Anyone who watched the launch of United Launch Alliance's Delta IV Heavy Rocket on Saturday was treated to an up-close view of the liftoff. This vantage point, showing the three-core rocket taking off under the blue skies, offered a distinct view of a fireball engulfing the racket during launch
This may be rather distracting if you have never seen it before- uhh – but in fact it's a byproduct of the RS-68 rocket engines that power each of the three core of the launch vehicle Delta IV Heavy
Developed during the 1990s by Rocketdyne , the expendable RS-68 engine was designed to be less expensive and more powerful than the RS-25 main engine's Space Shuttle. Like the Shuttle's engines, the RS-68 engine runs on a cryogenic fuel mix of liquid hydrogen and liquid oxygen
The fireball phenomenon manifests on the Delta IV Heavy Rocket because of the design differences between the RS-68 and the Shuttle main engines , and because the RS-68 fuel valve is open longer before the oxidizer starts flowing. Essentially, at engine start-up, only liquid hydrogen runs through the engine, because it is less chemically active than oxygen
This hydrogen flows out of the engine and, as hydrogen is very light compared to ambient air, it rises up the racket. When the liquid oxygen flow begins, the hydrogen is ignited into a fireball. This is in the last five seconds of the countdown, creating the arresting images of a fireball charging the orange foam exterior of the Delta IV Heavy Rocket.
This design trade was intentional, and the exterior of the rocket is configured to withstand the fireball. (Scott Manley has an excellent video on the subject that provides additional insight.)
As for the Saturday's launch, it went well, with the Delta IV Heavy rocket boosting the NROL-71 payload into orbit for the National Reconnaissance Office. The company's next launch is the WGS-10 mission, which will fly a geostationary communications satellite for the U.S. Air Force on a Delta IV rocket. Presently, the launch is scheduled for March 13 from Cape Canaveral Air Force Station, Florida.
Listing image by United Launch Alliance