A conceptual spacecraft radioisotope thermoelectric and heating unit (RTHU)

Spacecraft venturing to the outer planets and beyond-or onto the planetary surface where available solar energy is reduced-benefit from the longevity and consistency of electrical and thermal energy derived from radioisotope energy sources. A review of likely mission requirements and concept studies of small electrical generating units (<10W _e) reveals a potential opportunity for a unit with an electrical output of around 1W _e that can also supply some heat to the spacecraft to aid thermal control: a radioisotope thermoelectric and heating unit. This power requirement cannot be achieved with current US space-qualified modular radioisotope fuel assemblies. Additionally, new European programmes consider ²⁴¹Am fuel to be much more cost effective than ²³⁸Pu. Taken together, these factors provide the rationale for taking a relatively 'clean-sheet' approach to design of a radioisotope thermoelectric and heating unit fuelled with ²⁴¹Am. In this paper, initial requirements and performance targets for such a unit are developed, a simple concept design and thermal model is presented and the performance and mass are estimated. The results suggest that units generating 1-2W _e may achieve a specific power of around 0.7-0.9W _ekg ^-1 without the thermal inputs to spacecraft becoming impractically large. Such units can use a bismuth telluride thermoelectric material, which is commercially applied in terrestrial applications and is therefore likely to incur lower cost and development risk than more specialised compounds. This study may form the basis of a more detailed design effort.