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Structural Dynamics Observations in Space Launch System Green Run Hot Fire TestingThe Space Launch System (SLS) Core Stage (CS) Thrust Vector Control (TVC) system is comprised of eight mechanical feedback Shuttle heritage Type III TVC actuators and four RS-25 engines, each attached to a Shuttle heritage gimbal block/bearing. Two actuators are used to move each engine in two planes perpendicular to one another (i.e., pitch and yaw). The TVC system design leverages hardware from the Space Shuttle program as well as new hardware designed specifically for the Core Stage.

The Green Run Hot Fire (GRHF) of the SLS Core Stage provided a flight-like ground test environment for verification of integrated vehicle TVC performance. A TVC model coupled to a vehicle structural dynamic model has been developed previously and incrementally validated in subsystem tests and simulations. Still, some aspects of TVC performance in GRHF were not anticipated. The ensuing investigation demonstrated the need for well-instrumented test environments, various levels of modeling fidelity, test-representative structural models, and caution in reuse of legacy components.

This paper is the sixth installment in a seven-paper series surveying the design, engineering, test validation, and flight performance of the Core Stage Thrust Vector Control system. It introduces the salient structural dynamic phenomena uncovered in ambient and hot fire testing. During the Green Run test campaign, a comparison of ambient and hot fire step responses showed a significant change in apparent damping due to the presence of friction, challenging long standing assumptions that friction could be neglected. Additionally, the characteristic response of the engine and thrust structure during GRHF proved to be more complex than anticipated, as evidenced by the available actuator, thrust structure, and engine measurements. While the string-potentiometer based test instrumentation was intended to allow for reconstruction of the engine angles along the two control axes, the geometric placement, location uncertainty, and responses in overlapping frequency spectra revealed additional phenomena requiring further analysis and post-processing. The observations from both modal and frequency response testing during the Green Run ambient and hot fire configurations led to Engine and Core Stage FEM (finite element model) updates. When evidence of unexpected engine motion was found in engine section accelerometer data, the authors pursued additional structural analysis leading to FEM updates associated with the TVC gimbal and thrust structure. Through collaboration between structures, TVC, and flight control disciplines, the test-informed models and root-cause analysis led to confident flight rationale for the first flight of the SLS launch vehicle.
Document ID
Acquisition Source
Marshall Space Flight Center
Document Type
Richard Moore
(Jacobs (United States) Dallas, Texas, United States)
John Wall
(Jacobs (United States) Dallas, Texas, United States)
Abran Alaniz
(Jacobs (United States) Dallas, Texas, United States)
Stephen Ryan
(Manufacturing Technical Solutions)
Colter Russell
(Jacobs (United States) Dallas, Texas, United States)
Jeb S. Orr
(McLaurin Aerospace)
Robert Towner
(Jacobs (United States) Dallas, Texas, United States)
Jonathan Band
(Marshall Space Flight Center Redstone Arsenal, Alabama, United States)
Date Acquired
January 13, 2023
Subject Category
Launch Vehicles and Launch Operations
Meeting Information
Meeting: 45th Rocky Mountain AAS GN&C Conference
Location: Breckenridge, CO
Country: US
Start Date: February 2, 2023
End Date: February 8, 2023
Sponsors: American Astronautical Society
Funding Number(s)
Distribution Limits
Public Use Permitted.
Technical Review
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