# Will a rocket produce more thrust if fired in air, rather than vacuum?

I've been in an argument with a friend.

He claims that when a rocket engine is fired in air, it get significantly more thrust due to the rocket pushing gas into the atmosphere, and the atmosphere expands (due to there suddenly being more "air" in the same area of space), and that expansion helps to propel the rocket forwards (or upwards, whatever).

I claim that the only force that acts as thrust to the rocket is the response force from firing a large mass of gas out of the back, and so the gas exerts the same force on the opposite direction (namely, forwards of upwards), which propels the rocket.

He further claims that a theoretical rocket in honey, would fly much better than it would in vaccum. I claim that the resistance would nullify and theoretical benefit to thrust.

Which of us is closer to "the truth"? Do you get

• I think you are right, as every medium at a particular temperature will have a terminal velocity whereas in vacuum you are just bound by relativity. Jun 28 '14 at 8:16
• My guess would be that the air (or honey) drag would indeed nullify the effect. But it is still interesting to ask if there's a benefit in the first place. Jun 28 '14 at 10:15