# Eli5 how decreasing the diameter of the nozzle (of a rocket for example) makes the rocket go faster? It’s still the same amount of gas exiting the rocket no matter the diameter, so why is there a difference?

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Eli5 how decreasing the diameter of the nozzle (of a rocket for example) makes the rocket go faster? It’s still the same amount of gas exiting the rocket no matter the diameter, so why is there a difference?

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Because it’s not just the mass of gas that matters, it’s the momentum (mass * velocity) of the gas that affects the momentum (mass * velocity) of the rocket.

Look at it this way: guns have recoil, right? Do you get the same recoil if you just hold a bullet and “let go” of it? Same mass, completely different bullet speeds between shooting it from a gun and just letting it drop out of your hand. And the “recoil” is what makes the rocket go up.

Decreasing the diameter of the nozzle forces the gas to exit faster; it’s the same principle as squeezing a water hose to make it shoot water further away.

Note that for supersonic speeds the nozzles are shaped like “widening” bells because *increasing* the diameter makes the gas go faster, not decreasing it.

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Has an unbelievable mountain of info about rockets, rocket engines, spaceship design, warp drives, space stations, space warfare, … It’s intended for science fiction authors. 5 stars, would visit again.

A rocket is pushed by throwing hot gas out the back. From the rocket’s perspective, this means having a container full of high-pressure gas with a hole at one end. This hole means that the pressure does not push as much on that side and there is an overall force forward.

Even with the same amount of fuel being used, the higher the pressure in that reaction chamber, the more the rocket gets pushed. A smaller hole requires higher pressure to allow the same amount of gas to escape, and so it means more overall pushing in the end.

Turbines and rocket engines produce high pressure gas at the exhaust. Without a nozzle, that gas would be ejected and then expand in the atmosphere behind the craft without producing much thrust. A nozzle converts the high pressure gas into a stream of gas shooting out at or near ambient pressure, where most of the energy is now moving the gas out of the nozzle and thus producing thrust.

Think of a balloon.

If you blow it up and let it go without tying it, it will fly across the room.

Now if you take another balloon and cut the end of so it’s bigger, then blow it up and let it go, it won’t go as far.

A smaller hole creates more resistance to whatever’s under pressure.

Pop your balloon and it won’t go anywhere.

Why? No resistance.