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NASA Stennis S1C Test Stand  Apollo Saturn V
Hydrogen has a very broad flammability range—a 4 percent to 74 percent concentration in air… More often than not, fears about the potential for explosions were justified. Designers at Kennedy Space Center were concerned about using a hydrogen flare stack to manage waste hydrogen because they believed air could enter it and lead to an explosion. During the developmental testing of the SII stage of the Saturn V moon rocket at the Mississippi Test Facility (now Stennis Space Center), their concern was validated… A helium purge pushed gases from the system to the flare stack. When the gaseous hydrogen reached the top of the stack, it burned; however, because there was air in the system, the flame flashed back into the system to burn any flammable gases. Operational personnel having lunch in the nearby control room heard the flame flash back with a screeching sound. This flashback occurred several times until it found a hydrogen–air mixture that was detonable. The resulting explosion destroyed several hundred feet of the vent system.”
 Explosive Lessons in Hydrogen Safety By Russel Rhodes NASA, p. 1
 Explosive Lessons in Hydrogen Safety By Russel Rhodes NASA, p. 2
 Explosive Lessons in Hydrogen Safety By Russel Rhodes NASA, p. 3
 Explosive Lessons in Hydrogen Safety By Russel Rhodes NASA, p. 4
 Explosive Lessons in Hydrogen Safety By Russel Rhodes NASA, p. 5
Explosive Lessons in Hydrogen Safety By Russel Rhodes NASA, Red underline; Yellow highlight added. See Original here: http://appel.nasa.gov/wp-content/uploads/sites/2/2013/04/513855main_ASK_41s_explosive.pdf About Magazine: http://appel.nasa.gov/about-ask-magazine/