§ 3.4.1 Aerobraking

In space transportation, there is no such thing as a "free downhill" because there's no friction in space. It takes as much fuel to come down (and insert into a circular orbit) as it takes to go up.

An exception could be if we use the Earth's atmosphere for frictional braking. We do this when spacecraft like the Shuttle return to Earth. The Apollo program entrusted their heat shields and calculations for high speed encounters with Earth's atmosphere. We've just never done this to brake an object in a transfer from one orbit to a lower orbit, whereby the object would pass through Earth's atmosphere briefly one or more times in order to reduce fuel requirements in its orbital transfer.

This would require an "aerobraking vehicle", which would basically be a heat shield with the payload held safely within.

The heat shield would need to be able to withstand high temperatures and pressures. Unlike the reusable Shuttle, such heat shields may be "ablative", i.e., allow material to be blown off of them, this material taking away much of the heat as well.

Heat shields for aerobraking may be expendable. We may not want to re-use it because that would require we spend fuel to raise it up to another high orbit. The idea is to save fuel this way, not spend it. However, fuel from space resources may become cheap enough to warrant re-use. Or, the heat shield could be used to export products to Earth's surface.

Indeed, one of the applications of aerobraking is sending fuel to low orbit to raise things up to higher orbits.

Aerobraking shields can be made from asteroidal and/or lunar materials, as covered in the section on manufacturing refractory materials.

The main issue of aerobraking is safety. We would have to make it very safe so that there is practically no chance we will accidentally cause anything to hit a populated area on Earth, just like we have done for Apollo and every manned spacecraft over the past 35+ years. Objects must be small so that they are relatively harmless if there is a failure. Since the Pacific Ocean takes up almost a hemisphere, objects may aerobrake above that ocean rather than above land masses. A rapid deployment rocket must be available in case an orbital insertion rocket fails after a pass through the atmosphere, or if there's a bad error on the way towards Earth. Objects can have self-destruct mechanisms if there is any failure at any time, e.g., before entry, after the first pass, etc. For example, the payload can set to self-destruct automatically unless systems are operational and they get the command to not self-destruct. There can be multiple self-destruct mechanisms.

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