United Launch Alliance answered some questions for The Planetary Society about the science and engineering behind their rocket's ignition sequence.

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.

Is the Launch Table equipped with "sparklers" (similar to the space shuttle) that burn off some of the hydrogen before ignition?

Yes, prior to the RS-68 ignition sequence at approximately T-14.5 seconds, the Radial Outward Firing Igniters (ROFIs, or “sparklers”) start. As the hydrogen begins flowing during the engine start sequence, the ROFIs ignite the hydrogen. The ROFIs perform a critical function—they ensure the hydrogen burns in a controlled manner. Without the ROFIs, the hydrogen could mix with air before it can burn. Once pre-mixed with air, the heat of engine exhaust could cause a detonation of the hydrogen and oxygen mixture, with potentially damaging results. The ROFIs prevent this.

It seems like since the first flight of the Delta IV Heavy, the effect has been mitigated (but it's still present). Have efforts been made to decrease the amount of flames and charring?

The effects of the burning hydrogen at liftoff occur on all Delta IV vehicles to some extent; even Delta IV Medium vehicles with a single RS-68 engine experience the effect. But the Delta IV Heavy with three RS-68 engines produces more free hydrogen and a more noticeable flame. Beginning with a Delta IV Heavy mission from Vandenberg Air Force Base in August 2013, the Delta IV Heavy implemented a Staggered Engine Start sequence to significantly reduce the burning hydrogen. Staggered Engine Start involves starting the Starboard RS-68 engine two seconds prior to the Core and Port engines. The earlier start of the Starboard engine allows its thrust to establish a strong air current down the launch duct that sucks the hydrogen from the other two engines down and away from the vehicle. As a result, the hydrogen burning for the Vandenberg Delta IV launch last year was comparable to a Delta IV Medium vehicle with only one RS-68 engine. The Orion spacecraft's Exploration Flight Test 1 mission will be the first to use Staggered Engine Start from Cape Canaveral Air Force Station, and we expect much reduced burning of hydrogen and charring of the vehicle, similar to the launch from Vandenberg.

What kind of protection measures around the engines and rocket base are in place to protect the vehicle from these effects?

In addition to the ROFIs and Staggered Engine Start sequence, the design of the insulation systems throughout the launch vehicle accommodates the heat generated during liftoff. The design of these systems incorporates the brief high heating experienced during liftoff, and material selection minimizes the potential for sustained burning following exposure to the liftoff environment.

Does this happen for other launch vehicles as well, or is it unique to the Delta IV family?

Like the space shuttle, the Delta IV uses liquid hydrogen for fuel, and the start sequence begins by flowing hydrogen prior to ignition. The “hydrogen lead” and the buoyancy of the released hydrogen create the conditions for burning of hydrogen near the launch vehicle at liftoff. Other launch vehicles use different fuels that do not produce the same effects. The Atlas V, for example, uses refined kerosene (RP-1) for the fuel in the first stage and does not experience the same phenomenon.