Operational Analysis in the Launch Environment

Abstract

The launch environment is a challenging regime to work due to changing system dynamics, changing environmental loading, joint compression loads that cannot be easily applied on the ground, and control effects. Operational testing is one of the few feasible approaches to capture system level dynamics since ground testing cannot reproduce all of these conditions easily. However, the most successful applications of Operational Modal Testing involve systems with good stationarity and long data acquisition times. This paper covers an ongoing effort to understand the launch environment and the utility of current operational modal tools. This work is expected to produce a collection of operational tools that can be applied to non-stationary launch environment, experience dealing with launch data, and an expanding database of flight parameters such as damping. This paper reports on recent efforts to build a software framework for the data processing utilizing existing and specialty tools; understand the limits of current tools; assess a wider variety of current tools; and expand the experience with additional datasets as well as to begin to address issues raised in earlier launch analysis studies. INTRODUCTIONThe spacecraft launch environment is a highly complex and non-stationary event that is characterized by high amplitude input forces, highly variable loads, a wide spectrum of responses, constantly changing vehicle mass, active control interactions, staging, and limited instrumentation. At the same time, structural response analyses and loads estimations must be performed with models that are only partially validated using ground test data due to the fact that access to diagnostic and environmental ground tests are limited. To compound matters, projects are tending to use reduced uncertainty factors designed to protect for loads increases and model unknowns. As a result, the designs progress rapidly before loads and structural problems are uncovered. This means that there are very few tools available to recover from structural dynamics issues in such a dynamic environment without costly redesigns late in the design cycle or in early operations.

Progress in Operational Analysis of Launch Vehicles in Nonstationary Flight

“…The analysis of several recent data sets has shown that one effect (at least on the correlation-based processing approaches) is a "beating" or "blooming" phenomena which limits the amount of the correlation functions that can be used for processing. 30,35,39 Although the first low-lag points in the correlation functions are relatively unaffected, the higher-lag time data points are relatively useless for analysis. Figure 1 illustrates the beating phenomena as illustrated by a simple one DOF analytical model of a 10 Hz mode with 1% damping excited by random white noise using Newmark-Beta integration.…”

“…Hence, this approach needs a convergence metric to allow the analyst to know when to stop performing additional correlations. 30 A metric was presented in reference [38] and will be repeated here. However the ongoing effort has discovered that there are numerical issues with the recursive correlation approach that need to be understood as validation of the technique is proving elusive.…”

“…If the loading and system characteristics are fairly constant over each window, then estimates of the changing parameters can be obtained as a function of flight time. 28,29,30,31,32,33,34,35 Three of the references listed above show that one trajectory for NExT has been to act as a pathfinder for the development of operational analysis techniques to process launch data. 28,29,30 This effort is doubly challenging as modal damping is one of the most sought after parameters from the launch environment, which is difficult to extract even in well controlled stationary environments.…”

“…28,29,30,31,32,33,34,35 Three of the references listed above show that one trajectory for NExT has been to act as a pathfinder for the development of operational analysis techniques to process launch data. 28,29,30 This effort is doubly challenging as modal damping is one of the most sought after parameters from the launch environment, which is difficult to extract even in well controlled stationary environments. However, flight damping during launch becomes a critical part of the discussions during the design and operations of space vehicles due to the control over the response of structures and components.…”

“…Reference [30] is a recent study performed on the PA-1 test flight. This test flight provided a very challenging data set with a very short flight time and extremely rapidly changing modal frequencies.…”