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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

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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. – Iota 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. – Jonas Jun 28 '14 at 10:15

Rockets work better in a vacuum, for two reasons: Thrust is higher and drag is lower (non-existent) in a vacuum. The drag issue is obvious, and also a bit off-topic. The question is about thrust. A bit overly simplistic explanation is that there's nothing to impede the exhaust in a vacuum. For a given fuel, rocket exhaust velocities are typically 15% to 20% higher in vacuum than they are at sea level.

A more detailed explanation requires looking at the nozzle design. A rocket engine designed solely for use in the vacuum of space will not function well at sea level; it might even be dangerous to use such a rocket at sea level due to shock waves inside the nozzle. A rocket engine designed for takeoff conditions (one atmosphere) won't work as well in vacuum as that rocket designed solely for use in vacuum, but it will still work better in vacuum than it does at sea level.

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Related: what's the difference between the designs? – rob Jun 28 '14 at 14:47

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