Colonizing Luna, Part 2: Gravitats


Originally written: 8 August 2005. Previous | Overview | Next

Luna’s gravity is too low for humans to maintain long-term health. Therefore, like long-term residents of microgravity environments, lunar settlers will need to live in ‘gravitats’ where gravity is simulated by spin (centripetal acceleration).

The good news is that the design requirements for lunar gravitats and deep-space gravitats are very similar, so many lessons and techniques can be learned once and shared. Science-fiction authors have proposed that large types of gravitats such as O’Neill cylinders be used for multi-generational voyages. I think we can start small by working out all the problems and details of gravitats on Luna and in low-earth orbit (just as 1950s sci-fi artwork proposed). Once we have a reliable model, an engine cah be mounted on the spindle-axis of the gravitat to push humans out on very long-term missions. These might not be the first missions to Mars (that could be done using capsules, especially by a dedicated nation-state such as China). But a mobile gravitat could support a multi-year mission to the Jupiter and Saturn systems.

Initial gravitat:

This is a dumbell form, about 100 m (300 ft) radius.

A pair of pressurized vessels are spun around a central tower. The vessels probably will be the upper stages of heavy-lift vehicles. They will be suspended by cables or rigid swing-arms. Access/egress is most difficult, either by stopping the spin of the whole habitat or by an EVA climb up/down a ladder to the central spindle.

Full gravitat:

This is a “bicycle-wheel” form.

To eliminate vertigo caused by inner-ear sensation of the coriolis effect of spin, I think a full gravitat will have to have a radius of 500 m, and therefore a diameter of 1000m (1 km). The rim, 3142 meters in circumference, could begin as an open truss-work that gets filled in by opposing/balancing modules over time. The spin rate would have to be about 1 rpm, thus a speed of almost 200 kph at the rim.

The ring would be a sequence of links and modules. The links would be open trussworks attached to the radial suspension cables, and attached to each module. To access the ring from the Lunar exterior, one must board a “synchro railcar” located in a trench underneath the structure. The railcar is accessed via an underground passage network, which leads out beyond the rim and inward to the central hub. The railcar would accelerate up to matching speed with the ring and mate with an airlock in the bottom of a link-truss.

The suspension cable network consists of upper and lower planes of radial suspension cables, cross-tied for redundancy. Thus the overall appearance of the gravitat would be very similar to a bicycle wheel–except the tires would look like a string of sausages.

Pressurized Modules:

Modules will tend toward a generic, standardized construction. Habitable space will be enveloped by at least two if not three outer wrappings, held rigid by internal gas pressure. These sacks wrap around a rigid trusswork which is connected to the link at either end. Human-occupied space is built as decks within that truss.
Insack: The innermost sack is held rigid at human-comfortable atmospheric pressure, which I think is at least 800 mB of nitrogen-oxygen mix.
Midsack: I think the second sack would contain argon gas, at maybe 400 mB; it serves as an emergency backup if the inner bag fails.
Outsack: The outermost sack is coated with material to reflect alpha-rays, and very tear-resistant to absorb micrometeorites. Whatever it is inflated with should be a good beta- and maybe gamma-ray absorber, but the pressure must be low enough that the bag can deform to absorb impact energy without putting too much strain on the fabric.

The interior each module has three decks, listed from top to bottom:

1. Topside: built on top of the truss. This deck is open to the fabric roof of the insack. Hydroponics, recreation, informal meeting areas. Include a projector to show stars, images, movies. (slightly less than 1g)
2. Main deck: work labs/stations, sleeping and showering (1.00g)
3. The hold: hydroponics, water recycling, tanks and other heavy equipment (slightly more than 1g).

Assembling the first full gravitat

This is a massive structure, and the vast majority of the material needed to build it must me extracted and refined on Luna. (The main worry here is the flexible sacks. What can they be made from? Probably a woven metal mesh, coated to make it airtight.) This means the full gravitat will not be built until refining and manufacturing facilities on Luna are well-developed.

Protective Covering Dome:

Permanent gravitats should be placed under protective cover to shield from micrometeorites, hard X-rays, and mistargeted landing vehicles. As resources permit, the domes should be:

a. A lightweight, open geodesic framework which anchors the upper end of the gravitat-spindle. This initial dome can also serve as the construction-scaffolding for a second, heavier, outer dome.

b. A heavier dome which includes enclosing panels.

c. A very strong dome which is buried over with soil or refinery slag.

Locating the habitats in small craters will provide a head-start in terms of lateral protection, and reduce the amount of material needed for covering dome assemblies.

Long-Voyage Vehicle Development:

The design needs of a permanent Lunar gravitat are almost identical to those of a long-voyage vehicle. Either the dumbbell or bike-wheel design would serve, with fuel tanks, engines, and communication equipment attached to the central spindle. Given a large enough, powerful enough launcher, the whole vessel could be flung out from the Lunar surface at escape velocity in one or two pieces; minimal in-flight assembly required.

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