It has been revealed in a presentation at the International Astronautical Congress by Michael Berglund of the United Launch Allance that unexpectedly high temperature readings were recorded on the centrebody soon after ignition the NRO L-49 flight of the Delta IV Heavy vehicle. The flight took place on 20 February 2011 from the Vandenberg launch site in California, USA. The flight was the first time that the rocket had flown from the Western Test Range. All previous Delta IV Heavy flights had occured from Cape Canaveral in Florida.
The temperature readings exceeded modelling predictions. Part of the extra heating was thought due to the splitter flame duct configuration (which split the flow into north and south ducts) at the Vandenberg launch pad rather than the single flame duct configuration at Cape Canaveral. After research, it was decided to use a staggered engine start for the three RS-68 engines, rather than having them fire all at once. If this works at Vandenberg, then the technique will also be applied at Cape Canaveral where high centre body heading is also apparent.