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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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