The design, construction, and operation of a low-Earth orbit space station poses challenges for development and implementation of technology. One specific challenge is the development of a dynamics test program for defining the space station design requirements, and identifying and characterizing phenomena affecting the space station's design and development. The test proposal, as outlined, is a comprehensive structural dynamics program to be launched in support of the space station (SS). Development of a parametric data base and verification of the mathematical models and analytical analysis tools necessary for engineering support of the station's design, construction, and operation provide the impetus for the dynamics test program. The four test phases planned are discussed: testing of SS applicable structural concepts; testing of SS prototypes; testing of actual SS structural hardware; and on-orbit testing of SS construction

Bugg, Frank M.

Ivey, E. W.

Moore, C. J.

Townsend, John S.

English

NASA Technical Reports Server

N87-22751
SPACE STATION STRUCTURES AND DYNAMICS TEST PROGRAM
Bugg, Ivey, Moore, and Townsend
Marshall Space Flight Center
April 1986
PRECEDING PAGE, BLANK, NOT FILMED
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https://ntrs.nasa.gov/search.jsp?R=19870013318 2020-03-20T10:33:06+00:00Z
INTRODUCTION
The design, construction, and operation of a low-earth
orbit Space station poses unique challenges for developement
and implementation of new technology. The technology arises
from the special requirement that the Station be built and
constructed to function in a weightless environment, where
static loads are minimal and secondary to system dynamics and
control problems. One specific challenge confronting NASA
is the devopment of a dynamics test program for (1) defining
Space Station design requirements, and (2) identifying and
charaterizing phenomena affecting the Station's design and
developement. A general definition of the Space Station
dynamic test program, as proposed by MSFC, forms the subject
of this report.
The test proposal, as outlined herein, is a comprehensive
structural dynamics program to be launched in support of the
Space Station. The test program will help to define the key
issues and/or problems inherent to large space structure
analysis, design, and testing. Development of a parametric
data base and verification of the math models and analytical
analysis tools necessary for engineering support of the Station's
design, construction, and operation provide the impetus for the
dynamics test program. Four test phases are planned:
1) Phase I - Testing of Space Station Applicable
Structural Concepts
2) Phase II - Testing of Space Station Prototypes
3) Phase III- Testing of Actual Space Station
Structural Hardware
4) Phase IV - On-Orbit Testing of Space Station
Construction
The philosopy being to integrate dynamics into the design
phase through extensive ground testing and analytical ground
simulations of generic systems, prototype elements, and sub-
assemblies. On-orbit testing of the Station will also be used
to define its capability.
PHASE I - TESTING OF SPACE STATION APPLICABLE STRUCTURAL CONCEPTS
MSFC recognized the need for static and dynamic testing of
Large Space Structures applicable to Space Station design at an
early stage. The purpose of these tests was to gain insight into
the pecularities and requirements of not only the support fix-
tures, modal excitation, and data acquisition system, but also
for the investigation into the static and dynamic behavior of
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truss structures exhibiting linear and nonlinear characteristics.
Tasks have already begun to perform additional testing, modeling,
and analyzing of these type structures. Some of the Large Space
Structures that have been tested or are in the process of being
tested are:
1. 46' Rockwell deployable truss
2. 50'-30" diameter super astromast
3. 20'-30" diameter super astromast
4. SASP truss
5. 16_ meter Grumman beam
46 FOOT ROCKWELL DEPLOYABLE TRUSS TEST - The Rockwell truss is
a box structure with telescoping diagonal joints with a housing
frame assembly at one end as shown in Fig. 1. The truss has
joints at each corner of the 10 bays where the diagonal battens
and longerons meet. These joints contain a certain amount of
free-play and hence the structures will exhibit nonlinearlty with
varying stiffness and damping. To reduce the amount of free-play,
cables paraiiel to each of the longerons through _ joint
be pretensloned up to 400 pounds each.
A static and dynamic test of this truss has been planned and
is currently in progress. The truss will be tested in the hori-
zontal with housing assembly at the free end. The bays will be
supported in the vertical such that the loading at the joints
will be reduced as much as possible. Static tests will be run
laterally, axially, and torsionally. Deflection curves, with
incremental loading of the four cables from 0-400 pounds, will be
...a.=+=A to characterize the ]inearity/nonlinearitv of the truss.
After completion of the static tests, a modal survey test
will be run to characterize the modes of the truss. Frequencies
and damping will be determined for various preloads in the cables
and with varying amplitudes to assess the nonlinearity effects.
These dynamic tests of the truss will be for both cantilevered and
free-free boundary conditions.
20-FOOT 30-INCH DIAMETER SUPER ASTRONAST TEST - The Astro-Research
Corporation astromast consists of three continuous glass longerons
with glass/epoxy batterns and diagonals. A finite element model
of the truss structure and a test setup schematic are shown in
Figures 2 and 3. Each end of the truss is attached to a 36-inch
thick aluminum plate. The astromast is supported in the vertical
by an overhanging beam/plate which is bolted to the top plate of
the astromast.
The joint test performed was dynamic, with the truss sup-
ported vertically. A single point random testing technique was
used to calculate the bending, torsional, and axial modes of
vibration. Good agreement between the analysis and test was
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obtained except for the axial mode (see Table i). The predicted
frequency of 59.8 Hz did not agree with the test frequency of
37.7 Hz. After considerable analysis and some additional testing,
it was found that the i/2-inch aluminum plate at the free end,
where the shaker was attached, had an inplane frequency of 37.7
Hz. As a result, the test personal were unable to shake out the
higher first axial mode. A 2-inch plate was subsequently bolted
to the i/2-inch plate and the test was repeated. For this con-
figuration, the first axial mode was easily obtained and compared
well with analysis.
The static test followed the dynamic testing. Using the
same configurations and boundary conditions, as in the dynamic
test, axial and lateral static tests were conducted. For the
axial test, loads up to 1200 pounds in increments of 200 pounds
were applied in the center of the i/2-inch plate attached to the
free end. Several interesting results arose from this test:
a) The three longerons which ideally carried the total
axial load in fact did not. The load in the longer-
ons was also unsymmetric which caused seemingly ran-
dom deflection patterns as shown in Fig. 4. This was
caused by the following factors: i) bowing of the
bottom base plate introduced loads in the diagonals,
2) small off-set c.g. loading of the plate, 3) bend-
ing of the truss support beam, and 4) deflections
observed between the I/2-inch aluminum plate and the
support beam/plate (A random bolt pattern was used for
the attachment of the top plate to the support plate
on the beam).
b) Due to the above problems, the stiffness of the long-
erons were recalculated by determining the load and
deflections in each longeron separately. The results
are plotted in Fig. 5. Although this technique
required additional instrumentation, the findings
are conclusive.
For the horizontal test, loads up to i00 pounds were applied in
20- pound increments. The bending of the upper beam support was
again a factor and resulted in a 25 percent larger deflection
than predicted.
For future testing of truss support structures, having proper
boundary conditions and understanding these boundaries is a must.
Static tests should always precede dynamics test. It is also
recomended that the diagonals and longerons be instrumented with
strain gages to better understand the load path.
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PHASE II - TESTING OF SPACE STATION PROTOTYPES
The second phase of the test program will address the
structural development and testing of candidate Space Station
prototypes and concepts. While Phase I characterized and ver-
ified pre-Space Station structures and math modeling techniques,
Phase II will characterize specific Space Station structural
concepts. For example, the following structural investigation
is anticipated.
1) Static and dynamic analysis and testing of Space
Station erectable and deployable prototype hardware.
2) Static and dynamic analysis and testing of truss mem-
bers and joint concepts.
3) Experimental and analytical characterization of the
truss member buckling phenomenon.
4) Investigation into the applied usage of composite
materials for Space Station joint and truss design.
s) Experimental and analytical simulation of Space Station
dynamic events such as construction, payload handling,
berthing, docking, etc.
The objectives of the phase two test program are as follows:
1) Continued investigation and characterization of non-
linear phenomena on prototype concepts and hardware.
For example, define nonlinear load deflection curves
and evaluate the influence of structural preloads on
these curves.
2) Characterize the amount and types of damping inherent
to these structures and investigate methods of increas-
ing damping.
3) Access the effects of nonlinear stiffness and damping
on the systems dynamic response in the zero-g environ-
ment.
4) Develop and verify with testing nonlinear dynamic
structural math models which are suitable for control
system studies and evaluation.
5) Develop and verify isolation systems and mechanisms
for the low frequency zero-g environment.
1329
6)
7)
Develop and pursue dynamic simulation of construction
and other dynamic events, which will be encountered in
the Space Station working environment. This ground
based analysis and experimental simulation capability
will verify procedures and loads and develop potential
ground capability for mission support.
Develop analysis methods and construction methods
for the use of composite materials in truss structure
joints and members. (It should be noted that the
current state of the art for composite structural
analysis is based on the Plane Stress assumption,
which is not valid for truss joint design. Also, the
nature of truss structures is that they produce large
concentrated loads. Current composite material con-
struction methods deal well with distributed loads,
but a concentrated loads is at this point in time the
Achilles' heel of composites.)
PHASE III- TESTING OF ACTUAL SPACE STATION STRUCTURAL HARDWARE
Hardware verification is Phase III of the Space Station
test program. The objective being the characterization of
flight hardware, verification of math models, and anomaly
identification. Three test classifications are envisioned:
Structural Elements - testing of truss elements,
for example, short rods, diagonal rods, and
joint clusters.
Truss Subassemblies - testing of one bay of the
truss structure, five bays, and power boom/keel
intersection.
Large Assemblies - testing of keels/end booms/partial
power boom and nested common modules with interface
structure.
The verification phase of the test program must proceed in
stages, as outlined. Since the truss structure is to be made
up of multiple elements of the same design, material stiffness
variations are expected. Variations in joint combinations for
freeplay will also influence modal stiffness. Detail testing,
static and dynamic, of individual structural elements and truss
assemblies will provide the database necessary for insuring
modal/model predictability of the Space Station configuration.
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PHASE IV - ON-ORBIT TESTING OF SPACE STATION CONSTRUCTION
Phase IV of the test program addresses on-orbit verifica-
tion and control of structural design, space construction,
ground based math modeling, growth evolution, and structural
damage and deterioation. The Space Station is unique in that
it will only be totally assembled on-orbit with no possible
ground experimental verification for its zero-g environment.
NASA's experience with on-orbit structural dynamic testing
began with SAFE (Solar Array Flight Experiment). This experiment
showed a larger than anticipated damping and also demonstrated
unexpected thermal effects. NASA's experience with the erection
of space structures began with EASE/ACCESS (Experimental Assembly
of Structures in Extravehicular Activity/Assembly Concept for
Construction of Erectable Space Structures). Although the EASE/
ACCESS mission demonstrated space construction, it did not attempt
to do any structural testing. NASA'S past experience with on-
orbit test and construction points the way for future effort.
The first step in on-orbit testing of Space Station appli-
cable structures is at this time envisioned to be a demonstration
of onorbit eractable construction of a 5-meter-square cross section
truss beam. The 5-meter construction experiment would be tested
on-orbit for its dynamic response in the zero-g environment. This
testing would allow determination of the Space Station structural
damping and the effects of joints in the zero-g environment.
Successful completion of the large construction and testing
experiment would allow the initial construction of the Space
Station. On-orbit testing would follow a natural progression at
the end of each construction phase of the Space Station. The
purpose of these tests will be to verify space structural assembly
and assembly techniques, and provied basis for verification and
revsion of ground based math models. These verified and revised
math models will provide a basis for updated control system soft-
ware and gains.
An on-orbit structural verification and acceptance test will
be performed on the Space Station upon completion of the main
structural assembly. This testing will be performed prior to the
addition of payloads. Also, as the Space Station grows there will
be continued on-orbit structural dynamic testing to verify the
system capability and to access structural damage over the life of
the station. This continued testing will allow verification and
update of ground based math models and revision of control systems.
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Space station structures and dynamics test program

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