Why do rockets launch in an arc, and not just go straight up?
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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)
Low earth orbit require a speed of 7.4 km/s parallel to the surface of the earth. You can compare it to the required acceleration upward that would be equivalent to what is required to accelerate to a speed of 2km/s. So 80% of the required acceleration is parallel to the surface.
Part of the arch is earth curvature because at constant altitude moving around earth looks like an arch down from your perspective.
If the goal is to leave earth and to into an orbit around the sun where the goal could be another planet or something else you could just accelerate upwards. But most acceleration is to earth orbit and even if the goal is to leave earth going via an earth orbit is quite common because you do not lose energy doing it that way. You can gain control of exact
You ever see those coins well things at a mall or something you’d put a coin in and it spins around the outside, slowly falling down? See image [here](https://i.ytimg.com/vi/XTipCQxJ6Ak/maxresdefault.jpg)
Now imagine the center hole is instead the earth, how do you get a coin spinning around the well? Pushing it up just makes it come back down. If you launch it at an angle, it would make a sort of arc, whole spinning around the well a couple of times. Now imagine at the top of that arc, the coin accelerates again. Now its just moving in a circular path around the well, never falling down towards the center (assuming fiction isn’t a thing, which is true in space)
If you went straight up, you’d come straight back down.* **
*minus stuff like the earth spinning, the wind blowing, and you not going exactly straight up.
**Also if you went straight up fast enough you’d fly so fast you’d take infinity years to come back down.
Rockets go up high to where the air is thin/a total non-issue, then they start going side ways really really fast.
This is how a satellite stays up, it’s going so fast side ways that when it goes back down, it keeps missing the earth.
This process of going sideways so fast you keep missing when falling is called an orbit.
Now they don’t want to go straight up and then start turning.
Usually they’ll start turning a little bit immediately, just in case the rocket breaks and falls down. That way it doesn’t crash into the expensive rocket launching stuff/people.
They try to balance whether it’s better to go sideways more because that’s what most of the energy is doing, or whether it’s better to go higher so the air is thinner.
Rockets are also very hard to turn around, it takes a long time so they try to spread it out.
The thing about rocket is that going up is only a very small part of what they do.
We thing of things in orbit as being high above but the truth is that the added altitude is the least issue of getting there.
Space “begins” at 100 km high up according to some official designations. The International space station, a frequent target of rockets going into space, is about 400 km high up.
400 km may seem like a lot but if you consider that vertically, it is only about 4 hours worth of drive in a car. (2 to 3 hours in a fast car on a clear highway without any speed limits) it is maybe an hour and half on a good train, a fraction of an hour on a plane.
Distance wise there should be no issue getting there with a rocket. The 400 km up is the trivial part by comparison with the actual problem.
The ISS travel at speed of over 27,000 km per hour and so does anything else in a similar orbit. Those are speeds that would be over 22 times the speed of sound down here.
The main purpose of a rocket is not to go up, it is to go fast.
You don’t need to reach an altitude of 400 km above the ground, you need to be going insanely fast when when you reach there.
Rockets go up first to get out of the way of all the air we have down there. Once they reach an altitude where there is less air to get in the way they start to work on their true purpose of speeding up sideways to get as fast horizontally as they need to go.
There would be no need to go up all the way to orbital height, have your target zip by at speeds many many time faster than any speeding bullet and immediately fall back down again.
You need to accelerate enough that you stay up and can interact with stuff in orbit.
This cute, short (https://youtu.be/tJiAkBxuqfs) does a VERY good, ELI5 style, job at answering your question 🙂
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