NASA SLS Core Stage ready for high-stakes Green Run static fire
NASA’s Space Launch System (SLS) program and Core Stage prime contractor Boeing are finally ready… The post NASA SLS Core Stage ready for high-stakes Green Run static fire appeared first on NASASpaceFlight.com.
NASA’s Space Launch System (SLS) program and Core Stage prime contractor Boeing are finally ready for the big moment of the rocket’s Green Run campaign and the biggest moment in the 10-year old program, a static firing of the stage. After a year of test cases at the Stennis Space Center in Mississippi, visits from hurricanes, and waves of the COVID-19 pandemic, an eight-minute long, flight-duration Hot-Fire test planned for January 16 hopes to demonstrate the readiness of the new rocket stage for its first launch.
Four veteran Space Shuttle Main Engines (SSME), now equipped with upgraded computers and adapted to meet SLS performance requirements, are supporting actors in this crucial test of the rookie stage. The Hot-Fire test is planned as a full flight duty cycle to demonstrate the real-world performance of the large, complicated rocket; the renamed Aerojet Rocketdyne RS-25 engines will be put through a set of circular and sinusoidal gimbaling experiments that will help demonstrate operating margins predicted by analytical models.
Green Run design verification campaign a first and last opportunity before first SLS launch
Boeing is conducting the SLS Core Stage Green Run design verification campaign with the stage bolted into the B-2 Test Stand at Stennis. Core Stage-1 arrived at the test site almost a year ago in mid-January, 2020; the campaign is now ready for the culmination of testing with a flight-duration, nearly 500-second test firing of the stage.
Ignition is currently targeted a few seconds before T-0 at 4 pm Central Standard Time (2200 UTC) on Saturday.
The Core Stage is the all-new piece of the new launch vehicle designed around existing Space Shuttle liquid engines and evolved solid rocket boosters. The year-long test campaign is the only planned opportunity for the SLS Program to see the real interactions between the complicated stage systems in a test environment before committing the foundation of the vehicle to a first launch.
“I think some folks view Green Run as workmanship screening; I don’t view it as that, as a matter of fact that’s not what it is,” NASA SLS Program Manager John Honeycutt said early in 2020. “This Green Run and this Green Run only is for us to learn everything that we can about the Core Stage while we’ve got it here on the ground with us.”
(Photo Caption: Core Stage-1 during its first full tanking test performed as a part of the Green Run Wet Dress Rehearsal (WDR) on December 20. The liquid oxygen tank vents at the top of the stage through a port on the circumference of the forward skirt. Additional venting of oxygen from the four RS-25 engines can be seen near the bottom of the stage.)
“We need to learn everything we can about this stage before we start flying and it’s going to make us much better off in the long run.”
The Green Run Hot-Fire test on the ground at Stennis is a learning opportunity that won’t be available during flight on Artemis 1. Something less than a full-duration Core Stage firing in flight would likely jeopardize the full Artemis 1 mission, which would jeopardize the 2024 schedule that remains NASA’s top priority as an agency while the federal government is in transition.
Given the opportunity to fire the stage and run through a full mission cycle on the ground, the test has several design verification objectives. “It’s not a development test series, where you bring a new stage [or] a new test article out there and run it on all the corners of the boxes and see what it’ll do and see what it won’t do and then go make changes,” Marc Neely, Core Stage Green Run Test Operations and Execution Manager for NASA, said in 2019.
The different systems will be operated within their design parameters, leading to the “Green Run” labeling used for acceptance testing new hardware; however, the static-fire in the test stand allows Boeing and NASA to run experiments that will help demonstrate design requirements and operating margins.
The stage is filled with ground test instrumentation that will be watched and recorded throughout the Hot-Fire; those additional channels of sensor data augment both operational and development flight instrumentation data sets that will also be captured simultaneously. Ground test specific cameras are also set up inside the intertank and engine section to visually document the behavior of the equipment during the two fueled tests.
“I look at the team’s product as providing the data necessary to clear the test article, in this case the stage, for shipment to KSC for flight,” Neely said. “That’s our product is that data, that information.”
(Photo Caption: The four RS-25 engines are seen venting during the Green Run WDR on December 20. Above the nozzles are white, non-rigid engine mounted heat shields (EMHS) that provide thermal protection for the powerheads and the inside of the engine section from the heat generated when the engines are firing.)
The Green Run Hot-Fire is predominantly a test of the new rocket stage as opposed to the engines. The four engines installed in Core Stage-1 are flight-proven Space Shuttle Main Engines (SSME) that helped power multiple Space Shuttle launches. Re-certification of the SSME design was completed in 2017 to fly on SLS as RS-25 adaptation engines.
The Green Run was meant to demonstrate the Core Stage operation, which is the new, untested element of SLS. The RS-25 engines, the SLS Boosters, and the Interim Cryogenic Propulsion Stage (ICPS) upper stage have all been either extensively ground tested, flight tested, or both.Artemis 1 Updates
Decades of Space Shuttle flights and ground testing have established the engines’ interface requirements in terms of supplying propellant, hydraulics, and helium to run the RS-25 engine and point it for steering. The propellant must be fed to the engines at the right temperatures, pressures, and flow rates. The hydraulics actuate critical engine valves. The helium maintains equally critical purges within engine components during mainstage operation.
For this Stage Green Run, the engines are essentially test support equipment to evaluate how well the new Core Stage and its new Main Propulsion System meets those well-established but SLS-adjusted engine requirements. The new SLS flight software is in charge of vehicle management’s overall control cycle, monitoring systems health, and commanding the stage through the Green Run test plan via a brand new avionics system.
The B-2 position of the B Test Stand at Stennis was rebuilt and refurbished to support SLS Core Stage test firings. “Not to be sly, but if you want to [describe how Stennis is] involved, we’re actually a filling station, Bryon Maynard, Project Engineer for NASA’s B2 Green Run Project, said. “Literally, we’re the filling station. We’re going to fill it up, but after we fill it up it is Marshall and Boeing all the way.”
The test stand also provides the sound suppression water to protect the stage from the acoustics of an eight-minute long static firing and a water deluge to protect the flame bucket while the stage is firing its engines into it. In addition to critical stage operating parameters during the engine firing, the water supplies are also required to continue running the test to full-duration.
Boeing provides the ground control computer system to orchestrate the test, called the Stage Controller. As the test team monitors from the test control center, the Stage Controller will control vehicle purges, load the propellants from barges docked at the test stand into the Core Stage, manage heaters in the vehicle, manage the terminal countdown sequence, safe all the elements after engine shutdown, and capture many continuous data streams from start to finish to capture the results of the tests.
Hot-fire test objectives and event sequence
The Hot-Fire test, test case eight, is the eight minutes of the months-long campaign at Stennis, 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 design verification objectives.
As with the division of labor during a launch countdown, the ground-side Stage Controller’s terminal countdown sequencer is in charge of the final countdown; it orchestrates commands to the stage and to the stand while monitoring thousands of parameters of data being reported from the hardware and software of both the vehicle and ground systems. The Core Stage flight computers running the Green Run Application Software (GRAS) take over primary control of the vehicle for the final half minute of the countdown and throughout the firing.
The flight computers take over enforcing the large set of vehicle criteria needed to continue the countdown through ignition, but the Stage Controller also continues to monitor critical vehicle parameters and remains in charge of the health and operation of critical test stand systems. The vehicle computers, the ground control computers, or test team members can call for an abort if necessary.
(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.)
The Wet Dress Rehearsal (WDR) performed on December 20 was planned to proceed all the way through the terminal countdown from T-10 minutes to T-33 seconds, stopping a few seconds short of the point where the Stage Controller hands over authority of the vehicle during the test to the three Core Stage flight computers. The WDR countdown demonstration test ended early, but in the Hot-Fire test the countdown is planned to continue from T-10 minutes down to T-0.
The vehicle computers will begin the Automated Launch Sequence (ALS) at T-30 seconds and will run the stage through the eight-minute long test-firing. The Stage Controller will run test stand systems and it will maintain water flows to the B-2 position’s acoustic ring and flame bucket throughout the test.
The water flows will suppress some of the sound energy of the firing engines that might rebound off the flame bucket back up towards the Core Stage and protect the flame bucket from the engine exhaust. At T-12 seconds, the Stage Controller will start the hydrogen burn-off igniters, which prevent a build up of hydrogen gas that occurs during the hydrogen-rich engine start sequence.
“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 and the Stage Controller will then give that data to ALS that yes, we’re in the start box,” John Cipoletti, Boeing SLS Green Run deputy test director, explained in an interview early in 2020. Like the Exploration Ground Systems launch computer system in Florida, the Stage Controller 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 hundred percent 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-minus seven and six seconds, with all four engines coming up 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 control and monitor 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 during launch the engines will then throttle up post-liftoff to 109 percent RPL. “They’ll continue at that higher thrust level and then we’ll ramp back to a lower throttle and perform our first TVC (Thrust Vector Control) check,” Cipoletti noted.
During launch, the stage is accelerated off the ground by the vehicle’s twin Solid Rocket Boosters and is steering towards a precise position and speed in space at the end of the eight-minute long Core Stage engine burn. With the stage locked down in the stand for the test firing duration, NASA and Boeing plan to experiment with the interplay between the engines and stage systems.
Without the need to steer the stage in a static test, the NASA Green Run software running on the vehicle flight computers will use the stage’s hydraulic TVC system to move the engines in test patterns and collect engineering data that will be used to verify and calibrate analytical model predictions and also help to certify the vehicle for its first launch.
(Photo Caption: A water spray is seen activated in the flame bucket for the B-2 position of the B Test Stand at Stennis during the Core Stage Green Run WDR. The water deluge will protect the test stand flame bucket from the stage’s engine exhaust while a simultaneous high-rate water flow will help to dampen sound waves generated while the engines are firing as they rebound towards the stage.)
The first TVC check will be started at approximately sixty seconds into the test firing with the engines throttling from 109 percent to 95 percent 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.”
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 stage-based TVC actuators which point the engines to help steer the stage. While the engines are running, though, pressure from their hydrogen exhaust drives the CAPU, which power 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,” John Shannon, Boeing’s Vice President and Program Manager for SLS, said during a media briefing in 2020.
“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.”
“When [the TVC check is] complete then we’ll bring [the engines] back up to full throttle and then we’ll perform the frequency response test (FRT) 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 continue, it actually continues for a fairly long period of time.”
The frequency response test will measure the dynamic response of the Core Stage structure to the experimental TVC actuator movements. “As we continue at about two minutes and thirty seconds there’s what’s called a sine sweep of those nozzle engines so they’ll start moving in a sinusoidal pattern to check the structural response of those rocket engines moving,” Shannon said.
(Photo Caption: A presentation slide showing how the frequency response test (FRT) in the middle of the Green Run hot-fire is used to refine one of the SLS Program’s analytical models of vehicle dynamics. Three separate gimbaling tests of the four Core Stage engines will be conducted during the planned Hot-Fire test; the FRT would be the second test, starting about two and a half minutes into the engine firing.)
The FRT gimbaling lasts for about two minutes. “The FRT 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 in an earlier interview.
During a media teleconference on January 12, Shannon indicated that completing the first TVC check and the FRT would provide all the design verification data needed in case the engines can’t fire for the full eight minutes. Last year, there was some talk that perhaps as little as two minutes might be an acceptable minimum firing time after the first TVC check is completed.
“We have really all the engineering data we need to gather after about 250 seconds of that firing, but there’s no reason to stop the firing after 250 seconds,” Shannon said on January 12. “It just all depends on what the issues are and we have an excellent team between Aerojet Rocketdyne, Boeing, and NASA Marshall and Stennis to be able to work through any issues.”
There is no plan to intentionally stop the test early; if the vehicle and ground systems perform within limits the stage will run to a nominal shutdown. “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 noted last year. “It added so much risk to try and do a cut at some arbitrary time and then go drain the tank [that we decided to] just let it keep running.”
Near the end of the planned test firing a second TVC check will be run. “We’ll start to throttle the engines back to a lower throttle level and then at that lower throttle level we’ll do the same [TVC check] that we did at the high throttle, the circular response, at the low throttle to make sure that the whole system is working well together,” Cipoletti said.
The second check starts 456 seconds or seven and a half minutes after ignition, with the engines throttled at 85 percent RPL. This test would 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 FRT 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 engines drain the LOX and LH2 tanks and propellant levels get lower and lower. The 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.
“One of the main objectives of the Green Run from a propulsion standpoint is pressurization and understanding that,” Jonathan Looser, NASA SLS Core Stage Propulsion Lead, said. “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 that interaction of the hot pressurant gas and the liquid level surface.”
“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.”
“We want to get some steady state run time and just understand the thermodynamics inside the tanks and correlate those models,” Looser added. “The data that we want to get for the full duration is running at full power level (109 percent of rated power level), that’s the data that we’re really interested in and we can do that on the test stand.”
Assuming all went well the engines will remain throttled at 85 percent RPL after the second TVC check through 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 to run to propellant depletion in-flight to gain additional payload performance.
The engines require a hydrogen-rich shut down, so although both tanks need the low-level quantity sensors the system is biased to run out of oxygen first under normal conditions. “There’s a considerable amount of propellant in the downcomers, so when they flash dry there’s a timer that gets kicked off and when that timer ends the engines will be commanded to shut down,” Cipoletti noted.
Liquid oxygen is fed to the engines through two, long feedlines also called “downcomers” that run from the LOX tank at the top of the stage down to the engine section. “In parallel to that [timer], there’s another timer that starts at the beginning of the test and if we don’t see the sensors flash that timer is going to assume there was a problem in the system and shut the engines down based on a timer.”
Following MECO, the Core Stage flight computers will close its prevalves and hand control of post-cutoff safing of the combined vehicle and ground systems back to the Stage Controller.
Lead image credit: NASA/SSC.
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