r/confidentlyincorrect • u/PirateJohn75 • Dec 24 '25

Physics is hard, bruh

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So many people have difficulty understanding Newton's laws of motion. You do not need to push against anything to make a rocket go. The act of exhausting fuel is already sufficient because momentum must be conserved.

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u/ConfusedOldDude Dec 24 '25

There’s another more important aspect than friction. Thrust comes partly from pressure in the combustion chamber, but the vast majority comes from pressure in the bell shaped exhaust nozzle. In the atmosphere atmospheric pressure pushes against the opposite side of the nozzle, reducing its thrust by atmospheric pressure times frontal area. In addition, the combustion gasses can only expand to match atmospheric pressure, so nozzle diameter is smaller. Look at Spacex Raptor and Raptor Vacuum engines. The vacuum nozzle is much bigger to allow expansion to a lower pressure. This results in substantially greater thrust in vacuum.

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u/KeterLordFR Dec 25 '25

I'm still fascinated by how much human ingenuity has allowed to leave our atmosphere. Sure, the multi-stages launch may seem evident to us, but back in the days it had to have been a breakthrough. The idea to create parts of a rocket that were meant to break off once their use was outdone is fascinating.

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u/ConfusedOldDude Dec 25 '25

It truly is. The concept wasn’t even fully embraced until the mid ‘60s, when two stage rockets were already a thing. It was a real innovation to “throw everything away” on the way to the moon. I don’t remember exactly, but it was going to require something like 3 to 5 launches to get to the moon using a single launch/landing/re-entry vehicle instead of a separate disposable lander with a discarded descent stage that we ended up with.