Viewing a single comment thread. View all comments

fliguana t1_j33uv8a wrote

Rockets launch at full power. The effect of air resistance comes into play for some, more fragile rockets which cannot withstand maximum frontal forces, so there is a point during ascend (maxQ), when engines are temporarily dialed back to reduce acceleration.

A few seconds later, the max power is restored in less dense atmosphere.

Why the let-off is stepped rather than gradual is a mystery to me, perhaps some optimization is available. Also relevant the path of the rocket - the more vertically it launches, the faster it clears the atmosphere, but then longer takes to gain lateral orbital speed.

5

ncc81701 t1_j34xdbc wrote

I mean this is why rockets are staged and non single stage to orbit vehicles have been put into service. During early part of the flight you have massive engines/power and once you are at high speed and high altitude you drop your boost stage to dump the engine and Fuel/O2 tanks you don’t need anymore and continue to orbit using a much smaller second, 3rd or even 4th stage.

3

fliguana t1_j34yvei wrote

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.

2

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.

2

[deleted] t1_j34cc0a wrote

[deleted]

2

fliguana t1_j34nl3v wrote

Most engines are. Since liquid fuelrd rocket engines don't overheat, running them at less than top power would mean you are taking extra weight in orbit.

3