The cavity reactor concept consists of a core, generally of dilute fuel, surrounded by a moderating reflector. This concept has long been considered as a means of attaining very high temperatures in a gaseous core, since the fuel need not be in contact with structural material. A number of critical experiments have been performed on configurations of different core size and shape, with different structural material thicknesses between the cavity and reflector, with different coolant densities surrounding the core, and with various other alterations. The principal purpose was to evaluate the effects of engineering design variables for a cavity propulsion reactor concept. Experiments reveal that a power reactor with the required structure and a cavity size 182.9 cm (6 ft) in diameter by 121.9 cm (4 ft) long can be expected to have a critical core loading of between 20 and 30 kg of 235U.