About 25% of space-borne electro-optical (EO) sensor programs in both the civil and National Security Space (NSS) communities have experienced reduced on-orbit reliability, as well as cost and schedule overruns of 100% or more1-3. Many of these EO sensor program over-runs can be largely attributed to delays, errors, and inadequate communication that occur at the many handoff points between team members and contractors in the current design process. This leads to the late discovery of technical problems, making them more expensive and time-consuming to fix.

The design of EO sensors requires careful attention to the thermal and structural effects that adversely affect optical performance in terms of instrument pointing accuracy and image quality. Teams of domain experts, each focusing on a separate aspect of the sensor, work towards understanding and managing its complex behavior. The current engineering process is typically fragmented into silos of experts, tools and data. Separate models are constructed, one for each of the disciplines (mechanical CAD, thermal, structures, and optics). Analysis proceeds within each discipline silo with relatively infrequent interaction between groups. There is no organized way for individual domains, or the team as a whole, to manage their design and analysis models over the entire project.

Project design reviews are typically conducted using time-consuming “static” reports and PowerPoint summaries. There is no unified view of the engineering models that represent the product’s behavior, nor is there easy access to key system performance data.

Figure 1. The parts of the CAD model of structural interest are FEM meshed using rules that are iteratively developed by the structures engineer and captured in Comet.

An engineering project team at The Aerospace Corporation comprised of a lead structural, thermal, optical, and mechanical design engineer, directed by Senior Project Engineer Dr. David Thomas, has successfully implemented a new collaborative systems engineering approach on an actual flight hardware program, reducing each design evaluation cycle by over 50%, while providing better insight into the multi-disciplinary behavior of a space-borne sensor4. Using expertise captured in bi-directionally associative design/simulation process templates, integrated structural, thermal, optical analyses were performed in a matter of days vs. weeks/months in the traditional “silo” approach. The Aerospace Corp. has also recently participated in the contractor’s final thermal vacuum (TVAC) testing of the flight hardware to correlate and validate the STOP (structural-thermal-optical performance) process models and results. A significant reduction in the cycle time of the EO sensor design evaluation and validation was achieved while meeting overall sensor design reliability and optical system performance target levels.

The Aerospace STOP Project

The Aerospace team used this new approach on an independent STOP analysis of a critical lens subassembly in support of an instrument contractor’s final TVAC testing. A high fidelity STOP model was developed to calculate visible channel focus shifts and image quality impacts due to thermally-induced structural deformations and refractive index changes (Figure 1) to conduct an assessment of the contractor’s focus control method.

Figure 2. The temperature of lens is controlled by two heaters, one on the L13 side of the housing and one on the L16 side of the housing. Although the surface area of the L13 heater is larger than the L16 heater, equal amounts of power must be supplied to each heater resulting in a much higher power density near L16.

The design concept for the optical lens subassembly employed active heaters which were applied to the outer surface of the lens retainers (Figure 2). Three different on-orbit thermal equilibrium conditions were simulated in the STOP analysis: Hot, Nominal, and Cold Soak conditions.

The analysis details were captured in a system-level design/simulation template as well as in the individual discipline templates, allowing the team to easily run a large number of STOP analysis design studies while modifying both geometric and non-geometric model parameters, and the thermal environment conditions. After each STOP analysis iteration, key performance metrics were available for immediate visualization in the Project Dashboard and compared against the system analysis performance requirements.

The team performed STOP analyses using Comet’s Performance Engineering Workspace, combined with their current commercial CAD and CAE tools — Pro/ENGINEER®, Thermal Desktop®, Nastran, Abaqus®, SigFit, CODE V®, Excel® and Matlab®. The workspace provided a single, consistent view of all the data — models, environments, processes and results — allowing the team to create, share and access data easily. The workspace allowed domain experts to work within their own “domain sandboxes” to understand their aspect of the sensor, but also to work together with other domain experts to gain overall system performance insights. By automatically reusing the expertise captured by the experts in simulation templates, manual data handoff errors were eliminated and the confidence in the accuracy of each analysis iteration was significantly increased.

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