NASA will soon conduct SLS and Orion integrated modal testing at the agency’s Kennedy Space Center ahead of the Artemis I mission launch. Teams there will examine the considerable forces and vibrations the recently stacked rocket will incur on its way to space.
To safely control the rocket, flight software and the navigation system must distinguish the rocket’s natural frequencies from the vibration frequencies experienced during flight. SLS Chief Engineer John Blevins knows just how important these tests will be for success.
“We will use the modal testing data for multiple purposes,” he said. “We will compare the physical results with what computer models predicted. The information will also be fed into the flight computers so when SLS is flying, the computers know which vibrations are natural to the rocket and which are caused by external forces. The computers will use that information to steer the rocket, ensuring it is placed in the right orbit and does not unnecessarily deplete its fuel by reacting to natural vibrations the rocket should ignore.”
NASA’s Exploration Ground Systems and Jacobs team at Kennedy along with the SLS team from Marshall Space Flight Center will perform the tests on the mobile launcher and SLS stack in the Vehicle Assembly Building with support from personnel at other NASA centers. The assembled rocket comprises the solid rocket boosters, core stage, Interim Cryogenic Propulsion Stage, Orion stage adapter structural test article, and mass simulator for Orion. Engineers are using the mass simulator for Orion and the Orion stage adapter structural test article for the modal test series while the Orion spacecraft undergoes assembly of its launch abort system and CubeSat payloads are loaded into the Orion stage adapter for flight. The test hardware has the same weight characteristics as their respective flight components, which is important for this test.
“Approximately 300 sensors are attached to the rocket and mobile launcher to detect, record, and transmit the information,” Cliff Lanham, senior vehicle operations manager for Exploration Ground Systems at Kennedy, said. “Instrumentation is installed both internally and externally to the rocket including the boosters.”
The motion pattern of a system vibrating at its natural frequency is called the normal mode. To identify the SLS’s natural frequencies, the team places hydraulic shakers in seven locations on the rocket. A small hammer will deliver calibrated taps near key parts of the navigation system to understand the dynamics local to those spots. A hammer on a dolly will move to different locations on the mobile launcher to impart further vibrations.
For 10 hours a day, several days in a row, the team will conduct the test program. They will take advantage of the overnight shift in the Vehicle Assembly Building, when activity level is low. This quiet setting helps engineers verify the vibrations detected by the sensors are caused by the tests and not by other activities in the cavernous facility or on nearby roads. Some of the tests involve multiple vibration frequencies at the same time – like a guitar producing multiple notes at once – while other tests focus on a specific frequency. Following each round of tests, engineers will analyze the data and plan any adjustments needed for the next test.
While the official test series will end after the multiday program, the sensors will continue collecting data during rollout of the flight stack to Launch Complex 39B, which will include the flight version of the Orion stage adapter with CubeSat payloads and the Orion spacecraft. Sensors will also collect data during the wet dress rehearsal, a practice run of launch countdown that will include filling the core stage and Interim Cryogenic Propulsion Stage fuel tanks with super-cooled liquid hydrogen and liquid oxygen propellants.
“We will use the data collected from the modal sensors to cross-check, update, and validate the integrated SLS structural dynamic math models, which then provides confirmation and confidence of the vehicle design for guidance, navigation, control, and loads during launch,” Liliana Villarreal, operations flow manager for Exploration Ground Systems, said. “Bottom line: This test will help ensure we are ready to fly and safely navigate the atmospheric road to the stars.”
Editor’s note: Will Bryan, a Manufacturing Technical Solutions employee, supports SLS Strategic Communications.