Abstract: A thermionic reactor for providing electrical power and propulsion power. A reactor vessel has a plurality of thermionic heat pipe modules located therein and spaced apart on a hexagonal pitch. Nuclear fuel elements are positioned in the spaces between the thermionic heat pipe modules. Insulating material is provided between the reactor vessel and the array of thermionic heat pipe modules and nuclear fuel elements. Propulsion power is provided by a propellant storage tank and propellant nozzle in fluid communication with opposite ends of the reactor vessel. The thermionic heat pipe modules serve to produce electricity and aid in removal of waste heat.