Despite political drama, NASA, Boeing press on with SLS Core Stage structural loads tests at Marshall

by Philip Sloss

Planning margin expansion test, but not official yet

The intertank STA is the second of four articles to go through structural qualification testing; the first was the engine section STA. When that qualification test series was completed a “margin expansion test” was eventually run on the engine section test article where it was loaded above and beyond the required 1.4 factor of safety or 140 percent of expected flight loads up to where yielding of the structure was observed.

A similar “test to failure” is being considered for the other three STAs. “The margin test is not technically on contract yet but we are actively working that,” Haney said.

Once all the limit and ultimate load cases are complete, NASA and Boeing will review the data and eventually hold a “break of configuration” meeting to verify they have all the qualification test data they need. “Saying we’re done is easy, Sean sitting down with his analysts and making sure he’s got a path to verification is what will take him some time,” Cash noted.

Some of the imaging equipment on the floor of the test stand around the intertank STA. Credit: Philip Sloss for NSF/L2.

Sean Waninger is Boeing’s Lead Stress Analyst and Test Requestor for the SLS Intertank Structural Qualification Test. “You don’t take anything physically apart, you’ll just be given the approval from Boeing saying ‘yeah, you’ve got the data you need to do your qual test,'” Haney said.

“You’ve definitely got to do a ‘break of configuration’ because you’re going to damage the article, significantly,” Cash added. Once it’s agreed they have the necessary qualification data, that will lead into discussions about which margin test case to choose; since the article will be irreversibly damaged, margin expansion is expected to be the article’s last test.

“There a lot of options and a lot of smart folks are eventually going to get in a room and start weighing out those options,” Cash said. “You can’t actually test all of them because you are going to damage the article.”

He said he’s current aware of six cases on the table. “Our Chief Engineer’s Office is going to weigh in. Obviously NASA would like to protect for any booster or some booster upgrades I’m sure they have on their mind.”

“Sean and the Boeing team are obviously interested in first buckling mode, second buckling mode, crippling the thrust fitting itself,” Cash added. “It’s a risk-based discussion that upper NASA management is going to weigh in on what case they run.”

The engine section qualification article in the 4619 load test annex on March 4. The article is disassembled from its single simulator that was in testing in 2017 and early 2018. The metallic aft SRB attach fittings can be seen lower middle; during the margin test they were taken to 200 percent of expected flight loads. Credit: Philip Sloss for NSF/L2.

The engine section was taken well above ultimate load levels, but the intertank is not expected to have as much margin. “Engine section never went through an optimization, intertank has gone through two or three optimization cycles so there’s less to give,” Cash said.

Depending on the test case chosen, the article flanges would also be cooled to cryogenic conditions as with the limit and ultimate load test cases. For the engine section margin test, they chose the SRB attach points.

“Given that the next Block configuration of SLS were going to upgrade the boosters, they looked at the SRB attach points and took those to two-hundred percent while they took the rest of the vehicle to one-hundred forty,” Mike Lau, NASA’s Structural Testing Branch Chief at Marshall, said.

LH2 tank structural test preps continuing

In parallel with finishing the intertank structural load cases, NASA and Boeing are getting ready to test the other two STAs. The liquid hydrogen (LH2) tank STA arrived at Marshall during the government shutdown; transportation and installation into the outdoor 4693 Test Stand was an excepted activity during the workforce furlough.

Over one-hundred and fifty feet long, the article was lifted from horizontal to vertical and installed in the two-hundred foot tall stand in mid-January. “We did it at night to make sure the winds would be calm,” Sam Stephens, NASA manager for wet structures in the SLS Stages Office, said in a March 5 interview. “It was textbook, it just went off perfectly.”

The LH2 STA is lifted into the outdoor 4693 test stand in mid-January. Credit: NASA/Tyler Martin.

“It took a couple of days to get ready to lift. We had to put several hundred bolts on to get it ready to lift, that’s what the other crane was here for it was lifting the back end and then the front end there’s actually a hoist up in the middle of the stand and that’s what took a couple hundred bolts to attach, a big interface flange.”

Since the end of the shutdown, technicians have been working to configure the test article, connecting equipment that will impart loads, running wire harnesses on the article to the stand’s data acquisition system, and connecting the flex hose to the qual tank’s fill and drain line where liquid nitrogen will be loaded during some test cases to partially fill the bottom of the tank.

“We put a lot of external forces into the article, so what you see along the bottom here is twenty-four hydraulic cylinders each capable of two-hundred thousand pounds force,” Stephens noted. “Combined they could put over four million in, but we’re going to do about three, which is our max capability.”

The LH2 STA and 4693 test stand on March 5. The test article and stand are being prepared for structural qualification testing to begin in the Spring. Credit: Philip Sloss for NSF/L2.

The last of the twenty-four cylinders was installed that morning. “So they’ve been attached to our bottom interface ring here and attached down to the ground, plates on the bottom, and we’ll have additional cylinders coming from the sides of the test stand.”

In contrast to the dry structures that are open to the atmosphere, the cryogenic propellant tanks will be pressurized for tests. “Right now we have just a missile-grade air purge, we’re going to keep the inside of the tank dry,” Stephens said. “Industrial grade air is fine but eventually when we’re going to test we’ll have gaseous nitrogen that we can press with and then liquid nitrogen for our cryo cases.”

Similar to the type of data that will be recorded in the intertank tests, strain gauges, thermocouples and visual sensors are distributed in locations on the inside and outside of the test article and in locations on the stand. “All the internal instrumentation, all the internal strain gauges come out either through the aft dome, the aft manifold or on the forward dome up at the top,” Stephens explained.

“And then you’ll see on the outside the four bigger black squares with electrical tape and plastic over them, and then [in] places all around, those are for all the strain gauges that are on the outside of the tank underneath the foam.”

Some of the external instrumentation on the LH2 STA. On the left top, the black square is a sensor island where strain gauge wiring comes together. On the right, are studs where optical targets will be installed. The duct tape covers thermocouples on the tank skin. Credit: Philip Sloss for NSF/L2.

“We have started all of the instrumentation hookups, but when you’ve got over 2500 it takes a little while and you’ve got to make sure they’re all channelized correctly all the way through the data acquisition system,” he added. “It’s a long, tedious process.”

All of the preparation is pointing towards being ready to start running test cases in a couple of months. “Our test readiness review is towards the end of May right now and we’re on track for that,” Stephens said.

“I’ve got a couple thousand strain gauges I’ve got to get connected up, all the load lines, all the hydraulic lines, all the control systems for those hydraulics, the fluid systems, the liquid nitrogen, get all those activated and ready.”

Stephens said that Boeing is still doing the analysis to come up with a final set of test cases, but it should be a couple of dozen. There will be a similar ‘break of configuration’ review at the end of the test series and then NASA and Boeing will consider a margin expansion test case for the LH2 STA.

LOX STA preps at MAF

The liquid oxygen tank STA is the last piece that needs to go through structural qualification testing. It is currently going through final assembly at the Michoud Assembly Facility (MAF) in New Orleans where the other three articles were built.

The qualification tank was the last of the first flight hardware to go through the priming and spray-on foam insulation (SOFI) application [link] in Cells P and N of Building 131 at MAF, respectively. At the end of February, the qualification tank was set up in nearby Cell A in Building 110 at MAF for installation of its sump.

LOX tank qualification article being prepared for sump installation in the aft dome in Cell A at MAF on February 28. The article will return here for stacking with its two simulators prior to transportation to Marshall later in the year. Credit: Philip Sloss for NSF/L2.

Once that is complete, Boeing will navigate the tank through Core Stage hardware traffic and priorities at MAF in order to stack it with its two simulators. It may need to use both stacking cells in Building 110, so it would need to wait until the Core Stage-1 engine section finishes its initial stacking with its boattail assembly in Cell A and then for the Core Stage-1 forward join to complete its work in Cell D.

After the STA is stacked vertically, it will be placed horizontally on NASA’s Multipurpose Transportation System (MPTS) [link], rolled to the Pegasus barge [link], and transported by sea to Marshall. It will go to the 4697 Test Stand.

“The way it looks right now, [with] the LOX tank showing up late Spring, early Summer timeframe, [that’s] about the time we’re getting ready to start testing, so we’ll probably be shifting between testing here [4693] and then on the in between dates guys doing build up over there,” Stephens said.

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