In sci.space.station message -
september.org, Thu, 17 Jan 2013 07:25:27, Jeff Findley
posted:

In article om,
says...

Question: if a detached module is ordered to open a valve to let the
air
out, could this provide enough delta-V to cause it to drop sufficiently
to cause re-entry ? Since this module is to be empty, the amount f
energy required to lower its orbit would be less significant than a full
module.

You'd have to "do the math". One big challenge would be to insure it
was pointed in the right direction when it was deflated. If it's got
enough delta-V to reenter, it would have enough delta-V to impact the
station. Due to safety issues alone, I'm guessing it will either remain
completely inflated or would be only partially deflated before being
released by ISS.

You should be able to work it out for yourself. I've seen and forgotten
the necessary figures, but no doubt you can find them. I'll assume that
it, inflated, is roughly equivalent to a 4m cube of mass 2.5 tonnes.

Volume 64m^3, 64000 litres; air is 1.2g/L, so 77kg of air - mass of air
is 3% of total. Exhaust velocity will be of the order of Mach 1, 760
mph, so if the nozzle is efficient the delta-V of the module will be 3%
of that, about 23 mph.

IIRC, the delta-V of the re-entry burn of a Shuttle was of the order of
225 mph. Such a module therefore cannot power its own re-entry, though
it could lower the orbit to be noticeably below that of the Station.

Now re-do that with the correct figures for the module and a better
estimate of the nozzle efficiency!


I suspect they'll leave it at least partially inflated so that it
remains a "fluffy" structure which will reenter faster than if they
deflated it completely and caused it to become more dense.

It is the initial lowering of altitude which takes longest - think
"scale height" - so the effect of an initial reduction of orbit size may
be more useful than maintaining fullest fluffiness. But, as a pure
physicist's guess, I suspect that a great deal of deflation in pressure
will be needed before the cross-section of the module decreases much.


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(c) John Stockton, near London. Mail
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