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This paper describes the performance benefits and current technology progress of an active heat pump loop (HPL) thermal control bus for spacecraft and planetary thermal control applications. Having initiated this research more than 14 years ago, this paper also briefly highlights the technical developments and obstacles overcome during this 14-year development. This paper discusses the unique features of the HPL approach that make it an attractive design choice for future manned thermal control applications: the use of an heat pump to reject heat to space at a temperature above the heat acquisition temperature, the use of non-toxic thermally stable working fluids, and the use of high-performance lubrication-free (gravity independent) refrigeration compressors. The HPL approach has the performance benefits of a traditional two-phase pumped loop thermal bus coupled with the simplicity of a single-phase pumped loop.

We have successfully used techniques found in computational chemistry to identify a series of compounds for use as nontoxic heat transfer fluids for habitat two-phase thermal control systems. Our approach allows us to screen compounds, based on their molecular structure, where no thermodynamic data exists. Once compounds that have the desired properties are identified, their properties can be measured. The key problem in developing the most efficient and versatile two-phase pumped fluid system is the selection of the appropriate working fluid. Although many fluids are good candidates in terms of their thermodynamic properties, they cannot be used inside pressurized, habitat volumes due to toxicity, flammability, and thermal stability concerns.