[Space, Spaceships] Armor needed for Aerobraking/Re-entry

[thrash]

Originally Posted by DaltonS

There should be an equation to calculate how long it takes.

In reality, "orbital velocity" and "escape velocity" are situationally dependent -- the formulas in the books are generalizations. For any particular case, the only basic requirement is to change from an open, hyperbolic orbit (e > 1) to a closed, elliptical orbit (e < 1). That, in turn, depends on how close to parabolic (e = 1) your original approach orbit is. (All this relative to the body you intend to orbit.) The math is messy, since it's a variation on Kepler's problem with deceleration based on atmospheric density. You would normally solve it through numeric methods, rather than analytically.

Once you are in closed orbit, you may be able to use subsequent passes through the atmosphere to lower your apoapsis still further and circularize your orbit. But that's a different maneuver.