NASA likely to redo hot-fire test of its Space Launch System core stage
Following the unsuccessful completion of a Space Launch System hot-fire test, NASA is likely to conduct a second “Green Run” firing in February.
On Tuesday, three days after the first hot-fire test attempt, NASA released a summary of its preliminary analysis of data from the 67.2-second test firing. The report highlights three issues, none of which appears to be overly serious but will require further investigation.
The agency found that the test, conducted at the Stennis Space Center in Mississippi, was automatically shut down by an out-of-limits reading of hydraulic pressure in the thrust vector control mechanism used to gimbal, or steer, the engines. At 64 seconds into the test, the rocket began a pre-programmed sequence to gimbal its engines as if it were in flight. Shortly after, the pump-return pressure fell below the redline of 50 pounds per square in gauge, to 49.6. This pressure limit, the agency said, was more stringent than an actual flight redline and was set to protect against potential damage on the test stand.
Another issue, the “major component failure” in Engine 4, actually appears to be a relatively minor problem in which a redundant sensor failed. This is good, because it means that NASA would likely not have to swap out this engine from the core stage. There are four space shuttle main engines used on the SLS rocket.
Finally, the “flash” observed in the engine section seems to be fairly normal. A visual inspection of the thermal blankets between engines shows some scorching, but this is expected. There were no indications of a fire, leak, or other issue.
To repeat or not repeat?
The published NASA summary does not discuss whether the agency will press ahead with a second test of the core stage. Prior to the test, NASA had hoped to run the engines for at least 250 seconds to obtain a full suite of data about the core stage performance. Obviously, they only ran the engines for about one-quarter of that time.
According to sources at the agency, program managers are in fact leaning toward conducting a second hot-fire test in Mississippi. Due to the need to obtain more propellant at the test site, conduct minor refurbishment to the vehicle, and possibly change the erratic sensor on Engine 4, the agency estimates it will require about three to four weeks before conducting another test.
NASA and the core stage’s primary contractor, Boeing, did collect a lot of data about the test. The agency said that, of its 23 test objectives, 15 were completed with 100 percent of the data sought. Four others got most of the data sought, while three had partial data, and one no data. (This was a test of how the liquid oxygen tank pressure would respond when liquid oxygen was largely used up and the tank emptied.)
NASA engineers are now studying the data in greater detail and will soon perform a risk assessment to determine the hazards of skipping a second test. As it is standard practice to re-test in such circumstances, it is highly likely a second test will occur.
It remains difficult to quantify what this means to the schedule for launching this core stage as part of the Artemis I mission. Before this delay, NASA was already going to struggle to make its goal of launching an uncrewed Orion spacecraft around the Moon before the end of 2021. There was not much margin left in the schedule, and a launch this year assumed smooth operations in stacking and integration—a big assumption. Now it appears likely that NASA will lose at least a month.