Allen Meece wrote:
Apparantly the Russians are testing a reentry parachute and it makes me wonder,
is exotic high tech heat-shielding really necessary for reentry?
We all know that if the reentry angle is too shallow, the vessel will skip
off the atmosphere, cool down and come in for another skip. These skips could
get shorter and slower until the vessel had slowed enough for a steep dive into
the air.
What if this vessel were a very large disc with low weight/surface area
ratio and it didn't plow into the atmosphere but lightly skipped along the
uppermost air until it slowed down?
Don't forget, that's how the Rogallo Wing got started!

The lower the areal density, broadly speaking, the higher up in the
atmosphere the decelleration is done.
The peak G loading does not change.
What would be really nice would be if you could get the areal density
down low enough that relatively low temperature materials (say aluminium,
kevlar) could radiate out all the heat being generated, and not have
any of this nasty ablator or refractory mess.
Unfortunately, the numbers work against it.

A carbon-carbon heatshield can take around 2000K.
Kevlar around 600K.
Only a factor of three, but unfortunately thermal radiation is proportional
to temperature^4, so that's almost exactly a hundredth of the energy radiated
per square meter.

Neglecting quite a lot of stuff, this means that a kevlar heatshielded
vehicle needs to have a hundredth of the areal loading.

(I have not verified the 600K number, it seems high to me)

For stuff like polythene, or specta, then number is more like a thousandth.

--
http://inquisitor.i.am/ | | Ian Stirling.
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