Thermal

Overview 

This page and its subpages contain thermal system designs that are particular to LI Bus. Please refer to General Bus Design - Thermal for general information. To see more design details, analysis results and physical tests, please open the tree item.

Thermal Block Diagrams

Figure 2: LiBus System Block Diagram

Figure 3: LI Bus Thermal Control Block Diagram

Preliminary Thermal Design

As mentioned in the structural layout section, LI Bus's layout is designed to provide a favourable thermal gradient across the spacecraft. Equipment which may require cooling is separated from that which likely requires heating. This is done to ensure that, for example, heaters do not contribute to a cooling load in an adjacent module by bleeding energy through the structure. Functional and system block diagrams for the thermal subsystem can be seen in the first two figures in the section above.

As the exact thruster design and propellant for the LI Bus have not been determined yet, the propellant tanks may need heaters to keep them at appropriate temperatures to prevent the propellant from freezing. The batteries in the LI Bus power system may also require heating. Both of these systems will use the Datec electric heaters used by Iris. These heaters will be adhered to the batteries and propellant tanks along with thermistors to provide telemetry and control. 

Other thermistors will be placed to monitor the temperatures of the CDH computer, ADCS board, reaction wheels, communications equipment, and solar arrays. These thermistors will only be used for telemetry and health monitoring, as none of these components will have active thermal controls. A diagram of the thermal control system, including the thermistors, heaters, and flows of data and heat can be found in Figure 3.

Though Iris's structure was black anodized for thermal purposes, preliminary thermal analysis for LI Bus suggests that black anodization will have little impact on bus temperatures. As such, the structure will likely be clear anodized. Detailed thermal analysis also suggest that no special painting is required for the bus. 

Any payloads requiring active cooling or heating are expected to provide these systems themselves. The LI Bus can provide the payload with power for active thermal systems. Additional thermal equipment—like heaters, radiators, thermal straps, conductive or insulating films, or heat pipes—required by the payload to be mounted to the bus will need to be discussed with the LI Bus team.

Temperature Telemetry

The thermal subsystem collects temperature telemetry through thermistors, and all component mount thermistors are adhered with thermal epoxy.

Thermistors

Table 1 outlines the location, control and supplier of thermistors. 

Thermistor part number 445-174519-2-ND is automotive grade, it is reliable for the length of the mission lifetime. 

Thermistor part number 495-6716-ND is automotive grade, it is reliable for the length of the mission lifetime. 

Thermistors mounted to the batteries and propellant tank are fully epoxied to the battery surface, therefore all outgassing materials will be enclosed within the thermal epoxy.

Thermistors mounted on components other than the batteries and propellant are fully conformal coated, therefore all outgassing materials will be enclosed within the conformal coating.

To comply with requirement 7128, thermal components other than solar panel thermistors are shielded with structural aluminum shells, see the structure subsystem for more information. 

Solar panel thermistors are passive, solid-state components, therefore radiation effects on them are minimal.

All thermistors are sourced from Digikey, which comply with ITAR. 

Thermal Epoxy

Component mount thermistors require thermal epoxy to be secured to a component. Thermal epoxy needs to have high thermal conductivity to ensure that the thermistor measures the component's temperature truly, and have adhesive properties to ensure mechanical security. Furthermore, to comply to requirements 7133 and 7131 (see Valispace), the thermal epoxy needs to comply with outgassing parameters. 

Based on this sheet, I selected the product that is available in small volume, available from suppliers, have consistent properties, 100% solid, and have the highest thermal conductivity.

I selected Henkel Gap Filler 1500 from Mouser Electronics. Its datasheet can be found through the product link below. Currently (2024/08/20), the product is in stock.

This epoxy have TML = 0.41% and CVCM = 0.05%.

This epoxy is also a Certified Henkel Adhesives for high reliability applications product, which indicates its reliability.

This epoxy is a passive component, therefore radiation effects on applied epoxy is minimal. 

This epoxy is sourced from Mouser Electronics, which is ITAR compliant.