THERE IS NO SUCH THING AS A TRUE VACUUM… PERIOD!
(exerpt from a previous answer, again)
Contrary to popular belief, empty space is not empty, and it is certainly not a vacuum. In fact, a true vacuum has never been created, anywhere, where the atomic count is zero.
research it, its true.
“Empty” space actually contains an average of 1 hydrogen atom for every 32 cubic centimeters of space, this data is per NASA.

In our own Milky Way, space is more dense with 1 hydrogen atom for every 1 cubic centimeter.

One day the misinformation of space as a vacuum will finally die out.

So in addition to the principle behind Newtons 3rd law, there is actually atomic content providing a small but still resistive force to the Rockets thrusters.

spread the knowledge

basically, rockets are barely-controlled explosions moving cargo, crew, warheads, etc. from point zero to somewhere else. What happens when a grenade explodes next to a loose object? that object is pushed away by the explosive event, not by displaced air from the explosion. The same works in space. If a craft is in one place, not moving, and thrust is initiated, then that explosive force pushes the craft forward, creating a negative force in the expelled gas. But rockets dont even move like this only anyway.

Astrophysicists and mission planners use another, fuel saving method for their missions, its called slingshotting.
Basically, the gravitational influence from other planets and the sun can be used to build speed and basically be slingshot to a destination using very little fuel. That’s how a craft gets from the earth to Saturn or Jupiter, and its also why it takes so long. They have to move towards and inner planet, way away from the destination, pick up speed in orbiting that planet, and then using a little thrust, build their speed to the particular escape velocity of that planet, while staying on course, and head there. Many times with Mars missions using this method, the craft ends up coming up from behind the planet or ends up in a location in space where Mars will ‘run into’ it, and they just land.

If you look at trajectories of spacecraft from above the solar system, often times it looks like a warped spiral pattern with odd bends and loops around Venus or the sun or something. Their influence donates speed to the craft, and a little trajectory adjustment after escaping the pull can set them on course. Making straight-shot missions feasible would require much much more fuel to get their speed up and retrorockets to slow them down.

The moon missions could have been made a lot faster, instead of 3-day, one-way trips, it could have been cut down to a little more than a day, but slowing down something moving at 31km/sec is not easy and would need too much more fuel.

Combustion rockets work on earth as they work in space: by causing a chain-reaction in a fast-burning fuel. This makes the burned fuel to ‘explode’ out from the rocket with particles of high velocity. Since, according to Newton, action causes reaction, the momentum the particles gain in one direction, the rocket must gain in the exactly opposite direction. Rockets are actually faster in a vacuum than in the atmosphere due to air resistance. This has been known for several hundred years, even before Newton.

It relies on the principle of conservation of momentum. When you consider the rocket and the propellant inside it as one system, there are no external forces acting on this system, so the momentum of this entire system must be conserved. Now, rockets exhaust propellant. This gives the propellant velocity in the backward direction, so something must go forward to keep the total momentum the same. What is that something? It’s the rocket.

Yes they obey exactly as newton said. It’s not just air that can do the “push”, other things can push too. In rocket, they brings their own gas. For something to burn, you need oxygen. In space, there is no oxygen, so they take oxygen tanks to mix with other gas for burning. when those burning gas are released from the thruster, the thruster move forward. like you said “They push air backwards and air push them forward” but instead there is no air so they brought their own air (oxygen), and their own fuel. The rocket push the burning gas back and gas push the rocket forward.

The propellant is pushing against the center of gravity of the projectile, not against the ground or the air.