Preliminary work at Langley Research Center (LaRC) related to the design, analysis and testing of a space station scale model is reviewed. Included are some rationale for focusing the scale model program on space station and the utilization of the model to achieve the program objectives. In addition, some considerations involved in designing a dynamics scale model, such as ground test facilities, sub-scale component fabrication and model replication vs. simulation are presented. Finally, some related research areas currently ongoing at LaRC in support of scale model development are discussed

Mcgowan, Paul E.

English

NASA Technical Reports Server

N87-22711 :
215
https://ntrs.nasa.gov/search.jsp?R=19870013278 2020-03-20T10:34:08+00:00Z
INTRODUCTION
The paper reviews preliminary work at the Langley Research Center
(LaRC) related to the design, analysis and testing of a Space Station scale
model. Included are some rationale for focusing the scale model program on
Space Station and the utilization of the model to achieve the program
objectives. In addditlon, some considerations involved in designing a
dynamics scale model, such as ground test facilities, sub-scale component
fabrication and model replication vs. simulation are presented. Finally,
some related research areas currently ongoing at LaRC in support of scale
model development are discussed.
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LaRC SPACE STATION MODEL PROGRAM
The major objective of the LaRC scale model program is to develop
technology to improve our ability to predict the on-orbit structural
dynamics of large, flexible, multi-bodied, and articulated spacecraft. The
approach taken is to develop the technologies for using properly scaled
structural models to provide confidence for verification and control of full
scale structures which are too large to be tested in the earth's 1-g
environment. To this end, the project will fabricate a near replica scale
model of Space Station and conduct a comprehensive ground test/analysls
program to characterize the structural dynamics of the model. The resulting
analysis will then be used to predict the in-orblt behavior of the full
scale structure. Finally, via a correlation of the model results with
flight data obtained during on-orblt assembly and testing of Space Station,
verified analysis and ground test methodologies for other structutres of
this class will be developed.
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THECONTROL OF FLEXIBLE STRUCTURES (COFS) PROGRAM
The development of the scale model resides under the COFS III project
at LaRC. COFS Ill is the third in a series of three major projects under
the Control of Flexible Structures (COFS) Program. The COFS program
consists of a series of detailed ground and flight test/analysis projects on
a variety of large space strucrures. COFS Ill emphasizes multi-body
dynamics and control and focuses on a class of structures which are too
large to be properly tested beyond the component/subassembly level in the
earth's 1-g environment. Technology derived from COFS I and II wlll
transfer directly to COFS III, especially in the areas of developing testing
techniques and characterizing multi-jointed structures.
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MOTIVATION FOR SPACE STATION AS COFS III FOCUS
The Space Station is an excellent example of future generation multi-
bodied structures requiring validated structural dynamics and control
analyses, thus it serves as a natural focus for the COFS III project. A
scale model ground test/analysis program addresses the key technologies of
the COFS Program including development of verified analyses, ground test
methods, and spacecraft vibration suppression methods. Unlike COFS I and
II, COFS III does not contain an orbital test as part of the project;
however, the expected availability of flight test data from the full scale
station in orbit will allow the correlation between ground and flight tests
necessary to verify analyses. Furthermore, the scale model provides a means
for conducting technology development tests and examining the response of
evolutionary configurations and/or alternative component combinations prior
to flight.
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JUSTIFICATION FOR SCALE MODEL DYNAMIC GROUND TESTS
A major justification for ground tests of scale model hardware is the
reduction of the effects of gravity and size on full-scale ground tests.
For many large space structures, scale models offer the only opportunity to
achieve fully mated test data prior to flight, which could help uncover
potential problems or verify designs and mathematical models. Scale models
can also be useful in determining instrumentation requirements and optimum
placement on the full scale vehicle and for studying anticipated flight
maneuvers and investigating flight anomalies. Finally, as specifically
related to Space Station, the model provides a test bed for robotics
experiments and for studying potential growth configurations.
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APPROACH FOR OBTAINING QUALIFICATION OF MATH MODELS
An approach for obtaining verified analytical models is shown in the
adjoining chart. The scale models are used to develop methods for
correlating analyses with tests of model components and mated
configurations. The full scale hardware is limited to component and element
testing and perhaps flight tests of some hardware. Once the full scale
vehicle is in orbit, flight data can be used for the final adjustments
to the scale-model-verified analyses to obtain a fine-tuned representation
of the full-scale vehicle on-orbit behavior.
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COFS Ill GROUND TEST SCENARIO
The ground test scenario for the COFS III project includes three
levels. First, the scaling of elements such as structural joints and truss
members along with analytical characterization of these elements. Second,
major subassemblies of the full structure will be tested and systems
identification performed to identify the characteristics of those
subassemblies and make adjustments of analyses. The scale model will be
modular in design such that all anticipated full scale assembly scenarios
can be duplicated on the ground prior to flight. In addition, the scale
model will undergo the same types of tests anticipated for the full-scale
structure (i.e., static/dynamic tests of major subassemblies) in order to
obtain as many comparisons between model and full-scale test data as
possible. Third, fully-mated scale model ground tests will be performed on
both IOC and any growth versions.
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DYNAMIC MODEL SCALING
The optimum scale factor for a dynamics model is a trade-off between
how well the individual model components can be manufactured at sub-scale
and how large a model can be tested inside the available test facility.
Manufacturing limitations would tend to increase the model size (provides
lower bound) while simultaneously the limited size of conventional test
facilities tend to decrease the model size (provides upper bound). Another
major factor in dynamic model scaling is the identification of components
which require replication and those for which only require mass and inertia
simulation. For the COFS III model, components such as the joints, truss
members as well as interfaces between major subsystems or payloads and the
structure will likely require replication: however, model cost and
complexity can be reduced by simulating modules, major subsystems, and
payloads.
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EFFECT OF SCALE FACTOR ON MODEL FREQUENCY TO
PENDULUM FREQUENCY RATION
Pendulum suspensions on long cables are the most likely technology for
supporting models during tests, especially in complete system configura-
tions. Examining the effect of scale factor on the model-frequency-to-
suspension-pendulum-frequency ratio provides a means for identifying a
reasonable range for the scale factor. Shown on the adjoining chart is the
variation of frequency ratio with model scale factor for an initial and a
growth configuration of the scale model tested verticallly. The acceptable
design region is the upper right quadrant such that the scale factor is
above the assumed manufacturing limitation of .25 and the frequency ratio is
above the desired value of 5.
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ASSESSMENT OF SUSPENSION CABLES ON
SCALE MODEL TESTING
A key issue in the area of ground test methods was addressed in a study
of the interaction of the scale model with a candidate suspension system and
proposed test facility. Depicted in the adjoining chart are the 3 steps
used to study these effects. Note this study was initially performed on a
single keel configuration; however, the conclusions reached are expected to
be similar for the dual keel station. A first step is to suspend the model
from cables which connect directly to the top of the test facility (assumed
to be rigid). Next, a shadow structure concept is used whereby the model is
suspended from the rigid shadow structure, which covers the model planform,
and the shadow structure is in turn suspended from the test facility (also
assumed to be rigid). This concept provides maximum versatility in locating
cable attachment points. Finally, step 3 involves accounting for the
flexibility of the dome in the analysis procedure. Initial analyses have
shown the effects of the dome flexibility on the model dynamics to be
negligible, thus most analyses to date have focused around step 2.
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MODAL DENSITY OF I/3 SCALE SPACE STATION MODEL
A comparison between the modal density of a I/3 scale model analyzed in
a free-free condition and the same model analyzed as suspended in a pendulum
configuration is shown in the adjoining chart. The results of the supported
case represent a superposition of suspending the model both vertically and
horizontally in order to extract both in-plane and out-of-plane modes. The
free-free rigid body modes become pendulum modes in the supported case.
The natural frequencies of these models are well separated from those of the
structural modes, thus there were no interactions evident. Furthermore,
there were no significant changes in structural frequencies due to the
suspension system and all free-free modes were identifiable.
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RELATEDRESEARCHAREAS
The Structural Dynamics Branch, Structures and Dynamics Division, of
the Langley Research Center is involved in a variety of research areas
directly applicable to the scale model program. One such area is the study
of active member dampers which could be placed in the truss members of a
truss structure to provide vibration suppression. This is one concept
currently envisioned for introducing a vibration suppression mechanism into
the scale model. Another effort is an on-golng evaluation of In-house
capability to manufacture model components such as structural joints and
graphlte/epoxy tubular members. In addition, ground test methods for large
multl-jolnted structures are being developed by conducting tests (static and
dynamic) of prototype Space Station hardware.
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SPACE STATION SCALE MODEL RESEARCH HARDWARE
Some of the research hardware used in LaRC's in-house scale model
efforts are shown in the adjoining photo. Two seven-bay erectable truss
structural models have been built under contract, one at full scale (15-foot
bays) and one at I/4 scale. These models have been assembled in various
configurations to demonstrate the versatility of the erectable concept.
Currently these structures are undergoing testing at LaRC. These tests
include static tests of the joint components to characterize joint stiffness
and modal testing to characterize frequencies and mode shapes.
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SCALING OF GRAPHITE/EPOXY TUBES
In addition to fabricating subscale joint components, recent efforts
have focused on studying the manufacturability and scalabillty of
graphite/epoxy (gr/ep) tubes currently proposed for Space Station truss
members. The photo shows a series of gr/ep tubes fabricated at full (2 in.
O.D., .060 in. wall), I/2, I/3, and I/4 scales such that the aspect ratio of
each tube remains constant. The tubes will undergo a series of static and
dynamic tests to examine the degree of replication provided by the
manufacturing process at each scale.
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A summary of LaRC's plans for the development of a Space Station scale
model has been presented. The purpose of this program is to develop tech-
nology for better predicting the structural dynamics and control of large
multl-bodied spacecraft. The focus, Space Station, provides an opportunity
for comparing the ground test/analysis results with full-scale orbital data
which will be obtained for other purposes in the Space Station development
effort. Several issues which affect the scale factor determination were
discussed alone with research aimed at each of those issues.
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Considerations in the design and development of a space station scale model

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