[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?

189 views

[ad_1]

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

In: Physics
[ad_2]

[removed]

Pointing directly toward the moon and turning as the moon orbits would use soooooooo much fuel. There is no drag in space, so if you can accelerate in a way that gets you to where the moon will be by the time you get there, you can turn the engine off and just wait.

Also there are no squiggly maneuvers, I’m not sure what you mean. The process to get to the moon is

1. Orbit Earth

1. Accelerate away from earth at the right time so that you go where the moon will be

1. face backwards and “accelerate” to slow down so that you orbit the moon instead of shooting past it.

[Here’s a gif](https://thumbs.gfycat.com/AbandonedPassionateJunebug-size_restricted.gif)

We already have quite a lot of speed from the earth’s rotation, you can use that to help you go fast, easier, by accelerating in the same direction you were already going, which is parallel to the Earth’s surface.

Then the moon is also already moving, so if you can find a time when just speeding up a bit (a lot) from the direction you were already going will get you in front of the moon, that saves a lot of effort.

Now when you get to the moon, you’re going too fast and will crash, so you want to slow down safely. It’s easier to do that in a controlled way when orbiting the moon, because if you don’t slow down enough you won’t become moon dust, and if you slow down too much you won’t miss. Once you’re in orbit you have much more control and leeway to choose where to land and how quickly.

All of this works on the way home too, but then you’ve got earth’s atmosphere to deal with. As it gets thicker it slows you down, so by coming in at a shallow angle you can use the air to slow yourself down rather than heavy fuel.

Every pound of extra fuel you bring makes the whole rocket heavier and harder to lift (which then needs more fuel), so the less fuel they can bring, the better.

They could but it’s harder to do.

One of the biggest problems with rockets is that fuel weighs a lot. Any fuel you want to use during the flight has to be lifted up off the earth. If you want to lift more weight then you need more fuel. But that extra fuel adds weight so you need more fuel to lift it.

For every kg of fuel you want to get into orbit you need 5kg more fuel at the start of the flight. (Not an exact number, the maths behind the exact increase gets complicated). More fuel means a bigger, more expensive rocket.

If you want to keep the size of the rocket under control you want to minimise the fuel used once you are in space. Those curvey line routes use less fuel because they let gravity do a lot of the work.

Edit-A quick Google indicates that it’s closer to 10kg of fuel at the start for every extra kg of fuel/people/ship you want to get into orbit.

It’s not always about just getting to a certain point in space in the fastest manner. Mostly because all things are in constant motion relative to each other, and always moving. So going to the moon, if you take a sling shot and shoot it will miss because by the time it got there, because the moon is no longer there.

The next step is that you don’t even want to get there in a straight shot, but you also want to get there and hang out right? If you build up too much speed taking a direct line approach, that velocity could very well take you very much past the moon. So the trick is to put yourself into such precise motion that you leave on a path that will intersect where the moon will be at a specific time, while moving at a proper speed such that you can interact or land at the moon and not go hurtling by.

The third and final piece of the puzzle is the concept of the orbit. You don’t just fly straight up and out into space. You fly up, and then you fly sideways, and gain and incredible amount of lateral velocity. By the time you hit orbit, you are not flying up, you are flying around. This ‘flying around’ is the exact same thing the moon is doing around us every day (albeit, much further away). Once you are up, and flying around, there are procedures; transfers as I believe they are called, that govern the rules behind how your movement in orbit affect your orbit and movement around the planet.

For further study, please see Kerbal Space Program. Get yourself into orbit, dammit!

You’re thinking of the moon and the earth as two stationary objects in space. The earth and moon are moving really fast relative to each other, faster than a rocket can catch up. If you just point the snoot of the rocket directly at where the moon is and blast off, the moon is probably going to be on the other side of the earth by the time the rocket gets out to the moon’s original location.

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.

The moon orbits at a speed of 3700 km/h. If you just went to where the moon is going to be it is either going to fly past you or smack into you. Since you need to match the moons speed it just make sense to go into an orbit around earth first.

First of all: You can go directly to a lunar orbit (or lunar landing) without going into an Earth orbit first. If I’m not mistaken it actually requires slightly less fuel.

However, it’s a very bad idea to just point your rocket towards the Moon and accelerate until you get there. First of all, the Moon moves. So you’d have to plan ahead and target the point where it’s going to be.

Then the next question is: How and when do you decelerate? Do you want to enter a Moon orbit first and maybe leave a return vehicle there?

If you play PC video games pick up Kerbal Space Program. It is an excellent compromise between a fun game and a rocket science simulation.

You can try the point at the mun and go method and see how it turns out.

I spent a full day trying to rendezvous 2 ships before looking it up. Then learned it the NASA 2 tries to get it right.

To simplify: firing your engines downwards or upwards is incredibly inefficient – you want to fire your engine sideways (and along the path you’re moving at) as much as possible.

Youll notice in the gif you’ve posted the rocket fires it’s engine downwards only when landing and launching. Even during those sequences the engine is firing along the path the rocket is travelling.

If you point at the Moon and go, you will miss the Moon, because by the time you complete the journey, the Moon will have moved.

You have to aim for where the Moon will be at the time you reach its orbital distance.