93   Table 3. Average Pallet Accelerations for 12 Hz Dwell Tests   G input Test  Sequence Input as  Measured  (g) Average  Pallet   Response  (g) Average Pallet  Transmissibility Average Pallet  % Reduction  Compared to  Input* Average Pallet  % Reduction  Compared to  TS1 (Rigid)  0.2 TS1 0.198 0.281 1.42 -41.92 N/A  TS2 0.197 0.106 0.54 46.19 62.28  TS3 0.199 0.042 0.21 78.89 85.05  0.35 TS1 0.347 0.52 1.50 -49.86 N/A  TS2 0.35 0.122 0.35 65.14 76.54  TS3 0.344 0.073 0.21 78.78 85.96  0.5 TS1 0.507 0.776 1.53 -53.06 N/A  TS2 0.5 0.147 0.29 70.60 81.06  TS3 0.492 0.105 0.21 78.66 86.47  *Note: Negative reduction values indicate increase in input. 

During Test Sequence 3, the average reduction in acceleration or loading remains relatively constant over  all the input levels tests. Test Sequence 2 on the other hand seems to experience a greater percent reduction as the input level is increased. After the completion of Test Sequence 2, the Isolators were removed and the friction damper force was tested. The friction dampers for Isolators 7 and 8 were within the 111 – 156 N (25 – 35 lbf) target range while struts 5 and 6 had static/kinetic friction values of 267 N (60 lbf) and 67 N (15 lbf), respectively. During low level dwell and sine sweeps of Test Sequence 2, strut 5 was not fully released which resulted in a shift in the system dynamics.    The test demonstrated that the TO Isolators do an effective job at mitigating loads due to a TO event, reducing pallet accelerations to 20 - 40% of that of the input. The Delrin® friction dampers however, are overly sensitive and altered the dynamics of the system making correlation of pre-test analyses with the dampers difficult. This prototype damping system should be replaced with a more reproducible, controllable damper for a flight system.   

Lessons Learned 

Passive Friction Damping  The Isolator design solution utilized a passive, friction-based damping scheme. The inclusion of damping into the Isolator was not introduced until after fabrication of the Isolator parts, which partially drove the solution. Friction, by nature, is difficult to control and get repeatable results. The end result of this is that the friction force applied to the Center Rod, and therefore the damping coefficient of the Isolator, changes slightly from test to test. In addition, the Center Rod was turned down on a lathe, leaving a concentricity and roundness tolerance greater than desired. This caused variations in friction clamping force throughout Center Rod travel, which was observed in Isolator friction testing. A more appropriate finish on the Center Rod for a passive friction damping system would be to centerless grind the shaft, which would result in less surface variations and more consistent friction forces.  Structural Adhesive under Impact Loads  At both the forward and aft hard stop of Center Rod travel, a Delrin® piece would contact the Center Rod to prevent like-material contact. Hysol® EA 9394 structural adhesive epoxy was used to adhere the Delrin® to the substrate metal. This was selected due to the fact that it would require minimal modification to the components, ease of assembly, low-profile installation, and common usage in space industry. The