Orbital skydiving—possible?

[InigoMontoya]

Once upon a time I used to get paid to study re-entry trajectories for certain sub orbital vehicles. I'll say this: When you're coming in from space, the atmosphere at about 250,000 seems to hit you like a brick wall. You can very easily attain fatal G levels if you're not managing your energy prior to hitting that altitude (read: density) regime.

 

On a more helpful note, I recall reading some studies way back wherein the idea for an "orbital parachute" as a pretty sizable plastic sphere/balloon that was inflated to some magical value that was quite large. Gas volumes were managable because it was in space, blah blah blah. As I recall, the idea was that this thing would bounce along the upper reaches of the atmosphere like a beach ball bleeding energy with each bounce. Because it had such a low ballistic coefficient, it would slow down very quickly even in the very outter reaches of the atmosphere.

 

Not that the balloon was supposed to get you to the ground, mind you. It was just supposed to get you to alive and below 150,000 feet (or something like that).

 

Of course, there's still the problem of how to get from orbit to the upper atmosphere....

[CaptainPanic]

The problem is actually simple. The solution is not.

 

Objects in low earth orbit (200 km altitude) have a velocity of about 7800 m/s.

The total energy they have (kinetic and potential) is therefore:

 

[math]E=m\cdot{g\cdot{}h}+0.5\cdot{m\cdot{v^2}}=m\cdot({g\cdot{h}}+0.5\cdot{v^2})=1\cdot({9.81\cdot{200000}}+0.5\cdot{7800^2})=32 MJ[/math] (per kg)

 

Now, that's a lot of energy. This number clearly shows that you have to dump the heat (absorbing all the heat will cause you to evaporate). And whether you like it or not, you have to get rid of it - and, unless you use thrusters (rockets) to slow down, the only place you can dump it is in the atmosphere, through heat transfer and/or radiation. You have to heat the air by friction.

 

p.s. cool thread to necro

Edited November 16, 2009 by CaptainPanic