A mechanism called a deployment/retraction assembly (DRA) which provides not only a stable, but a deployable platform for the high gain antenna system (HGAS) aboard the Solar Maximum Mission (SMM) spacecraft is described. The DRA also has the capability to retract the system upon command

Bennett, N.

Preiswerk, P.

English

NASA Technical Reports Server

DEPLOYMENT/RETRACTION MECHANISM FOR SOLAR 
MAXIMUM MISSION HIGH GAIN ANTENNA SYSTEM 
Neal Bennett, Sperry Flight Systems 
Peter Preiswerk, ASTRO Research Corporation 
ABSTRACT 
Accurate steering of a spacecraft communication antenna requires a stable 
platform. A mechanism called a Deployment/Retraction Assembly (DRA) which pro- 
vides not only a stable, but a deployable platform for the High Gain Antenna 
System (HGAS) aboard the Solar Maximum Mission (SMM) spacecraft is described. 
The DRA also has the capability to retract the system upon command. 
INTRODUCTION 
The SMM spacecraft scheduled for launch into a 357 mile orbit in October 
1979 will have aboard a high gain S-band antenna system capable of communicating 
with and tracking the TDRS system. This antenna system, called HGAS, must be 
stowed within a required envelope in the aft end of the spacecraft and withstand 
launch by a Delta launch vehicle. The spacecraft attitude does not allow the 
antenna to view the relay satellites when stowed. Consequently, once in orbit, 
the HGAS must be deployed to a position that allows the antenna to communicate 
with and track the relay satellites. The HGAS is shown in the deployed condition 
in Figure 1. When deployed, the deploy mechanism must maintain accurate support 
alignment for the antenna and articulation system while being exposed to the 
orbital space environment. Space Shuttle recovery of the SMM is planned and to 
facilitate this the HGAS is required to retract within its launch envelope so 
that the SMM spacecraft can fit within the Shuttle bay. If retraction is not 
possible, all portions of the HGAS outside the recovery envelope must be jetti- 
soned from the spacecraft. 
The DRA design described in this paper was selected based on the flight 
experience of the concept and its potential to satisfy the stringent HGAS require- 
ments described above. Similar structures have flown successfully on the Air 
Force S-3 satellite and NASA's Voyager 1 and 2 as magnetometer booms of 20-foot 
length, 7-inch diameter and of 43-foot length, g-inch diameter respectively. 
The deployable portionof the DRA, the Astromast, provides an ultralight, low 
profile structure with the deployed stiffness and stability required of HGAS. 
DRA DESIGN DESCRIPTION 
The DRA is a major subassembly mechanism of the High Gain Antenna System, 
weighing less than 23 pounds. The total deploy stroke is 60 inches, which 
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positions the antenna at a point relative to the spacecraft that allows a view of 
the TDRS system. It has overall stiffness properties that yield major HGAS de- 
ployed resonant frequencies in excess of 8 Hz. Deployed alignment stability is 
expected to be better than .2 degrees over the required temperature range and 
deploy/retract cycle life. The required cycle life for ground operation will be 
approximately 30 cycles and for space, 1 cycle. 
The DRA itself consists of five major subassemblies: (1) An Astromast 
Assembly which is the basic deploying and retracting support structure for the 
antenna and articulation system, (2) A servo assembly, which restrains the 
Astromast during deployment in a controlled manner, and provides the force 
required for retraction, (3) hardware that interfaces the HGAS with the SMM 
spacecraft, (4) a jettison mechanism that is capable of jettisoning certain 
portions of the HGAS, and (5) an antijettison caging mechanism that inhibits the 
jettison mechanism in the stowed condition. 
ASTROMAST 
The Astromast provides the key function in the DRA of structural support to 
the deployed antenna and articulation system. Its construction provides for 
maximum stiffness, minimum weight, and minimum volume. 
Figure 2 is a layout of the Astromast showing the truss type construction. 
The basic members are the three main longitudinal members (longerons), triangular 
frames separating the longerons (batten frames), and pretensioned diagonal 
members connecting adjacent longerons and batten frames. The diameter through 
the longerons is 18.5 inches and the length between longeron pivot points is 
62.16 inches. There are 5 batten frames, forming 6 bays, each 10.36 inches long. 
All members are fabricated from unidirectional S-glass/epoxy laminate to take 
advantage of the inherent high stiffness-to-weight ratio and thermal stability. 
The total weight of the Astromast is 3.7 pounds. 
Stowed, the Astromast is coiled into a height of only 2.3 inches. The 
longerons develop, like coiled springs, a force tending to deploy the system 
and are restrained by a central lanyard. Figures 3 through 6 show the deploy- 
ment sequence, as demonstrated by an engineering model, beginning with the 
fully stowed condition. Two different stages during the transition from 
stowed to deployed are shown in Figures 4 and 5. Since all or some portion 
of the longerons still form a helix, the Astromast is relatively weak 
during the transition phase. The maximum stiffness and strength properties are 
not achieved until full deployment, shown in Figure 6. Note the restraining 
lanyard located in the center. The top plate, representing the interface to the 
antenna and articulation system, rotates about the longitudinal axis a total of 
382.5 degrees as the Astromast deploys. 
In the deployed state, the longerons provide axial load capacity and bend- 
ing stiffness, the battens stabilize the structure while in an elastically 
buckled condition, and the diagonals provide shear and torsional stiffness. 
Though the DRA will not be exposed to direct sunlight, the thermal alignment 
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stability is designed to remain within .2 degrees with as much as 270 degrees R 
temperature differential between the two diagonals of each bay panel. 
SERVO MECHANISM r 
The DRA servo mechanism controls the rate of deployment and provides 
the retracting force. A pully containing the restraining lanyard is attached to 
the output shaft of a simple worm gear assembly. The worm is casehardened steel 
and the driven helical gear is cast bronze; the mesh as well as the gear bearings 
are lubricated by Braycote 3L-38RP grease. The worm is driven by a brush type 
DC gear head motor, producing a total speed reduction of 2433:1. Motor brushes 
are redundant and of the longlife dry-lube type consisting of 85 percent silver, 
12 percent molydisulfide, and 3 percent carbon. Figure 7 shows the location of 
the servo mechanism on the bottom of the DRA. 
For launch the Astromast is not held in position by the lanyard, but by a 
pyrotechnic pin puller which absorbs the load directly. The pin puller is shown 
in Figures 7 and 9. Once the spacecraft is stabilized in orbit, the dual initi- 
ated pin puller is fired and the deploy sequence is started by commanding the 
DC motor on. The lanyard is in turn played out at a controlled rate allowing 
the DRA to deploy. Redundant microswitches are used to indicate the deployed 
state and are used to switch off the motor. A special bridle system, shown in 
Figure 8, is used in conjunction with a change in effective lanyard pully radius 
to prevent the DRA from "snapping" into the deployed state. This bridle system 
also serves to generate the initial rotation of the DRA about the longitudinal 
axis when retraction is commanded. Redundant stowed status microswitches, lo- 
cated so that they sense contact of the top structure with the base plate, are 
used to turn off the servo mechanism once stowage is complete. 
INTERFACE HARDWARE 
The interface between the DRA mechanism and the SMM spacecraft consists of 
a lightweight, stiff aluminum cylinder 28.3 inches long and 21 inches diameter, 
called the canister, shown in Figure 9. Three hat section stringers run the 
length of the canister carrying loads from the three Astromast longerons 
directly to three I-beams in the aft end of the spacecraft. The canister not 
only provides the static and dynamic interface with the spacecraft, but also 
acts as a guide tube during jettison. 
JETTISON MECHANISM 
The DRA is capable of jettisoning the antenna, articulation system, Astro- 
mast, servo mechanism, and control electronics. The jettison mechanism will only 
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be activated in the event the DRA cannot be retracted to the fully stowed condi- 
tion for Shuttle recovery. This capability is provided by three ball release/ 
jettison spring assemblies, shown in Figure 10. A lightweight but structurally 
sound mechanism has resulted through extensive use of 7075 aluminum. A pyro- 
technic cable cutter, shown in Figure 7, severs 3 stainless steel cables that 
release spring loaded plungers, unloading a set of steel balls in sockets. 
Once the ball loads are released, 3 jettison springs eject the base plate and 
all assemblies attached to it out of the canister. The spring stroke is 
2.5 inches providing a terminal velocity of 12 in./sec. Spring forces are de- 
signed to yield a net force through the HGAS center of gravity to minimize the 
tendency to rotate. In addition to the jettison release cables, the pyrotechnic 
cable cutter severs all electrical and RF cables interfacing the HGAS to the 
SMM spacecraft. 
CAGING MECHANISM 
To prevent inadvertent jettisoning of the HGAS after Shuttle recovery, the 
jettison capability is inhibited by a unique but simple caging mechanism shown 
in Figure 9. Caging occurs only in the stowed condition, but the mechanism 
allows the DRA to deploy and retract normally. It consists of a pivoted wedge 
which, when the DRA is fully retracted, will not allow the base plate to move 
relative to the canister. Consequently, if the cable cutter is accidentally 
fired, the jettison springs are inhibited from forcing the baseplate out of the 
canister. After approximately .75 inches of deployment, the caging wedges are 
released, and jettison is possible. 
CONCLUDING REMARKS 
A unique mechanism, called a Deployment/Retraction Assembly, has been de- 
scribed that is capable of deploying and retracting the S-band antenna and asso- 
ciated articulation system aboard the SMM spacecraft. Once deployed, it provides 
a stable and stiff structure from which the antenna can track and communicate 
with the TDRS system. 
At the time of writing, engineering model tests are underway and the proto- 
flight DRA is being fabricated. Engineering models of the worm gear assembly, 
the jettison mechanism, and the Astromast have been tested and the following has 
been demonstrated: 
o Worm gear design has load/cycle capability in excess of that 
required for ground and space operation. 
o Jettison mechanism ball release concept works successfully. 
o The Astromast has adequate cycle life and stiffness to meet 
mission requirements. 
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Acceptance testing of the protoflight DRA will begin in late January. At 
the time of the 12th Aerospace Symposium in April 1978, DRA acceptance tests 
will be complete and resulting data will be available. 
205 
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Figure 1 
HGAS Deployed 
Figure 2 
Astromast 
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Jettison Mechanism 
210 


Deployment/retraction mechanism for solar maximum mission high gain antenna system

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