The Mars Science Laboratory (MSL), developed at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, CA, was designed to determine whether the Gale Crater on Mars ever had conditions favorable for life. The Curiosity rover is equipped with a robotic arm that can drill into rocks, scoop up soil, and deliver samples to internal analytical instruments.
While experience with previous Mars rovers, including Spirit and Opportunity, played a role in the development of MSL’s thermal control system, there were major differences in this project that posed new challenges for JPL.
Curiosity’s power generator, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), is constantly generating a substantial amount of heat, so JPL had to add more capability to the heat-rejection system to accommodate it during the cruise phase. Also, Curiosity’s payload is larger, with much higher heat dissipations. The larger heat load on the rover influenced the need to add a rover heat-rejection system. But an even bigger difference is that Curiosity’s heat-rejection system has to operate on the surface of Mars. While the cruise heat-rejection system operates in a single mode to remove waste heat, the rover heat-rejection system must perform both heating and cooling on the Martian surface.
The design of the MSL’s thermal control system involved more than just the heat-rejection system. It included all the typical thermal control hardware (heaters, thermostats, thermal control coatings, and thermal blankets) that maintains the payload and the spacecraft subsystems within their allowable temperature requirements, for all operating modes and in the wide range of thermal conditions the MSL will experience throughout the mission lifetime.
The highest temperature that portions of the MSL flight system will experience is estimated to be 1447 °C during entry into the Mars atmosphere. The coldest environment it will experience is the coldness of deep space (-2 degrees Kelvin/- 275 °C) during the cruise phase to Mars. The thermal environment on the Mars surface will range from -135 to +50 °C.
Nearly a decade ago, JPL started to put together a technology infrastructure aimed at meeting the more aggressive schedules and leaner budgets it had started to experience. A key element was establishing seamless software interfaces from conceptual design through manufacturing. This would allow JPL to minimize transcription errors, manual processes, and interpolations between meshes. Minimizing errors and rework was critical to maintaining design and fabrication schedules.
To address these issues, JPL implemented NX software as an end-to-end mechanical design platform. With NX, JPL got a fully integrated computer-aided design (CAD)/computeraided engineering (CAE)/computer-aided manufacturing (CAM) system. This is the system JPL used to develop the mechanical portions of the MSL, including the thermal control system.
JPL’s mechanical designers modeled the entire MSL using NX. There are digital assembly models of the rover, the cruise stage, and the descent stage. Analysts used the NX geometry, simplifying it as necessary, as the basis for their finite element meshes. Having design geometry and the analysis meshes in a single environment improved collaboration between the design and analysis teams, and also reduced the time and effort spent creating analysis models. The integrated NX environment also allowed the engineering teams to rapidly re-evaluate designs as the mechanical hardware evolved.
JPL engineers started with small simulations (as this was the pilot program) to validate modeling assumptions, and eventually gained confidence that their models correctly replicated the physics involved. Then they used the NX CAE solutions for thermal analysis to simulate a variety of physical effects, such as fluid flow in the rover, heater control of the propulsion system, and solar loading of the cruise stage. Analysis results were used to update the design geometry.
The ease and efficiency of going from the design to thermal analysis and then back to update the design geometry accelerated the development of the MSL’s thermal control system considerably. Saving time and keeping to the schedule was critical, although an equally important benefit of using NX was the ability to evaluate the thermal control system’s performance under conditions that JPL could not simulate with physical testing.
In addition to tighter design-analysis integration, use of NX enabled integration between different types of analysis, such as thermal and mechanical distortion and stress analysis. Prior to adopting NX, engineers would have run a thermal solution and then manually mapped temperatures to the structural mesh. Use of NX eliminated this manual process.
Use of NX also enabled easier access to multiple types of analysis. For example, designers also needed to know whether any moving components would interfere with any other components or rover operations. This would have been difficult to determine by looking at static drawings or digital models. Using NX Motion made it possible to answer questions such as these without the costs and delays of physical testing.
The MSL flight system is the most complex Mars mission that JPL has implemented, involving new technologies and a new approach for entry, descent, and landing. As such, the development lifecycle is very difficult to compare to previous missions. It is clear that the MSL program had less manual work and more efficient upstream and downstream modeling and simulation interfacing compared to previous programs. And not having to re-enter data into multiple applications ruled out a potential source of error, giving JPL a higher level of confidence in the MSL design than it would have had otherwise.
JPL also uses Teamcenter, which enables a single source of structured product and process information management throughout the digital lifecycle.
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