In addition to predicting system response, during the early development cycle, additive manufacturing  models served as advantageous visual aids to assist with communication of concepts. Handheld models  provide a foundation for quickly grasping concepts with increased levels of engagement during collaborative  sessions.   Dynamics Simulations  Multi-body simulation of the transportation route has proven to be a useful tool to ensure the mechanisms  will remain within the specified travel ranges, and to identify the locations on each route that are likely to be  most challenging. General analysis tools for multi-body dynamics were already available, but a process for  turning road survey data into boundary conditions needed to be developed from scratch. The MPTS  inherently relies upon contact and various types of nonlinearities that cannot be linearized away. Adding  control systems for dozens of wheel modules, with many contact definitions between a road surface and  the tires, would make a miles-long roadway simulation strain solvability and still require a lot of guesswork.  As an alternative, the as-built survey data was used to create splines for the center, and each side of the  road. The splines incorporate features such as cross-fall, crown, and grade. A simplified mechanism of links  and joints spans between the splines in order to be pulled along the path without binding. The resultant  pitch, yaw, roll, and elevation for each location is imposed on a dynamic model of the MPTS mechanisms.  Thereafter, the model is able to illustrate points on the surveyed road, the system will have the least  available range-of-motion margin, and operators can respond to this information appropriately.   Since the MPTS mechanism model is built using both dynamically-reduced finite element models and  realistic joints, it is also possible to capture the effects of structural flexibility, friction, and gapping. Through  simulation, it was discovered that tolerances on a number of mechanisms needed to be kept extremely tight  for the system to operate properly. A degree of freedom that is designed to be released at a particular  location requires it to be released there, and nowhere else. Simulating the effects of tolerances revealed a  loose fit of certain interfaces would result in redundant joints and indeterminate states for the system. This  simulation resulted in a changed design, resulting in tight or adjustable interfaces at critical locations.   Friction Testing  In the first complete, end-to-end test of the integrated system, it was discovered that the friction at a critical  joint was higher than anticipated. Moreover, this friction was high enough to impede proper operation.  Initially, there was no plan in place to quantify or monitor this friction, but the Test Load Facility, designed  to statically load the structure, proved flexible enough to perform this task. This led to the development of  a means to test friction levels for critical functions (mechanism joints) which are essential to how the system  operates. This testing should also be included for periodic maintenance during the design life of the system.  Another lesson associated with the friction test was the need to fully understand a vendor product, even  something considered intuitively simple like a friction liner. A specific liner was selected in the design, based  on the specification provided by the vendor. This liner was expected to have a coefficient of friction range  that was crucial in one of our released degrees of freedom, and be capable of supporting the maximum  calculated load during our transport operation.   During testing, it was discovered the friction load was higher than expected, and the friction coefficient of  the liner was not as anticipated. Further research indicated the liner material is a specially designed product  where the friction coefficient decreases with increasing load as shown in Figure 12, which is counterintuitive with classic textbook problems.   NASA/CP—2018-219887 467