Boeing's Delta cryogenic second stage (DCSS) has been selected as the interim upper stage for the first two flights of the Space Launch System (SLS), according to a justification on NASA's procurement website.
"Based on the responses to that sources sought synopsis, NASA determined that the DCSS is the only means available to support the immediate in-space propulsion needs of the SLS within the SLS manifest schedule constraints," says the agency. "The DCSS is the only known in-space stage requiring relatively minor modifications to enable full compliance with the requirements of the early SLS manifest."
DCSS is powered by a single Pratt & Whitney Rocketdyne RL-10B engine, fueled by liquid hydrogen and oxygen.
SLS is slated to fly in 2017 and 2021, but the Pratt & Whitney Rocketdyne J-2X upper stage will not be ready in time. In order to fly, SLS requires an interim stand-in. The launch vehicle will be capable of launching 70 metric tonnes into low earth orbit (LEO), and an upgraded follow-on will launch 130mt.
The rocket has divided the political and space communities, with supporters trumpeting its unprecedented capabilities and detractors blaming the programme for taking precious dollars from NASA. In any case, NASA is now legally required to build SLS, earning it the derisive name 'Senate launch system' in some corners.
Boeing declined to comment until the award is formally awarded. NASA did not immediately respond to questions.