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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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.
You do not change the diameter of the nozzle coming out of the combustion chamber of a rocket! It is a specific size. Any larger, and the engine will hiccup and die due to lack of pressure. Any smaller and the pressure in the combustion chamber gets too high and explodes.
The nozzle exits into what’s called a bell. I think this is what you are confusing for the nozzle. The size of the bell has nothing to do with power output. It’s about efficiency. When rocket exhaust comes out of the nozzle, it wants to expand and reduce pressure. It does this by going not just backwards, but side to side as well. The side to side expansion is just lost energy if there is no bell. The bell transfers the sideways cimponent of expansion into forward thrust. The size of the bell has to do with the difference in pressure between combustion chamber and ambient. Larger bells are needed in space due to the larger differential (and therefore greater expansion) the bells used for 1st stage boosters are smaller due to smaller pressure differential.
The subject is a bit complex, but I’ll try…
At a given atmospheric pressure, there is an ideal nozzle size – one that allows the exhaust to expand just enough so the pressure at the edge of the nozzle is exactly the pressure of the atmosphere.
The nozzles for second stages are pretty simple as they generally only run in vacuum. The ideal size would be very big but they are limited to practical sizes, but are pretty big in general.
Unfortunately, rocket nozzles on boosters are a compromise as the start out at sea level and must operate in vacuum as well. If you run a full vacuum nozzle it sea level the air pressure from the outside will cause a disruption of the exhaust flow inside the nozzle and will likely cause the nozzle to break apart. Which would be bad.
So you design an engine with a nozzle small enough so that doesn’t happen at sea level, and therefore booster engines have nozzles that are pretty tiny. They are good at the start of the flight, but when they get partly out of the atmosphere the nozzles are too small to be effective. If you watch a SpaceX Falcon 9 launch you’ll see the rocket exhaust is going straight down at the launch, but right before the first stage is done, you can see that a lot of the exhaust is going out to the side – that is because the nozzle is too small for that altitude. A nozzle that is too small is less efficient but it’s not dangerous the way a nozzle that is too large would be.
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