Chair Force Engineer

Saturday, April 22, 2006

Something old, something new, and something borrowed

Until recently, the "J-2X" which will power the upper stages of Ares I and Ares V has been a very nebulous concept. Will it be a ressurrected version of the Saturn's J-2? Will it be the upgraded J-2S that was developed but never flown during the Apollo era? Would it be an all-new development in the same class as the old J-2?

Some answers are coming to light in the March 27 issue of Aviation Week. The engine will use the J-2's gas generator cycle (which burns a small amount of proellant to drive the turbopumps) instead of the J-2S's less mature tap-off cycle, which uses hot gas from the thrust chamber. However, the turbopumps themselves will come from the J-2S, because they were tested extensively for the XRS-2200 aerospike. The thrust chamber and nozzle of the engine will essentially be new. Looking to the Russians for guidance, NASA wants a channel-wall nozzle and milled channel thrust chamber. The resulting engine should put out 294,000 pounds of thrust and have an Isp of 448 seconds.

I'm a little disappointed that NASA won't use an unmodified J-2S, citing the development time for the tap-off cycle. Still, the current plan only forsees a six-month delay versus the SSME upper stage from the original ESAS report. It's also unknown if there will be competition for the new engine, because Rocketdyne owns the original J-2 and J-2S blueprints. Perhaps there can be a competition for the new thrust chamber and nozzle (although the number of engine competitors has narrowed after Pratt & Whitney bought Rocketdyne.) Hopefully, the J-2X that emerges will be a reliable engine that will be produced in large quantities and flown for a long time to come.