A total of 16 tests are scheduled and are expected to conclude by the end of this year. The first test was this past week.
The J-2X is being developed by Pratt & Whitney Rocketdyne for NASA's Marshall Space Flight Center in Huntsville, Ala. It is the first liquid oxygen and liquid hydrogen rocket engine rated to carry humans into space to be developed in 40 years.
The J-2X will provide upper-stage power for NASA's Space Launch System, a new heavy-lift vehicle capable of missions beyond low-Earth orbit.
NASA conducted an initial round of sea-level tests on the engine last year, then removed it from the Stennis test stand to prepare both stand and engine for the second round of simulated high-altitude testing.
NASA officials say such testing is critical; the J-2X with nozzle extension needs to prove it can perform at altitude as needed for SLS.
The SLS will use J-2X engines on the second stage of flight after the first stage is jettisoned.
"The first round of testing helped us get to know the engine, how it operates and its basic performance characteristics," said Tom Byrd, J-2X engine lead in the SLS Liquid Engines Office at Marshall Space Flight Center.
"Now, we're looking forward to testing J-2X in the SLS flight configuration, collecting nozzle data and continuing to learn about the performance of the engine itself."