This book has been written within the framework of the IAEA's innovative reactor and fuel cycle technology development activities. It elucidates the role that peaceful air & space related nuclear power research and development could play in terrestrial innovative reactor and fuel cycle technology development initiatives. This book gives an introduction to Direct Cycle Nuclear engines which would resemble a conventional jet engine, except that there would be no combustion chambers. The air gained from the compressor section would be sent to a plenum that directs the air into the nuclear reactor core. An exchange takes place where the reactor is cooled, but it then heats up the same air and sends it to another plenum. The second plenum directs the air into a turbine, which sends it out the exhaust. The end result is that instead of using jet fuel, an aircraft could rely on nuclear reactions for power. Dr.Ugur Guven and Rohan M Ganapathy, both specialists in the field of Nuclear Propulsion discuss and explain how nuclear power plants and nuclear propulsion devices work by describing the fundamental physics of these devices and their conceptual application on UAV's.