The design, construction, and testing of a passive radiative cooler that provides solutions to the design problems of withstanding mechanical stress, achieving the required thermal isolation, and maintaining optical alinement, cleanliness, and integration with the spacecraft are described

Mccullough, J. E.

English

NASA Technical Reports Server

RADIATIVE COOLER FOR SPACECRAFT
By John E. McCullough*
ABSTRACT
The design, construction, and testing of a passive radiative cooler that provides
solutions to the design problems of withstanding mechanical stress, achieving the re-
quired thermal isolation, and maintaining optical alinement, cleanliness, and integra-
tion with the spacecraft are described in this report.
INTRODUCTION
The passive radiative cooler provides a simple, reliable means of maintaining a
detector in a satellite-borne optical system at its required cryogenic operating temper-
ature. This device is able to cool a low-power-dissipation optical detector to tempera-
tures of approximately 100 ° K by virtue of its ability to radiate more thermal energy
to deep space than it receives from other parts of the spacecraft, the sun, and the
earth.
For maximum thermal isolation, mechanical connections between the radiator
and the spacecraft must be minimized. This is normally achieved by the use of guy-
wire supports or nonmetallic structural elements with a large length-to-cross section
ratio. However, these traditional types of supports for cryogenic components are not
appropriate for supporting radiative coolers because they cannot withstand the dynamic
environment of launch and they cannot provide the precise alinement of the detector
relative to associated optical components. Additionally, nonmetallic materials are
sources of optical-system contaminants like water vapor or volatiles that evolve under
space-vacuum conditions.
DESIGN CRITERIA
The complete design specification for a passive radiative cooler suitable for
spacecraft use must include the following requirements as a minimum.
1. Mechanical requirements
a. Survive launch and orbit-insertion mechanical environment
b. Maintain optical alinement of cooled elements and orientation with respect
to the remainder of the optical system
c. Use dimensionally stable materials
*Arthur D. Little, Inc., Cambridge, Mass.
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https://ntrs.nasa.gov/search.jsp?R=19730010146 2020-03-17T08:22:12+00:00Z
d. Operate reliably
e. Have minimum weight
2. Thermal requirements
a. Be able to reach required temperature levels while viewing deep space
b. Reject heat inputs from the spacecraft (both radiative and conductive) as
well as radiant thermal inputs from planetary and solar sources
3. Optical requirements
a. Maintain optical element spacing and alinement at operating temperatures
b. Position optical elements to allow focus adjustment and secure locking in
final position
c. Maintain cleanliness of optical elements before and after orbit insertion
d. Duplicate the properties of an optical-quality mirror with the radiative-
energy reflecting surfaces
4, Vacuum and cryogenic requirements
a. Use construction materials suitable for space environments
b. Consider the material properties at cryogenic temperatures
c. Use thermal-insulation techniques that are equivalent to multiple-
reflective-layer insulation systems
DESIGN
The design philosophy of the cooler described in this report is that a surface
thermally isolated from its mounting will radiate thermal energy to deep space, eventu-
ally reaching cryogenic temperature levels. However, to reach these temperatures,
the surface must be shaded from sunlight and be oriented such that it cannot "see" por-
tions of the spacecraft or earth that act as sources of energy radiated back to the cool-
ing element.
The basic right conical configuration and size of the cooler discussed in this
report were derived using thermal-analysis techniques from basic requirements of
operating temperature, bias-power heat load, location on the spacecraft, spacecraft
configuration, and orbit parameters. The mechanical design of the cooler started with
the subsequent shape and size requirements so generated as well as limits on the allow-
able heat leak or flow of thermal energy between the mounting surface and the cooling
element. The design presented in this report results from consideration of all of the
design criteria.
7O
Support Element
From previous experience in the design of cryogenic equipment, it was known
that the design of the support elements was the most critical aspect of the whole cooler
design problem. The support elements must be rugged enough to survive extreme
mechanical environments yet have very low thermal conductance. Additionally, they
must maintain precise alinement of the cooler stages, such that a detector mounted on
the innermost element of the cooler would maintain accurate alinement with respect to
the associated optical system attached to the cooler's outermost element. Three sup-
ports are used to locate each stage. The supports are located in the plane of the sup-
ported component center of gravity to eliminate eccentric dynamic loading. All major
components of the cooler were designed as bodies of revolution because this shape was
most convenient for the reflective interior of the outer stage and also because this con-
figuration is easily manufactured by standard fabrication equipment, has high stiffness
per unit weight, and can be readily analyzed to determine deflection under load and
natural frequency modes.
Radiative Cooler
The radiative cooler (fig. 1) consists of three major components: an outer stage,
an inner stage, and a mounting ring. The outer stage is a truncated cone, the large
end of which is open and flanged around the rim. The inner stage, which is mounted
inside the small end of the outer stage, consists of a structural disk and a cover plate
that is bolted to it. The mounting ring surrounds the outer stage and provides a means
for mounting the radiator to the associated optical system. The inner stage is sup-
ported within the outer stage at three equally spaced points around the perimeter, with
its center of gravity in the plane of the support elements. The latter unique design
provides strong mechanical support at room temperature, so that the cooler will with-
stand the shock and vibration of launch, and high thermal resistance in the space envi-
ronment. The outer stage is positioned within the mounting ring by three similar
support elements in the plane of its center of gravity.
Optical Design
The optical detector is attached to the structural disk of the inner stage, facing
toward the small end of the conical outer stage. The detector can be inserted into or
removed from the cooler assembly simply by removing the inner-stage cover plate.
The lens system used in conjunction with the detector is attached to the outer stage
with the aid of a lens holder. Threads on the exterior surface of the lens holder mate
with a tapped section in the outer-stage structure; for focusing adjustment, the lens
holder can be moved axially with respect to the detector by rotating it in the appropri-
ate direction.
Thermal Design
The interior surface of the outer stage is highly polished to minimize radiative
thermal-energy transfer to the inner stage and to reject solar energy that strikes the
cooler at angles nearly perpendicular to the axis of the cooler. The outer flange area
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of the outer stage serves as a heat radiator by virtue of its special _/e thermal con-
trol coating. Except for this flange, the outer stage is covered by a multiple-reflective-
layer blanket, which passes between the outer stage and mounting ring to reduce
radiative heat transfer to the outer stage from the cooler surroundings.
When the radiative cooler is exposed to a low-temperature heat sink in vacuum,
it radiates thermal energy to this sink from the inner- and outer-stage surfaces. Tem-
perature reductions in the two stages cause the support elements to decouple, resulting
in an additional temperature drop. The outer-stage element reaches an equilibrium
temperature when its total conductive and radiative heat inputs balance its total radi-
ative output to the heat sink. Similarly, the inner-stage equilibrium temperature will
reflect a balance between its total radiative and conductive heat inputs and its radiative
output.
To achieve an inner-stage temperature of approximately 100° K, heat inputs to
both stages must be minimized in every way possible. For this reason, multiple-
reflective-layer radiative insulation is used around both the inner and outer stages, and
very fine 0.10 to 0.13 millimeter (0. 004 to 0. 005 inch) electric wiring is used to con-
vey signals from the detector to the terminal strip on the mounting ring. These wires
are typically stainless steel and are encased inTeflon tubing in such a way that they can
withstand severe mechanical stress without breaking.
All construction materials used in the cooler are space qualified, and the quantity
of nonmetallic materials is minimized. In addition, all parts are scrupulously cleaned
before final assembly to maintain the cleanliness of the optical system contained within
and to minimize the evolution of adsorbed contaminants from component surfaces.
Volatiles emitted by cooler components could easily render the optical system inopera-
tive by coating the elements with condensed vapor or ice.
FABRICATION TECHNIQUE
The major components of the radiative cooler (fig. 2) were machined from solid
billets of 6061-T6 aluminum. This material was chosen because it has good mechani-
cal properties, can be readily machined to precise tolerances with standard equipment,
is readily available in large billets of uniform temper and homogeneity, can be made
dimensionally stable through heat treatment, and is a suitable substrate for nickel
plating and polishing. Machining these components from solid billets has proved to be
the best way to avoid encountering material nonhomogeneities during fabrication, es-
pectally in the conical reflector surface.
After rough machining to within 2.54 millimeters (0.10 inch) of final size, the
inner- and outer-stage components were subjected to a thermal stabilization to guaran-
tee dimensional stability during repeated thermal cycling from room temperature to
cryogenic temperatures (less than 100° K (-280 ° F)). After thermal stabilization and
final machining of the major parts, the parts were positioned relative to one another in
their final configuration and holes for the three support elements were line bored in the
inner and outer stages. In this way, precise alinement of the two stages and the mount-
ing ring was achieved and proper functioning of the support element was ensured.
72
The conical interior surface of the outer stage was prepared for optical polishing
in the following way. First, it was polished to remove the majority of machining
marks. Next, it was electroless nickel plated. Finally, it was polished with standard
optical lapping materials; the technique used was an automated process that was de-
veloped especially for polishing conical surfaces to a quality approaching that of an
optical mirror.
The evolution of this polishing technique was guided by visual comparison of the
polished surface with reference surfaces having the desired reflective qualities. This
comparison was made by "replicating the polished surface and comparing this to repli-
cates of high-quality optical surfaces (mirror or flats) as well as replicates from sur-
faces with known thermal control or reflective characteristics. This comparison was
made by visual examination of photomicrographs of the replicates made at magnifica-
tions as much as 25 000 times, using electron-beam microscopy techniques.
ASSEMBLY
Cooler assembly started by building up the inner stage. Following this, the inner
stage was positioned within the outer stage by three support elements so that its con-
centricity relative to the outer stage was held within 0. 038 millimeters (0. 0015 inch)
T .I.R.
After a multiple-reflective-layer blanket was placed over the outer stage, the
mounting ring was positioned by its support elements so that the concentricity of the
mounting ring relative to the outer stage was maintained within 0. 025 millimeters
(0.001 inch) T.I.R.
The internal electrical wiring of the cooler was connected to a terminal block
bolted to the mounting ring; pins on the terminal block are readily accessible for con-
nection to the associated optical-system wiring.
After the detector was inserted in the central counterbore of the inner stage and
bolted in place, the detector leads were soldered to adjacent terminals on the inner-
stage structure. Finally, the inner-stage cover was bolted in place, completing the
cooler assembly.
TEST PROCEDURE AND RESULTS
The fully assembled radiative cooler (fig. 3) was thermally tested in a vacuum
chamber equipped with a liquid-nitrogen-cooled shroud. The cooler was oriented so
that only its forward portions viewed this heat sink, while the rear portions were sur-
rounded by a shroud maintained at room temperature. With this apparatus, successful
operation of the cooler can be verified and temperature achievement measured. These
data can be extrapolated by thermal-modeling techniques to predict orbital thermal
performance.
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Data on temperature as a function of time were taken for the inner and outer
stages during the cooldown process. Thermal equilibrium was reached approximately
24 hours after the chamber was evacuated, and the shroud temperature was reduced to
liquid nitrogen levels. At equilibrium, the temperatures of the inner and outer stages
were 102° and 178° K, respectively.
Following thermal testing, the cooler was mounted to a test fixture and subjected
to mechanical environment testing, which consisted of flight-acceptance-level sine and
random vibration. The random vibration portion of this specification produces 11.3g
root mean square acceleration for 2 minutes in each axis. During this testing, the
cooler was covered with a large, clean plastic bag to protect it from ambient contam-
ination. The cooler was free to move in all directions without contacting the bag or
causing undue motion of the bag.
After this acceptance-level vibration test, the cooler was again mounted in the
thermal:vacuum chamber for final optical focusing adjustment and system character-
ization. The lens mounted on the outer stage was focused by rotating the lens-holder
component with a special drive tool mounted on a shaft passing through a rotary feed-
through in the vacuum-chamber wall while the cooler was at operating temperature.
After focusing, the most sensitive axis of the detector optical system was established
using a special translatable infrared source and commensurate optical-measuring
equipment.
During these measurements, it was established that the maximum radial move-
ment of the detector axis relative to the lens axis was less than 0. 025 millimeter
(0. 001 inch) as the cooler went from room temperature to operating temperatures.
Axial motion of the detector relative to the lens vertex was found to be approximately
0. 025 millimeter (0. 001 inch) over the same temperature range. No contamination was
detected on the optical system after extensive thermal and mechanical testing of this
unit.
CONCLUDING REMARKS
The radiative cooler described is a simple, rugged device with proven capability
for maintaining satellite-borne optical detectors at temperature levels of 100° K. It
is a passive device with intrinsic high reliability that can be readily integrated and
alined with precise optical systems. The design and the materials used in the con-
struction of this cooler promote long trouble-free operating life in the space environ-
ment with maximum protection of the low-temperature optical elements from
contamination.
74
DISCUSSION
A. L. Wade:
What information do you have about the criticalness of the contamination of the
cold surfaces in orbit? Also, what material was used for support between the inner
and outer stages ?
McCullough:
The orbital performance information that we are able to talk about is not com-
plete enough to draw conclusions about possible contamination phenomena. We have
been cleared to talk about only those aspects of the cooler design that I have presented.
R. L. Samuels:
Was surface-finish deterioration during orbital lifetime investigated?
McCullough:
Yes, the finishes that we selected are considered to be as good as any available
for the maintenance of the required thermal properties during extended exposure to the
space environment.
75
11.88
Dia,
9.95
D
Outer Stage, Ref.
/ Multilayer Insulation, Typical
f Mounting Ring, Ref.
___ ounting Surface
[ -- -- -1 I I
\
8.38 +'10 X 8.500 //
Diameter \ Diameter"
I ._ /
Clear Field of "" /
View of Deep
Space (101° )
Center of Gravity
Weight = 3.15-+0.15 Ib)i./ ,
J , Detector
/ .1
Inner Stage, Ref.
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DIMENSIONS ARE
IN INCHES
fj _ 7 Lens Holder
4.60-+.10
Diameter
Optical Axis
- "-_...- i_._ within
-'-J_ .... +5 milliradians
of [_-_]
---- Outer Stage Heater
(2.5W at 28 V DC)
Inner Stage
Cover
Removable
for Access
to Detector
2.36 2.94
i
Clear Plastic [
Protective Cover
Removable from
This side
2.78 3.34
Figure I.- Radiative cooler assembly.
76
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•s_uouodmoo ao.f_m .relOOO eAT:_Tp_veI -"g o_r_!_I
Figure 3.- Passive radiator for detector cooling.
78


Radiative cooler for spacecraft

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