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 and 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.
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 FE 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.
NASA worked with Altair’s Product - Design group to develop the 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. The Crew Module, instrumented with several data-collecting devices such as accelerometers, strain gauges, an inertial measurement unit, and pressure sensors, was primarily built from steel with reinforcements so that it could be analytically treated as a rigid body.
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, freshwater 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.
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.
The team then analyzed the sensitivity of parameters such as interface stiffness, mesh density, fluid pressure distribution, and boundary conditions, each in relation to acceleration data. The 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 engineers 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 dimension.