NASA and Space Launch System (SLS) Core Stage prime contractor Boeing are in the middle of a busy period of testing structural test articles (STA) at the Marshall Space Flight Center in Huntsville, Alabama.
One team is busy running test cases on the intertank STA in a large, indoor test facility while another is preparing the liquid hydrogen (LH2) tank STA for its test runs in another large, but outdoor stand down the road.
The continued testing shows a program that has not been impacted by the uncertainty created by comments made by NASA administrator Jim Bridenstine over recent days, pointing to evaluations that dampen SLS’ viability for missions it had already been tasked with.
The latest potential change of plan was noted on Wednesday morning, per options to launch Exploration Mission -1 (EM-1) on two commercial Heavy Lift rockets.
We need to consider all options to meet the Exploration Mission-1 target launch date of June 2020, including launching on commercial rockets. pic.twitter.com/fR5b2NzPtg
— Jim Bridenstine (@JimBridenstine) March 13, 2019
However, that remains just an evaluation, set to be concluded in April and not an official change of plan. As such, and as expected, SLS is continuing to press through its test cycle unabated.
Per the latest milestones, the test articles are being squeezed, stretched, twisted, and bent after several hours of freezing to simulate the forces and environment that flight structures are expected to see during launch and ascent into space. The test cases will help qualify the structures for first flight and validate the fidelity of computer models that can continue to be used for future analysis of virtual cases.
The team is also testing the structures to verify they are designed with the required factor of safety above and beyond expected launch conditions, and is planning on taking advantage of the structures to evaluate how much additional margin there is above the required safety level.
Current plans are for the last article, the liquid oxygen (LOX) tank STA, to be delivered to Marshall and set up for testing somewhere in the middle of the year while LH2 tank STA test runs are being executed.
Halfway through Intertank STA test cases
The team running the structural qualification test cases on the intertank STA was about halfway through them in early March. “We’re about halfway through,” Matt Cash, NASA Test Conductor for the SLS Intertank Structural Qualification Test, said in a March 4 interview. “We have gone past the influence load cases, where you’re really working on model correlation and systems checkouts and we’re into the flight-like or flight-level load cases.”
The structural loads tests run on the SLS Core Stage structural test articles are grouped into three sets, beginning with influence load cases. “Influence are half of the flight magnitude, so they are fifty-percent of the flight load and they are typically unidirectional,” Cash explained. “So we’ll torque clockwise, we’ll torque counterclockwise, we’ll apply uniform compression, we’ll apply uniform bending.”
“At the SRBs (attach points), you’ll go uniform compression, uniform tension, pinch. You’ll run through a pinch case. So you’re not combining axial moment, shear, bending, torque,” he continued.
“You want to do those first so you see that [the article] is going to react the way you think before you really add in some loads,” Heather Haney, Dry Structures Test Lead for NASA’s SLS Stages Office, added.
After the influence cases, then dynamic cases using limit loads and finally ultimate loads are run with combinations of different types of loads. For the intertank, a total of forty-one tests are planned currently. Twenty-one influence cases were completed last year, they are in the middle of eighteen limit load cases, with two ultimate load cases at the end.
Limit loads are one-hundred percent of expected flight loads and ultimate loads are one-hundred forty percent to test that the structure meets the requirement of having a factor of safety of 1.4.
“We’ve done twenty-nine cases so far,” Cash noted. “Right now I think we’re looking at the first of April to be done with that qual testing part,” Haney added. They are typically running tests twice a week.
The STA arrived at Marshall in March, 2018, and after being configured in the test stand the influence load cases were run in July. Moving into limit testing was then delayed.
“We took a break. We had some technical challenges that we had to work out,” Cash explained. “You want to make sure you’re meeting all your requirements so we took time to do that, so that’s why you’re seeing a lull between influence end and the limit start,” Haney added.
And then the government shutdown for five weeks, which forced the live test system normally maintained in a ready state to be powered off and safed. “We safed all of our systems,” Cash said.
“The load plates will actually move, you’ll get drift in all of these cylinders so mechanical stops are installed, you turn off purges, you shut down systems.” After returning from furlough, it took an additional week or so get back to a test-ready state.
“Every time you turn very, very expensive data systems off something doesn’t come back on,” he added. We had some of those challenges in there, we had some hardware failure to go work out, but we fixed them relatively quickly.
The test article is set up in its test stand inside the Central High Bay in the Building 4619 load test annex (LTA) at Marshall. “We can test articles up to sixty feet in diameter and up to thirty million pounds of compressive loads, they are very few facilities on the planet with this capability,” Cash noted.
The test article is comprised of an intertank qualification article, which is structurally identical to a flight intertank article, with a simulator attached on each end. It is oriented upside down in the stand, with each simulator attached to a load ring.
“The original drop of intertank loads we got exceeded the crosshead capacity, so we inverted the test article,” Cash explained. “I think on our highest loaded case, we’re at thirteen million pounds of compressive load. We’re going to do 4.8 [million pounds] on the crosshead and about 4.2 [million pounds] on each SRB load plate.”
Loads are applied to the article through hydraulic load cylinders attached at locations around the test article. “We’ve got a hundred different actuators we’re using to apply various load combinations to the intertank,” Cash noted. “There’s forty-eight hydraulic actuators that bolt to the crosshead. Those will do the engine loads, compression and bending and tension.”
“And then you see these two orange plates on the east and west, those are Solid Rocket Booster attach points that you can see that are attached to the plates,” he continued. “Actually the hat, the round piece. That is actually a simulator that bolts to our load plates, the actual thrust fitting is inside that hat. Twenty actuators on each side attach to a plate that will apply loads to a single point.”
“One of the real challenges for this test is you’ve got to apply loads into the actual thrust beam, which is basically a ball joint,” he explained. We’ve got twenty actuators that attach to a load plate that goes into a ball joint so it can swivel about all three axes. So we have taken painstaking care to make sure that we get that four million pound load at the perfect angle.”
Additional actuators are connected at the top of the inverted test article. “Eight actuators attach to those orange weldments, so we can do pure torsion, too, ” Cash added. “We can do shear in any clocking and pure torsion, both clockwise and counterclockwise.”
For the limit and ultimate load test cases, the mechanical interfaces at the intertank qualification article’s flanges are also chilled down with cryogenic liquid nitrogen to more accurately simulate flight conditions. In addition to being bolted to ends of the stage’s cryogenic tanks, most of the volume on the inside of the intertank is taken up by the aft dome of the liquid oxygen tank and the forward dome of the liquid hydrogen tank.
“Just one of the influence cases had cryo and it was independent from mechanical loads, that way you’re understanding each joint,” Cash said. “When you take joints to minus three-hundred and twenty degrees materials behave differently and joint geometries actually change.”
Test days now start with flowing LH2 through ducting at both flanges. “We bring in first shift at 3:30, 4 am,” he said.
“I’ll bring in a skeleton crew to get the article cold and then we’ll bring in the rest of the test team at 7 or 8. It takes six hours to get thermally stable.”
Over two-thousand channels of data are being recorded during the current tests, with additional instrumentation added after the influence cases were completed bringing the total data channels up over 2800.
“Primarily strain gauges, but we’ve also got thermocouples,” Cash noted. “So when you’re making things cold, you’ve got to thermally correct the strain so there are hundreds of thermocouples, there are twenty or thirty CLTSs (Cryogenic Linear Temperature Sensors), we’ve got over a hundred deflection measurement points that we’re doing with an optical [system].”
“First test I’ve ever worked where we’re completely reliant on an optical deflection measurement system, that’s Boeing proprietary PhotoG system. We’ve got a control room adjacent to the high bay where we’ve got all our load control operators, cryo control operators, PhotoG operators, the data system operator. Obviously we record video and audio live for all of this.”
“We brought in two and a half million pounds of steel in addition to the facility and all of that is bolted together with over 25000 fasteners,” he added. “So we’ve got accelerometers located strategically to help us determine if any of our bolted joints slipped. We want to understand if we’re seeing test article related stuff or test equipment related stuff.”
Structural qualification test cases also calibrate, update analytical models
In addition to qualification of the different Core Stage structures, the loads tests also help to “anchor” Boeing’s analytical models that predict how the structures will respond them. The structural loads tests on live hardware are much more limited in the number of cases than virtual models that can analyze how the structures should behave in a broader set of conditions.
“An analyst when he’s designing or doing the analysis on say the intertank, they’re looking at eight-hundred or a thousand load cases,” Cash said. “We can’t physically do a thousand test cases because we’d damage the hardware, so what they do is they pare those cases down.”
“Say for rollout, for instance — rolling out from the VAB (Vehicle Assembly Building) to the pad — they will look at all of the high-strain states at different conditions and will combine those into a test case. So we will have a limit case that is a combination of different rollout environments to hit all of the peak stress.”
“Some cases you don’t have to test because they don’t overstress the article, some do and you combine a subset of those flight cases,” he added. “But we’re really looking for a strain state in the article.”
The limit and ultimate load cases that are being tested on the different STAs should qualify the Core Stage structure to fly both in current Block 1 and future Block 1B configurations.
“[Boeing] took the worst cases from Block 1 and the worst cases from Block 1B and they combined them into a combined load set,” Cash explained. Believe it or not, even though Block 1 is a different vehicle configuration as your trajectories change and your flight path changes and your payloads changes there are actually loads from Block 1 that are driving design and there are Block 1B loads that are driving design.”
“Boeing’s intent was to take a combined load set, an enveloping load set from both packages and qualify the intertank for that, so it’s supposed to meet Block 1 and Block 1B [requirements]. It wasn’t truly as simple as saying ‘we’re going to go qualify to Block 1B and ignore Block 1’, because it’s not that simple.”
The bigger, higher-performance SLS Block 1B configuration employs the Exploration Upper Stage (EUS), both still in design. The Trump Administration recently proposed cancellation of Block 1B, EUS, and related Exploration projects, but they remain funded for the current fiscal year.
If EUS and Block 1B survive current and future cancellation attempts, then the new upper stage and associated structures would go through structural qualification testing — the four Core Stage structures going through qualification testing now would not need to be retested.