The 5-meter technology antenna program demonstrated the overall feasibility of integrating a mesh reflector surface with a deployable truss structure to achieve a precision surface contour compatible with future, high-performance antenna requirements. Specifically, the program demonstrated: the feasibility of fabricating a precision, edge-mounted, deployable, tetrahedral truss structure; the feasibility of adjusting a truss-supported mesh reflector contour to a surface error less than 10 mils rms; and good RF test performance, which correlated well with analytical predictions. Further analysis and testing (including flight testing) programs are needed to fully verify all the technology issues, including structural dynamics, thermodynamics, control, and on-orbit RF performance, which are associated with large, deployable, truss antenna structures

Dudeck, M. P.

Dyer, J. E.

English

NASA Technical Reports Server

N87- 16021
DEPLOYABLE TRUSS STRUCTURE
ADVANCED TECHNOLOGY
J.E. Dyer and M.P. Dudeck
General Dynamics Space Systems Division
San Diego, California
First NASA/DoD CSI Technology Conference
November 18-21, 1986
_F.C.X_|N_ P_l_ BLANK NOT FILMED
111
https://ntrs.nasa.gov/search.jsp?R=19870006588 2020-03-20T12:05:11+00:00Z
FUTURE SPACE ANTENNA REQUIREMENTS
DRIVE DEPLOYABLE STRUCTURE TECHNOLOGY
Deployable space structures can be divided into three classes: linear support members, platforms, and antennas. Although
each class has unique technology issues, future large, space antenna systems, with severe stiffness, stability, weight, and
packaging requirements, are the major drivers of deployable structures technology (Figure 1).
Commercial and military communication, radiometric, and surveillance satellites increasingly require reflector antennas
with RF apertures 5 meters or larger to satisfy future mission requirements. Due to launch vehicle payload envelope con-
straints, reflectors that can be deployed in space are required for the 5-meter or larger aperture antenna.
Typically, deployable reflectors use a flexible RF reflective surface supported by a deployable back structure. Deviation
of a reflector surface from that of an ideal paraboloid results in degradation of its radiation pattern. Several factors contribute
to on-orbit surface error, including surface geometry approximations, fabrication and adjustment tolerances, thermal and
dynamic distortion due to the orbital environment, gravity effects, and deployment repeatability.
Current large, deployable RF reflectors are typically designed to operate at low frequencies where large surface errors can
be tolerated without significant performance loss. However, the trend of future advanced space applications is toward the
utilization of frequencies above 14 GHz where sidelobe degradation and gain loss due to surface error can severely penalize
antenna performance. To meet future requirements, the large deployable reflector will be required to achieve on-orbit surface
error approaching that of a solid-surface reflector (3-10 mils rms).
In addition, requirements to maximize gain performance and radiation pattern purity are dictating the use of offset
reflector geometry to eliminate blockage by the feed array and support structure. Collectively, these requirements drive future
antenna systems toward high stiffness and thermal stability. These characteristics are best provided by deployable composite
truss structures.
ANTENNA SYSTEM REQUIREMENTS
• LARGE APERTURE
• HIGH OPERATING FREQUENCY
• HIGH RF PERFORMANCE
STRUCTURE REQUIREMENTS
• HIGH STIFFNESS
• THERMAL STABILITY
• HIGH SURFACE ACCURACY
• OFFSET GEOMETRY
Figure 1.
TECHNOLOGY
LARGE, PRECISION,
DEPLOYABLE STRUCTURES
667o-1
112
DEPLOYABLE TRUSS STRUCTURES FOR FUTURE SPACE ANTENNAS
The deployable truss structure, when integrated with a precisely contoured mesh reflective surface, possesses the features
needed to meet future space antenna requirements. Truss characteristics shown in Figure 2 that provide enhanced perform-
ance capabilities include:
• High Specific Stiffness -- High inherent truss stiffness permits structure and control system frequency separation and
minimizes active control needed to suppress structural vibrations excited by slewing torques and other dynamic distur-
bances. High stiffness also reduces deflections and reflector surface contour errors due to gravity effects during manufac-
ture, adjustment, and ground test.
• Low Thermal Deformations -- Low thermal deformations result from the use of simple axial truss structural members in
an array manufactured from low-expansion, advanced composite materials.
• Good Packaging Efficiency -- Folding tetrahedral building blocks provide an efficient structural package. This repeating
modular truss design provides versatility by permitting simple scaling to accommodate varying geometric requirements.
• Edge-Mount Capability -- For offset reflector applications, the tetrahedral truss reflector and feed structure are both
deployed from one, simple edge mount thereby simplifying deployment, reducing weight, and improving system stiffness.
]
FEATURES
I DEEP 3-D GRAPHITE/EPOXYTRUSS SUPPORT STRUCTURE
I R,.NPACKA_,.OEFF,C,ENCYI I .,_xSTRUCTURALEFF,C,ENCYI I LowTRERMALCOEFF,C,ENTSI
1
CUSTOMER
BENEFITS ! SMALL STOWED VOLUME [2_ ,
I LOWREFLECTORWE,"NTI I LOWDEFLECT,ONSIi IGH NATURAL FREQUENCY J
IEOOEFEEOSUPPORTI
'I'
I LOWREFLECTORMOMENTI
OF INERTIA J
CONT. SYST. BW; REDUCED
REQM'T FOR MOTOR TORQUE
IN SCAN
I NoDLOCNA_EI I LOWSURFACECONTOURD,STORT,ONI
I REOUCEDSCATTERI .IC.OA,NI
;j .....
SIDE LOBE REQUIRED; ELECTRICAL
SUPPRESSION IMPROVED POWER SYSTEM
SENSITIVITY
i, ,I,
IMPROVED S/C PERFORMANCE 6670.2
Figure 2.
113
TECHNOLOGY DEMONSTRATION ANTENNA
A 5-meter antenna (Figure 3) was recently fabricated by General Dynamics Space Systems to demonstrate precision deploy-
able, tetrahedral truss reflector technology. Design requirements representative of a space-borne communications antenna
operating over a frequency range up to 30 GHz included a 5-meter projected aperture with an F/D ratio of 1.3, an offset of
2.13 meters and manufacturing rss surface tolerance of 7.7 mils, and total on-orbit rss surface tolerance of 11 mils.
On-orbit errors are created by thermal, dynamic hygroscopic, and visco-elastic effects. Thermal distortion is minimized
by setting an overall thermal expansion coefficient goal (near zero), and then designing the individual structural elements to
meet this overall goal.
The deployable truss was fabricated from graphite/epoxy composites for high stiffness and low thermal expansion. The
reflective surface was fabricated from gold-plated molybdenum wire knit into a tight mesh for low transmission losses.
2.13 METERS
J_
VERTEX
PARENT
PARABOLOID
5 METER
PROJECTED
DIAMETER
FOCUS
6.5 METERS
• DEPLOYABLE GRAPHITE/EPOXY
TRUSS STRUCTURE
• PRECISION MESH SURFACE
• EDGE MOUNT
ORN
ARRAY
Figure 3.
UPPER SURFACE
STRUTS (TYP)
MESH ',
(TYP) i
\ i
i ,,"
r k
\
5.6M (18.4
LOWER SURFACE
STRUTS (TYP)
i
FT)
ERROR BUDGET FOR 5M DEPLOYABLE REFLECTOR
RMS SURFACE ERROR
ERROR SOURCE (MM) MIL
FABRICATION SOURCES
PROBE POSITION 0.054 (2.10)
PROBE GEOMETRY 0.064 (2.50)
MESH MEASUREMENT 0.051 (2.00)
MESH ADJUSTMENT 0.064 (2.50)
GRAVITY PREDICTION 0.051 (2.00)
DEPLOYMENT REPEATABILITY 0.076 (3.00)
SURFACE APPROXIMATIONS 0.114 (4.50)
ALIGNMENT 0.051 (2.00)
RSS TOTAL 0.194 (7.63)
ON-ORBIT SOURCES
THERMAL DISTORTION 0.0787 (3.10)
DYNAMIC DISTORTION 0.0025 (0.10)
HYGROSCOPIC EFFECTS 0.0051 (0.20)
VISCOELASTIC CREEP 0.0127 (0.50)
RSS TOTAL 0.0800 (3.15)
TOTAL 0.255 (10.87) 667o-3
114
TRUSS STRUCTURE DESIGN
The truss structure design was based on the deployable tetrahedral truss configuration with geometry and members selected to
meet stiffness and surface precision requirements. A computerized tetrahedral truss analysis program (Figure 4) was used to
generate the complete three-dimensional truss geometry and define the required truss element properties.
All structural elements, including tubular truss members and node fittings, used graphite/epoxy laminates designed to
have ah overall coefficient of thermal expansion (CTE) near zero. Zero tolerance titanium carpenter tape hinges were used to
fold the truss members for packaging and to provide stored spring energy for deployment. The large, positive CTE of the
titanium hinges was considered in the strut design so that when combined with the negative CTE graphite/epoxy tubes, the
overall CTE of the member was near zero.
TRUSS PARAMETER: 4 BAY 30.00 DEG AN 5M (16.0 FT) RF DIAMETER
0.00 EF 283 L-RHO ROUND .400 FID 20.00/0 CONTINGENCY
COMPONENTS
UNIT WEIGHT NUMBER WEIGHT
KILOGRAMS (POUNDS) REQUIRED KILOGRAMS (POUNDS)
STRUTS 0.138
UPPER SURFACE 0.167
TUBES 0.099
HINGE 0.015
END FITTING + PINS 0.027
DIAGONALS 0.076
TUBES 0.073
END FITTING + PINS 0.001
LOWER SURFACE 0.182
TUBES 0.112
HINGE 0.015
END FITTINGS + PINS 0.028
SPIDER ASSEMBLY 0,125
MESH INSTALLATION 2,241
CONTINGENCY
REFLECTOR WEIGHT
(0.305) 102 14.1 (31.1)
(0.369) 42 7,0 (15.5)
(0.218) 42 4.1 19.1)
(0.032) 42 0.6 (1.4)
(0.060) 84 2.3 (5.0)
(0.167) 36 2.7 (6.0)
(0.161) 36 2.6 (5,0)
(0.003) 72 0.1 (0.2)
(0.402) 24 4.4 (9.6)
(0.247) 24 2.7 (5.9)
(0.032) 24 0.4 (0.6)
(0.061) 48 1.3 (2.9)
(0.275) 31 3.9 (8.5)
(4.941) 1 2.2 (4.9)
4.0 (8.9)
24.3 (53.5)
X-C.G. CENTIMETERS (INCHES)
Y.C.G. CENTIMETERS (INCHES)
Z-C.G. CENTIMETERS (INCHES)
Ixx KILOGRAM METER SQ. (SLUG FT2)
lyy KILOGRAM METER SQ. (SLUG FT2)
Izz KILOGRAM METER SQ. (SLUG FT2)
Ixy KILOGRAM METER SQ. (SLUG FT2)
Ixz KILOGRAM METER SQ. (SLUG FT2)
lyz KILOGRAM METER SQ. (SLUG FT2)
NOTE - MOI ABOUT CENTER OF GRAVITY
MISCELLANEOUS GEOMETRY DATA
LOAD (PCR) NEWTONS (LB)
DIAMETER CM (IN.) STROT
THICKNESS CM (IN.) STRUT
DES STRUT LENGTH CM (IN.)
HINGE LENGTH CM (IN.)
STANDOFF LH CM (IN.) 20.2
MESH AREA M| (FTL_ 22.5
PACKAGE DIA CM (IN.) 61.2
0.00000 (0.00000)
0.0O000 (0.00000)
17.60790 (6.9322_
56.98889 (42.00081)
56.98889 (42.00081)
105.50880 (77,75999)
0,O0000 (0,00000)
0.0O000 (0.0O000)
O.0O00O (0.000O_
tiPPER StIR DIAGONAL LOWER SUR
096.0 (200.0)
1.69 (0.67)
0.051 (0.020)
154.4 (60.8)
12.8 (5.0)
(8.0)
1242.2)
(24.1)
1013.0 (226.0) 784.0 (175,0)
1.27 (0.50) 1.72 (0.68)
0.051 (0.020) 0.051 (0.020)
102.3 (40.3) 169.3 (66.7)
O.0 (0.0) 12.8 (5.0)
SPIDER DIA CM (IN.) 13.1 (5.2)
TRUSS DEPTH M fiT) 0.2 (0.0)
HEIGHT CM (IN.) 170.2 (67.0)
Figure 4.
CG + Z MESH
+X- I
STRUT ANGLE
NUMBER
OF BAYS
3_. x_'_ _v_._
DIAMETER
PACKAGED ANTENNA
CONFIGURATION
6670-4
115
MESH REFLECTOR DESIGN
Mesh system design used a computerized geometry analysis that optimally subdivided the mesh surface into small, triangular
facets whose positions are controlled by lines attached to the truss structure node fitting (Figure 5). Consideration of illumina-
tion taper permits facet geometry to be varied so that the facet sizes progressively increase from the center to the periphery.
This non-periodic geometry reduces grating lobes that may result from an equally subdivided surface. To meet the high
precision required, the 5-meter diameter surface was divided into 1,944 facets varying in size from three to six inches on a
side.
MESH GRID LINES
MESH SYSTEMWEIGHT SUMMARY
ITEM NO.REQ WEIGHT
MESHTRIANGLES 1944 2.975
GRID LINE SEGMENTS 2970 1.194
DROPLINES 1027 1.369
BUTTONS 1027 1.046
CLIPS 1027 0.136
MESH SYSTEMTOTAL 6.719
MESHAREA(SQYDS) 25.1
TOTALLINE LENGTH(FT) 3749.0
MESHDIA (FT)
NO. OFBAYS
FID
NO. OFDIV.IBAY
X OFF SET- °/oDIA.
Y OFFSET- °/o DIA.
HOLERADIUS (IN.)
DB TAPER
WEIGHTING
O-NONE,I-AR, 2-ILL, 3-BOTH
MESH LOADLBIIN.
GRID LINELOADLB
GROSSWEIGHT LB
X-ARM
Y-ARM
Z-ARM
Ixx-SLUG FT2
Iyy-SLUG FT2
Izz-SLUGFT2
Ixz-SLUGFT2
IxY-SLUGFT2
Izy-SLUG FT2
Figure 5.
16.404
4
1.300000
9
0.000000
0.426000
0.0000
10.00000
3
0.06000
1.50000
6.719
- 0.022
183.383
36.337
3.763
3.768
4.006
0.000
0.000
1.183
DROPLINES
NUMBEROFPOINTS 65124
NORMALRMS 0.00451
HALF PATHL RMS 0.00453
X DEFLECTION - 0,16197
Y DEFLECTION - 0.07396
Z DEFLECTION 0.04038
DELTAFOCALLENGTH 0.33303
ROTATIONABOUTX-X -0.00063
ROTATIONABOUTY-Y 0.00031
6670-5
116
TRUSS STRUCTURE FABRICATION 
A simple but highly precise modular approach was used to fabricate the truss structure. Assembly was accomplished with the 
aid of 19 tooling stanchions (Figure 6) that precisely located the truss node fittings. Individual truss members and fittings were 
assembled on this tool so that any tolerances were removed by simple, on-assembly length adjustments. 
BWBGMAL PAGE E3 
OF PQQR QUALITY 
Figure 6. 
117 
MESH SURFACE FABRICATION 
The mesh surface was fabricated using 20-gauge, two-bar tricot knit fabric. The pattern results in a 0.045-inch maximum 
dimension hole size, which extends RF performance into K-band and reduces cross-polarization response significantly. The 
fabric is knit from 1.2-mil, gold-plated molybdenum monofilament wire. 
The mesh assembly is formed to the parabolic contour of a precision plaster mold (Figure 7). The initial contour of the 
mesh is maintained by a system of grid lines interconnected to buttons located at each apex of the triangular facets subdividing 
the mesh. The buttons, which are attached and bonded to the mesh, also provide the attach points for the 1,027 mesh control 
lines. These lines run from the truss node fittings to the mesh and are used to adjust the mesh surface. 
ORffitNAL PAGE E3 
OF POOR QUALlW 
Figure 7. 
118 
CONTOUR MEASUREMENT AND ADJUSTMENT 
ORIGINAL PAGE tS 
OF POOR Q U A W  
A computer-controlled profilometer contour measurement system incorporating a non-contact sensing probe (Figure 8) was 
used for measurement and adjustment of the reflective, mesh surface. The probe uses a digitally driven, capacitive transducer 
accurate to f l/2 mil for proximity detection of the mesh surface. The 1,027 control lines between the mesh surface and the 
truss structure were loosened or tightened to adjust the mesh surface. Four iterations were required to complete adjustment of 
the mesh surface to the required accuracy. After the surface was set, the mesh control lines were locked out and the adjustment 
devices removed. 
Photogrammetric measurements of 315 random targets on the surface were used to independently verify contour accu- 
racy. After four adjustment cycles, the measured normal surface error was 8.3 mils rms using the profilometer and 8.6 mils 
using photogrammetric results. 
Figure 8. 
6670-7 
119 
PACKAGED REFLECTOR 
When folded, the truss reflector forms a compact package whose dimensions are overall height - 1.8 meter, truss height - 1.1 
meters, mesh diameter - 1.4 meters, and truss diameter - 0.9 meter. Packaged diameter is set by the number and size of node 
fittings used, and height is set by the length of the truss diagonal struts. The packaged reflector shown in Figure 9 shows one 
of three alternate packaging approaches, i.e., with all surface struts folded in. The other two approaches are with top struts 
folded out and with both top and bottom struts folded outward, which produces a longer, smaller-diameter package. 
120 
Figure 9. 
ORIGINAL PAGE E3 
OF POOR QUALITY 
0RIGSr:AL $AGE 6 
REFLECTOR DEPLOYMENT OF POOR QUALITY 
Three deployments of the reflector were performed as shown in Figure 10. The truss reflector is normally a freely deploying 
system employing stored energy in folded spring hinges (carpenter tape hinges). These hinges are incorporated in all truss 
strut members of the geotruss structure supporting the reflector surface. When the package restraints are released, each strut 
begins to unfold. This action automatically drives the node fittings apart until the surface struts lock in their deployed position. 
The hinge provides a nearly constant deployment force until just before lock-up. At that point, the hinge force increases 
sharply to lock the truss member into its final position. This final lock-up force also ensures that the antenna surface mesh is 
stretched into its desired configuration. 
CV1851392 CV1851395 CV1851398 
CV1851402 CV1851411 6670-11 
Figure 10. 
I 121 
ELECTROMAGNETIC (RF) TESTING 
RF performance tests of the 5-meter tetrahedral truss reflector were performed at the Martin Marrietta Near Field Measure- 
ment Laboratory in Denver, Colorado. Linearly polarized on-focus feeds supplied by NASA LaRC were used to measure 
gain, far field patterns and cross-polarization at 2.27, 4.26, 7.73, and 11.6 GHz. Installation of the reflector on a 20-foot 
tower in the near field facility is shown in Figure 1 1. Results of these tests are being evaluated at NASA LaRC and General 
Dynamics Space Systems. Preliminary results indicate good RF performance that correlated well with analytical predictions. 
"Qpical results at 4.26 GHz are shown in the plot in Figure 11. 
ORIGINAL PAGE B 
0 4.26 GHz OF POOR QUALITY 
E-PLANE - 5  - 
Figure 11. 
122 
DEPLOYABLE TRUSS TECHNOLOGY VERIFICATION PROGRAM
The 5-meter technology antenna program demonstrated the capability to successfully fabricate and test a precision deployable
truss antenna. Deployable truss structure technology issues associated with on-orbit performance requirements need to be
verified next by an integrated program of analysis and ground and flight testing. These technologies can be divided into five
general categories: structural dynamics, active control, thermodynamics, electromagnetics, and structure/mechanisms de-
sign.
A 5-meter deployable truss reflector integrated with a deployable truss beam (Figure 12) can serve as a low-cost testbed
for initial technology verification testing. Subsequent testing of a larger 15- to 20-meter article could be used to verify scaling-
sensitive technology issues.
./
5-METER GROUND TEST /_
5-METER FLIGHT TEST FLIGHT TEST
15-T020-METER GROUND TEST
5670-12
Figure 12.
123
CONCLUSIONS
The 5-meter technology antenna program demonstrated the overall feasibility of integrating a mesh reflector surface with a
deployable truss structure to achieve a precision surface contour compatible with future, high-performance antenna require-
ments. Specifically, the program demonstrated:
• The feasibility of fabricating a precision, edge-mounted, deployable, tetrahedral t/'uss structure.
• The feasibility of adjusting a truss-supported mesh reflector contour to a surface error less than 10 mils rms.
• Good RF test performance, which correlated well with analytical predictions.
Further analysis and testing (including flight testing) programs are needed to fully verify all the technology issues, including
structural dynamics, thermodynamics, control, and on-orbit RF performance, which are associated with large, deployable,
truss antenna structures.
124


Deployable truss structure advanced technology

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