In the aerospace industry, CubeSats have emerged as a low-cost, easily manufacturable solution for space-based optical systems. They offer a unique opportunity to develop a production line approach for a space-based product through the manufacture of a constellation of smaller, more affordable systems.
Companies that manufacture CubeSat optical systems need an accurate and reliable method for developing an optical design, opto-mechanically packaging the system, as well as modeling structural and thermal impacts that the system will experience in-orbit. This article series will walk through the high-level development of a CubeSat system by leveraging the Ansys software suites. We will illustrate how an integrated software toolset can streamline the design and analysis workflow.
Authored by Flurin Herren & Vincenzo Maria Vitale
See chapter: References for article attachments
Introduction
For decades, optical systems have been developed for operation in low, medium, and high Earth orbit. For many optical systems, the packaging form factor and the opto-mechanics that stemmed from this form factor were designed on a system-by-system basis. CubeSats are a class of lightweight nanosatellite that can house optical systems for applications ranging from laser communications to earth imaging. They are unique in that they use a standardized size and form factor.
For this article series, the paper Optical Design of a Reflecting Telescope for CubeSat1 was used as a reference for developing the CubeSat optical design.
In Part 5 of this series, we will cover a fully automated mission-level STOP analysis using various solvers of the Ansys software suite. While traditional STOP analysis often depends on manual data transfers and isolated physics models, a mission-level approach integrates thermal, structural, and optical simulations into an automated workflow. Additionally, it enables real orbital load cases—such as solar flux derived from mission parameters—to be applied directly to optical performance analysis.
Solver and workflow overview
To include mission level information in the STOP analysis, several Ansys solvers need to be coupled into a workflow. To orchestrate and automate the workflow, Ansys ModelCenter (ModelCenter) is used. Illustrated below are the individual workflow steps with the according simulation tool or solver.
Figure 1: Overall automated mission-level STOP Analysis; Including handovers and connections
The workflow steps are aligned with the chapters throughout this article, starting with “Workflow step 2. Calculate mission orbit” as the subsequent chapter. “Workflow step 1. Mission architecture set up” is covered in the previous parts of this article series. Each chapter also addresses the specific automation of the workflow step through ModelCenter, the overall automation is summarized in the Conclusion chapter.
Ansys STK - Reference: Ansys STK | Digital Mission Engineering Software
With Ansys System Tool Kit (STK) complex systems can be analysed and visualised within the context of an operational environment, also called a mission. The engineer can create satellite orbits and determine environmental effects such as solar flux, albedo, or IR flux based on said orbit.
Ansys Thermal Desktop - Reference: Ansys Thermal Desktop | Ansys
Ansys Thermal Desktop (Thermal Desktop) is a modelling environment that integrates geometric definitions and thermal analysis, which enables engineers to build, solve and post-process detailed thermal models.
Ansys ModelCenter - Reference: Ansys ModelCenter | MBSE Software
Ansys ModelCenter (ModelCenter) is a model-based engineering integration platform that links various simulation tools and workflows into a single, automated and traceable process. It enables engineers to orchestrate multidisciplinary analyses.
Ansys Mechanical (Mechanical; finite-element analysis platform) and Ansys Zemax OpticStudio (Zemax OpticStudio; optical design and simulation software), have been used in the previous parts of this article series.
Workflow step 2. Calculate mission orbit
To set up the CubeSat mission with STK, a simple LEO (Low Earth Orbit) was defined, and a fixed imaging payload was added. Orbital element parameters such as Semi-Major, Eccentricity, Inclination, and Period were used as input to STK Propagator.
STK contains various propagators to help users account for different perturbations in the orbit propagation. Short-duration or ideal orbits can be simulated by J2 Perturbation or Two Body Propagators respectively. In this case, a J4 Perturbation Propagator was used, this allows the consideration of secular drift.
As soon as the STK Satellite is propagated, metrics that are a function of position or altitude of the spacecraft can be calculated. The calculations also take into account the sun and other celestial bodies. The main metrics of interest for this workflow are energy sources such as Solar Flux, Albedo or Infrared Flux.
Figure 2: Satellite scenario with incident fluxes over time in simulation (left) and graph (right).
Automation node: Mission Planner
The satellite cartesian orbit position and its Eulerian Angles are the initial inputs in the Ansys ModelCenter workflow. In the first Automation node, ModelCenter uses the connection to STK to automatically calculate and pass these values to the next node, which is the thermal calculation with Thermal Desktop.
Workflow step 3. Map solar flux on geometry
In Thermal desktop, the orbital scenario from STK is connected with the geometry of the optomechanical system.
A combination of Finite-Difference (FD) and Finite Element Method (FEM) objects is defined throughout the geometry. This approach allows higher resolution to be focused on the points of interest. An example of this is the primary mirror and the primary mirror retainer. The mirror itself is modeled with a FEM Mesh, allowing a higher resolution, while the retainer is a FD component. The geometry itself is used as a scaffolding for the FD and FEM objects.
- The FEM and FD objects must be aligned with the FEM model in Mechanical analysis. This process is called Thermal Mapping*1. It needs to be done in parallel to this step and is further explained in the next chapter.
Figure 3: CAD scaffolding to FD surfaces (top left); FD bricks and FEM elements (bottom left); Thermal Model (right)
Once the model has been created, materials are assigned and thermal contactors are defined. The orbital data from the STK scenario can be imported via an Orbit Plug-in.
Based on the orbital data over the thermal model, specific time steps corresponding to the highest and lowest mirror temperatures for an orbital cycle can be isolated via a Steady-Cyclic Transient Solution. These time steps can then be converted into Mapping Files, containing element numbers and thermal values.
Automation node: Heat Transfer Modelling
As there are certain unique “engineering decisions” in the fundamental optomechanical design, the mechanically cleaned design and the thermal mapping (alignment with the Ansys Mechanical Model) need to be done manually and saved prior to the start of the automated workflow. In the second Automation node, ModelCenter is then able to calculate the mapping files based on the automatically received orbital data and can pass them onto Ansys Mechanical.
Workflow step 4. Calculate FEA Deformation
Before bringing in the mapping results from Thermal Desktop, the thermal model needs to be aligned with the mechanical model (Thermal Mapping *1). This ensures that element numbers which have mapped temperature values coming directly from the solar flux are applied as thermal load cases on the right geometrical location in Mechanical. Another step that is also taken in parallel is the structural pre-processing of the geometry. It mainly involves the mechanical cleaning of the system, meaning reducing the complexity of the model by removing any parts which are not essential for the calculations. For the CubeSat, this includes bolts, screws, and certain side panels.
Next, the model can be prepared for the mathematical calculation of the FEA load cases in Mechanical. For this, parts can be grouped, this simplifies the assignment of contact points and specific mesh density and structure. An example of this is the front side of the mirrors, which reflect the light and therefore have the densest mesh.
- More detailed explanation of the steps of mechanical cleaning and the assignment of contact points and mesh can be found in Part 3 (From Concept to CubeSat: Part 3) of this article series.
With the mechanical model set up, the thermal loads from the Thermal Desktop mapping can be imported. In this example this is done via the External Data Tool in Workbench, which allows the user to point to the Thermal Mapping files. Based on those thermal mapped values Mechanical applies a thermal load via an Imported Body Temperature Simulation. The loads are the sources of deformation which can be solved and visualized within Mechanical.
Figure 4: Workbench item (top left); Meshed geometry (bottom left); Combined Loads with Imported Body Temperature (right)
The fact that the load cases are based on the solar flux of the orbital position of the CubeSat itself, is the key aspect and fundamental difference between a mission-level and a “traditional” STOP analysis (Such as covered in Part 3 and 4 of this article series; From Concept to CubeSat: Part 3, From Concept to CubeSat: Part 4)
The loads are causing the front surface of the primary and secondary mirror to deform, which will degrade the performance of the optical system. To analyse the performance degradation, point clouds of the deformation mesh can be generated and imported back into Zemax OpticStudio.
Automation node: Structural Deformation
This step is the third Automation node defined in ModelCenter. The previous ModelCenter node (Thermal Model) is handing over the thermal mapping results. These result files are then used as the basis for a full finite-element analysis inside Mechanical. Finally, the resulting deformation point clouds are saved for the handover to Zemax OpticStudio. Geometry- and system-specific steps which are not changing such as material data, mesh or connections need to be set up manually prior to starting the automated workflow.
Workflow step 5. Analyse Optical Performance
The FEA data from Mechanical is imported back into Zemax OpticStudio. To do this, the FEA data represented as 2D surface deformations are imported as a Text file into the STAR module of Zemax OpticStudio via the Multiphysics Data loader.
Figure 5: Multiphysics Data loader and 3D Layout (top left); MTF (top right) and Image Simulation (bottom) both with and without FEA data applied.
As soon as the FEA data is applied, Zemax OpticStudio automatically fits the surface deformation over the native equation driven surface definition, and the results are visible in all analysis tools. The user can toggle the effects of the FEA calculation on and off in the STAR tab. As imaging systems like this CubeSat have a digital sensor, the primary metric to analyse is the Modulus Transfer Function (MTF). The MTF describes how effectively an optical system transfers contrast from the object to the image as a function of spatial frequency. The figure above illustrates both the MTF as a function of spatial frequency with and without thermal loads as computed in Zemax OpticStudio, together with the corresponding Image Simulation illustrating the effects of such a contrast drop on an image.
Automation node: Optical Performance
This is the fourth and final Automation node in the ModelCenter workflow. It receives the FEA Data as text files, after which the node calculates the nominal and degraded performance of the optical system of the CubeSat. Finally, an analysis window is generated, showing the Modulus Transfer Function and an Image Simulation for both nominal and degraded performance (See graphic in the next chapter: Conclusion).
Conclusion
By adding mission-level information into the STOP analysis, the user is able to base the thermal and structural effects on a realistic impact of the environmental conditions. Necessary for this is a physics-based modelling environment for the mission, which in this case here was STK, and to then further apply the mission-level impacts onto the geometry in a thermal model solver, which in this case was Thermal Desktop. After that, an FEA and an optical simulation are necessary to provide insights on the impact on the optics. Given the number of interacting physics domains and simulation tools involved, the next logical step is the automation of the overall workflow. In this case, the workflow automation was done with ModelCenter.
Summarizing the automated workflow, ModelCenter is getting the orbital parameters as input and calculates the orbit cartesian positions (Mission Planner Node), running them through a thermal model (Heat Transfer Modeling Node), calculating the thermal load cases (Structural Deformation Node), and then applying the data to the optical system in Zemax OpticStudio (Optical Performance Node). Finally, it outputs both the nominal performance and the performance under orbital load cases.
Figure 6: Workflow schematic including nodes, directory, output (bottom right) and ModelCenter UI (top right)
References
- Jin H, Lim J, Kim Y, Kim S. Optical Design of a Reflecting Telescope for CubeSat. J Opt Soc Korea. 2013;17(6):533-537. doi:10.3807/josk.2013.17.6.533
Article Attachments: If you are interested in receiving further information, example files and a live demo, please reach out to your Ansys, Part of Synopsys, representative.
Back to Part 1: From Concept to CubeSat Part 1: Using Ansys Zemax Software to Develop a CubeSat System – Ansys Optics