Dynamics and control of orbiting flexible structures exposed to solar radiation

Expressions for thermal deflections of uniform thin beams and plates exposed to solar heating are obtained as a function of the properties of the material and solar incidence angle. The major effect of the solar radiation pressure interacting with the thermally deformed structure is found to give rise to disturbance moments on the structure. The thermal deformations of the structures are assumed to be within 0.1% of the characteristic length of the structure. With the assumed thermal deformations, the resulting uncontrolled transient responses of these geosynchronous orbiting structures to the solar radiation pressure induced disturbances are simulated. The resulting rigid modal oscillations are found to be an order of magnitude larger than for those cases previously considered in which only the solar radiation pressure effect oh vibrating structures was treated. Modifications of control laws and/or the feedback gain values are considered in order to improve the transient response characteristics under the thermally induced disturbances. HE major environmental disturbances on proposed or- biting large space structural systems are expected to be due to solar radiation pressure and solar heating. The dynamics and control of a flexible beam and a flexible plate in the presence of disturbances due to the solar radiation pressure^ acting on the vibrating structure were considered previously.1"3 It was seen that the major effect of the solar radiation pressure interacting with an elastically deformed (vibrating) orbiting structure was to produce moments on the structure resulting primarily in rigid modal oscillations. For the case of extremely flexible structures the amplitudes of these modes may be appreciable, even in the presence of both active and passive control.1"3 In some situations the control laws previously developed by ignoring environmental effects may have to be redesigned/For simple low order systems the feedback gain values may be suitably adjusted; however, for large order systems the versatility of the linear Gaussian technique can be used to redesign control laws which provide a compromise between transient performance and the required control effort.1"3 An important environmental effect is heat due to solar radiation that results in thermal gradients in a structure. The deformations caused by the thermal gradients can be very large and can result in the dynamic instability of a structure.4"6 Another form of environmental disturbance is when solar radiation pressure interacts with a thermally deformed struc- ture. The deformations caused by solar heating depend on the thermal properties of the material and the geometric shape of a structure. The selection of materials with desired thermal properties and the careful selection of a structural design are required to minimize the thermal deformations of a structure to an acceptable level. The thermal deformations of a struc- ture will occur as long as the structure is in a sunlit orbit. Fur- thermore, the continuous removal of this deformation by us- ing active control may not be practical. The thermal deforma- tions will have to be minimized by careful consideration of the