Solar thermal process reactors can convert intermittent solar radiation and reactants into energy dense, storable and transportable chemical fuels. This method uses concentrated solar energy as the source of high temperature process heat for the production of many commodities such as zinc, cadmium, magnesium, hydrogen and carbon with zero or minimal emissions. Transient inefficiencies due to natural fluctuations in solar radiation degrade product throughput in all solar reactors. It is therefore important to design a system that allows the reactor to respond to environmental factors in order to maintain semi-constant temperatures inside the reactor. Maintaining reactor operating conditions stabilizes process efficiency. Previously, the effects of various aperture geometries have been investigated through the use of ray-tracing and discrete ordinance numerical simulations. In this paper, a more complete concept for the aperture mechanism entitled the "sliding variable aperture" is presented. This variable aperture allows the solar reactor to dynamically respond to changing flux conditions. Optical simulations have been carried out in conjunction with a numerical method for determining the output of the reactor with a dynamic aperture and changing flux conditions. Historical weather data was gathered from the National Renewable Energy Laboratory (NREL). Convective losses were modeled based on a desired isothermal temperature and static standard operating environmental temperatures. Results from the optics simulations, reaction kinetics and the heat transfer model were used to find the range for the area of the gap for the aperture mechanism, which resulted with ranges between 13.8 to 52.3 cm2 for beam normal insolation amounts between approximately 165-1100 W/m2 in order to maintain a minimum 1500 K internal cavity temperature inside the solar reactor.