# eli5: Trajectory of rockets

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Why do rockets curve when leaving earth?

Is it the cause of natural forces or a predetermined path by scientist before launched?

Do most rockets follow a general curve or does it depend on thrust, lift (when rocket is horizontal), height position, etc.?

Is it required or could it theoretically just go straight up assuming infinite fuel?

In: Physics

99% of rockets are trying to deliver something in to orbit. If you shoot a rocket straight up… that’s cool, but it’s going to just keep going out, away from the earth. If it doesn’t have enough force, it’ll just get pulled back down by the earth and crash. If it *does* have enough force, it’ll just keep going on its merry way out into space.

So you *need* to have an arc in the trajectory to help ensure the payload gets delivered at the right speed and altitude and angle to stay in orbit after it’s put there.

Go straight up, come straight down.

Orbit isn’t about altitude, it’s about velocity. You have to be going fast enough (parallel to the ground) that can stay in your orbit. Because orbit is the art of falling without ever hitting the ground. You have to go so fast that the Earth curves away under you as you fall. Shoot, if we ignore air resistance and obstacles you could orbit at head height and really make a nuisance of yourself!

So the UP part of the rocket’s path is really just to fuck off out of the atmosphere and get away from all that annoying air slowing you down. Then the SIDEWAYS part is what gets you into orbit.

Because for almost all of them their objective is to launch something in to orbit, so as it gets closer to its destination altitude it will want to change its trajectory from vertical to horizontal relative to the planet surface so it can give the satellite it’s launching the speed it needs to stay in orbit.

For this you need to have some basic knowledge of orbital mechanics and newtonian physics. Say you are in an elevator, and the elevator begins to fall. While the elevator is falling, you feel weightless because there is no floor pushing up on you. Orbiting the Earth is like that. The space station isn’t actually ‘weightless’, it’s just falling sideways constantly, and instead of splatting on the ground, it just misses the planet and goes around again, so it *feels* weightless. If it begins to slow down sideways, then it will keep falling, but it won’t miss the planet, and it will crash. If you somehow held it in place with magic, then the astronauts inside would feel about 90% the gravity we feel down here. When rockets curve, it’s so that they can pick up that sideways speed so that they can fall and perpetually miss the planet. In theory, if you just wanted to go up and back down (like with Jeff Bezos’ New Shepard rocket, that rocket goes up, then comes right back down), then you wouldn’t need to go sideways, but most rockets want to get into orbit, so they curve.

>Is it required or could it theoretically just go straight up assuming infinite fuel?

Going straight up actually isn’t helpful if you want to put something into orbit. The ISS is only about 400 km up which really isn’t far, but its moving at about 27000 kph

>Is it the cause of natural forces or a predetermined path by scientist before launched?

Its predetermined for modern launches so it follows an optimized path. If you launch a dumb rocket at a slight angle with no guidance you will end up with [a similar path known as a gravity turn](https://en.wikipedia.org/wiki/Gravity_turn), modern rockets know how gravity will effect them and steer accordingly.

There is going to be an optimum path for a rocket to follow based on its thrust, weight, and fuel consumption. Rockets need to get going fast horizontally to stay in orbit, but they want to get out of the thick atmosphere before speeding up so they don’t waste a ton of fuel fighting air resistance. The rocket also gets lighter as it burns fuel and because the thrust is the same it begins accelerating faster and faster.

Because it’s less like [the way we think about it](https://i.imgur.com/LVjQGnU.jpg) and more like [this](https://i.imgur.com/G8Xa4Xt.jpg). I’ll update this comment once I find the actual diagram online; this is just a quick drawing I made. Basically, what this diagram shows is that going straight up will make you go straight down, because gravity is still about 90% up there of what it’s like on the surface. You have to curve so that you wouldn’t fall.