"When NASA's Orion spacecraft begins its maiden voyage next week, you may notice something alarming at liftoff: The rocket carrying Orion sort of catches itself on fire. Not to worry, says United Launch Alliance, the rocket's manufacturer. Orion's launch vehicle is a Delta IV Heavy, a mammoth, three-core rocket normally used to heave classified military satellites into space. Just before the Delta IV ignition sequence starts, valves open that control the flow of liquid hydrogen to the engines. Some of that hydrogen seeps out of the engine bells and lingers around the rocket. When the engines roar to life, the excess hydrogen ignites, creating a fireball that chars the booster cores. Occasionally, the insulation on the booster cores smolders as the rocket lifts off." - Mark H

"What causes the charring phenomenon? The start sequence for the RS-68 main engines on the Delta IV booster must ensure that the pump speeds and combustion processes remain within limits. This complex task requires opening the main hydrogen valve at T-5 seconds, approximately two seconds prior to opening the main oxygen valve that begins the ignition process. This “hydrogen lead” start sequence results in unburned hydrogen exiting the engine prior to engine ignition. Since hydrogen is lighter than air, the hydrogen gas rises and drifts with the prevailing breeze. As the powerful RS-68 engines reach full thrust, the high speed engine exhaust flowing into the flame ducts creates a strong suction of surrounding air and hydrogen into the launch duct. As the air mixes with the hydrogen during this liftoff event, the hydrogen burns, creating heat that can also char the insulating materials on the launch vehicle." - Mark H