At T-4 minutes, the hydraulic systems that help operate and steer the engines will begin to be activated by spin-starting the Core Auxiliary Power Units (CAPU). The CAPUs are highly-modified Shuttle APUs; in Shuttle, the APU turbines were driven by burning a supply of hydrazine fuel internal to each unit. In the Core Stage, the hydrazine fuel elements were removed and the turbines are now driven by exhaust pressure from the RS-25 engines while they are running; prior to engine start, the CAPUs are driven by a helium spin-start ground system.
The next major milestone occurs at T-3 minutes, when the four engines will go into purge sequence four: the final purge sequence prior to ignition. The purges ensure the engines are free of contaminants, which is another criterion that must be satisfied to start them.
With the four hydraulic systems up and running, a slew test of the engines will be performed at about T-2 minutes 30 seconds. The hydraulic Thrust Vector Control (TVC) actuators will be commanded to move the engines in a canned pattern to verify the TVC system is ready to position the engines as commanded during flight.
The last milestone where the countdown could be held is at T-90 seconds when the Core Stage transfers to internal power. During its mission, the stage is powered by four batteries in the intertank which are conditioned to be at full charge at that point in the count.
“At that point we have no capability to hold. The only option that the test team has is they could recycle back to 10 minutes and try and resolve the problem, but there’s no ability to hold and continue forward once we’re on internal power,” Cipoletti said. “And that’s true for launch day. Once we’re inside of 90 seconds, we’re either going to go or recycle.”
During the WDR, the countdown will continue until just before engine ignition when it is aborted. This will allow a first look at how the stage behaves as an integrated whole leading up to the point where the engines would be started. The tanks will be filled and pressurized, the hydraulics will be activated and running, the vehicle will be running on its own battery power, and the flight computers will take control of the countdown from the Stage Controller at T-30 seconds.
Once the countdown is stopped before ignition, the Stage Controller will safe all the ground and vehicle systems, and the stage will be slowly drained of its propellants. Engineers will go over data collected during the first countdown while the test team at Stennis top off their propellant tanker barges with LOX and LH2 to proceed into the hot-fire test.
Credit: NASA/SSC.
(Photo Caption: Another view of the B Test Stand on July 14. Core Stage-1, which is being used for the Green Run and to fly Artemis 1, arrived at Stennis in mid-January. After losing two-months time due to COVID-19, the stage is expected to be at the center until almost the end of the year.)
After wet dress, teams will review all the data to address any issues that come up during the test and to clear the vehicle and the facility for the hot-fire. According to Mark Nappi, Boeing Green Run Test Manager, “While the Stennis folks are replenishing the commodities, we would give ourselves a go for going into hot-fire.”
“The plan is to go from one to the other with no entry into the vehicle,” Cipoletti said. “If everything goes well, the only thing we’ll be holding for between the two is replenishing the ground side propellants.”
Hot-fire test objectives and data will calibrate modeling
The hot-fire test, test case eight, is the eight minutes of the months-long campaign at Stennis that is often referred to as “The Green Run.” During the eight-minutes it takes for the four engines to empty the stage of propellant, the stage will be commanded through different test sequences to meet development verification objectives.
As during the WDR, at T-30 seconds in the countdown the Stage Controller will hand over authority of the vehicle to the three Core Stage flight computers. The Automated Launch Sequence (ALS) begins at that point, and the vehicle computers will run the stage through the hot-fire demonstration test. The Stage Controller will continue to be in charge of test stand systems; in the lead up to engine ignition, hydrogen burn-off igniters will be fired and water flows to the B-2 position’s acoustic ring and flame bucket will begin.
“In broad strokes once we get into ALS there’s some final comparisons that the Stage Controller will do to look at the data to assure that we’re getting into the engine start box. The Stage Controller will then give the ALS a ‘yes, we’re in the start box,’ [clearance]” Cipoletti said. Like the EGS launch computer system in Florida, the Stage Controller will continue to watch data from test stand ground systems and monitor vehicle systems as a backup; it will give the Core Stage ALS a “go for main engine start” at T-10 seconds if everything is still operating within expected ranges.
“Then [ALS] will start working with the Core Stage engine controllers to get the engines ready for ignition. And so they’ll ignite, and they’ll build up to a 100% throttle at the zero point. Then they’ll immediately ramp up,” he noted. ALS will start the stage engines in a staggered fashion beginning between T-7 and T-6 seconds, with all four engines coming to 100 percent of their early Shuttle-era rated power level (RPL) of 375,000 pounds of sea-level thrust.
During the final countdown and throughout the firing of the stage, the Stage Controller will continue to be in control of commanding and monitoring ground-side test stand systems. Although the stage is virtually isolated from ground equipment and operating on its own power and supplies, ground control must maintain contact with both vehicle and ground systems throughout the firing, with the flame bucket and acoustic ring water flows being among the critical items to continue.
T-0 is the simulated liftoff point, and as they will at this point during launch, the engines will then throttle up to 109% RPL. “They’ll continue at that higher thrust level, and then we’ll ramp back to a lower throttle and perform our first TVC check,” Cipoletti noted.
Credit: NASA/SSC.
(Photo Caption: Sound-suppression water flows through an acoustic ring in the B-2 position of the test stand. The Core Stage engines will fire down through this spray of water throughout the test. The flat plate of water created by the continuous spray will help deflect and dampen the sound waves from the firing engines that rebound off the test stand’s flame bucket back towards the vehicle.)
With the stage locked down in the stand for the duration of the test firing, NASA and Boeing can experiment with the interplay between the engines and stage systems.
There are four hydraulic systems in the stage, one for each engine. And the engines and the hydraulic systems are interdependent. The hydraulics drive engine valves which control overall engine operation and actuators which point the engine to help steer the stage. While the engines are running, though, pressure from their hydrogen exhaust drives the CAPU, which drive the hydraulics.
Running engine exhaust also keeps the propellant tanks pressurized while they are simultaneously being drained. “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.
“We’re going to really aggressively gimbal all four of the engines and it’s going to put a lot of pressure on the ability to continue to feed those hydraulic units and provide sufficient pressure to the LH2 tank.”
That “TVC check” will be started at approximately 60 seconds into the test firing with the engines throttling from 109% to 95% RPL. “For that check, the engines are going to be moved in a very tight circle, the radius of that circle being just a degree. But we’re going to move them at fairly high speed around that circumference of that circle,” Cipoletti noted. “All the engines will be moving in slightly different directions to make sure that the loads into the stand are balanced.”
Completing the TVC check early in the firing would satisfy minimum stage run-time requirements for the hot-fire test. “We looked at it from an engineering standpoint and said if we have just over two minutes of run-time on the vehicle, that will satisfy all of our engineering requirements to button up and go to the Cape. And so we’ll be happy,” Shannon noted.
“The reason we wanted to find that number is that if the vehicle has a problem, we may have to repeat [the hot-fire], we’d have to go fix something and repeat. If the facility has a problem where we go do [an early] cut, we wanted to know where we were good to be able to say ‘we’re done’ and so that’s just over the two-minute time-frame.”
“We talked about ‘do we want to cutoff at that point and then drain the tank?’ And everybody’s assessment of that was ‘no,'” Shannon added. “It added so much risk to try and do a cut at some arbitrary time and then go drain the tank, just let it keep running.”
Credit: NASA.
(Photo Caption: A presentation slide showing how the frequency response test in the middle of the Green Run hot-fire is used to refine one of the SLS Program’s analytical models of vehicle dynamics.)
The space center ran a stress test of all the test facility systems late last year to verify they can support the needs of loading and static-firing the stage. “The high-pressure water, the nitrogen, the helium, the oxygen, the hydrogen, all those commodities, we went off and did a stress test on the entire integrated system to make sure we had ample commodity and capability to handle the hot-fire,” Honeycutt said. “You’re talking about a significant amount of water and nitrogen and helium in order to pull this test off. Granted once we have the cryos onboard the vehicle will take care of itself there, but Stennis is in good shape.”
Back to the test fire, “When [the TVC check is] complete, we’ll bring [the engines] back up to full throttle and perform the frequency response test suite that Level 2 (the SLS Program) has requested,” Cipoletti said. “And those are moving actuators individually in one direction at a frequency sweep starting at low frequency and then going to very high frequency, once again limiting the actual deflections to less than one degree. That’ll actually continues for a fairly long period of time.”
The frequency response test (FRT) will measure the dynamic response of the Core Stage structure to the experimental TVC actuator movements. “The FRT stuff is to see how much the tail wags the dog; as the gimbals and the big engines are moving around, how much [are] the forces on the Core Stage going to be,” Shannon noted.
The frequency response test suite starts at about 150 seconds into the firing, and the gimbal tests for that last for about two minutes. “Then we’ll start to throttle the engines back to a lower throttle level and at that lower throttle level we’ll do the same [TVC check] we did at the high throttle — the circular response, at the low throttle — to make sure the whole system is working well together,” Cipoletti said.
The second TVC check starts near the end of the full-duration firing, about 455 seconds or seven and a half minutes after ignition, with the engines throttled at 85% RPL. This test will provide another data set showing how the stage balances the demands on the gas being tapped off the running engines to keep the propellant tanks pressurized as they near empty while simultaneously supplying power to the CAPU turbines under a high TVC gimbal load.
In between the frequency response test and the second TVC check, steady-state engine run data can also be collected on behavior of the stage’s tank pressurization system as the propellant levels in the LOX and LH2 tanks get lower and lower.
“The data that we want to get for the full duration is running at full power level (109% of rated power level), that’s the data we’re really interested in and we can do that on the test stand,” said Jonathan Looser, NASA SLS Core Stage Propulsion Lead added. “We want to get some steady state run time and just understand the thermodynamics inside the tanks and correlate those models.”
The Core Stage tanks must be kept pressurized to maintain their structural integrity, and getting enough data to verify the modeling of the interactions between the hot gas filling the top of the tanks and the cold, cryogenic liquid at the bottom can only be done in a Green Run hot-fire.
“There’s some uncertainties there with the different sized tank and the different dome shapes, and so that’s something that we really want to anchor that model with some test data.”
Back to the test, assuming all goes well, the engines would then stay throttled at 85% RPL after the second TVC check until main engine cutoff (MECO), which will provide an opportunity to test the stage’s low-level cutoff system. “We’re planning on having the test run until the cutoff sensors on the oxygen tank flash dry,” Cipoletti explained. The low-level cutoff system is typically a safety feature but also allows a vehicle in-flight to run to propellant depletion.
“One of the main objectives of the Green Run from a propulsion standpoint is pressurization and understanding that,” said Looser. “Understanding all the thermodynamics that are going on inside that tank is something we really want to do a full-duration hot-fire for.”
“The pressurization model is the one that we would like to get the most data at Green Run because you can model tank thermodynamics, but this tank is larger, it’s thicker materials,” Looser explained. “[We want to see] how the four engines flow hot-gas pressurant into the tank and how that interaction of the hot pressurant gas and the liquid level surface [works].”
Following MECO, the Core Stage will close its prevalves and hand control of post-cutoff safing of the combined systems back to the Stage Controller.
Lead image credit: NASA/SSC.