83 It was concluded that an isolation frequency of 5 Hz or less is required to reduce the crew acceleration  responses to 0.25 g. A 4.5 Hz isolation frequency was selected as optimal, as it represents a good balance between deflection and transmitted dynamic load. This isolation frequency of the pallet is achieved by placing linear springs in series with the 4 X-axis struts.  Strut Deflection  During the TO event and while isolated at 4.5 Hz, the pallet will translate between 0.5 cm – 1 cm  (0.2 in – 0.4 in). Therefore, the TO Isolator stroke was designed to allow for slightly more than the predicted max of 1 cm before hitting a hard stop. In order to eliminate Isolator deflection during ground operations, the mechanism was designed to be preloaded through launch until the TO event occurs between 3.25 g’s to 4.5 g’s quasi-static load.  

Volumetric Constraints  Volumetric constraints were imposed by the available strut length and the crew size. It was determined  that, being in-line with the CM struts, a maximum length of 39.4 cm (15.5 in) was not used for the landing event and allowable for the TO Isolator design. The cylindrical diametric size had to be kept to a minimum to ensure that it did not contact a crew member’s shoulder. Computer Aided Design models proved this to be less than 11.4 cm (4.5 in).  

Isolator Impact on Landing Loads  LS-DYNA®, kinematic analysis software, was used to investigate the effect from the TO Isolator on  landing loads experienced by the crew and compared to the baseline.    The initial model consisted of CM struts in series with an isolation spring; subsequent models included a damper to address potential issues with LA. With a 30g input, it was found that introduction of the TO isolation springs does not significantly or detrimentally affect accelerations transmitted to the pallet. However, the isolation springs have a detrimental effect on the stroking of the baseline struts, which ideally should be held to a minimum. Preliminary analyses showed that the Isolators need to be locked out during a landing event to minimize CM strut stroke while keeping the Brinkley model’s injury risk probability to an acceptable level.   Isolator Impact on LA Loads  Although other launch analyses were done using NASTRAN®, the effect of the Isolators on crew loads in the contingency case of a LA were evaluated with LS-DYNA® software.   This was done so that crew response of the abort event could be determined while keeping the Isolator kinematic motion in the model. Without damping, the seat accelerations were found to be unstable and grew without bounds. With just 44.5 N (10 lbf) of Coulomb friction in parallel with the Isolator, the accelerations were sufficiently attenuated and decayed once the abort loadings ended. Since actual damping levels throughout the crew module are unknown and difficult to determine it was not useful to attempt a detailed damping study for the isolation springs. Instead, it was decided to utilize a small amount of Coulomb damping in the model and then insure that the design of the isolation spring had a mechanism for providing a deterministic level of damping.     TO Isolator Strut Design   Design Overview  The TO Isolator strut was designed as a passive spring and damper system that would be active during ascent and locked out during landing. It is mounted in-line with the X-axis CM struts. For hardware testing, the Isolator was mounted to the wire bender via a 1.905 cm – 16 (.750”-16) threaded interface, see Figure 2.