Sowers said the RL10C will become the standard upper stage engine for all of the company's Atlas 5 and Delta 4 launches. An exception will be for the two-engine version of the Atlas 5's Centaur upper stage, which will continue flying with the RL10A-4-2 version of the engine. The shape of the RL10C's bell-shaped nozzle prevents two of the engines from being placed side-by-side in a dual-engine configuration, Sowers said. ..."The original plan was to go to all RL10Cs, but when the commercial crew program came along, it had some unique requirements that drove the need to retain the RL10A capability," Sowers said.
Some questions, as the NROL-35 mission in December 2014 will be the first Atlas-V mission featuring a Centaur upper stage with a RL10C-1 engine (http://www.spaceflightnow.com/news/n1406/04rl10c/): Has anyone found the performance specifications of the RL10C versions of the Atlas-V rocket?Will these versions get a new designator to destinguish between RL10A and RL10C versions?
But further upgrades to ULA's rocket upper stages, including concepts to build long-duration deep space tugs and propellant depots, may take a back seat as focus grows on developing a powerful U.S.-built booster engine to end reliance on Russian propulsion.
The trade is shorter burn time (short higher thrust needed for LEO insertion) for longer burn times with better ISP (larger payloads to HEO/GTO orbits).Not surprising... more interesting is they will need to put ACES on hold due to the RS-180 issue...
"a contract from Stratolaunch Systems Corporation (SSC) to provide six RL10C-1 production engines, with an option to provide an additional six RL10C-1 production engines at a later date, for the third stage of a revolutionary commercial air-launch system. The inaugural launch of Thunderbolt, the air-launch vehicle designed and developed for SSC, is scheduled for 2018."
