Thermal Control
Thermal control methodologies vary widely by spacecraft payload, orbit, and size. Some payloads require precision temperature control due to cryogenic or optically sensitive hardware; some require materials with high thermal conductivities to minimize thermal gradients across isolation structures. Thermal control components such as thermoelectric coolers, patch heaters, heat pipes, thermal straps, and deployable radiators are all weapons in the arsenal for preventing excessively high or low component temperatures while on orbit. Particle radiation is included on the ABEX thermal team because particle radiation contributes to spacecraft thermal energy by Charged Particle Heating wherein particles deposit energy in spacecraft structures. However, particle radiation can also cause electrical charging on the chassis exterior surface, Non-Ionizing Energy Loss in the solar arrays leading to a loss of power generation, Single Event Effects in the computer hardware, and ionizing radiation damage in all spacecraft circuitry. The Space Radiation Environment is complex, but ABEX students continually impress reviewers with their grasp on the subject.
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TPM |
Rationale |
Units |
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Minimum/Maximum Thermal Conditions |
To correctly define the thermal environment, the lowest and highest heat flows into and out of the system must be found so that a quantitative analysis may be made of the potential highest and lowest temperatures in the satellite. This TPM represents a custom array of absorbed heat per spacecraft face and operational ohmic heat; this calculation can be performed for spacecraft Beginning of Life (BOL) or End of Life (EOL). |
W |
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Required Heater Wattage |
The passive heat radiating and conducting away from spacecraft elements must be met with active heat so that components remain within their operational and extreme temperature limits. |
W |
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Radiator Area |
Spacecraft using photovoltaic generation can feature Sun-pointing faces with high temperatures. Radiators physically interfacing with the high-temperature surfaces can be used to radiate heat into space before the heat negatively affects spacecraft electronics. |
m2 |
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Analysis Tool |
Description |
TPM |
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SPENVIS |
The SPace ENVironment Information System (SPENVIS) is the European Space Agency's interface to various space radiation environment and effect models. The ABEX Radiation Team, considered part of the ABEX Thermal Control Team, uses SPENVIS to access models for five radiation effects of interest: Charged Particle Heating, Total Ionizing Dose, Non-Ionizing Energy Loss, surface charging, and Single Event Effects. With particle flux outputs from SPENVIS and stopping power values provided by NIST's PSTAR and ESTAR platforms, Charged Particle Heat absorbed per face at every time step is calculated in the MATLAB model to assist in the isothermal estimation of radiator area and patch heater wattage. The other four radiation effects are analyzed by the ABEX Radiation Team to advise on avionics-protecting structural design considerations like radiation shield areal density and solar cell coverglass thickness. |
Minimum/Maximum Thermal Conditions |
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MATLAB |
A programming language primarily used for numeric computation while also including numerous extensions and add-ons for specific analyses. The ABEX Thermal Control Team imports STK data for a specific orbit and calculates absorbed heat per spacecraft face due to incident radiation and thermal generation. From these absorbed heats, an isothermal spacecraft model is generated to provide a "first guess" heater wattage and radiator area, though the values are overestimated due to the high area-to-mass ratio of the solar array. MATLAB was chosen for this computation due to the ease of importing data and calculating an orbit-specific thermal radiation environment. The "first guess" values and calculated environment parameters are provided to Simulink as a basis for non-isothermal analysis. |
Minimum/Maximum Thermal Conditions Required Heater Wattage Required Radiator Area |
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Simulink |
Simulink is an interactive, graphical interface that acts as an extension of MATLAB. Through university-provided toolboxes, a wide array of dynamic systems can be transiently modeled, including thermal analysis. With the Simscape toolbox, the ABEX Thermal Control Team constructed a thermal resistance network of the entire spacecraft to analyze different combinations of heater wattage and radiator areas. The pre-programmed blocks can be oriented to successfully represent the spacecraft while providing a relatively high speed, code-based approach to location-specific analysis as opposed to the Finite Element Analysis in Thermal Desktop. Simulink analyzes a range of heater wattages and radiator areas based on provided isothermal values to create an operational envelope of acceptable heater wattages and radiator areas for both minimum and maximum thermal conditions. The coincidence of hottest and coldest operational envelopes represents a subset of acceptable heater wattages and radiator areas for evaluation in Thermal Desktop with physical interfacing and internal radiative view factors considered. |
Required Heater Wattage Required Radiator Area |
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Thermal Desktop |
Thermal Desktop is a three-dimensional (3D) computer-aided design (CAD) environment for thermal and fluid simulation. It functions as an add-on to Autodesk's AutoCAD program and excels as a tool for heat conduction calculations, radiative view factor assessment, and orbital definition utilizing 3D finite element analysis (FEA). With the built-in case set manager, the ABEX Thermal Control Team uses Thermal Desktop to quickly iterate and compare different combinations of heater wattage and radiator area that the Simulink thermal resistance network deems acceptable. The primary advantage of performing this analysis in Thermal Desktop as opposed to Simulink and MATLAB is the 3D FEA solver. Obtaining results in three dimensions, even with errors incurred by FEA grid dependence, allows for a higher-fidelity model and a more informed decision about the selection of radiator area, radiator mounting locations, heater wattage, and heater locations. With the radiator area and heater wattage design verified by MATLAB, Simulink, and Thermal Desktop analysis to be compliant to operational temperature requirements, the design is deemed acceptable for integrated qualification testing. |
Required Heater Wattage Required Radiator Area |
The ABEX Thermal Control team determines operational envelopes of acceptable heater wattages and radiator areas to maintain a thermal energy balance in space. The models' input data are self-consistent, meaning all models are using the same input data and can be accurately compared. The result is an amount of heat required to keep the spacecraft warm and an amount of area needed to keep the spacecraft cool at a given surface emissivity. To warm the spacecraft, ABEX has opted to use low-outgassing, surface-mountable patch heaters. Some heaters are mounted to exterior features of spacecraft components while others are mounted directly to electronics, but the purpose remains the same. For the radiator, ABEX is currently evaluating a variety of design options based on first-principle analyses. Knowing the required area and emissivity is sufficient for an energy balance, but achieving the required area using a low-mass system requires thought. Radiator design elements facilitating low-mass results include embedded heat pipes, ultrasonic additive manufacturing, thermostructural elements, anisotropic thermal conductivity, and flexible physical interfaces between the Sun-pointing face and the radiators. As ABEX proceeds through PDR and CDR in 2023+, the design will be analyzed for physical integration and thermal functionality, integrated with the spacecraft structural prototypes, and subsystem-level qualification tested in preparation for system-level qualification at Goddard Space Flight Center.