Edit: I didn’t understand how a jet engine worked, but now that I do, the question has been amended to this…
“Why does a rocket have to travel faster and faster the higher up it goes? Shouldn’t it require less and less speed as it is further from the earth it gets because there is a non-zero number(very small) of negative gravity change the higher you are?”
Edit #2: I think I suck at asking this so I’ll ask it like a 5 year old.
We have all seen videos of rockets taking off. They start very slowly, and then build in speed. Although, at first, they build up in speed. It’s not as if they torque off the earth at 20,000mph, although that would be ASTOUNDING to see. So here’s my super drawn out really dumb question that I cannot wrap my head around the answer for the life of me.
Let’s say you have a rocket going 100mph going 90 degrees straight up from the surface of the earth. Why can’t it just keep going 100mph straight up. Just keep going and going. Up, straight up. Up up up and away? Why can it move up starting from zero miles an hour? If it can move up at 5mph even for an instant, why can’t it continue at that velocity all the way up.
All the answers have been wonderful if I was asking how to get something in orbit. I’m asking why 100mph 90 degrees going straight up works down here, but not up there? I cannot find a straight answer to this question no matter what I google. I appear to be bad at research or this is just a stupid ass question. I really just don’t understand the physics of this at all.
Let’s try this another way. Say I threw a magic baseball that whatever velocity it was tossed at, it maintained until it hit a object. It doesn’t disregard gravity. It just has a magic anaerobic motor that maintains the speed. Like cruise control. Say I throw it 90 degrees straight up at 35mph. Will it leave Earth? Why or why not?
In: Engineering
Getting to space is easy. But to stay in space and not fall immediately back down when your rocket shuts off, you have to start moving sideways fast enough that you continuously miss the planet when you fall toward it.
To escape earth’s gravity completely, or to achieve orbit, you have to reach certain speeds. Starting from zero you have to accelerate to those speeds. But also you’re starting at the bottom of an ocean of air. The air is thickest down here at the earth’s surface. You’ll actually waste fuel and cause unnecessary friction and heating if you try to go too fast at low altitudes. So if you wait to pour on the gas until you are high altitudes you’ll encounter less air resistance, less heating, and be more efficient. Try playing Kerbal Space Program!
There is a concept called ‘escape velocity’. This is basically how fast you have to go in order to escape the gravitational pull of the earth. You have to reach escape velocity before you are able to fly into space. So it’s not really that you have to go faster and faster, technically if you started at your escape velocity you wouldn’t need to increase your speed, but it’s usually very fast so it makes more sense to gradually build up your speed. There’s some math and stuff but this is my first answer to ELI5 and I’m not sure a 5 yo would understand the math.
As other people have pointed out, rockets need to travel fast enough to maintain an orbit. The velocity is not about how fast you are going “up”, but rather “across’. People have explained that part well enough.
The ELI5 is that your rocket will run out of fuel, so your rocket engine will shut down. The question is not “how fast does the rocket need to go the higher it gets”, the question is “how fast do you need to be going when your rocket engine runs out of fuel”.
If you had a magic rocket engine that had a limitless source of fuel and enough thrust to lift the rocket, you could fly up into space at a nice, leisurely pace, like as slow as a person walks if you really wanted to take your time.
Then you could stay above the Earth at any point regardless of whether you were in atmosphere or above the atmosphere. You would just use the always-on rocket to keep you in position. At that point the rocket is keeping you from falling back to the ground.
If you were to go straight up and then come to a stop you’ll just fall back down because of gravity. To stay in space above Earth you need to orbit the Earth. You can think of an orbit as constantly falling to the planet, but going fast enough to the side that you keep missing it. The lower your orbit the faster you have to go to keep missing the planet, the higher the orbit the slower you have to go to keep missing the planet. The reason people float in space while in orbit isn’t because there’s no gravity, it’s because they are falling.
Kerbal Space Program is a great game to learn about how orbits work. It shows you your orbit in real time, and helps explains different orbital mechanics. Here’s a video of a person showing how to achieve orbit in KSP. [https://youtu.be/_q_8TO4Ag0E](https://youtu.be/_q_8TO4Ag0E)
To you last question yes your magic baseball will leave earth but to the commenters that are talking about orbit is important. Let me explain:
Say you had your magic baseball. Also suppose for simplicity, that no other planets or stars or things with gravity existed. Just Earth. So that baseball keeps traveling and eventually it gets to space. Yay, Space! So that is the simple answer but what if that magic force disappears? The ball will fall back down to Earth (eventually) unless it gets out of earth’s gravitational field which reaches **4,500,000,000** light years away. The Sun is, on average, **0.00001581 light years** from Earth. So that ball would have to go a distance we simply can’t imagine if it wants to stay in space.
So that begs the question: “I thought we were weightless in space? I see astronauts floating in the space station?” So what’s going on here is a key to WHY the shuttles need to accelerate. What’s actually happening here is that they are in orbit so they are impacted by gravity. They are just going so fast around earth that they are falling and “missing” the planet as they fall. Its like the weightless feeling you get at the top of a roller coaster. But because they are in orbit, they are always at the top of the roller coaster.
So that’s why they need to add speed. Not because they need it to “get” to space. Because they want to **stay** in space. Unless they go really far away (way further than the sun) they will always be impacted by gravity on Earth (and the sun, and everything else!) and eventually fall back down.
Side note: If you find this interesting there is a great video game that teaches these concepts: Kerbal Space Program
>They start very slowly, and then build in speed.
rockets are very heavy because they carry all of their fuel onboard. But the longer they burn (and therefore the higher they get) the lighter they get (as they burn fuel and possibly dump stages) which allows them to move faster as they go higher. If you had to carry 100 lbs up 100 steps, but got to drop 1 lb on each step, you would get faster then higher you went too.
>100mph 90 degrees going straight up works down here, but not up there
it would – IF you had a spaceship with a magic engine that could produce a constant velocity without the need for fuel. That’s basically how Superman does it.
I think the answer you’re looking for is propellant efficiency. Let’s say you had a rocket (instead of a jet, because of the oxidizer issue as others have explained) and it could throttle its engine so that it only rose in the air at 5 mph. If it starts going faster than that, it throttles down a bit to maintain 5 mph. Too slow, and it throttles up a bit. And let’s say it never ran out of propellant (it had some magic engine like your baseball edit). Then yes, it would be able to climb into space and beyond. The further away it got from Earth, the more it would have to throttle down its engine as Earth’s gravitational influence became less and less. Eventually. once it was far enough from Earth’s gravitational influence, it could shut off its engine completely and would be able to continue at 5 mph away from Earth forever as long as it didn’t hit something or fall into the influence of some other body’s gravity.
So why can’t a non-magic rocket do this? Because it will run out of propellant long before it ever gets close to space. So why can’t you just add more propellant? Because then it will get heavier. There is a relationship between the propellant and the amount of weight it can lift: the more thrust you want (either in terms of overall force or duration of burn), the more propellant you need. The more propellant you add, the heavier your rocket gets. The heavier your rocket gets, the more thrust you’ll need to achieve the same speed or burn duration (and then there are other considerations as your rocket grows in size to accommodate more propellant, like structural and drag concerns).
So the rockets we use today must find the most efficient balance they can in working that out, and since most of them are trying to achieve Earth orbit, they also weigh in the need to get up to orbital speeds. It’s just not as efficient to go slower. Any amount you go slower than we do now would mean you’d need more propellant to achieve the same speeds, which means you’d be heavier, and so on and so on.
So could you go faster and require even less propellant? To a degree, but then you run into structural issues (can your rocket handle the higher g-force loads and air drag at lower altitudes? You’d probably have to add more structural mass to fight those higher loads, and now you need more propellant again to lift the higher mass). And then if your rocket is carrying people, humans can only stand so many Gs and be comfortable, and then there are maximum Gs where it becomes dangerous for humans.
So, if Im reading your question correctly, I think I might be able to shed some light on the rocket portion of the question. To maintain a low earth orbit, you have to have a lateral speed relative to earth of about 18,000mph and maintain that speed. A lot of it is way above my head and is literally rocket science, but if you were to just launch “straight-up” relative to the earths surface, you would never be able to get into a true complete and stable earth orbit, and once you ran out of fuel, your ship would not continue into space forever, but instead be pulled back into earths atmosphere and come crashing down or be taken into the gravitational well of another object if moving fast enough
>All the answers have been wonderful if I was asking how to get something in orbit. I’m asking why 100mph 90 degrees going straight up works down here, but not up there?
There is physically nothing stopping you from maintaining a slow vertical climb as long as you have enough fuel. It would just not be useful for anything that needs to actually get to space. I can’t think of any reason why you would actually do that.
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