System Study for a Solar Thermal Thruster with Thermal Storage

Abstract

Solar Thermal Propulsion (STP) is potentially interesting for the augmentation of launch systems payload mass, because of its high specific impulse, owhich is about 800 s. This paper discusses the interest of STP in « thermal storage mode » for a transfer to Geostationary Earth Orbit (GEO). In a solar thermal thruster with thermal storage, the thermal power obtained from the concentrated solar radiation is not immediately used for propulsion. At each orbital revolution, this power is first used to heat an absorber material (graphite), then the accumulated heat is used to accelerate the propellant during a short, but more efficient thrust arc. A drawback of this mode is the mass of the absorber sub-system, which is detrimental to the thrust-to-weight ratio. A system study † has been conducted in order to assess the feasibility of a 30-days transfer to GEO. Transfer time is crucial for STP since it has to be competitive with solar electric propulsion , which already offers a better specific impulse. The system study focuses on the design of key-parameters of the thruster, the most important ones being the area of the solar concentrator and the mass of absorber material. In order to get a realistic estimation of the total transfer time, we have developed a simulation model which includes heat transfer calculations for both the heat accumulation and the thrust phases. It allows a precise estimation of the speed increment obtained during each thrust cycle. Simulations have been conducted considering two kinds of initial orbits: circular LEO (Low Earth Orbit) and Ariane 5-like GTO (Geostationary Transfer Orbit). The study has emphasized the importance of a key design parameter: the thruster's full load altitude, which is the altitude where the thruster gets enough illumination time to heat the absorber to its nominal temperature. The search for the optimal design parameters has shown that a 30 or 40-days transfer from LEO to GEO is possible with reasonable values for the concentrator area and absorber mass, yielding a +35 % increase of the payload mass by comparison with a state-of-the-art cryogenic stage.