Advanced Modeling of Control-Structure Interaction in Thrust Vector Control Systems

The Space Launch System (SLS) Core Stage (CS) Thrust Vector Control (TVC)
system is comprised of 8 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.

During the development of the SLS TVC system, a family of advanced dynamics models were developed to extend and compliment the simplified quasi-linear “simplex” model historically used for flight control design and stability analysis. The importance of these advanced models became increasingly evident after ambient and hot fire testing of the Core Stage, which revealed a number of findings associated with the dynamic response of the TVC integrated system. Test responses suggested that the TVC did not meet its performance specifications and its step and frequency responses exhibited unexpected departures from prior lab tests and
modeled behavior. One driving factor for these results was a higher-than-expected
degree of coupling between the TVC system, the engine dynamics, and the Core
Stage structure.

This paper is the third installment in a seven-paper series surveying the design,
engineering, test validation, and flight performance of the Core Stage Thrust Vector Control system. In this paper, a new method of modeling rocket vehicle thrust
vectoring servoelastic dynamics is presented. In this approach, the load dynamics
are replaced by a detailed finite element model containing both the rigid body and
elastic modes. A partitioning technique is used to compute the effective compliance from the modal data and obtain accurate simulation results using a reduced
number of generalized coordinates. Coupled backup structure and nozzle attach
compliance effects on multiple engines are captured in higher fidelity than with
a spring approximation, eliciting novel effects due to the complex load paths involved in the Core Stage structure. Validation of the model is demonstrated using
a variety of structural/modal, laboratory, and full-scale hot fire test data.