Before proceeding to the prototyping and environmental testing stages of your pocketqube or cubesat you will be carrying out various FEA analysis to ensure the design can survive the forces during testing, launch and the mechanisms and structures are fit for purpose.

To ensure that your FEA results are representative of the real-life environment of your satellite you need to carry out a few checks on your FEA model. Without carrying out these checks you can’t 100% trust these results as they may be inaccurate and can’t be used to determine if your satellite, mechanisms or structures will survive in its environment. The FEA checks in this post focusses on the mechanical stresses experienced during launch. We will cover static, modal and a bit on random vibration.

There are a couple of simple checks you can do to test your FEA model.

Model representation

Model representation is a very easy check. Since FEA analysis can be very taxing on processing power you will need to simplify your FEA model in terms of removing various design features to reduce the number of nodes and elements. When simplifying your FEA 3D model make sure you use the same materials of the assemblies and components. In addition, make sure that the mass, centre of gravity and moments of inertia is similar to your actual 3D model.

It might sound obvious but also check that there are no interferences or random parts aren’t floating or incorrectly placed in your model. When I was carrying out checks, I noticed I was failing the modal and static checks. This was due to two parts interfering with each other.

Mesh convergence

A more refined and fine mesh will generally produce a more accurate result. However the finer the mesh the longer the analysis will take longer to calculate. You also want to have the results of your analysis to be repeatable and not largely affected by the size of the mesh. As the mesh has been converged you won’t see much difference in the results as you keep refining your model.

For more in depth tutorials in mesh convergence I have included some sources [1] [2] [3]in the references at the bottom of this blog post.

When it came to carrying out FEA on the Claymore 6P deployer and the vibration test rigs I recommended using Autodesk Fusion 360 is a powerful cloud based FEA software that I recommend as you can use the cloud processing to reduce analysis time.

Modal analysis check

For modal analysis model check you want to no constraints on your model and

You will know that your FEA model has passed this check if your first 6 modes are 0Hz.

Static check

For static force analysis check you want to carry out a 1G load to your model and analyse the reaction forces of your model. To know that your FEA model passes the check the reaction forces match the 1G load you applied. Do a static check on each axis separately.

Random vibration check

To find out the stresses on your satellite, systems or mechanisms will endure from the random vibrations during liftoff you can carry out random response FEA. Before you carry out a random response you will have to create a frequency response model to produce the modes of interest during random response. To ensure that your model has been setup correctly is that you are capturing at least 80% of the mass participation in your model. Andrew Jabola [4] from Structures Aero has a very good in depth tutorial in setting up you random vibration model with NASTRAN software that I recommend watching as he covers how to setup your model properly.  

References

[1] P. Asim Rashid, “Mesh Convergence: The modern way,” 24 August 2020. [Online]. Available: https://www.linkedin.com/pulse/mesh-convergence-modern-way-asim-rashid-phd/. [Accessed 1 January 2024].
[2] J. Gardiner, “Finite Element Analysis Convergence and Mesh Independence,” XCEED, 27 March 2017. [Online]. Available: https://www.xceed-eng.com/finite-element-analysis-convergence-and-mesh-independence/. [Accessed 1 January 2024].
[3] Autodesk, “How to Perform a Mesh Convergence Study,” 8 October 2023. [Online]. Available: https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/How-to-Perform-a-Mesh-Convergence-Study.html. [Accessed 1 January 2024].
[4] A. Jabola, “Frequency Response and Random Response (Dynamic Response in Nastran),” Structures Aero, 26 June 2017. [Online]. Available: https://youtu.be/VKF5bMM-KpQ?si=SoZfDSZ5O40h116Q. [Accessed 1 January 2024].