When we look into the space inflatable structure in detail, we realize that it is not only used for the construction of balloon-like structures deploying and inflating in space. By using these balloons’ inner pressure, they can even be utilized as actuators. We can build structures with high shape accuracy if we adopt a concept of virtual structural rigidization to achieve the required shape accuracy and use the inflatable structural factor as an actuator to deploy force. Rather than curing the material itself, the virtual structural rigidization method produces previously structural elements that have the ability to return to their original shape and stows them by, for example, winding them on a reel. When released from the stowing constraints, they return to their original shape. An example of this in our daily life is the tape measure. It returns to its straight shape when extended. With the virtual structural rigidization method, since there is no need to cure the inflatable part, we can use the structure repeatedly. Figure. 3a shows the extendable antenna SPINAR (SPace INflatable Actuated Rod) by applying the virtual structural rigidization method, where the inflatable part is used as an extendable actuator. Since an extension motor is unnecessary, we can omit motor-related components including reduction gears, electronic circuits to drive the motor, and heaters. Thus, we can reduce weight and minimize mechanical components for the extension mechanism. Figure. 3b shows the SPINAR’s spin extension experiment under microgravity condition produced by an aircraft’s parabolic flight. Figure. 3c shows the SPINAR’s extension experiment in a vacuum chamber. In addition to the inflatable structure’s merits such as ultra-light weight, high rigidity, high storage rate, simple extension mechanism and fewer mechanical components, it also enables repeated ground tests.
We believe structures that combine the virtual structural rigidization method and inflatable actuators can be applied to other structures, not just the antenna above. Figure. 4 shows a conceptual illustration of an extendable lunar tower standing in the polar region of the Moon. By raising solar cells to an altitude possible to receive solar light continuously, the tower can generate electric power day and night, thus defeating night on the Moon’s surface. Also, it can be used as a radio tower to relay communications. By introducing this method, we can easily build a tower 15 to 20m above the surface on the Moon, yet whose weight is almost the same as a single instrument onboard the lunar lander.
As discussed above, space inflatable structures have many advantages and unique features. The structure must evolve further propelled by various ideas. I believe that this is not a technology for the future, but one that is available and practicable now. The structure, however, is not widely known because of the few demonstration opportunities in space. We are now trying to increase demonstration chances in space to accumulate results. I think that the evolutionary process of applying space inflatable structures to wider applications is also a chance for us to test innovative technologies and ideas. I wish that many people will retain an interest in space inflatable structures and that we can expand our dreams and hopes in space using the structures. Space inflatable structure is an engineering term and has no relation with the physics of the space inflation theory on how the structure of the universe formed and evolved. Nonetheless, it is interesting that the keywords, space and inflation, are common to both although their implications differ. Does inflation fit with space structure?