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[i]The Delta IV is the only launch vehicle that, by design, sets itself on fire during its ignition sequence. Thousands of pounds of hydrogen are dumped through the vehicle's three RS-68 engines to condition their internal temperatures five seconds before oxygen valves are opened for ignition. The hydrogen forms a cloud around the vehicle that is burned off spark generators to avoid an explosive hazard. This causes a huge ball of fire that blackens the core and the 125-ft. liquid strap-on boosters. During climbout, free hydrogen continues to attach itself to the base of the vehicle, where it burns on insulation designed for that purpose. So while the fire is inconsequential, parts of the boattail remain ablaze until ascent into thinner air. Heat emerging from RS-68 fuel turbine exhausts for roll control can also cause flame on the insulation as with the center engine just after liftoff. All of this looks frightening, but is normal.[/i]
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