The open loop dynamic performance of a nuclear rocket engine with bleed turbine or topping turbine drive is studied with the aid of an analog computer. The dynamics are accurately described by a system of ordinary, nonlinear differential equations. A linear approximation to these yield a stability criterion that is a function of (a) the rate of change of reactivity with temperature at constant propellant density, (b) the rate of change of reactivity with propellant density at constant core temperature, and (c) the relation between states of zero time rate of change of core inlet pressure. An explicit prediction of (c) is given and enables a simpler criterion to be established. The engine is stable if (a) is negative. The system is remarkably insensitive to changes of the major coefficients and can safely withstand large perturbations. It is shown that the long term response, which is dependent on the mechanical inertia of the turbopump, is of the order of ten seconds for vehicles in the million pound thrust class and that reduction of the thermal inertia of the core does not improve the response. The simulation results are explained on the basis of physical considerations and analysis in which the root locus technique proves useful.