Precision Temperature Control with a Loop Heat Pipe

The paper describes the design, development and testing of a Heat Transport System (HTS) for a Laser Communication Terminal (LCT) on the TerraSAR-X satellite. The HTS is based on the technology of Loop Heat Pipes (LHP) with variable heat conductance capability. Which maintains for this specific LCT payload component a constant temperature with a very small tolerance within a ≤0.5K range. For this new and challenging application an actively controlled bypass with a flow control valve has been developed.

In a conventional application the valve works in a passive mode, i.e. it actuates automatically according to the saturation pressure of the loop's heat carrier fluid. With increasing evaporator temperature (or saturation pressure) the valve opens the bypass line in order to divert vapour from flowing to the radiator. If the evaporator temperature decreases then the valve regulates again an increasing flow to the radiator. With such a passively controlled by-pass valve a control accuracy of about ±5K can be achieved. In the mentioned LCT application a much tighter temperature control is needed, which will be achieved with an active design characterized by an electronically controlled bypass valve. Heating or cooling of the valve changes its actuation pressure, which subsequently controls the temperature of the evaporator as well as the temperature of the device to be cooled. Compared to other devices for active temperature control the control power is low.

Besides the detailed description of this novel LHP-design the paper reports on ground tests in a simulated environment demonstrating the LHP-ability to provide the specified temperature control for the LCT on TerraSAR-X. Critical test conditions are operation in gravity, extended temperature range, start-up with low power and dynamic behaviour with fast changes of load or environmental conditions.