Restraining Bolt The Restraining Bolt is used to prevent inadvertent actuation during launch vibration. In Figure 3, the  crew interface is on the left side of the Restraining Bolt, and the shaft to the right of the threads blocks the  Release Plunger if the TDA is actuated inadvertently. By design, a small gap is intended between the fully  engaged Restraining Bolt and the Release Plunger. This ensures that the Split Spool locks the  mechanism and the Restraining Bolt is truly a redundant feature. During initial on-orbit activation, the  Restraining Bolt is removed, which arms the TDA. The “Fully Locked” view of Figure 3 shows the TDA armed with the Restraining Bolt backe d out from the launch position.  Failure Investigation  The first indication of the TDA failure was the tripping of a power switch. Telemetry analysis found no  electrical faults, indicating a mechanical failure was the likely cause. Subsequent video review confirmed  the cable was cleanly cut, and the full TDA actuation sequence had occurred. The remaining TDA  mechanism failure modes were: Structural failure of the Restraining Wire, Split Spool, or T-plunger; tolerance stack-up effects; assembly or proce ss errors; and orbital debris impact.  Structural Failure Structural failure modes that were identified as a possible cause were compression yielding or creep of  the Split Spool material (Torlon), fatigue or creep failure of the Restraining Wire, or yielding of the Tplunger assembly components. Failure due to fatigue or creep of the Restraining Wire was considered low likelihood because the load on the wire is constant with small magnitude thermal cycle loads that are  evenly distributed. Fracture of the T-plunger was ruled out due to large cross sectional area and adequate  strength margins for a nominal design. Under nominal design loading conditions the Torlon Split Spool also has adequate strength margins.  Design and Tolerance Assessment An assessment of the design was performed to determine what effects the as-built hardware could have  on the position of the components within the mechanism. The results of the assessment indicated that  when the components were built to the nominal drawing dimensions a gap would exist between the Actuator Plunger and Split Spool. If the as-built components were biased to one side this could increase  that gap. The TDA assembly drawing omits this gap (see Figure 5).  During assembly of the flight units, the top portion of the T-plunger is installed and the Release Plunger  with Retaining Pins is compressed. Next, the Restraining Bolt is engaged to hold the Release Plunger in place and preclude release of the top portion of the T-plunger. The Primary Compression Springs are  then mechanically compressed, and the bottom portion of the T-plunger is installed and fastened to the  top portion with a single bolt. The final step installs the TDA Cover with Split Spool to the Housing.  After reviewing the assembly process it was clear that any gap between the Actuator Plunger and Split Spool  would go undetected during final assembly of the flight unit. That gap is maintained by the Restraining  Bolt until the unit is activated on-orbit. During on-orbit activation the Restraining Bolt is removed and the Release Plunger would move to close the gap (see Figure 5).  262