Armed with a large calibrated mallet and decades of experience, a partnered Boeing/NASA test team at Stennis Space Center in Mississippi successfully completed a major milestone in the Space Launch System (SLS) core stage’s test campaign. Called “modal testing,” the purpose of the test was to measure the stage’s natural resonance frequency in response to external forces.
“Everything has its own resonance frequency,” explained Boeing Test & Evaluation (BT&E) SLS Test Leader Paul Wright. “Data from this testing will validate our engineering models to ensure the core stage’s resonance frequencies won’t interfere with the guidance, navigation and control frequencies that drive the rocket during ascent.”
To get the data, the test team sent vibrations into the 212-foot-long, approximately 188,000-pound core stage, which was suspended by a crane within NASA’s B-2 test stand to get as close to a free-floating condition as possible. A variety of impact devices were used to create the vibrations, including the calibrated mallet, which is itself a sophisticated data collection device.
The team addressed a number of challenges to complete the testing, including competing noise from wind and rain, as well as from an aircraft engine test facility three miles away. Twice-daily reports from the National Weather Service provided the team with wind data at zero, 100 and 300 feet to help them plan their testing, and the engine test facility temporarily suspended its work so the team at Stennis could complete the testing.
Despite these complexities, the team completed modal testing 3.5 days ahead of schedule in January, putting SLS on track for the next phase of testing known as “green run.”
“Here we came into a new facility, onboarded more than 300 people, and within a month’s time with 9-10 days of weather impact, we were able to stay on schedule,” Wright said.
“For me, the biggest success was how well all the different teams worked together to accomplish this,” said BT&E SLS Qualification Test Manager Luke Denney. “I consider this test success a pathfinder on what we could foresee and expect with the rest of green run.”
The test team is now performing final production-related work on the stage, including leak and other safety checks. To stay on track, teammates will work shifts that cover 24 hours a day, seven days a week through the end of the 2020 test campaign.
The stage’s upcoming milestones include avionics power-on, a countdown demo, and a fueled “wet dress rehearsal” before its four Aerojet Rocketdyne RS-25 engines are ignited for the eight-minute hot-fire test later this year.
In its actual liftoff on the uncrewed Artemis I mission, the core stage will be integrated with its upper stage, solid rocket boosters, and NASA’s Orion spacecraft, and produce a total 8.8 million pounds of thrust to launch Orion on a historic journey around the moon.