September 21 was a very busy day for NASA’s PR team, as NASA conducted another tanking test of the SLS rocket. This test was not a launch rehearsal but focused entirely on loading the vehicle’s Core Stage and second stage with propellant ahead of a possible launch attempt early next week.
Exactly like last time, liquid hydrogen fueling was stopped after only a few percent when a leak at the same seal between the ground and rocket. Unlike last time, they used a new procedure to load the very cold propellant extremely slowly into the rocket under low pressure, before ramping back up gradually. The goal of the new procedure was to keep the seal happy, which worked well, and the core stage was fully tanked and moved to replenish while maintaining a 0.5% concentration of hydrogen inside the umbilical.
Partway through the tanking process, the kickstart bleed on the hydrogen side was completed on all four engines. This procedure also caused problems during previous countdowns. During the bleed, the pressure on the seal increased, but the leak remained below the 4% limit.
Once both tanks were full, the test team decided to proceed with filling the upper stage. After that process was completed, another chill down of the engines occurred, this time at higher pressure to fully prepare the rocket for a flight. Both objectives were completed.
Overall, the cryogenic loading test was a mixed bag. Some of the problems recurred and then fixed themselves for reasons the team doesn’t yet understand, so now they need to look at the data. Meanwhile, the fix they performed… worked as expected. The new liquid hydrogen loading procedure kept the seal tight and the leak mostly within limits. The team even pressurized the stage and got the engines ready for flight using the systems at launch pad 39B for the first time.
Other procedures which had worked before had problems this time. The launch team got all they wanted to do done, but they would have scrubbed a launch attempt because of a slightly high hydrogen concentration that they ignored during this test. That concentration would have caused an immediate abort for the day.
So far, September 27 is still on the table for the next launch attempt, but they still have not received their Flight Termination System battery waiver from the Range. Another major concern is the time needed to safe the vehicle and get it back into the VAB if (when) a hurricane comes close to Florida. The team needs about four days for all that work. Hurricane Fiona is, fortunately, curving away from the U.S., but there will be other storms. This is Scrubtember, after all.