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
While I only have a little knowledge on the subject, so I’ll need someone to help with some of the science verification, but hopefully I can help.
Your question seems like its two parts. To answer the part about a rocket or object ascending at a constant speed, Yes it would eventually leave earths atmosphere and eventually its significant gravitational pull, orbiting the sun in a similar orbit to earth due to the orbital momentum it still had. Depending on the direction it was going after leaving earth, there’s a lot that could happen.
For that object to maintain that speed, it would have to have a force pushing it upwards continuously, such as the engines on a rocket or the propeller on a helicopter. This is due to the a combination of things such as air resistance, (which doesn’t quite “push it down” so much as resisting its movement. Kind of like pushing your hand through mud, only with a lot less resistance.) and the gravitational pull of earth on it, (which does pull it down).
For the answer about practical rockets such as space craft, it gets a bit more complicated. For those, the goal is usually to get into an orbit. It’s usually easier to gain the speed necessary when there’s still atmosphere for the engines to push against. You want enough speed to escape the atmosphere, but not so much that the air resistance on the front of the craft generates excessive heat or begins to resist the craft in greater proportion than the thrust you’re generating to maintain that speed. The higher you go, the less air resistance, meaning you can go faster before achieving orbit. What’s interesting that we’ve discovered, is that its actually more efficient to go a little sideways so that you don’t have to fight gravity/air resistance as much, saving fuel, weight, and money. This is where it gets a little hard for me to explain through text. Once you begin tilting in a horizontal direction, not only do you begin building the horizontal velocity needed for orbit, but because the rocket is heading so fast, some of the air is pushed under the rocket, generating a (relatively) small amount of lift. Similar to how aircraft wings generate the lift needed to keep the craft flying, just on a much less wing-like surface. However, This needs an incredible amount of both speed and constant acceleration to maintain and continue getting higher. The speed is also needed due to the massive weight of the rocket, and relatively small surface area on the bottom for the lift.
Once you leave the atmosphere, this is where my knowledge gets very spotty. You no longer have to worry about air resistance slowing the craft or anything, just finishing achieving the orbit you want.
If you want so great videos on stuff like this, check out Scott Manley on YouTube and his KSP videos! Hope this helped!
Edit: I can’t seem to find any obvious videos by him on the subject, but there’s probably some out there that showcase this better than I can explain it.
> 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.
If you can find a jet fighter that can go 90 degrees up with infinite fuel and oxygen to burn it, then it would work. You only need to orbit if you don’t intend on fighting gravity.
It sounds like you’re asking why rockets have to go fast to get to space. They don’t! They have to go fast to stay in space once their fuel runs out. So if you could build a jet fighter with an infinite source of fuel, that was airtight and had some way of maneuvering, then yes. Point it upwards and have it travel at 1 mile per hour if you want.
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
> 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.
I feel as if nobody had answered this yet. There are two reasons. One has been hinted at by Kerbal Space Program players already, and that is that going too fast in the lower atmosphere is really bad for your fuel mileage. If you ever held your hand out of a moving car, you have felt the wind pushing it towards the rear. That same wind is on the rocket, and while they try to build them very pointy to play nice with the air, they still feel the resistance of air. There is less air the higher you get, so you can go fast unimpeded once that air is out of the way.
But there is a second reason. They only go fast high up because they only *can* go fast after having gone slow for a while. For one, every engine from a car to a rocket can only do so much acceleration, that is to say that building up speed takes time. This is why car testers always give you the 0-60: How much time does the car need to get up to highway speeds.
The thing with 0-60 is that the heavier your car is, the longer it takes to get up to speed. Now for a car, most of that weight is the body, the engine, the cargo, the passengers and such. Not so much for a rocket. For the rocket, almost all the weight is in the fuel tank. So, when the rocket is almost out of fuel, it is very much faster than it would be with a full tank. Which is also why they carry so many engines that are just thrown away on the way up – from a Saturn V’s stages to the Space Shuttle boosters. Once you are high up, you have spent a lot of fuel, the rocket has become much lighter, you do not need such big engines anymore. And you are still gradually going faster and faster because up there, there is not much that slows you down, and you can keep accelerating.
>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.
At the altitude of the international space station, gravity is still 90% as strong as it is on the ground, so you have to be going sideways fast enough that the planet curves away as fast as you fall down. That’s why you have to go super fast to get into orbit. You only go up first in order to get out of the atmosphere so it doesn’t burn you up and slow you down.
Once you’re in orbit, going faster in the same direction you’re moving will make the other side of your orbit go higher, because you’re adding energy for the distance you travel in the direction of the force applied. If you try to burn straight up while in orbit, your orbit will go higher 1/4 orbit in front of you, but it will go lower 1/4 orbit behind you.
As for gravity getting weaker as you go higher, that’s true, but you still need more energy to get that high than you do to go 17,000mph in low orbit.
Atmosphere aside, if you threw a ball straight up at escape velocity, it wouldn’t fall back down, but that’s more like 25,000mph.
As for the magic baseball, that depends on its exhaust velocity, it would have to be absurdly high in order to overcome the gravity losses for moving so slow.
Basically, if you’re hovering, 100% of the fuel you burn is wasted, 0% of it is helping you get where you’re going. holding a constant 35mph is very very close to just hovering. Even with the most efficient chemical rockets on the planet you’d be out of fuel in 10 minutes at that level of thrust, having only gone up 5~6 miles. The farther you move in the direction of thrust, the more energy you get out of that burn, so you want to be going as fast as possible as early as you can. For more information on this, search for “Oberth effect”.
I strongly recommend you play the game Kerbal Space Program.
I’m pretty sure this is a physics question about forces on an object. Think about an object on ice, the smoothest ice you can imagine (The main idea is to lower the friction on the item). If you were to give the hockey puck a single push, you would expect it to glide at a steady speed across the ice. This is the same concept as “An object in motion will stay in motion”. Without any other action on this hockey puck you expect it to maintain speed.
Now push on the hockey puck to get it going, but then give it a second additional push. It should now be going faster than the first hockey puck since it was given more force. This leads to the idea that each “piece of force” you add gives the puck more speed.
Now we want to get to a constant force. This is a little harder to imagine, but imagine you could keep pushing the hockey puck once per second. Each second you now keep adding more force which creates more speed. Each second the hockey puck will travel faster and faster because you give it an additional push.
This is what is happening in the rocket example. The rocket is creating the same amount of thrust (force) every second, so the first second the rocket has very very little speed because most of the thrust is working against gravity. Maybe 90% of thrust is just counteracting gravity, but that last 10% of thrust works like the pushes on the hockey puck. So each second 10% of the thrust adds more and more speed to the rocket.
Now onto your question roughly. So yes you could create a rocket that would maintain the same speed to get to space, and what this would mean is each push on the rocket would need less and less force behind it, but that would most likely be inefficient in either a fuel aspect or the engine thrust aspect. (This is the part I know least about)
>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.
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.
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