10 Heat Switches

Heat switches, known also as thermal switches, are devices that switch as needed between roles as good thermal conductors and good thermal insulators. When one is installed in the heat-conduction path between a warm, heat-producing component and a heat sink, the change in thermal conductance it affords can control the temperature of the component. Heat switches can passively control the temperature of warm electronics or instrumentation without the use of thermostats and heaters, thereby reducing power requirements as well as the need for heater control circuitry and software. Cryogenic applications use heat switches to minimize heat loads on cooling systems by disconnecting components when cooling is not required or disconnecting redundant refrigerators that are not providing cooling because they have failed or have been tumed off. In general, cryogenic systems include different types of heat switches than the ones found in applications closer to room temperature. Heat switches differ from thermostats; the latter control heaters by opening or closing electrical circuits at a given temperature, while heat switches open, close, or vary heat-conduction paths. Heat switches thus function similarly to diode or variable-conductance heat pipes; however, they achieve temperature control by modulating a conduction path rather than a two-phase flow process. Most heat switches that operate in normal spacecraft temperature ranges are passive devices that automatically self-regulate their conductance rather than react to signals received from a controller. The control they exercise is characterized by a variable adjustment of conductance between "on" and "off' values. The time constant of heat switches is usually short enough to handle rapid fluctuations in power, yet well enough damped by thermal capacitance and mechanical characteristics to avoid overshooting desired temperatures. The name "heat switch" is therefore somewhat misleading because these devices afford a more complex control than the simple on/off function provided by a switch. This chapter provides an overview of several types of heat switch, including paraffin heat switches and cryogenic heat switches. Other heat-switch designs have been proposed and/or prototyped, including designs for magnetostrictive, electrolytic, and electrostatic switches, as well as mechanical switches actuated by electric motor or liquid-to-gas phase change. Although the concept of heat switches was first devised in the early 1960s for use on the Mariner mission to the moon, their practical development is still relatively new. Switches based on more recent paraffin-based technology will fly on the Propulsive Small Expendable Deployer System (ProSEDS) tether experiment mission in late 2002, and some additional heat switches are currently in the baseline design for the Jet Propulsion Laboratory (JPL) Mars '03 Rovers. Recent