A folding three dimensional truss and tripod assembly which deploys the SEASAT Synthetic Aperture Radar Antenna is described. The folding structure with the antenna panels and rf components stows in an 8.5-inch-thick package. Upon deployment, the structure produced is a flat and rigid support for the antenna

Campbell, B. E.

Hawkins, W.

English

NASA Technical Reports Server

AN ll-METER DEPLOYABLE TRUSS 
FOR THE SEASAT RADAR ANTENNA 
Bruce E. Campbell 
Astro Research Corporation 
and 
Wesley Hawkins 
Ball Brothers Research Corporation 
ABSTRACT 
The recently developed Extendable Support Structure is a fold- 
ing three-dimensional truss and tripod assembly which deploys the 
SEASAT Synthetic Aperture Radar Antenna. The folding structure with 
the antenna panels and rf components stows in an 8.5-inch-thick 
package. Upon deployment, the structure produced is a flat and 
rigid support for the antenna. 
INTRODUCTION 
The Extendable Support Structure (ESS) is a 35-foot (10.7-meter) 
deployable truss developed by Astro Research Corporation, Santa 
Barbara, California. The structure provides launch packaging and 
on-orbit extension for the Synthetic Aperture Radar (SAR) Antenna 
developed by Ball Brothers Research Corporation, Boulder, Colorado, 
for the SEASAT Global Ocean-Survey Satellite. 
Scheduled to be launched during the spring of 1978 from Vanden- 
berg Air Force Base, California, the SEASAT Satellite will be 
inserted into a circular 430-mile near-polar orbit by an Atlas/Agena 
Booster system. The satellite will continuously circle the globe, 
covering 95 percent of the earth’s ocean area, every 36 hours. 
After ascent to orbit, upon command, the folded and stowed SAR 
Antenna will be deployed from the SEASAT Satellite and extended, via 
the ESS, into a planar array 35 by 7 feet. The array will provide 
valuable and precise realtime all-weather radar imagery of ocean 
topography such as wave patterns, ice heads, icebergs, coastal 
erosion and environmental management features. The on-orbit 
configuration of the SAR Antenna and Satellite is shown in Figure 1. 
The ESS furnishes a mounting bed for the eight honeycomb panels, 
which upon extension becomes the planar array of the SAR Antenna. 
77 
https://ntrs.nasa.gov/search.jsp?R=19790013189 2020-03-20T18:33:25+00:00Z
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The ESS provides the strength and rigidity necessary to withstand 
the launch vehicle environments. Upon extension, it supports the 
antenna panels to maintain precise flatness, including mechanical 
alignments and thermal distortions, throughout the mission-orbit 
environment. 
DESIGN REQUIREMENTS 
The ESS was challenged by several major design requirements: 
1. pointing of the array's planar surface to *0.2 degree, 
2. repeatability of the array's pointing to *O.l degree, 
3. fundamental resonant frequency (stowed 210 Hz and 
extended 20.5 Hz), 
4. weight less than 77 pounds, 
5. planar flatness within *0.25 inch over the orbit temperature 
ranges of 200 to -250°F, and 
6. fourteen-month program from start of development to delivery 
of the flight ESS, which meant the development unit was to 
become the qualified flight unit. 
Envelope 
The envelope for packaging the SAR Antenna was defined by the 
volumetric availability of the SEASAT Satellite sensor module. The 
dimensions of this envelope are 101.5 by 64.0 inches with a packaged 
thickness of 8.5 inches. Thus, the planar array length of 35 feet 
was required to be folded and restrained for launch within the 8.5- 
inch dimension. This alone was a major design parameter, in that 
the total thickness of the eight antenna panels accounted for 3.66 
inches, leaving only 4.84 inches for the development of a three- 
dimensional rigid and stable folding truss and launch fittings. 
Truss 
The ESS has to support the 130-pound weight of the eight antenna 
panels and associated rf system for launch. To achieve this goal, 
the truss work of the ESS provided mounting hardware for the panels. 
To withstand the 8-g axial launch-vehicle acceleration, the truss 
incorporated nesting and interlocking structural features so that 
panel and truss loads could be transferred to the spacecraft 
78 
structure by three launch restraint-arm mechanisms. The interlock- 
ing features of the truss consists of cups and cones and key-type 
fittings which engage each other in the folded configuration and 
release one another during the truss extension. The stacks of cups 
and cones are clamped and preloaded by the launch restraint-arm 
mechanisms. Three load paths are employed, one at the apex of the 
truss and two at the hinge line of the truss (Figure 2). Thus upon 
the satellite's obtaining a ready-for-SAR deployment status, the 
launch restraint arms are pyrotechnically released and the ESS freed 
for deployment maneuvers and extension. 
Deployment 
The envelope allocation also required the folded and stowed 
package to be rotated out-board of the spacecraft in two go-degree 
maneuvers. These maneuvers were required to achieve a position for 
array extension which would result in the long axis of the planar 
array being aligned with the velocity vector of the spacecraft. In 
addition, the normal of the array's planar surface was to be pointed 
20 degrees up from nadir, thus requiring the ESS to pitch-up the 
antenna plane during extension of the array. 
This deployment sequence was efficiently implemented by a BIAX 
mechanism, developed by Lockheed Missiles and Space Company, which 
performed the two go-degree maneuvers (Figure 3). The ESS performs 
the array extension concurrent with the 20-degree pitch-up rotation. 
Upon completion of the 7-minute powered extension, the ESS 
truss forms a three-dimensional structure providing a flat array 
surface (within *0.090 inch to a least square best-fit plane). 
DETAILED DESIGN DESCRIPTION 
The ESS consists of a tripod assembly and a truss assembly. 
They are discussed separately in the following. Also, the design 
of important joints in the structure is described. 
Tripod Assembly 
The tripod is the single mounting of the deployed antenna to 
the spacecraft and controls the orientation of the truss during and 
upon extension. The tripod consists of two folding arms and a fixed 
brace mounted to a short box structure (Figure 4). The location of 
the pivots of the arms and the single pivot at the tip of the brace 
79 
direct the motion of the truss from its flat package to its extended 
position. As the ESS unfolds, the orientation of the truss is 
rotated to the position 20 degrees from its initial packaged angle. 
Deployment is controlled by the extension of a dc-motor-driven 
linear actuator. The actuator drives a central lever which has two 
ball-ended connecting rods joining it to the tripod arms. The arms 
are pulled to full deployment where spherical stops seat and pro- 
vide support to the arms. 
The arms and the nonrotating brace are titanium members with 
aluminum fittings in the arms. Titanium is used to minimize the 
effect of temperature changes on the pointing angle of the antenna. 
Truss Assembly 
The truss is made up of repeating frames and members which 
connect in seven types of joints. Along its long dimension are 
three longerons; two are tubular and the third is a flat bar (Fig- 
ure 5). With the exception of the control linkage, each of the 
other members is tubular and connects the longerons. The control 
linkage is the second flat-bar linkage which mounts to the flat-bar 
longeron in seven places. The frames which lie in the plane on 
which the antenna panels are mounted consist of a section of the 
two tubular longerons, a perpendicular cross bar connecting them, 
and a diagonal. A vee-shaped, two-member frame connects the two 
tubular longerons to the single flat longeron, and two other straighi 
members complete the truss. 
The joints are designed such that the member axes' intersect in 
the deployed truss. Each of the joints is pin-connected with dual 
tangs in each titanium fitting through which the pin passes. Special 
fittings were also bonded into the tubes where they cross, in the 
stowed position, to permit the flat package to be obtained. 
The tubes and the flat bar are all graphite/epoxy composite. 
The fittings are bonded to the members with graphite/epoxy outside 
on the tube members and inside on the solid flat bar. The graphite/ 
epoxy composite was chosen primarily for its low thermal-expansion 
properties. The composite members also provide a beneficial 
stiffness-to-weight ratio, but require the use of the titanium 
fittings to reduce thermally generated stresses in their attachment 
bond joints. 
To synchronize the extension of the truss and assure full 
extension, two special types of joints were designed. These joints 
function to assist extension by torsion springs which drive to 
straighten the flat-bar longeron. 
80 
The synchronized joint connects two vee-shaped frames to the 
flat-bar longeron and control linkage (Figure 6). This joint uti- 
lizes a toggle linkage to control the spread of the vee-shaped frame 
in accordance with unfolding of the flat-bar longeron. The rotation 
of the central shaft of the joint drives opposing ball-ended link- 
ages to deploy the frames. The stud which protrudes upward in the 
upper view of Figure 6 is located in two places along the apex of 
the truss where the tripod arms join the truss. 
A spring-loaded toggle-type joint was designed into the flat- 
bar longeron hinges. To permit the longeron to fold 180 degrees, 
the hinge centerline is placed on the edge of the longeron as shown 
in Figure 7. The torsion spring on the hinge axis is greatly 
assisted toward the end of deployment by an "almost-over-center" 
latch which acts on the opposite edge of the hinge. The latch con- 
sists of two levers and a torsion spring and contributes approxi- 
mately 12 in. -1bs of torque to close the longeron hinge at full 
deployment. This lever action multiplies the torque from the tor- 
sion spring so as to assure that the hinge will remain closed, even 
with l-g loads, without requiring a lock. 
FABRICATION AND TEST 
The ESS has been integrated into the SAR System as shown in 
Figures 8 through 10. In these photographs, the thermal blankets 
and rf components are being installed and the assembly is mounted 
on a gravity-compensation fixture. 
The weight of the ESS, consisting of the tripod and truss 
assembly, is 64 pounds. This is less than one-third the total an- 
tenna weight. 
The deployed ESS produces a panel mounting surface which mea- 
sures to be flat within kO.09 inch peak from the least square best- 
fit plane. Extension of the structure and measurements of flatness 
were made with the ESS mounted on the gravity-compensation fixture. 
The stowed natural frequency was tested with the panels, rf 
components, and launch restraint. The fundamental frequency was 
greater than 10 Hz. The deployed frequency was determined to be 
0.9 Hz. 
SEASAT PAPER NOTE 
This work was performed for JPL, sponsored by NASA under 
contract NAS7-100. The folding truss concept was developed by 
Astro Research Corp. during internal proposal funding. 
81 
SPACECRAFT 
STRUCTTT'?p\ 
b 7FT4 WAR AHTEtIW 
LAUNCH 
RESTRAINT 
ARMS,3 
PLACES 
TRUS 
DED 
Fig. 1. Satellite Configuration. Fig. 2. Stowed Truss Load 
Paths. 
,r- STOWED ENVELOPE (FIEF) 
ES5 EXTENSION 
Fig. 3. Envelope Deployment Sequence. 
82 
l x 
I 
I 
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Fig. 4. Tripod Assembly. 
83 
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Fig. 6. Synchronized Joint. 
Fig. 7. Longeron Hinge. 
85 
Fig. 8. SAR Stowed. 
86 
Fig. 9. SAR Partially Deployed. 
87 
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An 11-meter deployable truss for the SEASAT radar antenna

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