Freezable Single-loop Thermal Control Architecture Assessment and Potential Key Enabling Technologies

Abstract

A space habitat thermal control system (TCS) keeps the vehicle, avionics and atmosphere within a specified temperature range. On the International Space Station, a water coolant loop collects internal heat loads for transfer to an external anhydrous ammonia loop via a closed heat exchanger. The ammonia loop then interfaces with the radiators to reject the heat. This requires sensors, active components and feedback control to ensure that the fluid temperatures remain within their allowable limits without freezing water. Further, toxic materials like ammonia impose constraints on design and require additional instruments to monitor for leaks. Together, these result in a complex architecture for spacecraft thermal control. Incorporating a single-loop, freezable water-based cooling system can offer numerous potential benefits to the TCS architecture: 1) removing the ammonia cooling loop eliminates this toxic material and reduces complexity, 2) freeze-tolerant components reduce the risk of structural damage posed by freeze, 3) selective freeze of the fluid loop can passively turndown the heat rejection rate and 4) can also provide thermal storage capacity. Under cold environmental conditions, the radiator temperature drops below the freeze point and water freezes along the tube. The buildup of ice then passively turns down the rate of heat rejection in proportion to the net thermal load from the spacecraft and the external heat sink environment encountered, as the ice layer both adds thermal resistance and forces fluid flow through a bypass. Similarly, as the heat load increases, the ice absorbs heat during thaw due to the latent heat of fusion. In this position paper, we describe a freezable single-loop TCS architecture along with potential enabling technologies, present strategies to integrate this concept into the architecture allowing self-regulaton of the spacecraft thermal environment, and discuss performance attributes for thermal control of orbiting spacecraft and habitats.