Thermal

The thermal control components used for the satellite will be mainly passive, with the exception of heaters.  The thermal control subsystem will largely consist of using different materials and coatings to control the radiation properties and flow of heat through the satellite.  All exterior structure surfaces that are made of aluminum will be black anodized to maximize heat absorbed, as the satellite body runs cold. The solar panels typically absorb much more heat than the spacecraft structure, and due to their low mass and thickness can heat up very quickly. Therefore, all solar panels will be made using a white solder mask to minimize absorbed heat.  A conductive path from the solar panels to the main spacecraft body will be formed by the deployment hinges. 

Two different thermistors are used in the thermal subsystem-one part mount thermistor with wire leads, and one board mount thermistor that is soldered directly to the mounting location.  The part mount thermistor is used in only one location, on the payload sample plate. All other thermistors are located on electronic boards where they can be soldered in place.  The part mount thermistor will be mounted using thermal epoxy (Stycast 2850 with catalyst 9) and then wired in to the payload control board. All other thermistors will be soldered to their appropriate mounting locations and then staked in place using thermal epoxy (Stycast 2850 with catalyst 9). 

Due to the requirement for 80% system reliability, we have decided to include redundant heaters on the batteries. This is based on our decision to use Datec 3D printed heaters as our primary battery heaters. These heaters do not have space flight heritage, and therefore it is difficult to prove that they will meet the reliability requirement in a space environment. Space-proven Minco heaters will be included in the design to provide redundant heating capabilities, and meet the 80% reliability requirement. A redundant battery thermistor is also included in the thermal subsystem design as accurate battery temperature readings are important to battery health. If both thermistors are operating properly, the average temperature of the two will be used to control the heater power. If it is clear that one thermistor has failed, the data from the failed thermistor will be neglected and only one thermistor will be used for calculations. 

The redundant heater will only be used if the primary heater fails, or if the spacecraft enters a low power mode where the C&DH is powered down. The power to the primary heater is controlled by a switch, while the power to the redundant heater is not switched. The flow of power to the redundant heater will be controlled by a mechanical thermostat wired in line with the power line from the PCU. 

During typical operating modes (any mode where the state of charge of the battery is above 40%) , the heating of the batteries is controlled by the C&DH, which reads the temperatures from the  battery thermistors and determines if the heater should be turned on or off. The control algorithm (temperature cutoffs) for the primary battery heater will be defined such that the thermostat  will be creating an open circuit (no power flowing)  at all times when the PCU is on, to ensure that both heaters are not activated at the same time. 

When the state of charge of the batteries falls below 40%, the C&DH is turned off to conserve power. When the C&DH is powered down, the PCU will set the switch so that the primary (Datec) heater is off. At this point the redundant battery heater will be relied on to control the battery temperature, with the mechanical thermostat opening and closing  as the battery temperature varies in order to power the heater on and off.  The mechanical thermostat will be mounted on the battery saddle, in  close proximity to the batteries.  The function flow for the thermostat operation is shown in the diagram below.

In the case where the primary heater has failed, as determined by telemetry, the switch to the primary heater will be turned off permanently and the redundant heaters with thermostat control will be relied on entirely to maintain battery temperatures. This process will also occur automatically if the primary heater switch fails open (no flow of power). A switch that has a higher likelihood of failing open will be used here, to minimize the chance of a fail closed resulting in constant power to the battery heater.

Design Trade-Offs

RTDs vs. Thermistors

We chose thermistors for temperature measurement on board Iris for a number of reasons. First of all, thermistors are generally cheaper than RTDs when expected temperatures are within the standard measurement limitations of thermistors. Since the expected temperatures experienced by Iris are fairly mild, we are well within the temperature range of standard automotive-grade thermistors. Thermistors also have a faster response time and a smaller volume when compared to RTDs, as they are often available with a pin head sized temperature sensing bulb, which has an extremely low thermal capacitance compared to larger RTDs. The one potential advantage of RTDs over thermistors for Iris is that RTDs have a linear relationship between temperature and resistance, while thermistor resistance response is non-linear. However, equations that define the resistance profile of thermistors are provided by manufacturers, and the non-linear behaviour can therefore be accounted for by the software. For these reasons, thermistors are chosen as the preferable temperature sensor.