This work improves the performance of a heat switch, or a thermal actuator, by delivering heat to the actuator in a more efficient manner. The method uses a heat pipe as the plunger or plug instead of just using a solid piece of metal. The heat pipe could be one tailored for fast transient thermal response.

A heat switch/thermal actuator works by using the expansion of a paraffin wax as it melts as a means of moving a piston/plug/plunger to perform a function. Typically, this function is to close a small gap and increase heat transfer across a boundary, but it also could be used to move a latch. These devices are usually slow, and the stroke of the piston/plunger is very small.

A device of this kind could replace the need for heat switches that require power to operate on a spacecraft in a safe-mode condition. This device would require no power to operate except for the waste heat of the device it is protecting. It may also be used as an energy-harvesting device by using waste heat to move a piston back and forth much faster than could be accomplished otherwise.

The device uses waste heat that flows through the plunger pedestal into the heat pipe and out towards the paraffin wax to cause actuation of the plunger due to phase change of the wax from solid to liquid. For use as a heat switch on a spacecraft, multiple devices may be permanently attached to a radiator via the plunger, and the body attached to a rigid structure. During a safe mode orbital maneuver if the radiator should face the Sun, the

device will then push off the radiator, disengaging it from the spacecraft bus. The device could be mounted as a pull device as well, pulling the radiator closer to the thermal bus to increase the thermal conuctance between bus and radiator.

Thermal actuators of this kind are somewhat common, except that this device uses a heat pipe as a plunger, so this is an improvement. Most other devices require heat transfer through the wax chamber body, not through the plunger itself. This device will have three distinct advantages over other versions:

  • Fast actuation due to quick heat transfer.
  • Large stroke and stroke velocity.
  • Mass savings as there is no need for thick metallic sections for conducting heat.

The actuation stroke could be designed to be large and quick enough to be used as an energy-harvesting device, converting waste heat into mechanical energy.

This work was done by Juan Cepeda-Rizo of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at under the Mechanics/Machinery category. NPO-46679

This Brief includes a Technical Support Package (TSP).
Quick-Response Thermal Actuator for Use as a Heat Switch

(reference NPO-46679) is currently available for download from the TSP library.

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This article first appeared in the April, 2010 issue of NASA Tech Briefs Magazine.

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