From: mheney@access.digex.com (Michael K. Heney)
Subject: Re: Inflatable Space Stations - Why Not ?
Organization: Express Access Online Communications, Greenbelt, MD USA
Date: Thu, 3 Sep 1992 13:41:58 GMT

In article <BtwzCC.852@news.orst.edu> katzr@ucs.orst.edu (Russell Katz) writes:
>This is a bit related to the topic...I think.
>
>I have read in several sf stories about using the shuttle main tanks as 
>modules for space habitats...just having the shuttle carry the tank with it
>into orbit.  Some authors stated that it would not take any extra fuel(??)
>to do so.
>
>Any of you experts out there have an answer as to why NASA doesn't use the
>tanks as at least a temporary habitation?
>
I hardly qualify as an expert, but there are several very good reasons why
NASA doesn't use the tanks.
 
1)  They tend to come down pretty quickly.  Empty, they're very low-density
structures, which causes their orbits to decay fairly quickly.  Note that 
the current direct-ascent trajectories used on shuttle launches has the
orbiter climbing to apogee and the tank re-entering after MECO, with no
burns to alter either's trajectory.  The tank *is* set tumbling by venting
LOX thru the top to produce a predictable cross-section and horrible
drag characteristics, while the orbiter keeps a minimum drag attitude.

2) There's residual LH2 and LOX in the tanks - close to 5,000 lbs LH2,
if I remember correctly.  They don't run the tanks dry because the
engines would "belch" as fuel flow became erratic the last few seconds -
not healthy for the engines.  Mission planners don't really want to 
carry that stuff around on the orbiter's belly any more than they have to -
a valve-closing problem or just plain leakage could allow both LH2 and LOX
to vent (the tank is built to be a throwaway; not space-certified orbital
hardware), so they want to drop it as soon as possible.

3) The foam insulation has trapped gasses in it (it was foamed on
in the atmosphere, after all), and there are concerns that it could
"pop" like popcorn over time in vacuum, contributing a fair amount of debris
in the same orbit as the orbiters like to use.  There have been suggestions
to wrap the tank in beta cloth or something similar once on orbit to
contain this.

4) The level of on-orbit operations needed to convert an ET are non-trivial.
There *is* an access "hatch" on the bottom of the LH2 tank - 3 feet in
diameter and bolted on *very* securely.  Martin Marietta (henceforth MM)
has determined that a spacesuited astronaut *could* get through the
hatch - it's *barely* big enough.  Now, you need to somehow add on a
*real* airlock, build internal structures, add power supplies, plumbing,
windows, etc,  all of which involves cutting and welding on orbit by 
astronauts on EVA.  Yes, it sounds like fun (!), I'd go and do it in
a minute, but it's more work and money than NASA can reasonably be expected 
to put out when it's easier and SAFER to build the stuff on the ground and
boost it to orbit with minimal EVA required.
 
MM has (at least as of 1989) an office which dealt with ET on-orbit 
applications.  The US gov't apparently in 1988 or 1989 said that for 
a 5-year period, anyone who wanted an ET could have it *free* after launch
provided they could assure controlled re-entry or maintenance of a 
stable orbit AND dealt with the insulation-popcorn concerns.  I
recall that it would require an extra 1800 lbs of OMS fuel for the 
OMS-2 burn to carry the ET all the way to LEO; this may fall within 
the "management reserve" allocated on a mission, but this reserve is for
contingincies (e.g., the Intelsat rescue - damn near ran the tanks dry
in the nose because of 3 instead of 1 rendezvous (rendezvouses?)
(and very fuel-efficient flying at that).

Bottom line - yeah, I'd like to see all that metal hauled up to 
orbit, too, even as just an orbital scrap yard.  After all, we *will* 
at some point be able to use tons of metal in oribital apps sooner
or later.  There's been talk of a "beam builder" crunching the tanks into
tiny pieces, melting them, and extruding truss members, for example.
But it's not gonna happen this year, or next.
-- 
Mike Heney                |                                  |  Reach for the
mheney@access.digex.com   |     * Will Work for Money *      |  Stars, eh?
Kensington, MD (near DC)  |                                  |

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