A space qualified, precision, large force, thermally-activated driver that has been developed jointly by the NASA Langley Research Center and PRC Kentron is described. The driver consists of a sealed hydraulic cylinder containing a metal bellows, a bellows plug, a coil spring, a spring retainer, and output shaft, a shaft guide, and a quantity of silicone oil. Temperature changes cause the silicone oil to expand or contract thus contracting or expanding the bellows/spring assembly thereby extending or retracting the output shaft

Kinard, William H.

Murray, Robert C.

Walsh, Robert F.

English

NASA Technical Reports Server

N87- 16337
A PRECISION, THERMALLY-ACTIVATED DRIVER FOR SPACE APPLICATION
Robert C. Murray*, Robert F. Walsh*, and William H. Kinard**
This paper describes a space qualified, precision, large force,
thermally-activated driver that has been developed jointly by the NASA
Langley Research Center and PRC Kentron. The driver consists of a sealed
hydraulic cylinder containing a metal bellows, a bellows plug, a coil
spring, a spring retainer, an output shaft, a shaft guide, and a quantity of
silicone oil. Temperature changes cause the silicone oil to expand or
contract thus contracting or expanding the bellows/spring assembly thereby
extending or retracting the output shaft.
INTRODUCTION
The primary objective of the first reflight of NASA's L__ong _Durati°n
E_xposure F_acility, the LDEF-IB mission planned for mid-1987, will be to
establish the abundance of the rare actinide group elements in cosmic rays.
These abundances, which hold clues to the origin and evolution of cosmic
rays, will be determined through analysis of energetic particle tracks in
stacks of thin plastic sheets after the sheets have been exposed in space
for 2-1/2 years on the LDEF.
When energetic particles such as the actinide cosmic rays penetrate
certain plastics, the molecular bonds in the plastic at the track site are
damaged, and the degree of damage is proportional to the energy that was
dissipated along the track. If the plastic sheets are etched after they are
penetrated by elements, conical pits will result at each track site. The
damaged plastic at the track site etches much faster than the undamaged
plastic, and the increase in etch rate is a precise indicator of the degree
of damage to the plastic molecular structure.
With proper information and control, scientists can determine the
element responsible for a given track from the observed dimensions of the
etch pits. By ratioing the number of tracks in the LDEF exposed plastics
which result from elements of known abundance (the platinum-lead group) to
the number of tracks which result from actinides, the abundance of the
actinide elements will be established.
The main problem in implementing this experiment, however, results from
the fact that the temperature of the plastic stacks at the time a track is
produced is a critical item of information in the analysis of etch pits. It
must be known to within I°C. The precision, T_hermally-A__ctivated D_river
*PRC Kentron, Aerospace Technologies Division; Hampton, Virginia.
**NASA Langley Research Center; Hampton, Virginia.
231
https://ntrs.nasa.gov/search.jsp?R=19870006904 2020-03-20T12:15:10+00:00Z
(TAD) which is described in this paper was developed to be the heart of the
Event Thermometer (ET) system that will be used on the LDEF-IB mission.
The ET system concept is functionally illustrated in figure I. Two
plastic sheets are mounted above the detector stacks as shown. The upper
sheet is movable and its position relative to the lower sheet is controlled,
as a function of the temperature of the detector stacks, by the TAD. During
the postflight analysis of the etched plastics, the temperature of the stack
at the time a given track is produced will be determined by aligning the
track etch pit in the movable sheet with the track etch pit in the fixed
sheet and noting the stack temperature that was required for the driver to
so position the sheets. The pits were obviously so positioned when the
track was originally produced.
The specific requirements which were imposed on the TAD design to
ensure that its performance will satisfy the ET requirements for the LDEF-IB
cosmic ray experiment are as follows:
(I)
(2)
(B)
(4)
(5)
(6)
(7)
(8)
(9)
Temperature range: -62°C(-80°F) to +49°C(+120°F)
Displacement over temperature range: 5.08 cm (2.0 in.)
All materials must be non-magnetlc
Fluids must be non-hazardous for Shuttle operations
Accuracy of .046 cm per degree C(.OI0 in. per degree F)
Adequate axial stiffness to restrain attached ET sheet during
launch and landing vibration environment
Minimum size and weight
Maximum dimensions: 3.332 cm (1.312 in.) wide, 3.510 cm
(1.382 in.) deep, and 109.38 cm (43.06 in.) long
Reliability
OPERATION
The operation of the TAD, as schematically shown in figure 2, is a
result of the expansion or contraction of an incompressible fluid as the
temperature of the fluid changes. A coll spring controls the position of
the output shaft by applying a resisting force to the pressure force on the
piston. The system will remain in equilibrium as long as the spring force
balances the piston pressure force.
The operation of the TAD is dependent on the thermal expansion of the
charging fluid. Referring to figure 3, potential energy is stored in the
bellows and coll spring when the TAD is pressurized at room temperature. As
the temperature decreases and the volume of fluid contracts, the bellows and
coil spring release some of their potential energy to keep the bellows
piston area in contact with the oii column. As the temperature increases,
potential energy is increased in the bellows and coll spring. Since the
output shaft is welded to the bellows plug, the output shaft tracks the
movement of the bellows and fluid.
232
HARDWARE
The sizing of the driver is dependent upon the coefficient of thermal
expansion of the charging fluid; therefore, the first component of the
driver selected is the fluid. DowCornlng 200, I0 centistokes silicone oll
was chosen for the following reasons: (I) large rate of thermal expansion,
(2) high flash point, (3) low pour point, (4) non toxic, (5) readily
available, and (6) relatively inexpensive. The outside diameter of the TAD
was constrained to be no greater than 3.332 cm (1.312 in.) in order to fit
in the allotted experiment canister space. Seamless, 321 stainless steel
tubing, 3.175 cm outside diameter x 2.565 cm inside diameter (1.25 in. x
1.01 in.) was selected for the pressure shell. A thin, flexible metal
bellows was selected as a means of storing and releasing potential energy
during temperature cycles while sealing around the output shaft to prevent
fluid leakage. The bellows was designed to function over the operating
displacement range for >106 cycles while mechanical stresses remain within
the proportional limit of the bellows material. A coil spring was designed
to increase the axial stiffness of the TADand to assist the bellows by
releasing potential energy at the lower temperature range where the bellows
has expanded to its "as-formed" length. The bellows material is 321
stainless steel, .0152 cm (.006 in.) thick. The coll spring material is
Inconel X750, .318 cm (.125 in.) diameter wire. The output shaft is madeof
321 stainless steel rod, .794 cm (.3125 in.) dia. The output shaft is
guided in the front support by a bushing, and supported in the oil reservoir
by a cylindrical guide. Both of these parts are made of phosphor bronze,
CDA54400. The phosphor bronze provides a good sliding surface on stainless
steel while closely matching the thermal growth rate of stainless steel.
The coil spring is captured between the cylindrical shaft guide and a spring
retainer made of 347 stainless steel. The front support and rear support
are madeof 347 stainless steel.
ASSEMBLYANDPRESSURIZATION
After machined parts are completed and the bellows is formed, the
bellows assembly is fabricated by loosely assembling the front support,
bellows, bellows plug, and output shaft. Weld joints at three locations
seal the bellows to the front support, bellows plug, and output shaft
(figure 4). The bellows assembly is placed inside the pressure shell and a
full penetration weld joins the front support and pressure shell. From the
open end of the pressure shell the remaining internal parts are assembled in
the following sequence: (I) coll spring retainer, (2) coll spring, (3)
shaft guide, and (4) capture nut (figure 5). Figure 6 shows the assembly of
the internal components; for clarity, the pressure shell is omitted. The
capture nut is torqued in order to stretch the bellows .96 cm (.38 in.)
which increases the effective length of the bellows. The rear support is
positioned in the pressure shell and a full penetration weld joins the two
(figure 7). Radiographic and dye penetrant inspections of each weld are
performed sequentially during the assembly process to ensure the integrity
of each joint. After the TAD componentsare assembled, a vacuum pump is
233
attached to the fill tube through a valving arrangement. The internal
cavity of the driver is evacuated and valved off. A valve attached to the
liquid sump/pump is opened, allowing the silicone oil to fill the driver.
The hand pump is then operated slowly to increase internal pressure to
~1723.8kPa (~250 psig). Since manufacturing variations in the bellows
and coil springs will result in slightly different spring rates for each
item, shaft displacement rather than internal pressure determines when the
pressurization process is completed; thus ensuring a practically equal
amount of fluid is pumped into each TAD. The fill tube is flattened,
pinched off (under pressure), and dipped in solder. The TAD assembly and
pressurization procedure is now complete.
TESTING
Three types of tests are conducted on the TAD: (1) pressure testing to
verify structure integrity, (2) thermal testing for calibration and
survivability demonstration, and (3) vibration testing to verify workmanship
and structural integrity for Shuttle induced accelerations.
Pressure testing of the TAD is an integral part of the pressurization
procedure. As the output shaft reaches its calculated extended position at
~1723.8kPa (_250 psig) internal pressure, it is restrained from further
movement while internal pressure in increased to _2585.6kPa (375 psig)
which is approximately 1.5 times nominal pressure. The shaft restraint is
necessary to prevent the bellows and coil spring from collapsing
excessively, which would exceed the material yield strength of one or both.
Thermal testing of the TAD is conducted in a NASA/LaRC thermal vacuum
chamber. The driver is suspended in a bath of Dow Corning 200 silicone oil
and a Direct Current Displacement Transducer (DCDT) is attached to the
output -shaft. -- Thermocouples are a_tached to the outer surface of the
pressure shell, suspended in the oil bath, and suspended in the vacuum
chamber. The chamber is cooled by liquid nitrogen which becomes gaseous as
it is injected into and circulated around the chamber. The chamber is
warmed by resistance heaters. Since the thermal chamber can be controlled
to only +2.8°C(+5°F), the oil bath is used to dampen the temperature
variations.
Vibration testing of the TAD to achieve desired acceleration levels is
conducted on an Unholtz-Dickie shaker fixture at NASA/LaRC. The driver is
mounted in a cavity of the experiment canister and accelerated to 5.0g's in
the longitudinal axis, 16.5g's in the axis orthogonal to the longitudinal
axis, and lO.0g's in the other orthogonal axis. These acceleration levels
generate loads which are 1.4 times limit loads due to Shuttle launch or
landing.
234
CONCLUSIONS
The TAD is a precision, large force driver that meets the very
stringent requirements of the ET system for the LDEF-IB cosmic ray
experiment. Forty-five TAD's will be flown on the LDEF-IB mission, and it
is expected that TAD's will also find use in a number of other space and
terrestrial applications. With properly selected fluids, fluid volumes, and
bellows sizes, TAD's can offer a wide choice in performance
variables--namely displacements, operating temperatures, driving forces, and
response times.
TAD's obviously have applications in fluid control systems where
precision valve controls are needed; for example, in manufacturing processes
and in safety control systems.
A single TAD may be used to drive a large bank of thermal control
louvers for spacecraft or for terrestrial buildings. The fact that part of
the fluid volume can be remotely located and piped to the driver allows
flexibility in the thermal control designs and, in some cases, performance
improvements.
TAD's may also be used to compensate for thermal expansion and
distortion of spacecraft structures where extreme dimensional stability is
essential.
In conclusion, the TAD features may be summarized as follows:
o Precision, thermally-activated displacement and position control
o Wide choice in performance and operating variables
o Rellablility, and
o No power required
235
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A precision, thermally-activated driver for space application

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