High-precision Control Method for Large Space Structure Subject to Thermal Deformation

Abstract

As the sizes of space antenna reflectors increase, the radio wave transmissions will become more susceptible to small structural deformations that occur in the reflectors. When the Engineering Test Satellite -VIII (ETS-VIII) communications satellite entered the Earth’s shadow, radio wave transmissions from the large deployable reflector (LDR) to the Earth were observed to change [1]. Moreover, the temperature of the LDR was observed to decrease for about 200 ºC. Therefore, it is conceivable that radio wave transmissions were significantly affected by temperature transition on the LDR. This phenomenon may become critical for the satellites because highly accurate beams are expected to be required for large space structures in the near future. Therefore, not only the means by using materials in which thermal deformation is hardly generated but also the means of active shape changing in orbit have been required to deal with various issues.In this study, the LDR model mounted on the ETS-VIII is constructed for investigation. As a result of numerical simulation of the deformation behavior under the actual thermal history detected on the ETS-VIII, the midpoint of the LDR was confirmed to deform by approximately 5 mm as the temperature decreased [2]. 5 mm deformation is equivalent to 65 km transition of the footprint of communication beam on the surface of the Earth, which explains the phenomena actually observed in the ETS-VIII. Based on these results, we developed an effective method for mechanically compensating the thermal deformation by adjusting the combined coefficient of thermal expansion of structural members which is calculated from the constituent ratio of CFRP and titanium alloy components, and by focusing on springs used to deploy a modular space structure. As a consequence, the thermal deformations at every apex that support the antenna reflector were all suppressed at a high correction rate. It is shown that the combination of these means is best for large space structure to actively suppress the thermal deformation in orbit.