"Dogleg" Trajectories Which Can Confuse Trajectory/Momentum Solution Procedures

General Crash Reconstruction Tecnhiques
Crash tests, Pole Impacts
MSI
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"Dogleg" Trajectories Which Can Confuse Trajectory/Momentum Solution Procedures

Post by MSI »

August 12, 2022: A post today on the NAPARS page of Facebook included a discussion of situation where there is a rollout of the vehicle at the end of rotation. It brought back memories as follows:

A long long time ago (like Don McLean American Pie song)
    • One of my first projects in 1979-1980 was a NHTSA research project working on improvements to the trajectory option of the CRASH3 program. The final report was published in 1981: As part of that research we came up with many option for handling many different types of trajectories. The main issue was that in most crashes the 'separation angle' assumed by simplified momentum solution procedures is not directly from the point of impact to the point of rest.
      Our research looked at a number of different scenarios and came up with several refinements, however...
      • we concluded that due to residual linear velocity at the end of rotation and the possibility of a 'dogleg' type runout that:
        • "Therefore, a realistic appraisal of the achieved results led to the recommendation that future developments of CRASH should focus attention on the existing step-by-step, time ­history forn of trajectory analysis, in which fundamental physical laws are applied directly. "
    • in 1997, we added and improved:
      • 1997 "CRASH-97 - Refinement of the Trajectory Solution Procedure", SAE 970949
        • We improved the scatter of the CRASH3 Momentum results (see below) However we never released a commercial version since working on many other projects and not time.
          It also had some sensitivities and we believed that the automatic iteration/optimization of SMAC would be the best solution to test and refine the scatter in simplified momentum results.
        Crash97 results.jpg
        Crash97 results.jpg (77.7 KiB) Viewed 4852 times
    • IN 2003 in response to the major advances in PC computers we worked on the automatic iteration/optimization of SMAC:
      • We developed an automatic iteration/optimization routine for the SMAC program:
        • 2003 SMAC2003 - The Automatic Iteration of SMAC SAE 2003-01-0486
          Which used the CRASH3 results form the trajectory collision and iterated/optimized to the following correlation with the full scale test result and measurements for damage, trajectory, impact speed and impact speed change
          • and note we verified the veracity and accuracy of the RICSAC full scale test result and deltaV measurements in our 1997 SAE paper RICSAC-97 :
          Smac2003 Results.png
          Smac2003 Results.png (58.84 KiB) Viewed 4852 times
        • which included in the discussion:
          • "The collision and trajectories of vehicles can be highly non-linear events. Minor variations in starting conditions (i.e., speed, impact location) can produce major changes in the resulting rest positions (X, Y, PSI) and discontinuities in the calculated error evaluation terms. For example, during decelerations of the linear and angular velocities, as a vehicle rotates while it travels from separation to rest, the vehicle may “shoot off” tangentially in what has been described as a “dog leg” type of trajectory at any time that the velocity vector aligns with the longitudinal axis."
Doglegs type trajectories can be a problem for simplified momentum solution procedures.
  • Obviously if you KNOW the answer you want or need it can solve the issue, however if you don't, HOW to you objectively vary the separation angles of the vehicle: what evidence do you use to justify the change?
  • That is why we recommend using a time history vehicle simulation program like msmac3D for such cases to allow an objective analysis of the damage AND trajectory evidence to evaluate what combination of speed, angles and offset project the proper damage areas and trajectory match.

i will post up some dogleg type crash trajectories, etc
i added this post because we really hadn't addressed 'dogleg' directly in any posts yet, now we have!

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