Static and dynamic neutronic analyses have been performed on an innovative burst mode (100's of MW output for a few thousand seconds) Ultrahigh Temperature Vapor Core Reactor (UTVR) space nuclear power system. This novel reactor concept employs multiple, neutronically coupled fissioning cores and operates on a direct closed Rankine cycle using a disk Magnetohydrodynamic (MHD) generator for energy conversion. The UTVR includes two types of fissioning core regions: (1) the central Ultrahigh Temperature Vapor Core (UTVC) which contains a vapor mixture of highly enriched UF4 fuel and a metal fluoride working fluid and (2) the UF4 boiler column cores located in the BeO moderator/reflector region.Reactivity feedback phenomena associated with the individual fissioning cores (such as vapor fuel density feedback of the UTVC and liquid-fuel volume feedback of the boiler columns), as well as the effects of core-to-core neutronic and mass flow coupling between the UTVC and the surrounding boiler cores were included in the dynamic models. The dynamic analysis of the UTVR reveals the existence of some unique and very effective inherent reactivity feedback effects that are capable of quickly stabilizing this system, within a few seconds, even when large positive reactivity insertions are imposed. It is found that when the vapor fuel density feedback coefficient of reactivity, , is suppressed the UTVR is still inherently stable because of the boiler core liquid volume feedback. In contrast, “conventional” gas core reactors tend to be inherently unstable for small . Due to the strength of the negative reactivity feedback in the UTVR, external reactivity insertions alone are inadequate for bringing about significant power level changes during normal reactor operations. Additional methods of reactivity control, such as variations in the gaseous fuel mass flow rate, are needed to achieve the desired power level control.