Autzoo.1545
net.space
utzoo!henry
Sun Apr 11 04:08:16 1982
external tanks, cont.
Actually, neither I nor A.exp@Berkeley was entirely right about whether
it's easier to use solids or the shuttle main engines to get the tank
into a suitably-high orbit.  It depends on how high the orbit needs to
be, something I'm not sure of.  The tradeoffs are like this:

1. Using the SSMEs to carry the tank higher requires carrying the orbiter
	higher too, which wastes fuel *unless* the orbiter is going that
	high anyway (say, to orbit a long-lived satellite).  Remember,
	though, that the ET is *heavy* -- I seem to recall it weighs as
	much as the whole orbiter -- and thus it's not as much of a
	waste as it sounds at first.

2. If you cannot get enough fuel for the operation simply by filling the
	ET full and cutting down shuttle payload, then using the SSMEs
	requires more tankage somewhere.  This is a pain because the
	cargo bay does not have plumbing for LOX/LH2, last I heard.
	Moreover, LOX/LH2 is much bulkier than solid rockets.

3. The SSMEs are not restartable, last I heard, so if you need more than
	one burn you cannot rely entirely on them.

4. The SSMEs, being very-high-pressure oxyhydrogen engines, have a much
	higher exhaust velocity than any solid.  So it is decidedly to
	your advantage to use them if you can.

I suspect the optimum approach, actually, is to use the SSMEs as far
as possible, getting the fuel for this by reducing shuttle payload and
draining the ET as dry as possible, and then use solids for the rest.
The solids preferably should be attached to the ET at launch, to avoid
in-orbit moving and arming and to keep the cargo bay clear.

Alternatively, if the Air Force's project to put Titan engines plus
tankage on the bottom of the ET to get very heavy loads into polar
orbit goes well, this might be a very handy propulsion system for
moving the tank to higher orbits.

It is reasonable that fitting the ET out as a station would be no harder
than building an apartment.  But I'm not sure this is encouraging;  ever
seen how long it takes to accomplish the latter?  It's a lot of work even
in a nice helpful one-gee field.  I am not quarrelling about the project
being worthwhile, but NASA will have to get used to extensive in-orbit
work projects before it is willing to seriously consider this.  When it 
comes to funding such projects (as opposed to funding studies of them),
NASA is very timid and conservative.  Part of this, of course, is because
with turkeys like Proxmire around, NASA daren't goof badly.  But NASA
has always been obsessed with safety and never doing anything untried
when it can be avoided;  arguably the space program would have gone
further and faster if a bolder approach had been adopted.  (It has even
been suggested that the lack of long-term emotional commitment to the
space program by the public was partly a result of lack of boldness:
it never really looked *hard*.)

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