# Using Laythe for Jool aerocapture

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21 hours ago, cephalo said:

But this gas begins in an extremely rarified state. There's almost nothing for your craft to compress against. In any case, as your craft presses against the gas, the gas is also pressing against the craft. For every action, there is an equal and opposite reaction. This model seems to defy the conservation of energy. Setting up a high orbit around Jool with a periapsis of 190k would give you a massive source of power that would last millions of years. 2000k of heat can be useful if you're prepared to harness it! It sure seems wrong though.

hmmm 8000 m/s is a definite load of kinetic energy. Don't forget that mass is not as big a factor than velocity, which is exponential and not linear as mass. The tiny atmosphere particles have so few mass but the same relative speed. The problem with the conservation of energy is that the loss of energy by slowing such a comparably huge mass as a spaceship down is paid off with extreme heating of the little air molecules - and your ship's hull. Heating means conversion of kinetic energy, so you won't sail on at a 190k periapsis orbit forever. That's a misconception. If KSP really doesn't simulate an appropriate velocity decay, it would be a bug.

But one single high-altitude pass-through won't take away much velocity. You simply don't hit enough air molecules, even if they get steaming hot.

Think "I am driving a Truck loaded with blocks of concrete through a wall of a single layer of paper". The paper is ripped apart (=air molecule heating), but your speed won't be affected at all. You would have to drive through some hundred walls of paper to slow the truck down just a little bit.

Now the truck rips through the paper in an instant, but airbreaking is done for some minutes maybe. The masses are different, and the density is different. It's only an illustration, not a physical example.

But think: "I want the truck to be slowed with only one hit of the wall". You would have to make the paper much thicker. First you use cardboard, but no big effect, either. Then you use several layers of cardboard, and the truck trembles a little bit, and maybe the windshield is damaged, but it's not as slow as you would like it to be. A concrete wall will probably not stop the truck completely, but will be enough to kill the driver.

Making the wall thicker and thicker (=going lower on periapsis) has bigger effects on the speed of the truck but will damage it more and more. It's better for the truck to drive through 100 walls of cardboard than driving through 1 wall of concrete.

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