Why things easily leave earths atmosphere but burn up while re-entering the same atmosphere
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Rockets go most straight up until they are out of the atmosphere and then accelerate to get to orbital velocity, so they aren’t going very fast when they go through the atmosphere.
Coming back you need to deal with orbital velocity, which is much much faster, and the amount of energy is proportional to the square of the velocity.
The velocity when it goes out of the atmosphere is around 1000 meters per second, while orbital velocity is around 7500 meters per second. That means the energy is about 50 times higher.
Rockets go most straight up until they are out of the atmosphere and then accelerate to get to orbital velocity, so they aren’t going very fast when they go through the atmosphere.
Coming back you need to deal with orbital velocity, which is much much faster, and the amount of energy is proportional to the square of the velocity.
The velocity when it goes out of the atmosphere is around 1000 meters per second, while orbital velocity is around 7500 meters per second. That means the energy is about 50 times higher.
Because it’s way way cheaper to careen into the atmosphere at ludicrous speeds and use air to slow down, than to use rockets
On the way up a rocket spends very little time in the atmosphere. This helps save a bunch of fuel fighting air resistance. To get the speed to enter an orbit it will accelerate sideways to high speeds only after it it gets high enough to be in very thin atmosphere/vacuum. You just don’t get going fast enough while in atmosphere to really need to deal with heat.
If you wanted to get down the same way you would have to spend fuel to decelerate while in vacuum/thin atmosphere, then calmly drop down lower. Basically you would need to double the amount of time the rocket burns. But that means you need to use even more fuel to lift that up there. And more fuel to lift that fuel See the “tyranny of the rocket equation” for more detsils, but as a very rough estimate to double the burn time means your rocket weight increases 10x.
Or you could use the atmosphere to slow down for the price of some heat shielding and a parschute and have a far smaller rocket.
Because it’s way way cheaper to careen into the atmosphere at ludicrous speeds and use air to slow down, than to use rockets
On the way up a rocket spends very little time in the atmosphere. This helps save a bunch of fuel fighting air resistance. To get the speed to enter an orbit it will accelerate sideways to high speeds only after it it gets high enough to be in very thin atmosphere/vacuum. You just don’t get going fast enough while in atmosphere to really need to deal with heat.
If you wanted to get down the same way you would have to spend fuel to decelerate while in vacuum/thin atmosphere, then calmly drop down lower. Basically you would need to double the amount of time the rocket burns. But that means you need to use even more fuel to lift that up there. And more fuel to lift that fuel See the “tyranny of the rocket equation” for more detsils, but as a very rough estimate to double the burn time means your rocket weight increases 10x.
Or you could use the atmosphere to slow down for the price of some heat shielding and a parschute and have a far smaller rocket.
When you lift a hammer it doesn’t make a hit, when you let it go down it does.
You use a massive rocket to push the thing up through the atmosphere, at a controlled, increasing speed.
Broadly, at re-entry, the vehicle needs to bleed the speed it accumulated in the way up, and the preferred method is by air friction. Now, you can’t really chose a shallow path as there’s much risk to bounce on the atmosphere and get back into space, then fall down again at a bad angle again. Instead, they chose the safest approach of going down sharp enough to pierce the atmosphere.
This comes at the expense of having quite rough minutes of very hot friction and other the atmosphere.
Once you lift the hammer, it has to impact something on the way down. The only way to prevent it is to put the hammer down gently, which in the space equivalent, would mean having a big rocket to slow you down before re-entry, which would require a lot of fuel, really a lot of fuel. And even more fuel at take off to carry up the extra load. Doable? Yea. Economic? Nope.
When you lift a hammer it doesn’t make a hit, when you let it go down it does.
You use a massive rocket to push the thing up through the atmosphere, at a controlled, increasing speed.
Broadly, at re-entry, the vehicle needs to bleed the speed it accumulated in the way up, and the preferred method is by air friction. Now, you can’t really chose a shallow path as there’s much risk to bounce on the atmosphere and get back into space, then fall down again at a bad angle again. Instead, they chose the safest approach of going down sharp enough to pierce the atmosphere.
This comes at the expense of having quite rough minutes of very hot friction and other the atmosphere.
Once you lift the hammer, it has to impact something on the way down. The only way to prevent it is to put the hammer down gently, which in the space equivalent, would mean having a big rocket to slow you down before re-entry, which would require a lot of fuel, really a lot of fuel. And even more fuel at take off to carry up the extra load. Doable? Yea. Economic? Nope.
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