The inflation process involves complex dynamics where flow passages are deformed by the flow of the inflating liquid such as gas and then the fluid runs through the deformed flow passages. Even if we were able to ignore the dynamics and inflate semi-statically, troubles may still occur such as failure to open the flow passages due to a bend in the fold or a twist in the passage and such troubles may eventually halt the inflation process. If we fail to raise the pressure of whole structure uniformly or if the pipe kink-points open suddenly, the structures may move unpredictably. If these movements differ in each inflation and deployment test, we cannot forecast behavior in an actual mission. Although this problem is partly attributable to the method of folding, a fundamental solution is required to inflate structures as designed.
Space inflatable structures cannot maintain their shape if debris or micro-meteoroids hit and break the membrane to cause gas leaks. We have to cure the structures in the early stage of construction. The first proposed ideas were thermal curing using heaters and chemical curing using solar light or ultraviolet present in space. On the ground, resin-based composites are cured by retaining the materials in the desired shape and controlling the temperature, pressure, etc. In space, however, where we cannot fully control conditions such as temperature and pressure, it is very difficult to implement a curing process using a chemical reaction that requires retaining the structure in a desired shape over a certain period so as to obtain uniform material characteristics. Thus,the elaborate curing methods derived from techniques used on the ground have never been used in space. Research, development and ideas on materials and processes to adopt for curing in outer space are the key to the broader use of space inflatable structures.
I presume that there are other rigidizing methods, apart from the conventional idea to cure chemically resin after inflating the membrane with gas pressure. We may employ, for example, a method using plastic deformation of metal film during inflation or a method using a foaming agent to simultaneously inflate and cure (Fig. 2a). The latter has the advantage of resistance to collisions with debris or micro-meteoroids. One idea is to return the structure to its original shape and keep it stable in space by partial use of shape-memory alloys or resins. Another idea is to dispense with rigidization and produce a structure with so many membrane cells that it is unaffected when some cells are pierced by debris, for example, and shrink. This idea has the advantage of easy removal, replacement and repair of the structure.
Future evolution of space inflatable structures
While there is an impression that we cannot expect high structural accuracy for inflatable structures, in fact inflatable solar cell arrays and radar surfaces have already been proposed and fabricated experimentally on the ground. Ten years ago, test of the inflation and deployment but not the curing process of an antenna in orbit was conducted (Fig. 2b). It is already possible to build structures without high structural accuracy in outer space using the space inflation method. It would be wise to step up our applications. First, we gain experience through the sunshade for in-orbit telescope, lunar tent, etc., and, then, prove the technology in applications with medium structural accuracy such as the surface of a solar cell array. After that, we should try applications with high structural accuracy such as antenna reflectors or light-concentration/reflection mirror surfaces. To assure structural accuracy, we must not just rely on inner gas pressure but also consider the introduction of active control methods, for example, the combination of cable network and membrane (Fig. 2c) or complementing shape-reproducibility using shape-memory alloys or resins.
Since we see many large-scale, air-membrane structures on the ground, we tend to think that a space inflatable structure is a technology for constructing large-scale space structures. In fact, it is suitable for large-scale space structures. Its advantages, however, are not limited to just this application. By taking advantage of its high storage rate and few mechanical components, we can utilize the structure to realize medium-satellite-level performance in a small satellite. By mounting inflatable solar cell arrays, inflatable extension antennas, inflatable radiators, and inflatable drag chutes, we can easily improve performance of small satellites without depending greatly on mechanical components that are heavy, complex, and prone to cause failure.