the explosion of combustion, and the reaction is the circulation of the rocket in the different direction. The pushing is finished by utilizing the rocket exhaust on the lower back of the engine, and the rigidity is provided by utilizing the enhance of combustion products in the non-provide up explosion. the reality that area is a vacuum yet rockets artwork o.k. up there could recommend to you that your assertion approximately wanting some thing to push against is incorrect.