Finite Element Modelling for CubeSats

Modelling the structural integrity of a CubeSat with an optical payload is an extremely important step during the design process. At Simera Consult, we followed a numerical approach to evaluate the structural capacity of the remote sensing payload. This payload is precision engineered to consist of a high-performance optical front-end, an embedded attitude determination and control system (ADCS), and high resolution focal plane electronics. The success of a remote sensing payload is very much dependent on maintaining the structural integrity during the launch process.

In the space industry, the simulated environment will depend on the launch vehicle as well as the specific orientation and mounting configuration thereof. These conditions are usually predetermined, and the simulations are set-up accordingly. However, the standardised CubeSat design features enables the opportunity to consider numerous launch vehicles. To address this possibility numerically, the structural analysis of the Simera Sense remote sensing payload was simulated using universal standards. These standards include the NASA-derived Mass Acceleration Curve and GSFC-STC-7000.

The NASA-derived Mass Acceleration Curve was used to determine the design limit loads for a quasi-static load cases in each direction. This method includes an uncertainty factor of 1.15 during the preliminary design and an ultimate factor of safety of 2.6. Ultimately, the numerical model had a mass of less than 4 kg and consequently it was subjected to quasi-static loads of 180 g’s in the x and y-axes and 188 g’s in the z-axis. Here g is the gravitational acceleration at 9.81 m/s2.

For dynamic loads, sine sweep, shock, acoustic and random response loads are used to simulate the launch environment. During the design phase, the random response environment was considered the most severe. The GSFC-STC-7000 random vibration input curve was used for the latter and is shown in Figure 1 along with the random response input curves used for other well-known launch vehicles. From this figure, it is evident that the GSFC-STC-7000 random response input curve surpasses the other launch vehicle input curves considered, and therefore implies a very conservative and generalised approach. These simulations were used to drive the mechanical design and determine the structural capacity of the optical payload. Illustrations of some of the numerical results are shown in Figure 2 and Figure 3:

Figure 1: Various random vibration input curves

Figure 2: FE results of the payload and ADCS system at the first modal frequency at 303.8 Hz constrained within a 3U frame. The frame has been removed illustrative purposes.

Figure 3: Von Mises stress results of the xSpace100 and ADCS frame within a 3U frame at 180 g applied in the negative X-axis. The frame has been removed illustrative purposes.

Although it is essential that the mechanical design is sound, the ultimate goal is to achieve the desired optical performance. This in itself is not a trivial task. One cannot simply verify the structural capacity of the assembly without taking into account the effects of the mechanical design and optical performance of the system. Therefore, structural analyses are usually completed in parallel with optical analyses to evaluate the optical performance of the system.

At Simera we typically use the numerical results obtained from the FEA along with optical software, such as Zemax, to elevate both the optical and mechanical aspects the design. The FE results are extracted from the numerical model and processed into a format that is compatible with Zemax. Here the resulting surface deformation of the relevant optical components, and its relative translation and rotation in the system, are imported into Zemax where the optical performance of the system is evaluated. This process continues in an iterative manner until the mechanical and optical design reaches the desired criteria. This method allows for a dynamic design environment where the mechanical and optical design can be addressed in parallel, while gaining great insight into both the optical and mechanical behaviour system.

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