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fliguana t1_j34yvei wrote

Reply to comment by ncc81701 in How do you optimize a rocket or space vehicle liftoff? by seriousnotshirley

Dropping unused fuel tanks is not unique to rockets, some military planes use it too.

In most cases, dropped stage also contains the engine(s), because replumbing inflight is hard, and because the next-stage thrust need is reduced. But not always. For example, the Space shuttle made do with oversized reusable engines and an add-on tank.

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aspheric_cow t1_j38aco0 wrote

Also, you want a larger nozzle at higher altitude (lower ambient pressure). An engine + nozzle optimized for low altitude would be very inefficient at high altitude, so you might as well drop it and switch to the high altitude engine. The Shuttle is an exception as you say, but actually the solid rocket boosters provided about 2/3 of the thrust at liftoff.

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