This paper describes the design of an advanced emergency power system meeting future More-Electric aircraft demands for starting at 50,000 feet (16,000 meters) and acceleration to full power in two seconds. This rapid starting requirement stems from the need for uninterrupted electric control power in the event of main engine shutdown at high altitude. Key to developing this power system is a flexible-cycle (flex-cycle) combustor. The combustor, with other shared auxiliary power unit (APU) components, constitutes an Integrated Power Unit (IPU). The IPU, gearbox, and stored energy components form the aircraft secondary power system. The flex-cycle combustor design accommodates a high exit temperature, a rapid temperature rise, and a high heat-release rate. High combustor performance is achieved with thermal barrier coatings, effusion cooling, and air-assist atomizers. The flex-cycle combustor design, IPU starting analysis, and proposed development test plan are discussed.