Hydrogen vapors which occur during the main engine
start sequence expel into the engine nozzles just before ignition. This action
results in a hydrogen-rich atmosphere in the engine bells. To prevent damage to
the engines, six hydrogen burnoff pre-igniters have been installed in the Tail
Service Mast. Just before main engine ignition, the pre-igniters activate. They
then ignite any free hydrogen in the area below the engine bells. This avoids
rough combustion at main engine start.