The Core Stage article used in the Green Run is also the Artemis 1 Core Stage, and NASA and Boeing set the test limits conservatively going into the first Hot-Fire to balance the desire to stress test some vehicle systems while also minimizing wear and tear on the stage before it flies the first SLS launch.
“Our primary goal is to not only get the Hot-Fire data, as much as we can, but also to protect the stage to be the first flight article,” Shannon noted. “So we’re kind of threading a needle here between keeping the vehicle safe and gathering this data.”
The first TVC gimbaling test will begin at T+60 seconds, when the stage’s hydraulic TVC systems will begin to move all four engines simultaneously in small circles at high speed. This first test had just started when the effect of the maximum TVC gimbaling demand exceeded thresholds for hydraulic system performance determined in part by analytical models and subsystem testing.
Parameters for hydraulic pressure and reservoir level have been adjusted since the first test.
Assuming the stage continues to operate within limits, the circular engine gimbaling would run for about half a minute. “That first gimbal profile [starts at] sixty seconds, and it’s basically a circular swiveling of all four engines,” Shannon said. “That will last until about a minute and thirty-three seconds. During that time we’ll throttle down the vehicle to 95% power level from 109%.”
Engine throttle control is also hydraulic, and the throttle down from 109% to 95% will occur at T+65 seconds, further stress testing the hydraulic systems while they are simultaneously performing the high-speed gimbaling.
The engines will then be throttled back up to 109%, which would mimic a throttle bucket that could be performed in flight if necessary while the SLS vehicle was flying through its maximum aerodynamic pressure region.
Credit: NASA/SSC.
(Photo Caption: The RS-25 engine shock diamonds are shown with the engines throttled at the 100% startup power level and at the 109% power level they are throttled up to at T-0. The four engines start one at a time in a staggered sequence beginning at T-6.6 seconds.)
The second gimbal test is part of a two-minute long frequency response test (FRT) suite that will begin at T+2 minutes 30 seconds. “[It’s] called a frequency response test sine-sweep where we’re going to go through a series of graduated movements in yaw and pitch that go slowly moving the nozzles and then faster and faster, and then we’ll combine pitch and yaw at the same time,” Shannon explained.
“This is all to determine the effect of those rapid movements in multiple axes of the nozzles on the rest of the Core Stage, basically figuring out what the resonant frequency of the vehicle is at different loading profiles. That sine-sweep lasts from two and a half minutes to four minutes and 37 seconds, so it’s about a two-minute test.”
Enough data would be captured for the set of ten primary objectives towards the end of the sine-sweep test, but the plan is to continue as long as the test commit criteria are being met. For the next three minutes of firing, with the engines held steady, additional data can be collected on the thermodynamics of the tank pressurization system as the engines drain the LOX and LH2 and propellant levels get lower and lower.
The tanks must be kept pressurized to maintain their structural integrity, and a longer Hot-Fire test will help provide more data for calibrating the modeling of the interactions between the hot gas filling the top of the tanks and the cold, cryogenic liquid at the bottom.
“The team fully understands where we want to get to really understand the control system and the thermodynamics in the [liquid] hydrogen tank and the liquid oxygen tank as we run through the pressurization process while the engines are running,” Honeycutt noted. “And then, obviously, the longer we run, the more data we get.”
Power to run the hydraulic systems is generated by engine exhaust from the running RS-25s; that exhaust is also used to keep the propellant tanks pressurized as the engines are draining it. “We take a bleed off of the hydrogen exhaust to be able to drive the auxiliary power units for hydraulic pressure; we also take that same bleed coming off the engine to fill the hydrogen tank pressure to keep it up and pressurized,” Shannon said in a 2020 briefing.
A third and final TVC gimbal test will be performed at the tail end of the engine firing, which will provide another hydraulic stress test data set when the tanks are almost empty and the engines are at a lower power setting similar to how they’ll be shut down in flight.
“We’ll go through another power reduction from 109 [percent] down to 85,” Shannon said. “When we’re at 85 we’ll do another sweep of the gimbals and that’s starting at seven minutes and 36 seconds and will go until eight minutes and one second, so it’s about a 25 second test.”
The engines will be put through the same circular motion as they were during the first test.
“You would not normally see this kind of nozzle movement on any normal flight,” Shannon noted. “You would see some nozzle movement at booster separation and just a very little bit as you’re coming up on main engine cutoff. But again, this is an opportunity for us to gather that engineering data on this vehicle and really understand how it responds.”
“It’s going to look dynamic, it is not [a] normal test that you see where we’re essentially sitting there firing for an extended period of time and maybe you’ll see a little bit of gimbaling movement. It won’t be that at all; it’s going to be very dynamic with the rocket nozzles throughout the test. We’re gathering as much data as we can.”
Post-test refurbishment expected to take a month
Following the second test-firing, the stage and the engines will be refurbished for their next use. NASA and Boeing will go through the same post-test process used after the first Hot-Fire test to evaluate the performance and health of the vehicle, whether the design verification objectives were met, and next steps.
Credit: NASA/SSC.
(Photo Caption: The four RS-25 engines installed in Core Stage-1 fire during the January Hot-Fire test. Prior to the January firing, the four former Space Shuttle Main Engines had not been fired for almost a decade when they helped finish the final launches of the Space Shuttle Program.)
If the second static firing meets the program’s requirements and after the vehicle refurbishment is completed, the Core Stage will be removed from the B-2 position of the B Test Stand, rotated from vertical to horizontal, and put back on the Pegasus barge to travel to the Kennedy Space Center in Florida for Artemis 1 launch preparations.
“Right now in the schedule we have 30 days, post Hot-Fire, to refurbish the vehicle and get it loaded on the barge to go to the Cape,” Shannon said in February. “We did learn a tremendous amount [about post-firing refurbishment].”
“Mainly what we learned is that the vehicle did not need a lot of refurbishment after the first Hot-Fire, it came out of it in great shape. We did a lot of work on the engines to make sure that they’re in good shape for another firing, and then it’s just some minor thermal protection system touch-ups.”
“So overall the vehicle came through in great shape and we would anticipate after a second Hot-Fire that the vehicle would continue to be in good shape,” Shannon added.
The bulk of the post-firing refurbishment work is a standardized set of procedures on the RS-25 engines, beginning with drying them out to make sure that no moisture is trapped inside. The engines will be purged with heated nitrogen gas after firing to warm them up from cryogenic temperatures and aid the drying process.
After the stage is inerted following the test, dry air purges will continue the drying, which once complete will be followed by engine hardware inspections.
“It’ll take us about a month to complete the refurbishment,” Jeff Zotti, Aerojet Rocketdyne’s RS-25 program director, said in the February 19 media briefing. “We did apply lessons not only from our Shuttle experience but from this last test, and we’re working as many tasks as we can in parallel to reduce that time-frame.”
(Lead image credit: Brady Kenniston for NSF)