As the rocket fuel burns, it releases energy and gases. These cause a rise in pressure in the engine's combustion chamber which causes a force that throws these gases out the back end. According to Newton's third law of motion, for ever force acting on one body, there is an equal and opposite force acting on another body. Therefore, the force pushing the rocket forward (the thrust of the engine) will be exactly the same as the force that is pushing the gases out the back of the rocket. This is not affected by whether or not the rocket is operating in an atmosphere or not. What the atmosphere does is create drag that will slow the the rocket's flight more deep in the atmosphere than it will higher up where the air is thinner. Again, this has nothing to do with the amount of thrust is produced by the engine itself.

Rockets work better in a vacuum - in fact about 10% better than in an atmosphere - because the flame emerging from the rocket's nozzle doesn't have to push any air out of the way. One of the great fallacies about rockets or any kind of jet propulsion is that they must have something to "push" or "react" against, so how can they work in space, where there is nothing to "push" against? Rockets are actually a perfect demonstration of Newton's Third Law of equal and opposite reaction. Once a rocket's flame leaves the nozzle, it has no further function. All the work is done inside the combustion chamber and nozzle, where the gases expand and build up their maximum velocity and are guided in one direction by the nozzle - it is the velocity of the gases going one way, that, by Newton's Third Law, makes the rocket go the other way, and once the gases leave the nozzle, they provide nothing more than spectacular fireworks. However, in the atmosphere, the gases emerging from the nozzle have to fight against the atmospheric pressure, which chokes the rocket and reduces its efficiency. As the rocket ascends higher, and the atmospheric pressure drops off, the rocket's efficiency increases. There is a question about jet boats a few questions down so I will add this: A very similar phenomenon is apparent with jet boats or personal water craft, Jetski, etc, that use a jet of water for propulsion. Because water is so much more dense than air, it is this very reason all of these types of boats are designed with the jet nozzle above the water surface at plane speed. Jet boat manufacturers learned very early, for a given horsepower input the jet produces much more thrust above the water compared to under it. This is the one and only reason why the jet nozzle exits above the surface of the water, not because of drag or any other factors. It is considerably more efficient this way because it does not have to push water out of the way, only air. It is mass and velocity that determine thrust of a jet or rocket. If something gets in the way like water or air the thrust is reduced.

To Glen I tried to comment on your answer but couldn’t figure out how to do it. I decided to just do an answer to you. I suggest you go to this NASA web site: http://www.nasa.gov/about/contact/ask_nasa_form.html Ask the rocket scientists if their rockets are more efficient in the vacuum of space as compared to earth's air pressure. Ask them if their rockets develop more thrust in the vacuum of space. They will give you the same answer. Any thing slowing or impeding the flow of gases from a rocket engine will decrease overall thrust. Air does slow the gases down, a vacuum doesn’t of course. I’m not talking about drag on the rocket which has no affect on the efficiency of the engine. I’m talking about thrust output of the engine which is affected by the environment it is operating in. I have taken this information from NASA web sites so I know it is correct and, what’s more, I truly understand it. You will understand it too when you get it straight from NASA I believe. Do you agree with my jet boat answer? It is along the same lines as the rocket in space question. Same principles apply.