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NASA s development of new concepts for the Crew Exploration Vehicle
Orion presents many similar challenges to those worked in the
sixties during the Apollo program. However, with improved modeling
capabilities, new challenges arise. For example, the use of the
commercial code LS-DYNA, although widely used and accepted in the
technical community, often involves high-dimensional, time
consuming, and computationally intensive simulations. Because of
the computational cost, these tools are often used to evaluate
specific conditions and rarely used for statistical analysis. The
challenge is to capture what is learned from a limited number of
LS-DYNA simulations to develop models that allow users to conduct
interpolation of solutions at a fraction of the computational time.
For this problem, response surface models are used to predict the
system time responses to a water landing as a function of capsule
speed, direction, attitude, water speed, and water direction.
Furthermore, these models can also be used to ascertain the
adequacy of the design in terms of probability measures. This paper
presents a description of the LS-DYNA model, a brief summary of the
response surface techniques, the analysis of variance approach used
in the sensitivity studies, equations used to estimate impact
parameters, results showing conditions that might cause injuries,
and concluding remarks.
Finite element analyses have been performed for two full-scale
crash tests of an MD-500 helicopter. The first crash test was
conducted to evaluate the performance of a composite deployable
energy absorber under combined flight loads. In the second crash
test, the energy absorber was removed to establish the baseline
loads. The use of an energy absorbing device reduced the impact
acceleration levels by a factor of three. Accelerations and
kinematic data collected from the crash tests were compared to
analytical results. Details of the full-scale crash tests and
development of the system-integrated finite element model are
briefly described along with direct comparisons of acceleration
magnitudes and durations for the first full-scale crash test.
Because load levels were significantly different between tests,
models developed for the purposes of predicting the overall system
response with external energy absorbers were not adequate under
more severe conditions seen in the second crash test. Relative
error comparisons were inadequate to guide model calibration. A
newly developed model calibration approach that includes
uncertainty estimation, parameter sensitivity, impact shape
orthogonality, and numerical optimization was used for the second
full-scale crash test. The calibrated parameter set reduced 2-norm
prediction error by 51% but did not improve impact shape
orthogonality.
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