There are no such thing as vacuums. I don't know much about rocket propulsion, what I do know is that a rocket is just pushing it's fuel through a nozzle. Air is an oxidizer, a rocket carries its oxidizer onboard. I know that propulsion is a function of the density of the gas, air is pretty low density and thus makes a lousy propellent. You have to increase the density of air for it to be a good propellent, and then you have to heat it and mix it with other stuff to further increase its density. The greater the density the greater the volume expansion when heated. The difference between a lit fart and a kg of uranium.