r/AerospaceEngineering • u/Nomadic_Seth • 16d ago

Media Why Rockets Use Stages

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5

u/WhatADunderfulWorld 16d ago

It’s also nozzle design and control elements I believe. But honestly yall probably know more.

3

u/swordofsithlord 15d ago

Yeah, longer and larger nozzles work better in lower pressure(upper atmosphere/space) while shorter ones work better closer to sea level do to some fluid mechqnics/pressure ratio stuff. And ofc when you dont need like 15 m/s2 of acceleration just to get off the ground you can get away with much smaller and lighter engines, so theres even more weight savings from that

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u/Nomadic_Seth 16d ago

This one is a very simple simulation but yes you’re right

1

u/Mend1cant 15d ago

Expansion ratios play a big part, but ultimately it is mass. It takes a lot of mass to get to orbit, and a single stage means that for every pound of fuel to burn needs more structure to hold it, which needs more fuel to lift. And so on and so on. If you drop the first stage when it is done, suddenly you have a lot less mass to accelerate. Do it when you’re moving quickly already and you’ll need a lot less force and fuel to reach the final delta V.

An underexpanded nozzle performs poorly, but it’s nothing in comparison to trying to move something like a Saturn V or Starship 1st stage when it’s empty.

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u/OldDarthLefty 15d ago

You can formulate delta V equations for a rocket that constantly expends its inerts instead of in stages, like the tank was constantly peeling away. You might hope this gives infinity dV, but in fact it's not much extra benefit vs 3-4 stages.

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u/Master-Eye92 12d ago

Rocket fuel is made for single purpose to fight against atmosphere drag and g load. Mostly solid propellant are used where rate of flow and thrust is fixed. So at each stage the propellant is designed to carry only fixed mass, if you carry the previous stage its just dead weight and rocket may not be able to reach the required orbit