Aerospace Planes are substantially different from both conventional aircraft and conventional rocket powered spacecraft and have a unique set of secondary power requirements. Although there are a wide variety of aerospace plane concepts incorporating both single and multiple stage to orbit, the various arrangements proposed have many common features.One of the primary common features unique to aerospace planes is the use of liquid hydrogen or liquid methane as the fuel. The cryogenic fuel storage temperature, while inconvenient from a fuel handling point of view, offers some synergistic cooling and power generation benefits unavailable to conventionally fueled aircraft.A second common, but unique, feature of aerospace planes is that the propulsion engines typically have no power extraction shafts, either because the propulsion engines are themselves shaftless (ramjets or scramjets), or because the core rotating parts are inaccessible (turboramjets, air turborockets). Accessory power is required throughout the flight envelope and in all flight modes. Accessory power may also be required for pre- and post-flight ground operations.A third common, but unique, feature of aerospace planes is that they operate with the engine off, both in space and during reentry. Accessory power is required in both modes although typically the magnitude of the loads are substantially reduced during space flight.This paper evaluates these unique features and recommends integrated cooling and accessory power configurations that can save substantial weight over nonintegrated systems. Since the payload fraction of aerospace planes is small, weight reduction of the secondary power system and other subsystems is extremely important. Figure 1 shows some estimates of the effect of subsystem weight and fuel consumption on aerospace plane weight. Subsystem integration can add a comfortable payload margin if done properly and can eliminate the payload entirely if not performed well.