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The SPACEHAB laboratory is designed to fly in the Space Shuttle. Shuttle has a flight rule that payloads must allow access to the payload bay door latches by a space-suited astronaut, or the payload must be jettisonable. The flat top provides an EVA access corridor to the entire payload bay door hinge line. For more explanation, go to: www.spacehab.com
We changed the reference mission to use SPACEHAB primarily because of cost. Remember, we're closing the loop on cost in this program, not on engineering elegance. And among available manned modules, SPACEHAB has the best weight:cost performance ratio for application to our lunar mission. Of course there's also an appealing altruism to SPACEHAB being the world's first and only commercially developed manned spacecraft, and it certainly helps that both the owner and builder of SPACEHAB are enthusiastically supportive of the Artemis Project.
Far from being "stuck with it" (as has been said) the flat top of the module is probably something we would do if we'd thought of it.
The weight impact of the structure to accommodate the flat top is much less than you'd imagine if you're thinking of a minimum-weight pressure tank. The structure is designed to be operational under normal Shuttle launch and landing operations (3 g's) and to survive Shuttle crash loads (9 g's) without destructive failure.
In fact, the flat top is probably a weight advantage when you add up the whole mission. The Artemis Project spacecraft will be launched with a lot of external equipment. Regardless of the launcher used, that flat area gives us a convenient place, with structure hard points, to mount that equipment for launch. If we used a circular cross section, we'd have to carry structural load paths through the pressure vessel and support them with additional external structure.
If we used a pressure vessel that fills the entire payload envelope, such as the space station nodes and habitat modules, all external equipment would have to carried separately and assembled in orbit. That means more weight, usually more than the weight of the spacecraft itself, in the parasitic weight of flight support equipment.
Another great advantage of the SPACEHAB flat top defies numerical evaluation at this point in the program: it looks neat. We're in show business. Looking cool counts for a lot, perhaps even billions of dollars. In that same vein, SPACEHAB has a very romantic history. It grew from plans to fly a commercial passenger service aboard the Space Shuttle. The commercial passenger service didn't work because there was no way to get the people out quickly in the case of an emergency on the launch pad, but its heritage still shows in the "can do" attitude of the people who built SPACEHAB -- people who not only said they could, but actually did.
To accommodate any kind of viewing port or scientific airlock on the side of a pressure vessel, we will require a break in the structure. It's lighter and less costly to do this on flat surfaces than on curves.
Windows will cost interior volume on a circular cross section, so unless we're going to have no windows at all, we're not losing much interior volume to having a flat section.
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