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Since kinetic energy have to be converted into heat during re-entry, the re-entry into the earth's atmosphere seems very violent.

Why most rockets or crew re-entry modules plunge into the earth's atmosphere at it's full orbital velocity, leaving at the mercy of the heat tiles? Why don't they come to idle velocity before re-entry? Will carrying more fuel for the re-entry burn more safer and easier than the heat insulation mechanism?

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  • $\begingroup$ An ablative heatshield is much simpler and thus more reliable than a rocket engine. No need to rely on heat tiles and their special problems. $\endgroup$ – Uwe Sep 9 '20 at 11:45
  • $\begingroup$ The amount of fuel to slow down the reentry vehicle is exactly the same amount to accelerate it, meaning you need a full size rocket. However you need another rocket to bring this one to orbit as well. As a result the cost is rocket equation squired. $\endgroup$ – user3528438 Sep 9 '20 at 12:15
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    $\begingroup$ Because everyone will die within five minutes! $\endgroup$ – uhoh Sep 9 '20 at 12:15
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    $\begingroup$ @uhoh I like that one very much; it just seemed like less of an exact duplicate. $\endgroup$ – Organic Marble Sep 9 '20 at 12:43
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The fuel requirements to slow down via rocket would be enormous.

In order to slow down from orbital velocity (which in LEO is ~8km/s) you need to reduce your velocity by the same amount, which means carrying enough fuel to do that. Which means you need to carry that weight up to orbit with you, which means you need to use more fuel to get your increased mass to orbital speed in the first place. Of course that extra fuel increases the mass of the vehicle, so you need even more fuel, which adds more mass and.. you see where I'm going with this.

As Randal Monroe put it:

This exponential increase is the central problem of rocketry: The fuel required to increase your speed by one km/s multiplies your weight by about 1.4. To get into orbit, you need to increase your speed to 8 km/s, which means you'll need a lot of fuel: $ 1.4\times1.4\times1.4\times1.4\times1.4\times1.4\times1.4\times1.4\approx 15$ times the original weight of your ship.

Using a rocket to slow down carries the same problem: Every 1 km/s decrease in speed multiplies your starting mass by that same factor of 1.4. If you want to slow all the way down to zero—and drop gently into the atmosphere—the fuel requirements multiply your weight by 15 again.

So, while it's not physically impossible to do as you suggest, it would be difficult and inordinately expensive for any meaningful spacecraft. It's far cheaper and easier to slam into the atmosphere and let the friction do it's thing.

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