Space Shuttle Orbiter Atlantis Liquid Oxygen Pre-Valve   Detent Roller Cracking Investigation Elizabeth Holleman*, David Eddleman*, Rebecca Jacobs** and James Richard*  Abstract  During routine inspections of the Space Shuttle’s Main Propulsion System Liquid Oxygen (LO 2) pre-valve,  the mechanism provided to maintain the valve in the open position was found cracked. The mechanism is  a Vespel roller held against the valve visor by a stack of Belleville springs. The roller has been found  cracked 3 times. All three instances were in  the same valve in the same lo cation. There are 6 pre-valves  on each orbiter, and only one has exhibited this problem. Every-flight inspections were instituted and the  rollers were found to be cracked after only one flight. Engineers at Marshall Space Flight Center, Johnson Space Center, and Kennedy Space Center worked together to determine a solution. There were several  possible contributors to the failure:  a misaligned visor, an out-of-specification edge with a sharp radius,  an out-of-specification tolerance stack up of a Belleville spring stack that caused un-predicted loads on  the Vespel SP-21 roller, and a dimple machined into the side of the roller to indicate LO 2 compatibility that  created a stress riser. The detent assemb ly was removed and replaced with  parts that were on the low  side of the tolerance stack up to eliminate the potential for high loads on the detent roller. After one flight, the roller was inspected and showed fewer signs of wear and no cracks.  Introduction  NASA’s Space Shuttle is propelled into orbit by two solid rocket boosters and three Space Shuttle Main  Engines (SSMEs). The SSMEs are liquid rocket engines that use liquid oxygen and liquid hydrogen as propellant. The propellant is stored in the non-reusable external tank. During loading, the propellants flow  through the orbiter by way of the fill and drain system on the Main Propulsion System. The propellants are  loaded in this manner in order to cool down the various Main Propulsion System fluid components and to chill the engines. This prevents gas ingestion by the engines during ignition.  The Main Propulsion System contains various sub-systems including the feed system. The feed system  allows flow of oxidizer and propellant from the external tank to the three SSME’s. After the feed system  manifold, there are three pre-valves that are used for isolation of the propellant supply from the SSME’s. These pre-valves are used to prevent catastrophic failures of the oxidizer turbo pump during nominal  engine shutdown as well as during a contingency situation when engine isolation is necessary. Because  of these important tasks, the pre-valves are inspected and tested routinely.   Valve Function  Each Space Shuttle Main Engine has two 30.5-cm (12-inch) diameter propellant isolation valves. One  valve is in the liquid hydrogen system and the other in the liquid oxygen (LO 2) system. They are referred  to as pre-valves and are located in the Main Propu lsion System. The pre-va lves are used during all  phases of the shuttle operation:  fill and drain of the external tank, ascent, and in contingency situations.  The primary purpose of the pre-valves is to stop the flow of propellant to the SSME’s in the case of an  engine failure or shutdown. The restriction of flow reduces the likelihood of an uncontained fire in the aft  compartment or engine. The LO 2 pre-valves also serve a critical purpose during Main Engine Cut Off  (MECO). During MECO, helium is injected from the SSME pogo accumulator into the area upstream of  the high-pressure oxidizer turbo pump (HPO T). This maintains the proper pressure for shutdown of the  HPOT and allows for safe engine shutd own. The pre-valves close, providing the sealing force to maintain  * NASA Marshall Space Flight Center, Huntsville, AL  ** Jacobs Engineering Science & Technical Services Group, Huntsville, AL  Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  97