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Space Shuttle Solid Rocket Booster Holddown Post Stud Hang-Up Root Cause Analysis
 


Problem:
Solid Rocket Booster (SRB) stud hang-ups have occurred over the life of the Space Shuttle Program. The SRBs are bolted to the Mobile Launch Platform by a Holddown Post (HDP) system. At the time of launch, these bolt studs are designed to quickly exit into the HDP allowing the Space Shuttle to liftoff. A stud hang-up at liftoff can increase loads at the SRB/External Tank attach points. NESC undertook an extensive hardware test program to aid in determining root cause. This included development of a high fidelity stud and frangible nut model that was calibrated with data from the test object. The NESC found that a number of often violent factors work to slow or interrupt the stud's descent and clearance from the SRB. The cause of stud hang-ups is a combination of contributing factors including: frangible nut pyrotechnic firing skew, nut half recontact, plunger seating and frangible link breakage, debris interaction, bore hole contact, frictional forces from the plunger, and movement of the SRB aft skirt prior to full stud ejection. At launch, the cumulative result of the factors' individual effects on the stud, most of which are almost always in play but take place at variable levels of intensity, add up to slow down the stud's descent enough that a hang-up occurs. Extensive testing, modeling and simulation was used to determine the cause of the stud hang-ups. the Space Shuttle SRB Project can now work toward modifications that reduce the number of these unwanted occurrences.

Lessons Learned:
Testing that includes firing pyrotechnics produces its own set of problems. Pyrotechnic devices produce large quantities of smoke and can interfere with photographic analysis unless smoke mitigation steps are taken. Pyrotechnic devices function very quickly and require special instrumentation to measure pyrotechnic explosion timing. Accelerometers may perform poorly in a blast environment, especially in a contained environment, due to shock and reverberation from the initial blast that can overwhelm the accelerometers.