24 Forcing Function Estimation for Space System Rollout 271 24.6 Conclusion The process steps detailed herein are the result of efforts to implement tools to estimate a rollout FRF for the purpose of separating out the harmonics from the flexible body effects to allow estimation of the system’s structural dynamic properties. Alternative paths are provided to assess the fidelity and robustness of different processing steps. This set of processing steps have been subjected to limited initial data sets to exercise, assess, modify as-needed, and verify the applicability to the systems that will be enhanced via operational testing. Systems that are difficult to test in controlled laboratory environments or subject to unusual boundary conditions and/or loading such as full launch stacks, spacecraft in-flight, or systems in transportation are targeted stakeholders. The results, processing updates, and assessments have not been finalized but do provide evidence that the processing tools can be successfully applied. A process has been setup as a framework for the data analysis. Within this framework the following are unique contributions: 1. Expanding traditional CG centric force reconstruction techniques to expanded input locations and boundary conditions; 2. The use of CG force transformation matrix null space vectors as basis vectors to create full rank forces; and 3. Constrained least squares force updating to maintain targeted force updating. Success criteria have been identified and initial but limited application data is available. The first success criteria (reproduction of the input data) appears to be met with the current processes as forcing function/FRF calculations can be fit to match the measured data with the full system data example covering this example. The second success criteria (proper estimation of forcing functions and transfer functions) has shown to have a positive potential based on a comparisons presented herein. A reasonable assessment of the third success criteria will need a more dynamically active data set as well as a more complete assessment of FRF calculation. In summary, the processes developed herein hold promise to allow operational data to be used to derive structural dynamic parameters for multiple uses including but not limited to system identification, forcing function development, fatigue spectra generation, design assessment, and structural health monitoring, while expanding the reach of operational modal analysis. Acknowledgements The authors with to acknowledge Curt Larsen for his original insight to support this work. The team is also greatly indebted to Joel Sills for the continuing support and efforts to keep this work integrated into the larger scope of exploration initiatives within NASA. Multiple groups and individuals supporting the Space Launch System including structural dynamic, ground operations, and rollout analysis specifically have contributed significantly to this work, yet are too numerous to name here. A Appendix A: Hardware and Data Background A.1 Crawler-Transporter (CT) Hardware Rollout forces are generated as the CT imposes a series of harmonic excitations (sine and cosine-like waveforms) onto the entire system under transport. Previous work during rollout of the STS system found two primary families of harmonics, which are characterized by a speed-dependent harmonic and integer frequency super harmonics [7, 12]. This loading is inherent in the tracked vehicle design of the CT. Figure 24.21 shows one of the four trucks on the CT and one of the eight tracks on the CT. The trucks contain the drive trains. Atrack is the continuous collection of shoes that transfers the motive force from the CT to the ground. The shoes are the structural contact between the CT and the ground. The rollers carry the transported weight to the shoes. The spacing between two of the 57 shoes on each track and the spacing between two of the 11 rollers on each track define the two harmonic families of the forcing functions of interest. Note that vehicle response can significantly increase when one of these forcing function harmonics created by the shoes and rollers is at or near one of the resonant frequencies of the transported vehicle. Previous work found that these forcing functions do not act as pure harmonics. The load paths through the trucks and rollers of the CT change over time during roll based on (as-yet) undetermined factors. Although harmonic frequencies of the forcing function may stay relatively constant at constant speed, other parameter changes (such as harmonic amplitudes and phase) make the forcing function difficult to model analytically. Each of the four trucks has a Jacking, Elevation, and Leveling (JEL) system and a guide tube system to properly support theCTchassis and the payload (launch platform and launch vehicle) as well as four electric track motors to provide motive force. The guide tube transfers all lateral loads from the CTchassis, the launch platform, and the launch vehicle into each of the four trucks. Figure 24.22 shows a schematic of a CTtruck with the tracks, track motors, and guide tube marked.
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