Abstract: There are multiple situations in which fires may occur at environmental conditions that are different than standard atmospheric conditions. Changes in ambient pressure, oxygen concentration, flow velocity, the presence of an external heat source or gravity may change the flammability and fire dynamics of materials. The objective of this work is to study the effect of external radiant heating on downward flame spread over cylindrical samples of polymethyl methacrylate (PMMA). In this work, experiments under normal gravity and atmospheric ambient conditions are conducted using a variable heat flux with peak values up to 13.2 kW/m(2). A forced flow of air with a mass-mean velocity of 10 cm/s is used during the experiments. Flame spread rates were measured from video processing of the experiments at different conditions. Results show that the flame spread rate measured depends strongly on the amount of radiant heating provided. An analysis is presented to correlate the flame spread rate with the energy applied to the surface of the sample and the surface temperature. The results provide a baseline for comparison with future microgravity experiments to be performed by NASA as part of the SoFIE/MIST project aboard the International Space Station. It is expected that the results will provide insight for what is to be expected in different conditions relevant for fire safety in future space facilities.