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United Launch Alliance and Pratt & Whitney Rocketdyne (PWR) are developing short- and longer-term solutions to counter the recurring formation of a cloud of burning hydrogen that envelopes the base of the Delta IV Heavy during every liftoff.

Seconds before launch, thousands of pounds of hydrogen are dumped through the three PWR RS-68 Common Core Booster first-stage engines to optimize their hydrogen/oxygen mix for ignition. Excess gas is deliberately burned off by radial outwardly firing igniters, or spark generators, prior to ignition. However, due to the large volume of hydrogen and possibly the configuration of the vehicle and its relatively slow initial ascent profile, some gas burns in pockets of flame (see video, just prior to lift-off) that remain close to the booster as it clears the launch tower.

While it is described as a harmless side effect of the standard launch procedure, the burn-off phenomenon has warranted attention and will be initially solved by igniting one of the three RS-68s slightly earlier than the others prior to liftoff. “The three boosters normally start together, but we will start one a couple of seconds early to get the burn-off established around only that one engine,” says Steve Bouley, PWR vice president for launch vehicle and hypersonic systems. “This will entrain the hydrogen for the other two in the start sequence.”

Further design changes are planned for the longer term, including alterations to the timing of the valves in the hydrogen system and alternate chill-down methods using cooled helium.