Thermal System

The thermal design of the IlliniSat2 bus is a progression based upon numerical simulation validated by experimental simulation. Currently, the Thermal system team is refitting the in-house TVAC chamber and developing the necessary control logic to run appropriate thermal vacuum simulations. A cooling system must be designed and implemented in order to simulate the temperature fluctuations the satellite will encounter. Also the current TVAC chamber cannot hold an ideal vacuum for an extended period of time. Sources of leaks must be identified and repaired before the successful function of the chamber.

Thermal Design Philosophy

It was important to first establish a design philosophy for the thermal control design process in the context of our project. The first step in the thermal design process is to develop a simple, yet accurate thermal model in simulation. This requires evaluating what thermal transfer modes are negligible and which modes are dominant. Once the model is simulated, it needs to be validated.

The thermal vacuum chamber serves to validate the model simulation experimental measurements. If there are discrepancies between the simulated model and the experimental results, the simulated model environment is modified to match the experimental test conditions and connections and constraints are changed to accurately replicate the experimental data. Prior to this step is the development of a reliable control system for the TVAC chamber cooling system as well as the ability to record calibrated instrumentation readings.

Once a validated model is developed, it is used as a test bed for modifying the thermal control logic of the satellite. This control logic pages the system to enter a low power mode if components become too hot, and will page the system to activate heaters to warm components which become too cold. Developing the control logic using the simulated thermal model will allow for more rapid development and less cost and risk intensive TVAC chamber testing.

Once the thermal control model is developed, it is validated in the TVAC chamber. In the final stages of prelaunch testing, the fully assembled bus - with the onboard thermal control logic - is tested in the chamber using on-orbit environment cycling. Once confident that all components of the satellite endure the TVAC chamber testing within acceptable ranges, the satellite becomes qualified for launch.

This philosophy applies to both our generic subsystem bus and mission specific payloads. The philosophy of combining numerical simulation with experimental validation using a thermal vacuum chamber allows for quick modification and testing of new structural and thermal design architectures while being gaining the assurances given by experimental validation.

Copyright University of Illinois at Urbana-Champaign 2008