Space Shuttle Orbiter Atlantis Liquid Oxygen Prevalve Detent Roller
Elizabeth Holleman', David Eddleman*, Rebecca Jacobs _, and James Richard*
inspections of the Space Shuttle's Main Propulsion System (MPS) Liquid Oxygen (LO2)
the mechanism provided to maintain the valve in the open position was found cracked.
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 location. There are
6 pre-valves on each orbiter, and only one has exhibited this problem.
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
There were several
to the failure:
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 LO2 compatibility
that created a stress riser. The detent assembly was removed and
replaced with parts that were on the low-side of the tolerance stack up to eliminate the potential
loads on the detent roller. After one flight,
the roller was inspected and showed fewer signs of wear and
NASA's Space Shuttle
is propelled into orbit by 2 solid rocket boosters (SRBs) and 3 Space Shuttle Main
The SSMEs are liquid rocket engines that use liquid oxygen and liquid hydrogen as
is stored in the non-reusable external
tank. During loading the propellants
through the orbiter by way of the fill and drain system on the main propulsion system (MPS).
propellants are loaded in this manner in order to cool down the various MPS fluid components and to chill