Inflatable Study Comments

Mike Eckardt

The use of inflatable habitats may not be as simple as Mr. Mook indicated in his study.

Accepting the structural properties of polyester film (25,000 psi yield), a wall or floor section composed of two layers of 12.5 mil film, separated by polystyrene (negligible strength), will accept a structural bending moment of 625 ft-lb/ft. If we assume a floor loading of 10 lb/ft² (Earthly design standards are 100 lb/ft² +), the maximum span we could bridge with this material would be about 22 feet. This maximum can only be attained if we stress our materials to the yield point, which is very poor engineering practice.

Similarly, the bubble structure of the habitat will be subject not only to internal pressure loading, but also to a significant dead load resulting from the cover of lunar soil over the structure. If we assume a weight of 150 lb/ft³ (Earth) for typical regolith, a two-foot cover over the dome would weight approximately 50 lb/ft². As described, the Inflatable would fail.

A modified design of the inflatable bubble could be used, however, that would avoid these flaws, as well as the problems that Mr. Bennett mentioned in his critique. To support a bending moment of 15,000 ft-lbs

1. we could use a bubble section as shown in figure 1. Here, the sphere is composed of a multiple layer shell. The outer layer is as described in Mr. Mook's study. A second, inner shell of two more layers of polyester with 2" of polystyrene filler is placed inside the first shell. Between the two layers is a 24" free space (vacuum is acceptable). Kevlar reinforced webbing of film/foam can be added every 3 - 4 feet. The resulting structure is similar to a standard I-beam, as used in construction here on Earth.

   The loading on floor elements is much less than the bubble. Here, we could get by with about 4,500 ft-lb in bending (2), which can be easily supported by a double layer structure as described above, except with a flange spacing of only 12".

   The empty space provided by this method of construction is typically used on Earth for utility services. Presumably similar uses could be made of it in a lunar habitat.

   Using the above method of construction, the habitat weights that Mr. Mook provided would be changed to the following:

                         3.75 psi        14.7 psi
        ------------------------------------------
        Shell            3,187 lb       12,750 lb (3)
        Air              2,225 lb        8,900 lb
        Flooring         3,000 lb        3,000 lb
        Vertical Boom      280 lb          280 lb
        Horizontal Boom    150 lb          150 lb
        3 Airlock Doors  1,440 lb        1,440 lb
        Airlock Tube       360 lb          360 lb
        ------------------------------------------
        SUBTOTAL        10,642 lb       26,880 lb

   While these figures are substantially higher than those Mr. Mook provided, they probably represent a more stucturally sound habitat. Other factors not included in this summary are materials required to make the habitant livable (utilities, dividing walls, etc.).

2. This figure assumes a dead load weight of 50 lb/ft² for soil cover on the habitat. Moment calculated at peak of bubble calculated as follows:

   Mb = 50lb/ft2 * 30ft * 30ft / 2 - 50lb/ft2 * 30ft * 20ft / 2 (approx. triangular section)
      = 7,500 ft-lb   *   2 (S.F.)
          
      = 15,000 ft-lb

3. Using a floor loading of 20 lb/ft² (allowing for dynamic live-loads), a 1ft section of floor spanning 30' would have a peak moment:

        Mf = 20lb/ft * 30ft * 30ft / 4
   
           = 4,500 ft-lb

   NOTE: Neither calculation has considered shear loading on structural member.

4. The 50 mil film used for the higher pressure envelope does not require the same structural section used for the 12.5 mil variation. These figures are based on the same structure, with the heavier material used.