Notional regolith bag covered Kevlar biosphere and bio-torus next to two solar powered cylindrical SLS propellant tank technology derived lunar regolith habitats on top of a microwave sintered lunar outpost floor.

by Marcel F. Williams

NASA's Space Launch System (SLS) scheduled to go into operation by 2020 or 2021. But large cargo landing vehicles are going to be required in order to utilize the SLS for the deployment of lunar outposts habitats. Large multilevel pressurized habitats derived from SLS propellant tank technology could be deployed to the lunar surface on top of cargo landing vehicles designed to fit within a 10 meter in diameter SLS payload fairing. Such multilevel habitats for the lunar surface could be 8.4 meters in diameter, with two to four levels available for habitation. The average apartment in the US provides approximately 82 meters of floor area. With each 8.4 meter in diameter level providing more than 55 square meters of floor area, a single multilevel SLS deployed lunar habitat could provide lunar astronauts with 105 to 210 square meters of habitation floor area.

X-Ray of notional SLS propellant tank derived Lunar Regolith Habitat

However, substantially larger lunar habitats would require the deployment of inflatable structures.

Various types of inflatable habitats have been proposed by NASA personal since the dawn of the space agency. In the 1980's, M. Roberts of NASA's Johnson Space Center, proposed deploying inflatable Kevlar biospheres to the lunar surface. Since the lower hemisphere of such biospheres would be underground, the radius of the inflated habitats would be limited by the depth of the regolith. Depending on the region on the lunar surface, lunar regolith can be as deep as eight meters or as shallow as two meters before encountering bedrock. Such depth constraints on the lunar surface would limit the diameter of a biosphere to just 4 to 16 meters. A 16 meter biodome pressurized with an Earth-like nitrogen and oxygen atmosphere of 14.7 psi (101.3 kPa) with a safety factor of four would weigh only 1.76 tonnes.

X-Ray of notional regolith bag shielded biosphere on the lunar surface (Credit: NASA)

However, the constraints of regolith depth could be easily alleviated by inflating a biosphere-- on top of the lunar surface-- and surrounding it with an inflatable bio-torus. An inflated Kevlar torus would be an inherently self supporting structure. So regolith could be deposited within the cavity between the bio-torus and the biosphere, providing structural support for the inner biosphere. Since the surround bio-torus would require substantially more Kevlar material than the biosphere, reducing the diameter of the torus to approximately half that of the biosphere could substantially reduce the amount of mass needed to be deployed to the lunar surface. A spacious cavity between the bottom of the biosphere and the surrounding bio-torus could accommodate additional living space in the form a smaller bio-torus about one third the diameter of the external bio-torus.

Inflatable torus extraterrestrial habitat (Credit: NASA, 1961)

Lunar Statistics

Diameter relative to the Earth: 27.3%

Surface area relative to the Earth: 7.4% (Land area not covered by water only comprises ~ 29% of the Earth's surface)

Surface gravity: 0.17g

Regolith depth: 2 to 8 meters

Annual amount of cosmic radiation on the Lunar surface during the solar minimum - 38 Rem

Annual amount of cosmic radiation on the Lunar surface during the solar maximum - 11 Rem

(Maximum amount of radiation allowed for radiation workers on Earth per year - 5 Rem)
(Maximum amount of radiation allowed for adult female during nine months of pregnancy -)

The biodome and the upper and outer exterior of the bio-torus could be covered with regolith bags that are either 2.5 meters or 5 meters in thick, depending on what level of radiation protection is desired for the habitat. At least, 10 centimeters of lunar regolith is required to protect humans from the cell killing heavy nuclei component of cosmic radiation. Thermal fluctuations of the lunar surface may also require as little as 10 centimeters of lunar regolith. Assuming an average regolith density of about 1.5 grams per cubic centimeter, at least 60 centimeters of lunar regolith would be required to protect the habitat from micrometeorites.

Its relatively easy to shield habitats and even humans in pressure suits from the heavy ion component of cosmic radiation. But most cosmic ray particles are composed of the smallest ionized atoms: protons (85%) and alpha particles (ionized helium atoms) which are much more difficult to shield against. Most protons and alpha particles streak harmlessly though the vacuous space between the atoms of the human body. But the relentless rain of these cosmic ray components inevitably results in impacts upon our body tissues.

On average, humans receive about 620 mrem per year of radiation due to a combination of sources from both cosmic and terrestrial radiation sources. The maximum recommended radiation exposure for a pregnant woman is 50 mrem per month which comes very close to the average radiation exposure that humans on Earth experience in a year.

The maximum level of radiation exposure for radiation workers on Earth is 5 Rem per year. And that would require approximately 2.5 meters of regolith shielding. But the maximum level of radiation exposure allowed for a woman during the term of her pregnancy is just 0.5 Rem. So lunar regolith shielding would probably have to be increased to 5 meters (the same level of radiation shielding provided for humans by the depth of the Earth's atmosphere). Inflated with an Earth-like atmospheric pressure, biospheres and bio-tori could easily support the weight of 5 meters of regolith.

Of course, there would be no shortage of available regolith on the surface of the Moon. Just one hectare of regolith on the lunar surface could provide between 20,000 to 80,000 cubic meters of shielding material (2 million to 8 million cubic meters per square kilometer) for large pressurized habitats. And the excavation and deposition of lunar regolith and even the production of regolith bags could be done by robots teleoperated by personal employed on the surface of the Earth.

During solar minimum conditions, the maximum radiation exposure on the lunar surface can exceed 3000 mrem per month. A hardened pressure suit designed to protect against the heavy nuclei component of cosmic radiation could reduce general cosmic radiation exposure by two thirds. But even 1000 mrem (one Rem) per month would exceed annual radiation levels for radiation workers in less than six months. Pregnant lunar colonist would probably have to remain inside the protective confines of their habitat during nine months of pregnancy. But even if lunar colonist spent only 10% of their time outside of pressurized habitats (less than 2 Rem of annual exposure within radiation hardened pressure suits ), that would still avail them to more than 16 hours a week of EVA time on the lunar surface. But I seriously doubt if most lunar colonist will spend more than 5% of their time outside of the comfort of their lunar habits.

So it seems likely that Lunar colonist will spend at least 90 to 95% of their time on the Moon within the confines of pressurized habitats. So living on the Moon will mostly be about living within the protective confines of pressurized habitats that are also designed to protect its inhabitants from the dangers of micrometeorites, extreme thermal fluctuations, and excessive radiation exposure.

So if future Lunarians are going to have to spend the overwhelming majority of their time-- indoors, such pressurized habitats should be as comfortably-- spacious-- as possible. Once large SLS propellant tank technology derived habitats are on the lunar surface, much larger (inflatable) habitats could be deployed by the SLS.

A single SLS Block I launch could deploy a 27.5 tonne biosphere, plus a 38 tonne external bio-torus and a 2.5 tonne inner bio-torus to LEO. So a total mass of 68 tonnes would be deployed by the SLS to Low Earth Orbit. A pair of reusable ACES-68 orbital transfer vehicles could transport the payload to NRHO. Reusable lunar cargo vehicles could transport the biosphere and the bio-tori separately to the lunar surface.

X-Ray of 40 meter in diameter lunar biosphere surround by two bio-tori

A second SLS Block I launch could deploy five 3 meter in diameter and 3 meter high airlocks: one to be connected to the bottom of the biosphere and two each to be connected the bottoms of the two bio-tori on opposite sides. Six 3 meter in diameter expandable tunnels will also be deployed to linearly connect the airlocks to each other and to allow astronauts to enter and exit the base of the inflatable habitats. Six expandable regolith walls will be included to provide a firm regolith base for the biosphere and the bio-tori. Six 2.4 meter in diameter ECLSS modules will be included: two to be attached to the a biosphere airlock and individual modules to be attached to each of the bio-tori airlocks. Piping will be provided to connect the ECLSS modules to external radiators. And wiring will be provided to connect the ECLSS to external solar, nuclear, and chemical power units. Again, these payloads will initially be deployed to LEO before be transported to NRHO and then to the lunar surface by reusable LOX/LH2 vehicles.

Once deployed to the lunar surface, the inflated Kevlar biosphere would be 40 meters in diameter. An 18 meter in diameter bio-torus would surround the biosphere. And an additional 6 meter in diameter bio-torus would be placed with the lower cavity between the biosphere and the external bio-torus.The pressurized biosphere and bio-tori would sit on top a regolith base. Airlocks beneath the biosphere and bio-torus would be connected to cylindrical metallic tunnels internally pressurized with cylindrical Kevlar bags would provide astronauts with easy access to the other sections of the habitat while also allowing them to exit the habitat or to connect to exterior habitats.

The atmospheric pressure within the biosphere and within the bio-torus would be the same atmospheric pressure as on Earth. And this will allow people working in the bio-torus to move easily back and fourth between the bio-torus and the biosphere without the need of to deal with differences in pressure.

Notional biodome recreational floor area of a 40 meter in diameter bio-torus

With a floor area of 1257 square meters within a spacious biodome 20 meters high, the upper hemisphere of the 40 meter biosphere could be used for a variety of recreational purposes (tennis, volleyball, basketball, gymnastics, swimming, etc). The biodome could also provide astronauts with a spacious area for relaxation if landscaped with grass and trees and other aesthetically pleasing foliage.

The lower hemisphere would be composed of four expansive habitat floors, 2.4 to 3 meters high, providing apartments, laboratories, and gyms and more than 1200 square meters of habitable floor space. The floors, rooms, and apartments will be composed of prefabricated sections manufactured on Earth and assembled within on the Moon within the pressurized biosphere. Ceiling, floor, and wall panels and beams and other structural components could be transported to the lunar surface by reusable and expendable commercial lunar transports. So the lower half of the biosphere should be able to provide at least four expansive levels for habitation, with the lower hemisphere alone far exceeding that of the floor area for SLS propellant tank derived habitat modules.

The surrounding 18 meter bio-torus would also consist of multiple levels that are composed of modular components. But, under this scenario, the bio-torus would be divided into five levels. The top level would be used for orchards (apple, orange, lemon, cherry, and peach trees) and also for raising large fauna: pigs, miniature cows, sheep, and possibly even ostriches. The second level would be used for poultry. The third and fourth level would be used for growing fruits and vegetables: bananas, pineapples, watermelons, tomatoes, carrots, lettuce, potatoes, corn, wheat, sugar beets, etc. The bottom level of the bio-torus would be used for aquaculture: brine shrimp, fish, oysters, etc.

The inner 6 meter in diameter bio-torus would be largely used for storage and for emergency habitation in case something serious should occur inside of the biosphere.

The entire facility would be designed to comfortably accommodate between 50 to 100 individuals.

Diameter and mass of Kevlar biospheres and bio-tori pressurized at 14.7 psi (101.3 kPa) with a safety factor of four without regolith shielding and structural support

40 meter in diameter biosphere: 27.5 tonnes
Surrounding 18 meter in diameter bio-torus: 38 tonnes
Surrounding 6 meter in diameter interior bio-torus: 2.5 tonnes

Mass of an M1-Abrams Tank - 62 tonnes

100 meters in diameter biosphere: - 430 tonnes
Surrounding 50 meter in diameter bio-torus: 759 tonnes
Surrounding 16 meter in diameter interior bio-torus: 44 tonnes

Mass of a Boeing 747 - 440 tonnes

200 meters in diameter biosphere: 3438 tonnes
Surrounding 100 meter in diameter bio-torus: 6071 tonnes
Surrounding 32 meter in diameter interior bio-torus: 348 tonnes

Mass of the Eiffel Tower - 7300 tonnes

300 meters in diameter biosphere: 11,600 tonnes
Surrounding 150 meter in diameter bio-torus: 20,512 tonnes
Surrounding 50 meter in diameter interior bio-torus: 1264 tonnes

Mass of an Ohio-Class atomic submarine - 16,764 tonnes

400 meter in diameter biosphere: 27, 500 tonnes
Surrounding 200 meter in diameter bio-torus: 48, 574 tonnes
Surrounding 60 meter in diameter interior bio-torus: 2478 tonnes

Mass of a cruise ship - 100,000 tonnes

1000 meters in diameter biosphere: 430,000 tonnes
Surrounding 500 meter in diameter bio-torus: 759, 000 tonnes
Surrounding 160 meter in diameter interior bio-torus: 44, 000 tonnes

Mass of the Golden Gate Bridge - 804, 673 tonnes

Much larger inflatable facilities will probably require the Kevlar material to be exported from Earth in small sections to be woven together by machines deployed to the lunar surface. And, eventually, Kevlar threads will be manufactured on the lunar surface from lunar materials mostly found at the lunar poles.

Biospheres that are 400 meters in diameter could be very attractive for human colonization of the Moon. The 200 meter high bio-domes of such facilities would be able to provide artificial lakes and lagoons at least 200 meters in diameter with surrounding sandy beaches where you could not only swim but also put on a pair of wings and fly under the low lunar gravity. The top half of the surrounding 200 meter in diameter bio-torus could also be used for housing familiar to that on Earth plus recreational parks and 100 meter lakes and lagoons. And with a 100 meter high rooftop, there should also be enough room in the bio-torus to strap on a pair of wings and fly at least 50 meters above the ground within the upper half of the bio-torus.

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