Electrochromic Radiator Coupon Level Testing and Full Scale Thermal Math Modeling for Use on Altair Lunar Lander

Abstract

In order to control system and component temperatures, many spacecraft thermal control systems use a radiator coupled with a pumped fluid loop to reject waste heat from the vehicle. Since heat loads and radiation environments can vary considerably according to mission phase, the thermal control system must be able to vary the heat rejection. The ability to "turn down" the heat rejected from the thermal control system is critically important when designing the system. Electrochromic technology as a radiator coating is being investigated to vary the amount of heat rejected by a radiator. Coupon level tests were performed to test the feasibility of this technology. Furthermore, thermal math models were developed to better understand the turndown ratios required by full scale radiator architectures to handle the various operation scenarios encountered during a mission profile for the Altair Lunar Lander. This paper summarizes results from coupon level tests as well as the thermal math models developed to investigate how electrochromics can be used to increase turn down ratios for a radiator. Data from the various design concepts of radiators and their architectures are outlined. Recommendations are made on which electrochromic radiator concept should be carried further for future thermal vacuum testing.

Variable‐emittance infrared electrochromic skins combining unique conducting polymers, ionic liquid electrolytes, microporous polymer membranes, and semiconductor/polymer coatings, for spacecraft thermal control

“…They are also increasingly inadequate for the greater thermal control needs of modern, energy‐intensive, large spacecraft. These latter include future manned missions where multiple coolant loops interface to crew quarters on one end and external radiators (e.g., louvers) on the other …”

“…Another, important property of thermal control materials is their solar absorptance, α(s), which is an absorptance that is integrated specifically over the solar spectrum, with predominant contributions in the 0.3–1.5 µm spectral region. It again varies from 0 to 1, with low values indicating low absorption of solar radiation.…”

“…The benchmark ε variation is that set by mechanical louvers, 0.15–0.55 (Δε ca. 0.4) . Additionally, in an ideal thermal control material, α(s) should remain constantly low, preferably <0.4, while ε is concomitantly varied from about 0.1 to 0.8, as needed, with Δε > 0.4.…”

“…Several years ago, we described an IR electrochromics system, an offshoot of a military IR camouflage technology, with some promise for space application, which used poly(aniline)‐based CPs and unique dopants that imparted enhanced IR electrochromism, and presented a number of advantages over other CP‐based electrochromic systems . This system, however, had a number of drawbacks that eventually prevented its successful implementation in space.…”

A Multi-Environment Thermal Control System With Freeze-Tolerant Radiator

“…A variation of this approach uses loop heat pipes to eliminate the circulation pump; however, this adds complexity during start-up and mass to the system. Other approaches to prevent freezing include (1) using survival heaters to prevent working fluid from freezing, (2) actively controlling radiator conductance by varing the radiator exposed surface areas or surface effective emissivity (Bannon et al 2010), (3) removing working fluid from some of the radiator tubes to reduce active areas and therefore the heat dissipation capacity (Ganapathi et al 2009), and (4) allowing working fluid to freeze on the internal surfaces of radiator tubes while maintaining a small liquid flow through the center of the tubes (Nabity et al 2008). However, all these methods incur some performance penalties in terms of mass, power, reliability, and recovery time, or pose technical challenges due to their low TRL.…”