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[The Following is an excerpt from a letter to Allen Meece]

[Updated 14 September 2002]

The following is a rough breakdown of the estimated mass for the crew cabin, airlock, and externally attached equipment, made using my plastic hab floorplan. The weights are based on volume-density calculations or guesses using common equivalents as a reference (equivalents which may or not be suitable to the task). Everything is to be considered “off the cuff” baring further elaboration.

CREW CABIN:

The crew cabin mass estimate can be further reduced by using a thinner inner pressure wall, which will require a custom cast tank. Note that the outer pressure wall must be reinforced accordingly, but it takes far less mass for the same load because it isn't subjected to the same stresses as the inner wall. Mass savings could be as high as 1800kg.

Using Ni-MH batteries for power instead of fuel cells reduces the Crew Cabin's H2/O2 regulation system to just 50kg by eliminating the need to crack water for air, though the batteries will weigh as much as the eliminated fuel cells. Half the water/antifreeze supply goes away. Also, emergency batteries become unnecessary, as does most of the emergency oxygen (because oxygen is delivered in an immediately usable form that requires little energy or waiting for preparation). Mass savings could be as high as 800kg for the crew cabin alone, not counting reductions in the solar array and other platform systems.

Accumulating less dramatic savings in other systems, the Crew Cabin total mass could be reduced below 5000kg.

AIRLOCK:

As with the Crew Cabin, the airlock mass could probably be pared down by judicious changes. However, it's simple design (an insulated tank with air cylinders and valves in it, with a compressor on top) limits these reductions. It will probably weigh at least 1000kg.

MISC. HAB STRUCTURE:

The Crew Cabin and Airlock are just two parts of the suspended deck design. Here is the rest:

Using a suspended hab instead of building it into or over the keel allows a unique emergency evacuation system: parachuting the entire hab down. My friend Floyd Moore, a former race car driver and pit crew mechanic, informs me that the drogues used on dragsters are rated for 600+mph and I think four or five should be sufficient to keep a 7 ton hab below 300mph and upright during its descent. Unlike the main chutes, they can be deployed almost immediately, as they will survive a fall through the stratosphere so long as they are able to prevent the Hab from exceeding the speed of sound. Thus, a recovery system is included in this mass estimate.

Composites are unsuitable for the flooring, but might replace aluminum in the deck frame. (They may also be employed in the crew cabin and airlock, which would decrease their mass still further.)

From this estimate, we can expect the total hab mass to be between 7700kg and 11000kg. The best way to get this mass below 7000kg is to use a smaller crew cabin. A 3.5m diameter cabin could be as short as 5m and still sleep 6, but there wouldn't be room in the cabin for them to do much except sleep. Eliminating a separate bath partition allows some further mass reduction. (Reducing the workroom size isn't a viable option because that's where people are going to be spending their time with a smaller crew cabin.) Further reductions in crew cabin mass can be had by limiting the instrumentation aboard, since that determines power system and other requirements.

That's my first guess as to the hab mass.

CME