Hank.Walker@UNH.CS.CMU.EDU (12/07/85)
Why do people keep talking about venting the leftover LH2 and LO2 from the ET once in orbit? This is now very expensive fuel or water we're talking about, perhaps more useful than the tank itself. I'd try hard not to waste any.
kcarroll@utzoo.UUCP (Kieran A. Carroll) (12/09/85)
*
The reason that I, for one, usually think in terms of venting
the leftover LH2 and LO2 from shuttle external tanks if they're
brought up to orbit, rather than saving this rather useful stuff,
is that the equipment required to scavenge it would be rather complex.
If it was too complex, it'd make the combined problem of
(bringing ETs to orbit and scavenging their leftover fuel) too
expensive to solve easily (and hence cheaply). Consider
some of the problems involved:
-the LH2 and LO2 are cryogenic fuels; they reamin in liquid form
so long as the tank doesn't absorb any heat. Any heat absorbed goes into
vaporizing these liquids. As liquid vaporizes, the pressure in the
tanks rises. If this pressure rise continues unchecked, the
tanks will eventually rupture, and you lose the fuel.
-a large fraction of the fuel could be saved by employing an evaporative
chiller: deliberately allow a small amount of the liquids to
evaporate, and be vented, keeping the remaining fuel cool enough
to remain in liquid form. This is the way your body keeps you cool,
by sweating. The same process keeps the Shuttle cool, by evaporating
water and venting the steam, while the payload bay doors are closed
and the heat-radiating panels stowed. This solution requires some
hardware to be attached to the tank, however, and the hardware may be
somewhat complex. For example, you have to make sure that only
gases are vented; in free-fall, however, the gases and fluids may
be difficult to keep separate. Also, you don't want your venting nozzles
to freeze (ie. collect deposits of solid oxygen, for example,
clogging them), as this could lead to an eventual tank rupture.
-you could "safe" the fuel by burning it to yield water, which
can be easily stored. However, this again requires extra hardware.
You'd have to get the fluids to flow to your burning unit, in
the correct proportions, in the absence of gravity. You might install a
low-thrust motor on the end of the tank to provide a bit of
inertial force, or you might have the shuttle fire its OMS engines
to do the same; both solutions would involve shuttle operational
constraints, however (ie. tie up the shuttle, until the burning was finished).
Or, you could re-design the interior of the tanks to be zero-g flow types
(like the fuel tanks on three-axis stabilized satellites), but
this would require a lot of extra work be done on the tanks.
Taking the tanks up to orbit initially seems attractive, because of
the fact that getting them there costs little or no extra fuel.
They arrive on-orbit practically for free. However, if you want to
keep them there, you ahve to spend some mass, and some engineering
development (eg. motors to keep the large, light tanks from re-entering
due to atmospheric drag; possibly attitude-control systems,
including power, computers, sensors, etc. to keep them
favorably oriented; hardware for scavenging unused fuel).
If you try to save the tanks and their contents, the problem may
become too expensive in its solution to be practical; ie.
you won't have the tanks on-orbit "for free" after all.
However, by venting the excess fuel (or by letting the shuttle use it,
rather than OMS fuel, to reach initial orbital altitude),
the problem's constraints relax considerably, and solutions become
much less expensive.
There! It felt good to get that off my chest; I've been mulling
these points over in the back of my mind for 2 years now!
--
Kieran A. Carroll @ U of Toronto Aerospace Institute
{allegra,ihnp4,linus,decvax}!utzoo!kcarroll