From: henry@zoo.toronto.edu (Henry Spencer)
Subject: use of external tanks
Date: Wed, 9 Sep 1992 23:00:37 GMT
Organization: U of Toronto Zoology

In article <1992Sep3.134158.3514@access.digex.com> mheney@access.digex.com (Michael K. Heney) writes:
>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...

For those who didn't read the above carefully :-), note in particular that
*it is no longer true* that the shuttle expends fuel just to get the tank
into a disposal trajectory.  Taking it into orbit, even low orbit, will
cost you.

To expand on Michael's general point, the tanks won't stay in orbit long
because they are big and light and very vulnerable to air drag.  This is
especially true because, left to themselves, they will stabilize with
the long axis pointing along the tank-Earth line, which puts them
broadside-on to orbital motion -- the worst possible case for air drag.

It is Politically Unacceptable to just let them come down anywhere.
You have to be able to either keep them up or deorbit them under control,
with a very high probability of success (i.e. you need redundant systems).

Another significant point is that any large, light structure has to count
on being punctured by space debris.  The Gamma-Ray Imaging Telescope
concept -- the most detailed study on ET use that I know of -- planned to
wrap an outer blanket around the entire tank once on orbit, both to act
as a "meteor bumper" and to contain popcorning insulation.

>There *is* an access "hatch" on the bottom of the LH2 tank - 3 feet in
>diameter and bolted on *very* securely...

In fact there are two, although one is obstructed by some of the internal
plumbing.  There is also one in the top of the LH2 tank (letting you into
the intertank space), one in the bottom of the LO2 tank (ditto), and a
door in the side of the intertank ring (giving you access to those hatches).
Getting access to any of them is going to require cutting away the spray-on
insulation, by the way.

>... 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...

Actually, you can shortcut this a lot by using the technique planned for
the original "wet workshop" Skylab concept:  once the hatch cover is off,
insert a cylinder 3ft in diameter and the length of the tank (actually,
you'd probably have to join two pieces end-to-end to get that length from
something fitting the shuttle cargo bay).  Bolt it down where the hatch
used to be.  The outside end of the cylinder contains your airlock,
which can stay *outside* and hence be any convenient diameter; only the
inserted part is limited to 3ft in diameter.  That part is packed as
solidly as you can with parts, plumbing, etc.; it becomes your utility
core, while things like structure can be assembled -- by people in
shirtsleeves, inside the pressurized tank -- from the parts.
-- 
There is nothing wrong with making      | Henry Spencer @ U of Toronto Zoology
mistakes, but... make *new* ones. -D.Sim|  henry@zoo.toronto.edu  utzoo!henry
Newsgroups: sci.space