The external pressure changes, but the internal physics remains the driving force; the rocket moves because it throws mass behind it, regardless of the surrounding environment. As the lower stages burn through their fuel, they are jettisoned, reducing the total mass that the upper stages must accelerate.
How Newton's 3rd Law Drives Rocket Thrust and Motion
Unlike a car engine that pushes against the road, a rocket carries both its fuel and its oxidizer, expelling mass rearward to generate thrust in the opposite direction. For a rocket to lift off, the thrust must exceed the combined weight of the vehicle and its payload.
In the context of a rocket, the action is the high-velocity expulsion of combustion gases out of the nozzle at the back end. This expulsion generates a rearward force.
How Newton's 3rd Law Creates Rocket Thrust with Rearward Gas Expulsion
The direction of this thrust is what steers the rocket. This reaction produces a massive amount of hot, high-pressure gas.
More About How does newton's 3rd law apply to rockets
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More perspective on How does newton's 3rd law apply to rockets can make the topic easier to follow by connecting earlier points with a few simple takeaways.
