There is a wide range of spacecraft thermal management applications that require variable conductance devices such as thermal switches. These switches are used to help maintain the heat source (electronics) temperature under varying thermal loads and varying thermal environments. These applications include satellites and Lunar and Mars landers and rovers, as well as future human spacecraft that may transit through both the cold environment of deep space and warm transient environments such as low lunar orbit.
A low-mass, two-phase thermal switch (TPTS) was developed to meet these requirements. The switching mechanism is passively triggered by the temperature of the heat source. The TPTS operates in a manner similar to a heat pipe with flexible wall, as shown in Figure 1. It consists of a sealed metallic bellows housed in a hermetic enclosure, with one end of the bellows fixed to the enclosure, which is in turn fixed to a heat source. Within the bellows is a small amount of saturated working fluid and a wick structure. Incoming heat vaporizes the liquid working fluid and increases the saturated vapor pressure within the bellows. This increase in pressure causes the bellows to expand until it comes in contact with the other end of the enclosure, which is fixed to a heat sink.
The saturated vapor temperature associated with the vapor pressure at which the bellows comes in contact with the heat sink can be considered the set point temperature of the TPTS. This set point temperature is determined by the design of the TPTS. One component in determining the set point temperature is the pressure of the gas inside the enclosure, which provides an opposing force to the expansion of the bellows. Changing this counter pressure provides a method for remotely adjusting the set point temperature of the TPTS.
The TPTS operates in two regimes. The first is similar to a conventional thermal switch, in that the TPTS switches from low to high conductance and back with the application or removal of a heat source. However, when the TPTS is in the On condition, it also operates as a variable conductance device to maintain the heat source at the set point temperature determined by the design. As shown in Figure 2, the TPTS can maintain the heat source set point temperature as the sink temperature changes widely. This variable conductance regime is the result of a complicated dynamic mode of operation that causes the bellows to oscillate and be in periodic contact with the heat sink.
A prototype TPTS has been fabricated and tested. All aspects of the TPTS operation have been demonstrated, including dependence of the set point temperature on enclosure gas pressure, On/Off thermal cycling, and variable conductance while in the On condition to maintain the set point temperature of the heat source.