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
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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