The two major factors that affect auxiliary power system design decisions are: the working fluid to be used, and vehicle mission requirements. It has been found that optimum turbine designs will be similar for the two working fluids (hydrazine or hydrogen-oxygen) considered for the shuttle orbiter system due to the constraints imposed by geometrical and mechanical design limitations. As a consequence, variations in power level and/or working fluid selection can be efficiently accommodated by relatively minor modifications to turbine nozzle design.Analytical techniques for optimization of turbine aerodynamic design parameters have been developed. These techniques can be extended to include transient-state simulation and design optimization of the other system components, including the turbine controller, in a manner similar to that used for development of control systems for multi-spool fan-jet engines. Transient-state simulation has demonstrated significant advantages in reducing development cost and risk for a number of systems, and similar advantages can be expected to accrue to the space shuttle auxiliary power system, regardless of the working fluid selected.