If NASA’s former Constellation Program suffered one deficit, it wasn’t technical but it was one of information–few got to see tangible evidence of progress for the $9 billion spent over 5 years. Recently, those elements surviving Constellation have had a coming out party of sorts. First was the Multi Purpose Crew Vehicle (MPCV, or Orion). Now it’s the J-2X rocket engine’s turn. The J-2X is about to undergo a series of 10 test firings, the first of which could occur on June 21st, next week, according to Aviation Week & Space Technology’s Frank Morring, as reported in First J-2X Hot-Fire Test Could Come Next Week.
Originally developed as the upper-stage engine in the Ares I crew launch vehicle, the J-2X has been tentatively selected as the upper-stage engine for the heavy-lift Space Launch System (SLS) that Congress has ordered NASA to develop as a government-owned capability to send humans beyond low Earth orbit. In the tentative design for the SLS, NASA chose a vehicle that would use one or two of the 300,000-lb.-thrust liquid-oxygen/liquid-hydrogen engines in its upper stage, with a throw-away version of the Space Shuttle Main Engine powering the main stage and twin solid-fuel boosters based on the Ares I first stage as strap-ons.
The J-2X is one of the most efficient rocket engines with a specific impulse of 448 seconds, bested only by the Shuttle Main Engine (RS-25) performance of 452.5 seconds. By contrast, the Delta V’s RS-68 comes in at 410 seconds and SpaceX’s Merlin 1a at 304.8 seconds. Specific impulse is a measure of efficiency in fuel burned for a change in speed. So, the higher the specific impulse, the faster one can go on a pound of fuel.