Why do rockets launch in an arc, and not just go straight up?


Why do rockets launch in an arc, and not just go straight up?

In: 4

Because in order to get into orbit, you need to be traveling around the planet, and therefore need to be going in an arc 🙂

They want to go into a circular orbit, and orbiting Earth means going parallel to its surface obviously, so the rocket needs to turn in an arc. Going straight up will just mean going straight down and crashing once all fuel is spent.

Because it is energetically beneficial. If you launch straight up, you have to overcome gravity to avoid falling back down all the time. If you launch up and keep tilting sideways, you’ll still fall back down but eventually be fast enough sideways to miss earth all the time. This is called orbit.

Space is up, orbit is sideways.

You need to go up so you get out of the atmosphere which slows you down. But more importantly, you need to pick up a lot of sideways speed so you stay in orbit instead of hitting the ground again.

Starting with the “up” portion is more efficient since you want to leave behind the pesky air as soon as possible. That gives you the arc. On a mountainless, airless planet, you could have the launch pad horizontally.

Getting to orbital height only takes about 10-20% of a rocket’s fuel. The other 80-90% of the fuel is used to get going really really fast, so that they have enough velocity to maintain an orbit, which requires moving sideways at a high speed. (the speed depends on the altitude of the orbit you’d like to maintain).

For example, the ISS is only about 250 miles up. Not very far at all. BUT, it’s travelling “sideways” at over 17,000 mph.

So the initial launch direction is straight up, to get to that 250 mile altitude, but as it approaches the desired altitude, it rolls sideways to continue to burn to the side, to pick up that immense speed that’s needed to maintain an orbit.

How an orbit works: Earth’s gravitational pull extends a LONG way out into space. Earth’s moon is about 240,000 miles away, and stays in orbit anyway. The ISS is only 250 miles away. This means that the ISS, and everything on it, is still experiencing around 99% of the full force of the Earth’s gravity. If it were to simply stop in mid-air, it would plummet to the Earth instantly.

In fact, it IS always plummeting to the Earth. The only thing that stops it from crashing is that while it’s falling down, it’s also moving very very fast to the side, so that it is always “Missing” the Earth.

A way to visualize that: If you go up to the top of a tall tall tower and fire a bullet horizontally, it travels sideways for a short distance and arcs down to the ground. Then, you fire a more powerful bullet, and it travels faster, and further to the side, before hitting the ground. If you fire larger and larger bullets, they go so far that they start to go around the curvature of the Earth before hitting the ground. Then, when you get to the just-right level of power, you fire the bullet and it arcs down towards the Earth at the same speed that it moves sideways, and it just falls forever. [Here’s an image of the thought exercise drawn out to help.](https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Newton_Cannon.svg/1200px-Newton_Cannon.svg.png)