ICES-2020-576 Gateway PTCS Integrated Thermal Math Model Jonah R. Smith1 Jacobs Engineering, JSC Engineering Technology and Science (JETS), Houston, TX, 77058 As part of the Integrated Analysis Cycle 4 (IAC4) tasks for NASA’s Gateway Passive Thermal Control Systems (PTCS) team, a simplified thermal model of the Gateway vehicle in its 9:2 Lunar Synodic Resonant Near Rectilinear Halo Orbit (NRHO) was developed within the Thermal Desktop environment. This paper details the methodology and assumptions that were used to model Gateway’s heating rate and orbital environments for the purposes of performing thermal analysis for the Gateway program. Discussion includes determining worst-case hot and worst-case cold environments for Gateway, approximating Gateway’s 9:2 NRHO orbit by a Keplerian orbit for purposes of thermal analysis, and the derivations of solar, albedo, and lunar IR heating rates experienced by Gateway. Nomenclature °C = Degree Celsius K = Kelvin m = Meter W = Watt 휖 = Emissivity 퐼퐿푊 = Longwave (IR) Irradiance 푖 = Solar-Incidence Angle 푆0 = Solar Constant (at Moon’s surface) 푎̅ = Albedo 휎 = Stefan-Boltzmann Constant 푇 = Surface Temperature θ = True Anomaly 휙 = Orbit Inclination AE = HLS Ascent Element BOL = Beginning of Life DE = HLS Descent Element DSNE = Design Specification for Natural Environments DSXR = Deep Space Exploration Robotics EOL = End of Life ESPRIT = European Structure Providing Infrastructure for Refueling and Telecommunications ETCS = External Thermal Control System GN&C = Guidance Navigation and Control HALO = Habitation and Logistics Outpost HLS = Human Landing System IAC = Integrated Analysis Cycle IHAB = International Habitat IR = Infrared Radiation ITCS = Internal Thermal Control System JSC = Lyndon B. Johnson Space Center 1 Thermal Analysis Engineer, L2 Gateway PTCS Model Development Lead, 2224 Bay Area Blvd./JT-5EA. L2 = Level 2 L3 = Level 3 LM = Logistics Module (a.k.a. Logistics Vehicle) MLI = Multilayer Insulation MMOD = Micro-Meteoroid Orbital Debris MPM = Multipurpose Module Airlock NRHO = Near-Rectilinear Halo Orbit OLR = Outgoing Longwave Radiation (a.k.a. Planetary IR) PPE = Power and Propulsion Element PTCS = Passive Thermal Control Systems RTD = Resistance Temperature Detectors SINDA = Systems Improved Numerical Differencing Analyzer TD = Thermal Desktop TE = HLS Transfer Element USHAB = United States Habitat I. Introduction HE NASA Gateway Passive Thermal Control Systems HLS T (PTCS) team has created a simplified thermal model of (TE/DE/AE) the Gateway space station in order to provide an accurate HALO radiative thermal environment for use by module, visiting PPE vehicle, and payload developers to evaluate the thermal impacts to their systems when attached to or in proximity of Gateway. The Gateway PTCS Integrated Thermal Math USHAB Model, shown in Figure 1, was developed in Thermal Desktop 6.0 within the AutoCAD 2019 environment. This model was created as part of the Gateway program’s fourth MPM Integrated Analysis Cycle (IAC4), and was distributed to IHAB NASA’s international, commercial, and government Orion partners in November 2019. The NASA Gateway PTCS ESPRIT- LM Refueler team had previously created their first integrated thermal DSXR model of Gateway for IAC3 in February 2019. The IAC4 Figure 1. Gateway PTCS Integrated Thermal version of this model redefines worst-case hot and cold Math Model (IAC4 Configuration 52) orbital heating rate environments, provides new geometry for each module, updates boundary condition assumptions, and introduces a new symbol-driven tool for modelling the different configurations of Gateway. The thermal geometry shown in this document is representative of the best- available geometry provided by NASA’s Gateway program at the beginning of IAC4 (August 2019), and does not reflect the most recent design decisions of the Gateway program. II. Thermal Model Development and Features A. Modelling Philosophy and Assumptions The Gateway PTCS Integrated Thermal Math Model is intended to be a simplified model that can be provided to Gateway stakeholders for integration with a detailed thermal model of their own element. The integrated Gateway model provides a radiative environment, accounting for shadowing effects, reflections, and IR heat exchange, and was built with consideration to minimize model complexity. Figure 1 above shows the integrated model when all Gateway elements are present. Thermal geometry is built exclusively with Thermal Desktop primitive thin shell surfaces with reduced nodalization to minimize node count and decrease runtimes. All thermal geometry, except for the Human Landing System (HLS) Ascent Element (AE), was created by the NASA L2 Gateway PTCS team (L2 means “Level 2” and refers to teams responsible for the integrated Gateway stack as opposed to an individual module, which would be L3 or “Level 3” work). The NASA L3 HLS PTCS team out of JSC provided a simplified thermal model of AE for use in the integrated thermal model. Developers of each Gateway module were contacted for optical and thermophysical property references and MLI placement whenever available, but many module developers had not yet determined what their optical properties 2 International Conference on Environmental Systems would be; in these cases, nominal assumptions were made by L2 Gateway PTCS. Given the lack of certainty in most module’s optical properties at the time, the IAC4 model did not distinguish between BOL and EOL optics. Future versions of the model in IAC5 and beyond include separate BOL and EOL optical property files, though EOL values for many thermal coatings are still being researched given the lack of flight heritage in Gateway’s NRHO environment. A full understanding of the optics and thermophysical properties assumed throughout the model is given in the model’s documentatation1, which was released along with the model to Gateway participants. Heater architecture for most Gateway elements is not yet defined, so in lieu of assuming some number of heaters/RTDs for each module, symbol-controlled boundary nodes are used to define temperatures of pressurized cabin walls. This also aids in reducing runtimes by eliminating the need to take small timesteps for use with heater logic, but comes with the consequence that a conservative hot/cold case temperature for pressurized shells had to be used. All MLI and MMOD shielding (if any exists) over interior cabin walls are left as diffusion nodes (where MLI is assumed arithmetic), typically with an effective emissivity of 0.03. Boundary temperatures on the cabin walls are controlled by 12 different symbols to allow for a high level of control by the model user, or the user can change the symbol “IHEATS” to a value of either 0, 1, or 2 to set cabin wall temperatures to 4 °C, 20 °C, or 27 °C respectively. Cold case analyses should set IHEATS to 0, and hot case analyses should set the symbol IHEATS to 2 to produce a worst-case environment. Radiator temperatures were set as boundary conditions (varying for hot and cold case analysis using the symbol IHEATS again) based on analysis using the minimum and maximum heat loads estimated for each module. Miguel Perez of the L2 Gateway ATCS team performed an analysis using a mock ETCS/ITCS loop sized to reject the expected maximum heat load on the HALO, IHAB, and USHAB modules to define worst-case instantaneous cold and worst- case instantaneous hot radiator temperatures. Body-mounted radiators were discretized, and FloCAD was integrated into a copy of the Gateway PTCS Integrated Thermal Math Model to model the ETCS/ITCS loops, as shown in Figure 2 to the left. Results from this analysis were implemented into the integrated PTCS model by taking the T4 area-weighted Figure 2. HALO Mock ETCS/ITCS Model average temperature of each radiator panel. Unlike the HALO, USHAB, and IHAB modules, the PPE uses a heat pipe radiator system. Operating temperatures for PPE’s radiators were provided by the L3 PPE thermal team. B. Modelling Different Gateway Configurations Given that the design of each Gateway module is still under development, the Gateway PTCS team decided to model each Gateway element in its own AutoCAD drawing file, and import each element into a master drawing as “external references” using the AutoCAD “XREF” command. This allows for ease of use on the model developer’s end when making changes to individual Gateway elements as the design process continues. This also makes it easy for a thermal analyst working with a detailed thermal model of some Gateway element to replace the simplified representation of that element with their own detailed model. An in-depth step by step instruction guide to importing Thermal Desktop models using XREF was provided to the Gateway thermal community within the model’s documentation1. A typical integrated thermal model like this, like the one used on the ISS program, would have the user disable the building of certain SINDA submodels, and exclude certain radiation analysis groups from radiation conductor/heating rate calculations, in order to model different configurations of the vehicle. Similarly for Gateway, if analysis was being performed on only Gateway’s PPE and HALO modules prior to any other module arriving, then all SINDA submodels referring to other modules could be turned off, and their corresponding radiation analysis groups excluded from radiation calculations. While this is an option within the IAC4 Gateway thermal model to analyze different configurations of the Gateway vehicle, users may also choose to simply unload drawings for those elements which are not present in their analysis by using the XREF Manager. 3 International Conference on Environmental Systems Figure 3 shows the XREF manager within AutoCAD being used to unload elements from Gateway to create a configuration where the HLS Ascent Element (AE), Descent Element (DE), and Transfer Element (TE) are not present at Gateway. Figure 4 shows IHAB, USHAB, MPM, ESPRIT-Refueler, and the robotic arm Deep Space Exploration Robotics (DSXR) being unloaded such that only Gateway Phase 1 (pre Moon landing) elements are present (under the IAC4 assumption that IHAB is not present in Phase 1).