William Mook has prepared a number of studies on space development, some of which relate directly to the Artemis Project's goals. This concept of an inflatable habitat could be a possibility for expanding the pressurized space of Moonbase Artemis for increased crew, or for lunar tourism. He has posted the following brief summary of his study:
Polyester film has a yield strength of about 25,000 psi. So a reinforced polyester film blown to form a 60-foot-diameter sphere would need to be about 1/80 inch thick to sustain a 3.75 psi pressure. You would need about 1/20 inch thick film to sustain 14.70 psi pressure.
Assume an outer shell 1/80 inch thick, and an inner shell 1/80 inch thick, separated by 2 inches. The film is bonded together every 2 inches or so by kevlar netting. The space between the two films is filled with lightweight polystyrene.
The whole sphere would inflate from a small pillbox type container. Assuming 3.75 psi working pressure, the sphere would weigh about 1,275 lb. The oxygen would weigh 2,225 lbs! More than the container!
Five floors would be formed the same way, and deployed along with the inflation of the spherical shell. They would consist of 2 sheets of polyester film separated by a kevlar reinforced polystyrene filler.
The poles of the sphere would be connected by a lanyard-deployed continuous longeron coilable boom. This would interconnect the five floors.
Starting at the south pole of the 60-ft-diameter sphere, the first floor is 5 ft above the pole. It is a circle 46 ft in diameter containing 1668 square ft of space. The second floor is 15 ft above the south pole. It is a cirle 53.6 ft in diameter and 2262 sq. ft. in area. The third floor is largest, with an area of 2,750 sq. ft. and a diameter of 59.2 ft. We then repeat the same sequence in reverse. The sixth panel is actually the ceiling of the fifth floor. The mass of these floors is 1,200 lbs.
The mass of the vertical shaft is 280 lb.
Assume the sphere is inflated on the lunar surface, from the nose of a landing craft. The craft is a cylinder 12 ft in diameter and 18 ft tall.
From the side of this cylinder is a 30-ft-long, 10-ft-diameter tube cut into two sections. The end of each section has attached to it an airlock door made of diffusion bonded/superplastically deformed titanium. The weight of each airlock is 480 lb. The weight of the outer tube, made of polystyrene foam inflated polyester film is 360 lb. Another lanyard-deployed continuous longeron coilable boom connects each of the airlock doors. The innermost door is attached to the airframe of the pillbox/spacecraft. This spacecraft contains all the environmental control systems as well as consumables. It is airtight, and forms a link between the airlocks and the station above. It also is a control center from which to control the station. The longeron coilable boom weighs 150 lb.
So, the total station weighs:
1,275 lb. Shell 2,225 lb. Air 1,200 lb. Flooring 280 lb. Vertical Boom 150 lb. Horizontal Boom (walkways) 480 lb. Titanium Airlock Door 480 lb. Titanium Airlock Door 480 lb. Titanium Airlock Door (attached to S/C) 360 lb. Horizontal Airlock Tube --------- 6,930 lb. SUB-TOTAL 4,620 lb. Deploying Spacecraft & ECS Equipment ---------- 11,550 lb. TOTAL
Once the balloon is inflated, 24 tethers bonded to the outer surface around the equator of the sphere would drop down. These would be anchored into the lunar surface. Netting would be attached between these tethers. Lunar soil would then be piled up around the netting, forming a radiation-proof area in vacuum under the sphere. As the soil is piled up it eventually covers the sphere, creating a pressurized radiation-proof area within. Access is by the 30-ft tunnel connecting the outer terrain with the central column.
There is a total of 10,610 usable square feet within the sphere. Illumination is via fiber optics. An inflatable parabolic mirror 1,516 sq ft. in area (44 ft diam.) concentrates sunlight into an optical fiber. This fiber makes its way into centerline of the sphere and up the central column. There light is projected through diffusers to illuminate the interior of the habitat.
The mirror is part of the equipment mass budget (it weighs less than 50 lb.) and is inflated upon arrival. The system provides illumination during the 2-week-long lunar day, and a high intensity bulb provides illumination at night via the same optical fiber setup.
Power can be supplied by a small PV array operating at 1000 solar intensity - at the focus of another 44 ft diameter mirror. This provides energy during the lunar day. Propellants are used in fuel cells to power the station during the lunar night.
Early inhabitants will occupy the station only 2 weeks every month. Up to 30 people may use the station at one time. Only a 3-person skeleton crew maintains watch at night. All others depart to minimize energy usage.
I think William Mook made an error in calculating the diamter of the first floor of the habitat. It ought to be 33.2 feet in diamter instead of 46 feet. (It looks like he used the diameter instead of the radius of the 60-foot sphere.) But nevertheless, his description of this habitat shows quite well how much habitable volume we can get from a simple inflatable.
The thickness he chose for the habitat pressure shell, and the reasons for it, are a bit worrisome. If we have a bioregenerative life support system based on terrestrial plant life, we'll want an atmosphere pretty close to Earth normal. Using a pure oxygen atmosphere, even at low pressure, for extended habitation is more of a cavalier risk than I'd be willing to take.
The strength of the pressure shell will be sized for impact loads more than retaining pressure. With nothing but that shell between you and oblivion, you wouldn't want it to be so flimsy that you'll puncture it if you drop a screwdriver.
The real value of a flexible inflatable pressure shell comes from its packaging efficiency rather than its mass compared to, for example, a rigid aluminum tank. The improved packing densitity might result in lower total launch weight because the parasitic weight of supporting structure, packing material, flight support equipment, and the payload shroud might be less for a given volume.