SLS Core Stage Green Run campaign moving into propulsion system checkouts

by Philip Sloss

The WDR and hot-fire follow test case six; the WDR will conclude by taking the vehicle through the terminal countdown sequence, and verify the stage can meet the pre-start requirements of all four engines before the countdown is aborted prior to ignition. The vehicle and test stand systems will then be safed, allowing the team to watch the performance of the safing routines at that point with the vehicle fueled and ready to fire.

After stopping just short of starting the engines at the end of the terminal countdown sequence in the WDR, the stage will be drained and turned around for the hot-fire test while NASA and Boeing review data.

The test team will then come back and fuel the vehicle again several days later for test case eight. The hot-fire test will proceed through the countdown past the cutoff point in the WDR, with the stage computers running through their pre-ignition sequence, starting the engines, and then executing a series of tests during the eight minutes it will take for the engines to empty the stage of propellant.

Following the flight-duration test firing of the stage, the final phase of the campaign will be to prepare it for transportation to the launch site. The engines will go through post-firing inspections and servicing, the stage systems will be refurbished, and then the stage will be lifted out of the stand and put back on the Pegasus barge for the trip to the Kennedy Space Center (KSC).

Test case four is next

This next iteration of testing is with the Main Propulsion System (MPS) elements and will bring in the rest of the control room test team that was kept away by COVID-19. “We’ve got a minimal test team for these first couple of test cases,” Nappi said. “We’ll step that test team up when we get into test case 4 to more of a normal test team.”

“We will, in a lot of cases, not being able to maintain the six feet, so we’ll have masks and face shields for the folks in the room (test control center), and of course real strict cleaning requirements. So that’s been our protocol to keep people safe and be able to staff to the point where we can be able to resume a normal schedule even though we don’t have all the people here.”

Before loading the stage with its liquid oxygen  and liquid hydrogen propellants, test case four will validate the cryogenic connections with leak tests and the pneumatic propulsion system’s moving parts will be checked out in ambient temperature conditions.

In addition to all the internal plumbing to distribute the low-temperature propellant to the tanks on the vehicle and eventually to the engines, there are four umbilical plates that connect services from the test stand to the stage, with multiple connections on each umbilical. The integrity of those connections will also be tested before proceeding into cryogenic loading later in the campaign.

The Main Propulsion System pneumatics use high-pressure gaseous helium from vehicle and ground supplies; the Core Stage has five large, composite over-wrapped pressure vessels (COPV) to store helium needed in-flight, and those will be brought up from a ground, “pad” pressure to their operating range for the tests.

“The configuration of the vehicle is pretty static now through the actual wet dress itself, with the slight exception that when we go into Green Run-4 (test case four) we bring up some of the higher-energy systems,” Cipoletti said. “There are some internal configurations we do during that test to make sure we’re isolating different pneumatic lines for leak checks and the like, but those are all controlled within the test sequence itself.”

Beyond test case 4:

Test case five will bring up all the previously tested systems again to support checkout of the hydraulics and engines. The four, veteran RS-25 engines will be firing for the first time since their last Space Shuttle launches, and their digital engine controller units (ECU) will be brought up to perform a flight readiness test.

Core Stage avionics boxes in a Marshall Space Flight Center test lab. Credit: NASA/Tyler Martin.

“We call it Green Run-5 (test case five), which is where we’re testing out the vehicle hydraulics, TVC, and also the Core Stage engines, the RS-25s,” Cipoletti noted. “That is our biggest integrated test. we’ll have our [Main Propulsion Systems] up, our hydraulic systems up, and the engines turned on and dealing with the pneumatics and the hydraulics and the pre-start checks and that kind of stuff.”

“We run the first part of that test on hydraulic power from the stand. The RS-25s will go through a series of tests to make sure that they can go through their start sequence, stop sequence, make sure the igniters work, make sure all the valving is working within their calibration parameters. For the engines to be able to do that, our [Main Propulsion System] system has to be up and providing proper pneumatic support to the engines.”

“We’ll then go through both the hydraulic and pneumatic shutdown procedure, and when the engines are finished with that, the whole sequence really is a flight readiness test,” Cipoletti added. “Then we’ll switch over to concentrate on the hydraulics on the Core Stage.”

“We’ll cycle all the actuators to make sure everything is ready and working properly for Wet Dress [Rehearsal]. Then we’ll fill up all the hydraulic reservoirs, disconnect from the ground, and at that point we’ll look to Mark and say we’re ready for Wet Dress.”

After test case five, all the stage hardware will have been checked out with Boeing’s ground control systems and will be ready to support the Wet Dress Rehearsal and hot-fire tests.

Test case six is an unfueled countdown simulation that provides an opportunity for the test team to do one more practice run of the countdown before committing to the first tanking of the stage. Prior to executing the test case, the flight and ground systems will get a software update.

“There [are] some changes coming our way both from our NASA counterparts on the Green Run Application Software, which is [the] version of the flight software that’s loaded on the flight computers for hot-fire in particular, and there [are] also some changes in the Stage Controller that needed to match that.”

The separate software packages have been in integrated verification and validation testing at the Marshall Space Flight Center, where a full set of stage and ground control avionics are connected together in a test lab. The software is run through automated simulations in an emulated environment; aside from the computers and software, the rest of the vehicle is emulated by other software.

Test case six will also provide an opportunity to run the software updates for the first time with the live flight and ground control systems. Over time, this test case has taken on some additional objectives.

“At one point it was just going to be a refresher, but when we saw the changes that were coming from [Stage Controller] Phase 2 and the updates to the [Green Run Application Software], and then also with the downtime we’ve had with COVID, we decided to turn it into a more robust, full, dry count,” Cipoletti explained. “It was just going to be a system test, ‘hey we’re here. Let’s just make sure everything lines up,’ but with the downtime we’ve had as a test team and with the new Phase 2 coming in and the new [Green Run Application Software] drop, we’re going to roll it into a full dry count.”

“And so it’ll be a little more rigorous than we had planned originally. But that’s a good thing, and it’ll give our team full familiarity with both the new [Green Run Application Software] on the flight computers and then the new updates to the Phase 2 software.”

The focus of the countdown simulation is to run through the critical, terminal countdown sequence. “The vehicle is not going to be live with propellant, so we’ll be simulating some aspects of what’s going on. But the focus is to really make sure we run through a set of conditions that get us to our terminal sequence,” Cipoletti said.

That starts at 10 minutes before we run the hot-fire, which corresponds to KSC’s terminal count sequence that starts at T-10 minutes before launch. That’s where a lot of the changes have been made to both the new [Green Run Application Software] version and the Phase 2 version of the Stage Controller. And so we really want to get to that point and run with the team through that T-10 minutes and down to make sure that we have all of the parameters that we expect to see when we get to Wet Dress.”

Nappi noted that the test is also an opportunity for the extended support groups to see the flow of the countdown. “At one portion of it we’re going to invite the senior management team in as well so that they get some exposure of how the test is going to go and how they’re supposed to behave during the Wet Dress and hot-fire,” he said.

The Green Run test team participated in full hot-fire simulations last year, and test case six will also take the role of a final simulation before proceeding into the critical phase of the test campaign.

“There’s a little bit of history that led us up to the sim (countdown simulation) that we’re talking about,” Nappi explained. “Last year, we had several training exercises with the control room test team here at Stennis where they came in for a couple of weeks at a time and ran portions of the software against emulators to make sure they understood their procedures and they understood how the system was going to work . And that ended with two full hot-fire runs, two separate sessions where they had hot-fire runs.”

Core Stage-1 boattail and engines in mid-May when essential personnel began returning to work. Credit: NASA/SSC.

“We made the decision that because there was so much time between that last hot-fire run and when we’d actually do Wet Dress that we would really try to do it during this simulation. The only difference being that instead of being hooked up to an emulator we would be hooked up to the vehicle and we would have to use red cards or green cards to throw failures at the test team.”

“So it really is very equivalent to those hot-fire training runs that we ran last year, except we’re doing it now closer to the actual event itself,” Nappi added.

As a part of the test, the team will be given issues to practice troubleshooting. Rather than create anomalies with the flight hardware, the problems will be simulated. “Once we connected to the vehicle, the ability to pull in a sim module went away, and we don’t want to actually drive the vehicle into a redline condition,” Cipoletti noted.

“So we’ll have live vehicle data, but when the failure comes up it’ll be a card. And we plan on making sure we do that as an integrated one and throw some trouble the stand guys’ way so they can practice [their reactions] as well.”

Preparations for the critical tests, cases seven and eight, will also be in work ahead of the first tanking . With the stage held down during the static hot-fire test, additional cameras that won’t fly with the stage during launch will be deployed in the intertank area to visually document how some of the hardware behaves when the stage is loaded for the first time and then while the stage is firing in the subsequent test.

Solid Rocket Separation on Artemis 1. (Credit: Mack Crawford for NSF/L2)

In addition to making sure additional ground test instrumentation is configured correctly, other activities will close the vehicle out for the WDR and hot-fire tests. “There’s a lot of reconfiguring of the vehicle to be ready to put cryogenics onboard,” Cipoletti explained.

“We’ll be removing internal platforms, we’ll be making sure that all the internal sensors for the Green Run are properly run outside and connected and making sure that whatever closeouts that we need to have [are completed], like doors closed and [Thermal Protection System] and the like. And we’ll have our engineers doing final walkdowns.”

“During the time that our hands on people are doing the work reconfiguring the vehicle, we will be updating the [Green Run Application Software, updating the Stage Controller to Phase 2, and running our sim in parallel. That’s doing all the work to get configured,” he noted.

Lead image credit: Philip Sloss for NSF/L2

Related Articles