Technicians for NASA Space Launch System (SLS) Core Stage prime contractor Boeing completed the first of many tests on the program’s first working article in the test stand at the Stennis Space Center (SSC) in Mississippi and are now getting it ready for its crucial pre-launch test-firing later this year. Core Stage-1, which serves as a test article for the Green Run development campaign at Stennis, is also the flight article for the program’s first launch forecast for next year.
By Philip Sloss, via nasaspaceflight.com February 11, 2020
The Green Run campaign at Stennis is the final, major pre-launch development test of the newest piece of the space agency’s launch vehicle and the first test performed collected data on the resonant frequencies of the assembled rocket. Teams at Stennis continue to work around the clock, now establishing work platforms in and around the stage in the B-2 position at the B Test Stand to provide access to internal and external equipment.
The next steps are to finish connecting propellant lines in the engine section and fully connect and wire the stage into the test stand. Once complete, Boeing will perform an extensive suite of integrated checkouts to establish that the stage is ready for the two planned countdowns that are the centerpiece of the Green Run test series.
After NASA and Boeing review the data, they will give the go-ahead to fuel the stage for the first-time and practice counting down to a point short of engine ignition. Following a review of that wet dress rehearsal (WDR) test, the plan is to refill the stage with its liquid oxygen (LOX) and liquid hydrogen (LH2) propellant in a second countdown for the hot-fire test. Test control will hand off the count to the stage’s flight computers, which will begin its simulated Autonomous Launch Sequence (ALS) thirty seconds before zero.
If all goes well, the stage will start its four RS-25 engines to begin an eight-minute long, flight-duration test-firing. The activation, checkouts, practice countdowns, and test-firing of the stage will provide data to calibrate analytical modeling and ultimately to help decide when the whole SLS is ready to fly for the first time.
Modal test completed
Core Stage-1 arrived at Stennis on January 12 after it was finally rolled out of its assembly plant at the Michoud Assembly Facility (MAF) in New Orleans. NASA rolled the stage out of the factory on January 8 and placed it on the agency’s Pegasus barge for the day-trip to Stennis.
The weather finally cooperated enough for the stage to be installed in the B Test Stand in the B-2 position overnight from January 21 to 22 and work began right away to perform the modal test on that stage. The stage was lifted by the stand’s derrick crane a few inches off the hold downs in the stand to perform the test.
Credit: NASA/Stennis Space Center (SSC).
(Photo Caption: A Boeing technician uses a hammer to ring the structure of the suspended Core Stage on January 30 at Stennis. Hitting the yellow “lift spider” fixture attached to the top of the stage allowed data to be collected on the resonant frequencies of the structure. A similar test was performed on the lift spider and individual Core Stage structural elements. The data collected will help refine some of the analytical modeling of how the vehicle structure resonates in flight, which is used as a part of the rocket’s guidance software to manage navigation and loads.)
Getting everything in position and synchronized to run the test was affected by weather again. “The modal test went very well,” Mark Nappi, Boeing Green Run Test Manager, said in a February 5 interview. “Of course we’re battling weather a lot in the stand because when you pick it up and try to put it in position where you can do the modal you’re limited by winds.”
“We also had a lot of rain during the event so it slowed things down and it had the potential for damaging equipment, so we had some delays but it was all part of the plan. We incorporated [the schedule] for those sorts of delays, so when it was all said and done we ended up finishing right on time.”
“So we did complete test case 1, the engineering community reviewed the data, they were happy with it,” he added.
Weather had also factored in the delays getting the stage lifted off the tarmac at the stand and into the B-2 position. “Some of the equipment that we were using was not completely waterproof and so when we got driving rain we stopped but most of our constraints was because of either a hardware failure with the lifting equipment or with the winds,” Nappi explained.
“The winds at some points had to be less than five knots and most points had to be less than 10 knots. You could see [the vehicle] was visibly moving with higher than 10 knot winds.”
The modal test measures the standalone structural frequency response of the Core Stage; similar measurements are planned to be taken on the fully integrated vehicle during launch preparations. While getting ready to perform the first test run, NASA and Boeing came up with an alternate plan.
“We did it a little differently than plan, we were planning to do three lifts and collect the data,” Nappi noted. “While we were waiting for one of the weather delays we did some testing with the vehicle in the stand and determined that we could probably get all this data in just one lift, so that saved us a few days and helped keep us on plan.”
(Photo Caption: One of the silver and white colored forward Solid Rocket Booster (SRB) attach fittings hangs with the rest of the Core Stage above one of the “yellow box” forward hold downs in the B-2 test stand. The stage was lifted out of the stand on January 30 to perform modal testing. A Boeing technician is posted on the yellow box structure to communicate positioning of the stage so that the lifting operation can be performed safely. There is only about eight inches of clearance between parts of the stand and the stage.)
For the test, the stand’s crane was once again rigged to the “lift spider” fixture that is bolted to the top of the forward skirt of the stage. Once the crane was carrying the load of the stage, it was unbolted from the stand’s hold downs. When locked down in the stand, the hold downs attach to the same attachments on the stage that the SLS Solid Rocket Boosters will attach to, two points on opposite sides of the intertank and two locations on the engine section.
The crane lifted the stage a few inches off the hold downs so it was hanging off the crane free of any other support points; external instrumentation cabling clipped to the outside of the stage ran down into the stand’s data acquisition system to record the test data. To generate the test data, a technician used a hammer to rap the top of the lift spider.
After the test run on January 30, the stage was placed back in the stand and locked down again while NASA and Boeing conducted a “break of configuration” review the next day. These reviews have been conducted throughout integrated and subassembly testing to make sure there is a consensus that all the data that needed is collected before moving on and reconfiguring the hardware.
“We held the break of configuration last Friday (January 31) and the engineering community was happy with the data so we started down the path of securing the vehicle in the stand and then taking the next steps, which is really starting to get access into the vehicle,” Nappi said.
Working towards power-up checkout
With the modal test complete, work to fully connect the stage into the stand’s infrastructure could begin. The vehicle remained in the configuration it was in when shipped from MAF in order to capture the modal test data with it outfitted more closely to how it would be for flight.
Now the stage can be opened up to move from the structural test configuration to a full functional checkout configuration, opening access doors, connecting air conditioning, installing internal and external work platforms, lining up umbilical plates.
“On Saturday (February 1) we started getting access into the forward skirt and the engine section, that’s currently ongoing and then once we get that access complete we’ll start on the critical path which is hooking up those liquid hydrogen feed lines in the engine section,” Nappi said. Boeing will take the next few weeks and months to get the stage ready to power it up and do a full functional checkout of both the stage and B-2 stand’s ground infrastructure to demonstrate everything and everyone are ready for the hazardous operations of fueling and firing the stage in the stand.
(Photo Caption: Core Stage-1, which doubles as the SLS Program’s working development test article and first flight article, in the B-2 position of the B Test Stand at Stennis on January 24. On the left, the B-1 position is used by Aerojet Rocketdyne for acceptance testing single RS-68 engines for Delta Common Booster Cores (CBC). A couple of single-engine RS-68 tests are expected while the SLS Core is in the stand. In the foreground, a second lift-spider fixture is being lifted by the stand’s derrick crane.)
“We have what we call critical path work, secondary critical path work, and non critical path work and the critical path work is the engine section,” Nappi explained. “That’s our critical path, getting these feed lines hooked up and getting ourselves in a position to do power-on testing goes through the engine section, so that is obviously the stuff we want to pay close attention to.”
Julie Bassler, NASA’s SLS Stages Office Manager, provided an update on the feed line work on February 10 at Stennis. “We hooked up the first one already and we did a leak test yesterday and that passed the leak test so we’re still on schedule,” she said.
“That was one of our risk areas because it was something new that we doing in the stand here but everything looked really good and we should be able to reduce that risk fairly quickly. In about two weeks we should be able to be done with all that work on the feedlines.”
Thermal protection system (TPS) spray-on foam insulation (SOFI) closeouts will be applied to the bolted flanges, but Bassler said if the feedline work stays on its current pace, the TPS work would not be on the critical path.
“There’s non-critical path work that you can do in other areas of the vehicle that allow us to continue to make progress and keep it off the critical path,” Nappi noted. “Those are things like removing all the modal sensors that were installed for the modal, doing some TPS repairs, hooking up some of the umbilicals. They will eventually come on the critical path if we don’t get them done, so we want to get all that stuff done as soon as possible and in parallel with the critical path work in the engine section.”
“The secondary critical path is the Stage Controller,” Nappi said. “We have two legs of that. One is the software that’s being finally verified and that’s being done at Marshall and KSC and that’s being done by our avionics group.”
“The second piece of that is getting the stand hardware ready to receive that software when it is ready. I’m watching the stand work very closely and that plan from now until March 11 is tracking on plan, and what they’re doing is they’re wringing out cables, they’re checking out all the equipment that is installed and they’re still installing some of the final pieces of equipment. So that’s what we’ll call the secondary critical path.”
The Stage Controller, short for Core Stage Controller, is a Boeing-developed package of software and avionics that manages the test stand side of the Green Run. It sits between the stage and the stand infrastructure, providing a command and control and monitoring interface to the team managing test operations from the B-2 test control center.
“What we’d like to see is the engine section work gets done right about the same time that Stage Controller of the stand work gets done at about the same time the software gets done, they’ll all come together and we’ll be able to power-up,” Nappi said.
Credit: NASA/SSC Video Services.
(Photo Caption: The engine section and foil-covered boattail at the bottom of the Core Stage is maneuvered into the B-2 test stand position at Stennis on January 22. After the modal test was completed, access doors like the red one in this image were opened to install work stands to provide access inside for finishing connection of the LH2 lines inside and future power-up testing.)
With the stage outside in the elements for the duration of the Green Run campaign, weather will continue to play some role in the pace of work. Work areas lower in the stand and inside the vehicle have more shelter from weather conditions.
“It’s different for the different levels,” Nappi said. “Once you’re inside the vehicle, particularly the engine section, you’re pretty safe. We can work inside the engine section whether there’s lightning or thunder or high winds or rain because you’re inside the vehicle it’s like being inside the vehicle at MAF.”
“That work is uninterrupted; however, getting from the interior of the soft core of the stand into the vehicle you have to go outside,” he added. “So if there’s lightning warnings in effect than the folks have to stay inside the hard core and they can’t go out, so that affects us.”
“When you’re working at other levels exterior to the vehicle, if there’s high winds or lightning warnings or heavy rain you know where it’s just too uncomfortable to work then obviously we have to shut down work,” Nappi explained. “So what we did was we walked through all the work that we have in those areas and we looked at the history of weather here in Stennis and we said based on the last several years of history, we’re probably going to be impacted by this much and we applied that to our schedule.”