[ELI5] Why do rockets have to go around in orbit and do a bunch of squiggly maneuvers to get to the moon? Why cant they just go in a straight line?

Question

And by straight, i mean, just… Point the nose of the rocket towards the moon and keep steering till you get there….

In: Physics

Answer 1

Imagine that you had a rocket with enough fuel and power to hover in the air for weeks or months at a time. That would be some feat of engineering right? Considering that most running engines, especially rocket engines, go through their fuel supply very quickly.

It would even be wasteful to use the fuel to just hover there instead of flying up and out of Earths gravity well. Except that Earths Gravity well is huge. The distance you’d have to move out from the Earth, not to fall back to the Earth is farther than we’d want to fly. And even if we fly directly towards the moon, so that at some point we fall towards the moon instead of back to Earth, that is still a hell of a waste of fuel.

It is not fuel efficient to use a rocket like an elevator. Instead you can save more fuel by not constantly fighting the Earths gravity the entire time, by using ballistic arcs of increasing speed and diameter, until you are moving so fast sideways, that the Earth falls away from you at a faster rate than you fall towards the Earth, meaning your orbit is increasing in altitude.

Once in a stable orbit, you can shut the rocket motor down completely, and you won’t fall back to Earth (or more precisely you won’t fall in such a way that you will ever hit). Once in one of these orbits you’re not moving in a straight line relative to anything. It’s like riding a merry go round, so if you want to go from point A to point B, you have to do it by changing your orbit, which is a series of arcs or ‘squiggly lines’.

What prevents us from flying directly from point to point in a rocket, like an elevator, is the fact that fuel weighs a lot, and it’s very wasteful to fly like that. If we ever developed anti gravity, or super efficient futuristic scifi thrusters that gave us unlimited thrust and runtime, then it would be possible to fly direct. It would also be possible to hover in place over a planet, not orbiting it, but using the thrusters to hover in place against the pull of gravity.