system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : Americans went to the moon, but how did they come back with so little fuel and a much weaker engine ?
gpt
A few reasons:

* For multi-stage rockets, each stage needs to carry the weight of the stages before it, and its' own weight in fuel; this is a fundamental problem of rocket science and is sometimes called the tyranny of the rocket equation: basically, the more mass you have, the bigger an engine and more fuel you need to propel yourself, but that extra fuel *also* has mass, and so you need even *more* fuel... but that's why we need such big rockets to get to the moon: we're carrying everything we need to get back, plus everything we need to get up in the first place.

* Landing on the moon requires fuel because there is no atmosphere, and so you need to burn fuel to slow your descent onto the surface. However, there's also no atmosphere to slow down your lift-off, and much less gravity to boot, *and* much less mass carried, so you overall need much less fuel to lift off from the moon.

* Because the moon orbits the earth, if you lift off from the moon and point away from the way it's headed around the earth, you effectively A) escape from its' gravity well, and B) get captured by the earth's gravity well at the same time, saving fuel.

* Remember that gravity that was making us have problems lifting off at the start? Well it's helping us come home now. But it does pull us helluva fast, which is why...

* Remember that air friction that was giving us problems on liftoff? Well, it's slowing us down a bit now, so instead of using fuel, we can use parachutes and ablative heat shields (that is, special materials that absorb heat and burn off some material as the friction from re-entry heats up the space craft, to avoid having charbroiled astronauts and explosive re-entry).

* There's also a note here: in atmosphere/on Earth, we need big inefficient boosters that provide lots of thrust in a short time, to have a positive *thrust to weight ratio*, so that we can move at all; once in orbit, we can use much more efficient engines with less thrust, since there isn't really another force pulling against us at that point. Similarly, landing on the moon will require a positive thrust to weight ratio, but your weight goes down by 1/6 when on the moon, so you're ultimately going to end up not needing a big heavy booster like you need to get off of earth.

Basically though, the big thing is that we could do with much smaller rockets if it were a 1-way trip, since we wouldn't need to pack for the return trip.