SIRTF Controller Simulation: Instability Masked by Numerical Integration

In the course of a simulation study of a candidate design for the Space Infrared Telescope Facility, an interesting phenomenon was observed. The uncompensated control system was unstable, but a numerical simulation with the fixed step-size, classical fourth-order Runge-Kutta method gave a stable response. This study describes this phenomenon in the setting in which it occurred. Also the Runge-Kutta simulation model is analyzed as a discrete, linear system and shown to be stable, thus corroborating the numerical results.