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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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.
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