What is an inflatable structure?
Inflatable means literally “can be inflated.” Air-inflatable membrane structures that are inflated by internal gas pressure have a long history as ground constructions. At the Osaka World Exposition in 1970, a number of air-inflated membrane pavilions were constructed. Today, Tokyo Dome is a well-known inflatable structure. Air-inflated membrane structures have many features such as a large open interior, short construction leadtime, low cost and high resistance to earthquake. These features are also advantageous and suitable for space structures. At the end of the 1960s, the U.S. adopted air-inflated membranes for its satellite structures and launched a series of ECHO satellites (Fig. 1). These were passive communication satellites that reflected radio signals from the earth on their spherical surface. Later, however, active communication satellites carrying transponders became dominant, and demand for space inflatable structures declined for some period.
In recent years, interest in and demand for large-scale space structures have increased and inflatable structure features such as light weight, high storage rate, few mechanical components are now recognized again as suitable for fabricating large-scale structures in space. Thus, research on space inflatable structures has been re-activated.
Features of inflatable structures
Space inflatable structures need three fabrication processes: deployment, inflation, and cure. Here I will outline the three processes and consider the research and development required for each process. I hope that readers of this article will join me in this consideration.
Technology to deploy the membrane structure from its stowed, folded state is required for all large-scale membrane structures, not just space inflatable structures but also solar sails. With a large membrane structure, it is better to simultaneously fabricate and fold because it is difficult to fold a large structure after fabrication. However, the simultaneous method makes it impossible to comply with our established reliability-management concept for launching structures whose deployment is fully tested on the ground in advance. Further, even if we were able to conduct deployment tests on the ground, since membrane structures are extremely flexible and light, we cannot completely exclude the effects of atmosphere and gravity on the deployment test. Thus, tests simulating the space environment are difficult. In addition, it is difficult to guarantee the reproducibility of the stowing process after deployment. We have to change our way of thinking if we are to establish a verification method on the ground for ultra-large membrane structures.
There are many interesting studies on folding geometry for membranes. Since the currently proposed folding geometry depends greatly on the post-deployment shape, however, it lacks universality and flexibility for design changes. The development of a folding method that can assure a high storage rate and a firm, stable deployment with universality will in turn promote the research, development, and utilization of space inflatable structures.