As part of the effort to extend the diameter of three Deep Space Network large earth antennas from 64 meters to 70 meters, a slim profiled quadripod structure was designed to support a 7.7 meter diameter subreflector for the 70 meter antenna. The new quadripod design, which particularly emphasizes reduced radio frequency blockage, is achieved by means of a narrow cross sectional profile of the legs. Buckling analysis, using NASTRAN, was conducted in this study to verify the safety margin for the quadripod structural stability

Chian, Chian T.

English

NASA Technical Reports Server

N87-27245
BUCKLING ANALYSIS OF THE QUADRIPOD
STRUCTURE FOR THE NASA 70-METER ANTENNA
CHIAN T. CHIAN
Jet Propulsion Laboratory, Pasadena, CA
SUMMARY
The diameter extension of the three NASA DSN (Deep Space Network)
large Earth-based antennas from the existing 64 meters to 70
meters will provide a needed increase in space communication
capability for the Voyager 2 - Neptune encounter in August 1989.
As part of the upgrade effort, a slim-profiled quadripod
structure was designed to support the 7.7-m-diameter subreflector
for the 70m antenna. The new quadripod design, which
particularly emphasizes reduced radio-frequency (RF) blockage, is
achieved by means of a narrow cross-sectional profile of the
legs.
Buckling analysis, using NASTRAN, was conducted in this study to
verify the safety margin for the quadripod structural stability.
INTRODUCTION
The upgrade of the three NASA/JPL 64m diameter antennas will
provide a needed increase in Earth-based space communication
capability at all three Deep Space Communications Complexes:
Goldstone, California (DSS-14)I Canberra, Australia (DSS-43); and
Madrid, Spain (DSS-63). In addition to the increase of the
antenna aperture area from 64m to 70m, a number of significant
improvements in the quadripod, surface panels, subreflector
positioner, and microwave aspects are included in the design.
The upgrade objective is to increase the radio-frequency (RF)
gain/noise temperature (G/T) by about 1.9 dB at X-band (8.q5
GHz).
As part of the upgrade effort, a new, high-precision 7.7-m
(25.4 ft) diameter subreflector and positioning mechanism are
needed. Consequently, an entirely new quadripod structure is
required to support the subreflector. The new quadripod design
particularly emphasizes reduced RF blockage, which is achieved by
means of a narrow cross-sectional profile of the legs. The
profile adopted provides about 0.32 dB of gain improvement in
comparison with the existing 64 meter design (Ref. I). This
report addresses the stability analyses performed on the new
quadripod design to ensure that it has an adequate safety margin
for buckling and that the minimum natural frequency is compatible
214
https://ntrs.nasa.gov/search.jsp?R=19870017812 2020-03-20T09:38:39+00:00Z
with control system requirements.
QUADRIPOD DESIGN
The quadripod assembly is a tabular space-frame steel structure
with four trapezoidally shaped legs connected to another large
space frame at the apex, as shown in Figs. I and 2. The four
legs are supported at the corner points of the rectangular truss
system of the main reflector structure as shown in Flg. 2. The
final sllm profile leg cross-section envelope selected is shown
in Fig. 3. Also, the quadripod will be used occasionally for
hoisting the cassegrain feed cones, the subreflector, or other
heavy equipment which may be removed and reinstalled.
The finite element model of the 70m quadripod truss structure is
a pin-jolned frame (3 translational degrees of freedom per node)
comprising 156 nodes, 445 axial bars, and 28 m_mbrane plates.
The JPL/IDEAS (Iterative Design of Antenna Structures) computer
program was used for analysis and design (Ref. 2). The program
employs the optimality criterion to minimize the structural
weight (objective function) with a constraint placed on the
lowest natural frequency. A subsequent analysis of the 70m
model, accounting for bending and torlsonal stiffness at the
Joints (6 degrees of freedom per node) using NASTRAN (Ref. 3),
showed only a small increase in the torsional natural frequency
(Ref. 1). Outrigger braces (Fig. 2) were added thereafter to
increase the lowest natural frequency.
Due to the slimness of the quadrlpod legs, the followlng
requirements had to be considered:
Static buckling stability: The occasional use of the quadripod
as a derrick required a check on the possibility of buckling
instability. A factor of safety of at least 1.5 was recommended.
The smallest elgenvalue found from a structural buckling analysis
is equivalent to this factor of safety. Since the IDEAS program
that was used for design and natural frequency analyses does not
perform buckling analysis, the new quadrlpod design was optimized
for the frequency requirement using IDEAS and then analyzed for
buckling using NASTRAN (NASA Structural Analysis Program).
NASTRAN was used to determine the buckling loads of the natural
frequency-constralned quadrlpod design. Two versions of the
NASTRAN program were used because of possible different finite
element formulations: the NASTRAN-COSMIC (NASA's Computer
Software Management and Information Center) (Ref. q) and the
proprietary NASTRAN-MSC version (MacNeal-Schwendler Corporation)
(Ref. 3). The two versions were used both for the buckling and
natural frequency analyses, and the results obtained were
compared.
215
RESULTS OF BUCKLING ANALYSIS
The Rigid Format No. 5 of the NASTRAN program was used to perform
the buckling analysis. The results of the buckling analysis for
the quadripod pin-joined model are presented in Table I. Both
the COSMIC and MSC versions of the NASTRAN program were used and
compared. Four antenna configurations, each subject to the
maximum loads anticipated to be hoisted when employing the
quadripod as a derrick, in addition to the quadripod weight, were
considered in the quadripod buckling analysis:
(1) Zenith look with outriggers.
(2) Zenith look without outriggers.
(3) Horizon look with outriggers.
(4) Horizon look without outriggers,
Table I shows that the outriggers tend to at least double the
buckling load capability.
FINITE ELEMENT PLATE STIFFNESS REPRESENTATION
Comparisons of the COSMIC-NASTRAN and MSC-NASTRAN results on the
quadripod model in Table I show that the plate element CQDMEM2 in
the COSMIC version gives a different stiffness matrix
representation compared with the CQUAD4 plate element in the MSC
version, or with the IDEAS plate element CQDMEM.
A parametric study was conducted to readjust the moduli of
elasticity of the COSMIC CQDMEM2 plate elements to produce
results similar to those of the MSC elements. Comparison of the
results of the stiffness parameterization study is shown in Table
2 for the quadrlpod buckling analysis. The plate element used in
the NASTRAN-COSMIC employs the constant stress formulation, while
the elements used in the NASTRAN-MSC or IDEAS permit a stress
variation. As a result, the COSMIC element generates a stiffer
structure than the other elements.
CONCLUSIONS
A structural stability study was conducted for the 70m antenna
quadripod. The quadripod was found to be stable in buckling when
the outrigger braces were included. One computer program used in
the investigation was found to give an over-estimate of the
stiffness. In order to correct the excessive stiffness, a
parametric study was conducted to derive empirical coefficients
to adjust the plate stiffness for future use of this program.
216
I •
2•
.
•
REFERENCES
Cucchlssi, J. J., "A New 70-Meter Antenna Quadripod With
Reduced RF Blockage," TDA Progress Report 42-82, pp. 24-30,
Jet Propulsion Laboratory, Pasadena, CA, Aug. 15, 1985.
Levy, R., and Chal, K., "Implementatlon of Natural Frequency
Analysis and Optimality Criterion Design," Computers and
Structures, Vol. 10, pp. 277-282, Pergamon Press Ltd•, Great
Britain, 1979.
McCormick, C. W., Editor, MSC/NASTRAN User's Manual, MSR 39,
The MacNeal-Schwendler Corporation, 1983.
The NASTRAN Theoretical Manual (level 15.5), NASA SP-221
(06), Scientific and Technical Information Office, NASA,
Wash. DC, Jan. 1981•
ACKNOWLEDGEMENT
The author acknowledges the assistance given by his JPL
colleagues, Dr. R. Levy and J. Cucchissi, during the various
execution steps of this work.
217
Case
Table ]. The quadripod buckling analysis results
Antenna configuration
NASTRAN NASTRAN
COSMIC MSC
version version
Zenith look, with outriggers 22.81 12.27
Zenith look, without outriggers 9.23 (not run)
Horizon look, with ouui_ers 12.87 3.77
Horizon look, without ouUiggers 4.12 1.66
Table 2 Comparison of the plate element stiffness and the smallest elganvalues for the
qusdripod buckling analysis
Young's modulus, Shear modulus,
Program Plate element N/m 2 N/rn 2 Ami n
(psi) (psi)
(s) Zenith Look Antenna Configuration, with Outrigger Braces:
NASTRAN-MSC CQUAD4 20.0 × 10 ! 0
(29.0 × 106)
NASTRAN-COSMIC CQDMEM2 10.7 × 101°
(15.5 × 106)
(b) Horizon Look Antenna Configuration, with Ouuigger Braces:
NASTRAN-MSC CQUAD4 20.0 × 1010
(29.0 × 10 e)
NASTRAN-COSMIC CQDMEM2 5.2 × 10 I°
O.S x 10 6)
8.3 x 101°
(12.0 x 10 6)
4.4 x 101°
(6.4 x 10 e)
8.3 x 101°
(12.0 x 10 6)
2.2x 101°
(3.2 x 10 6)
12.27
12.30
3.77
3.97
218
REFLECTOR
Fig. .
SURFACE
SUBREFLECTOR
Antenna quadripod and reflector system
219
QUADRIPOD
OUTRIGGE
OUTRIGGER$
SECTION
A-A
Fig. 2. Plan view of 70-m qu_:lrlpod with outriggers
220
70-m
/ /---84.rn
I"- 27.9 cm / /
45.7¢m _ .J. I 508_ ,1.4_'
,,8"_ t_._____ J.. - ..L / _ (20) (as..)
- 243.8 cm (96")-- -- I
Fig. 3. Ouadrlpod cross-sectional profile
221


Buckling analysis of the quadripod structure for the NASA 70-meter antenna

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server