The ST8 Thermal Loop technology advance has two basic elements:

• A closed-loop method of communicating heat from one place on a spacecraft to another using only the capillary forces and vapor pressure available from a two-phase working fluid to effect the necessary transport;

• An analytical model capable of predicting the steady-state and transient behavior of this closed-loop thermal-control device.

1. To test and measure the steady-state and transient performance of a two-evaporator, closed-loop heat-pipe thermal control system capable of controlling the temperature of two independent heat sinks—representative of any device on a spacecraft needing heating or cooling—capable of generating up to 75 W and needing cooling or requiring up to 75 W of heat to maintain its required temperature;

2. To measure the effects, if any, of gravity on the performance of this heat-pipe thermal control system by testing it in different orientations;

3. To develop an analytical model of this thermal-control system, validated by the test data, capable of predicting the steady-state and transient behavior in a 1-g environment or in zero-gravity of this thermal control system or of similar thermal-control systems of different size and thermal capacity.

The ST8 Thermal Loop validation experiment will consist of a series of tests conducted in a vacuum chamber where the Thermal Loop test article will be installed. Heat sources will be simulated by electrical heaters, and excess heat will be radiated to surfaces maintained at approximately 75 Kelvin by liquid nitrogen. These tests will allow the performance of the Thermal Loop to be measured and provide data with which to validate the analytical model of the system. The principal tests are:

1. Start up under several different conditions;

2. Power ramp-up test;

3. Operating temperature control;

4. Heat-load sharing;

5. Flow regulation test;

6. Control heater power test; and

7. Test in different orientations with respect to gravity.

In addition, scaling rules will be established to validate the scalability and applicability of the flight results when extending to other loop heat pipes with different geometries and configurations, and/or under different environments.