Orion 낙하 시험팀구성원들의 맨토로써, 저는 알테어가 이 프로젝트의 성공에 대단히 중요한 역할을 했다는 것을 인정합니다. Altair ProductDesign 팀은 정밀하고 신속한, 그리고 정확하면서 통찰력 있는 모델구성, 시뮬레이션을 통해서 Orion 프로젝트에 중요한 기여를 했습니다. – Thomas K. Mattingly II, Former Lunar Astronaut and Director of Space Enterprise
NASA가 차세대 우주선에 남, 여 승무원을 승선시켜 다시 우주로 보내게 될 때는 우주 비행사가 Orion Crew Module를 탑승한 상태에서 지구로 돌아오게 될 것입니다. Orion Crew Module은 태평양에 밝은 오랜지 빛깔 낙하산 아래서 모습을 선보인 세계의 관심을 집중 받았던 Apollo Crew Module 과 비슷한 외관을 가지고 있습니다. NASA는 Orion이 아폴로와 비교하여 기술, 성능 면에서 엄청난 발전이 있었다고 강조 했습니다. Orion은 새로운 디자인과 생명유지 장치, 추진시스템 , 열 보호 및 항공 전자 시스템을 갖추어서 먼 우주에서 오랫동안 임무를 수행할 수 있도록 설계 되었습니다.
Engaging proactively to help NASA avoid potential issues, the NASA Engineering and Safety Center (NESC) performs value-added independent testing, analysis and assessments of NASA’s high-risk projects to ensure safety and mission success.
The Challenge: Simulating Splashdowns
The water landing of a craft like the Orion Crew Module is a very complex and changeable event, subject to the dynamics of the vehicle’s structure and sub-structures, such as its heat shield, as well as atmospheric and water conditions. To maintain the spacecraft’s structural integrity and increase safety of the crew, a clearer understanding of the dynamic loads generated during water impact was required Creating a computer simulation of this event is difficult and especially sensitive to such input variables as mesh density, boundary conditions and contact interfaces. To ensure that simulations reflect real-life conditions as accurately as possible, physical test data is needed to correlate to and anchor the finite
element simulation models. Once engineers can be certain that the physical and virtual models correlate well with each other, they can confidently use finite element models as a predictive tool for conditions encountered during water landings and for refining structural designs.
The NASA Engineering and Safety Center (NESC) sought to establish a clear understanding of the specific modeling methods needed to perform dynamic simulations of the Orion Crew Module water landings. Specifically, the work was focused on determining the critical simulation variables, methods and physical testing needed to create an accurate computer simulation finite element analysis (FEA) model. With an accurate FEA model, accelerations, loads and trajectories could be used to evaluate and develop astronaut safety systems during water landing as well as predict the structural stability of the Crew Module structure itself.
Solution: HyperWorks and the Altair ProductDesign Team
NASA called on Altair’s Product Design group to develop this critical simulation model, working as part of the larger NESC assessment team. NASA built a full-scale boilerplate Crew Module to perform the required physical testing. This Crew Module was primarily built from steel with reinforcements so that it could be analytically treated as a rigid body. It was instrumented with several datacollecting devices, such as accelerometers, strain gauges, an inertial measurement unit and pressure sensors. The team placed photogrammetric targets on the outside surfaces to accurately measure the Crew Module trajectories, along with high-speed video cameras at strategic locations.
At a still, deep fresh-water lake, NESC performed more than 60 physical drops of the module at slightly different impact angles and velocities. The raw data from each drop test was supplied to the simulation team to aid with the correlation of the FEA models.
“We realized that determining precisely the right mesh sizes and ratios would be key to producing a model that correlated well with the physical test results,” said Mahesh Patel, NESC Crew Module Water Landing Simulation team member and engineering manager for Altair ProductDesign, “so we used HyperWorks’ pre-processor, HyperMesh to model the Crew Module structure as a rigid body.”
Altair ProductDesign positioned accelerometers in the virtual model to replicate those from the physical test. Additionally, the model incorporated 25 feet of water depth and 13 feet of air height to match the drop test conditions.
Once the simulation team members received the physical test data, they adjusted the model by varying input parameters, finding that acceleration data was the most reliable factor. They then analyzed the sensitivity of such parameters as interface stiffness, mesh density, fluid pressure distribution and boundary conditions, each in relation to acceleration data. This analysis showed that mesh density proved by far to be the parameter that most significantly influenced the correlation of the simulation with physical tests.
The team created a matrix consisting of 20 separate models with different combinations of mesh density for the Crew Module and the fluid mesh. They discovered a very good correlation when applying the smallest mesh dimension to the fluid mesh in three directions and only varying the Crew Module mesh size to be the same or larger than the fluid mesh dimensions.
Results: Confidence in Future Spacecraft Design
The results from the mesh sensitivity study matrix revealed that mesh size ratios of Crew Module to fluid are very important in obtaining good correlation. Altair ProductDesign’s efforts proved so valuable that the NASA Engineering and Safety Center presented its Group Achievement Award to the Crew Module Water Landing Modeling Assessment Team on Nov. 1, 2011 in Williamsburg, Va. The team was commended for its “outstanding contributions…on an accelerated schedule” for the coordination of efforts that contributed substantially to the achievement of the NESC mission. The NESC Honor Awards are given each year to NASA Center employees, industry representatives, and other stakeholders for their achievements in engineering, leadership, teamwork and communication. The awards formally identify individuals and groups who have made outstanding contributions to NESC’s mission, demonstrate engineering and technical excellence, and foster an open environment.
“The findings from this study will prove invaluable in ensuring accurate simulation data for the rest of the ongoing Orion Crew Module project,” Mahesh Patel said. “Moreover, now that NASA is equipped with a tried and tested simulation methodology, it can be confident that the simulation results from future development activities are an accurate representation of real-world impact behavior, leading to the development of improved landing safety systems.”