Critical sizes are determined for a variety of fast assemblies appropriate for nuclear rocket reactors. These are based on cores of UO2-W cermets or UC-metal carbide solid solutions reflected by beryllium. Rocket reactors weighing as little as 200 lb are possible, and, in larger sizes, either high power density or high exit gas temperature can be achieved. The fast spectrum allows the use of the most refractory materials (such as HfC) in bulk to obtain high performance. Use of U233in place of U235 can lead to substantial improvements in reactor weight, specific power, and/or temperature capability at the cost of the radiation associated with U233. The core sizes are generally quite small, which is valuable where shielding may be significant. Nuclear aspects, including control, uranium investment, power distribution, and reflector materials, are briefly discussed.