GH2 flow control valve and poppet. Each valve has a poppet that resembles a tiny sprinkler head, which pops up and down regulating the flow of gas through the valve. The valves have two rates: high and low. During a launch they switch between the two rates approximately fifteen times. Image Credit: NASAOn November 14, 2008, as Space Shuttle Endeavor rocketed skyward on STS-129, flight controllers monitoring data during the ascent noted an unexpected hydrogen flow increase from one of the shuttle's main engines. Despite this in-flight anomaly, the launch proceeded smoothly—since three flow control valves (one per main engine) work in concert to maintain proper pressure in the hydrogen tank, one of the other valves reduced flow to compensate for the greater flow from the valve that malfunctioned. This issue would require immediate attention as soon as STS-126 landed safely on November 30.