The work done in designing a high speed starter-generator for an aircraft engine will be presented in this paper. The electrical machine is of the variable reluctance type, which was chosen due to its inherent robustness, ease of manufacture and reliability. The Starter-Generator function requires the machine to initially act as a motor, spinning the aero-engine through the starting sequence, till light-off speed, (maximum torque condition). It then continues to assist the engine till idle speed is reached, following which the machine acts as a generator. The machine described was designed to deliver its maximum torque of 54Nm at 8000rpm, and will be required to supply a 45kW load at shaft speeds up to 32000rpm. The design envelope is the same as that used in sizing starter-generators for use in the latest models of regional jets. This paper details the challenges in designing such a machine to meet the wide constant power speed range required, and proposes a novel rotor structure as a possible solution.