Jet Propulsion Laboratory

Jet Propulsion Laboratory (JPL) began as an informal group of students and staff from the California Institute of Technology (Caltech) who experimented with rockets before and during World War II; evolved afterward into the Nation’s center for unpiloted exploration of the solar system and deep space, operating related tracking, and data acquisition systems; and was managed for NASA by Caltech.[890] Dr. Theodore von Karman, then head of Caltech’s Guggenheim Aeronautical Laboratory, shepherded this group to becoming a center of rocket research for the Army. Upon NASA’s formation in 1958, JPL came under NASA’s responsibility.[891]

Consistent with its origins and Caltech’s continuing role in its man­agement, JPL’s orientation has always emphasized advanced experimen­tal and analytical research in various disciplines, including structures. JPL developed efficiency improvements for NASTRAN as early as 1971.[892] Other JPL research included basic finite element techniques, high-veloc­ity impact effects, effect of spin on structural dynamics, geometrically nonlinear structures (i. e., structures that deflect sufficiently to signifi­cantly alter the structural properties), rocket engine structural dynam­ics, flexible manipulators, system identification, random processes, and optimization. The most notable of these are VISCEL, TEXLESP-S, and PID (AU-FREDI and MODE-ID).[893]

VISCEL (for Visco-Elastic and Hyperelastic Structures) and TEXLESP-S treat special classes of materials that general-purpose finite element codes typically cannot handle. VISCEL treats visco-elastic prob­lems, in which materials exhibit viscosity (normally a fluid characteris­tic) as well as elasticity. VISCEL was introduced in 1971 and was adapted by industry over the next decade.[894] In 1982, the Shell Oil Company used VISCEL to validate a proprietary code that was in development for the design of plastic products.[895] In 1984, AiResearch was using VISCEL to analyze seals and similar components in aircraft auxiliary power units (APUs).[896]

JPL has been leading research in the structural dynamics of solid rockets almost since the laboratory was first established. TEXLESP-S was specifically developed for analysis of solid rocket fuels, which may be polymeric materials exhibiting such hyperelastic behavior. TEXLESP-S is a finite element code developed for large-strain (hyperelastic) prob­lems, in which materials may be purely elastic but exhibit such large strain deformations that the geometric configuration of the structure is significantly altered. (This is distinct from the small-strain, large – deflection situations that can occur, for example, with long flexible booms on spacecraft.)[897]

System Identification/Parameter Identification (PID, including AU-FREDI and MODE-ID) is the use of empirical data to build or tune a mathematical model of a system. PID is used in many disciplines, including automatic control, flight-testing, and structural analysis.[898] Ideally, excitation of the system is performed by systematically exciting specific modes. However, such controlled excitation is not always prac­tical, and even under the best of circumstances, there is some uncer­tainty in the interpretation of the data. The MODE-ID program was developed in 1988 to estimate not only the modal parameters of a struc­ture, but also the level of uncertainty with which those parameters have been estimated:

Such a methodology is presented which allows the precision of the estimates of the model parameters to be computed.

It also leads to a guiding principle in applications. Namely, when selecting a single model from a given class of models, one should take the most probable model in the class based on the experimental data. Practical applications of this prin­ciple are given which are based on the utilization of measured seismic motions in large civil structures. Examples include the application of a computer program MODE-ID to identify modal properties directly from seismic excitation and response time histories from a nine-story steel-frame building at JPL and from a freeway overpass bridge.[899]

Another system identification program, Autonomous Frequency Domain Identification (AU-FREDI), was developed for the identification of structural dynamic parameters and the development of control laws for large and/or flexible space structures. It was furthermore intended to be used for online design and tuning of robust controllers, i. e., to develop control laws real time, although it could be modified for offline use as well. AU-FREDI was developed in 1989, validated in the Caltech/ Jet Propulsion Laboratory’s Large Spacecraft Control Laboratory and made publicly available.[900] This is just a small sample of the research that JPL has conducted and sponsored in system identification, control of flexible structures, integrated control/structural design, and related fields. While intended primarily for space structures, this research also has relevance for medicine, manufacturing technology, and the design and construction of large, ground-based structures.