Evaluation of High Velocity Wear

The HHSTT located at Holloman Air Force Base conducts hypersonic testing in a unique way. Rather than perform cost prohibitive flight testing or hypersonic wind tunnel testing, a rocket-powered sled propels test articles down a track. This test setup has been used to test at speeds up to 2885 m/s (~Mach 8.6). The sled is kept on the rails by utilizing slippers, fabricated to wrap around the rail [1]. This slipper design keeps the sled from separating from the rail during a test due to the airflow producing lift, in a designed effort to minimize the wear that occurs during the test run. Different sleds and rails are used at the HHSTT and thus different geometries exist [2]. The slipper material is carefully chosen in an attempt to minimize rail wear as well as to include a safety margin so that the slipper can maintain the designed load carrying capability despite material loss due to wear. Through the years, different materials were used as test velocities increased and a better understanding of the wear environment occurred. This research investigated the wear phenomenon using three different methods, incorporating finite element mechanical modeling as well as heat transfer thermal modeling. One of the methods incorporated a three-dimensional FEM consisting of a portion of the slipper colliding with micron sized asperities to simulate slipper-rail contact and included assumptions pertaining to that scale. The second method was a one-dimensional heat transfer model that simulated the entire time that a test run occurs. The third method was another three-dimensional FEM. As opposed to the previous FEM described, it used friction as the mechanism by which the slipper and rail interacted. These last two models were at a larger scale and their results, along with the asperity collision results, were compared to experimental data acquired from a worn slipper from an actual test at the HHSTT in 2008