Some recent experiments on an open-cycle gaseous-fueled nuclear rocket engine concept have been performed. In such an engine, the gaseous fuel and the propellant are in direct contact. This feature removes the requirement of using solid fuel elements. These experiments have shown there is a relatively large central volume into which the simulated fuel expands. The fuel-to-cavity volume fraction depends on the propellant to fuel-flow-rate ratio. Fuel volume fractions varied from 0.5 to 0.3 as the mass-flow ratio varied from 25 to 1 up to 100 to 1, respectively.