49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition
4 - 7 January 2011, Orlando, Florida
AIAA 2011-256
Insulation Materials Development for Potential Venus
Surface Missions
Mike Pauken1, Lin Li2, Dannah Almasco3, Linda Del Castillo4,
Marissa Van Luvender5, John Beatty6, Mike Knopp7, and Jay Polk8
Jet Propulsion Laboratory, California Institute of Technology,
4800 Oak Grove Dr., Pasadena, CA 91109 USA
818-354-4242; Email: [email protected]
Any potential mission that operates on the surface of Venus requires a unique thermal
protection system unlike those found on traditional spacecraft. This paper describes the
results of thermal and mechanical testing of insulation materials that may be used to protect
a conceptual Lander for Venus surface operations. The thermal control strategy for a
conceptual Venus Lander does not rely upon developing new technology; it utilizes the
efforts pioneered by the Soviet Venera missions. A new investigation of insulation materials
is warranted because many of the practical design and implementation details of the Venera
thermal system are unavailable and the present generation of thermal expertise lacks
specific knowledge to claim heritage with validity. Thermal conductivity testing of three
different classes of insulation materials was made at earth-like and Venus-like conditions.
The material classes were porous silica, TiO2 filled aerogel, and silica fiber blankets. Earthlike test conditions were carried out in an oven at ambient pressure air at 470C. Venus-like
test conditions were performed in a heated pressure vessel capable of providing a CO2
atmosphere up to 470C and 92 bar pressure. In all material classes, the thermal
conductivity under Venus-like conditions increased significantly over the earth-like values.
Data from these tests can be used in thermal design to determine the required insulation
thickness to protect the Lander for the mission operating life. Mechanical testing of the
insulation materials was performed because the atmospheric entry and landing of the vehicle
can generate significant deceleration forces. Insulation bonding techniques have been
developed for attaching insulation to the exterior surface of a Lander pressure vessel. Data
on shear and tensile loading capacity have been collected for various adhesives and bonding
techniques. Furthermore, insulation restraint using an exterior skin of stainless steel foil has
been developed to enable the insulation material to handle up to 150g’s of deceleration
forces. Data from these tests can be used in the mechanical design of the insulation system to
ensure it would survive the high body forces of atmospheric entry and landing.
1
Senior Engineer, Thermal Hardware & Fluid Systems Engineering, 4800 Oak Grove Dr M/S 125-123, Pasadena
CA 91109, AIAA Member.
2
Student, Thermal Hardware & Fluid Systems Engineering, 4800 Oak Grove Dr, Pasadena CA 91109.
3
Student, Thermal Hardware & Fluid Systems Engineering, 4800 Oak Grove Dr, Pasadena CA 91109.
4
Senior Engineer, Advanced Electronic Packaging Engineering, 4800 Oak Grove Dr, Pasadena CA 91109.
5
Staff Engineer, Propulsion Flight Systems, 4800 Oak Grove Dr, Pasadena CA 91109.
6
Staff Engineer, Structures and Configuration Engineering, 4800 Oak Grove Dr, Pasadena CA 91109.
7
Staff Engineer, Materials & Contamination Control Engineering, 4800 Oak Grove Dr, Pasadena CA 91109.
8
Principal Engineer, Propulsion and Materials Engineering, 4800 Oak Grove Dr, Pasadena CA 91109, AIAA
Associate Fellow.
1
American Institute of Aeronautics and Astronautics
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Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Go
Nomenclature
ASTM
JPL
VEXAG
VMTF
=
=
=
=
American Society for Testing and Materials
Jet Propulsion Laboratory
Venus Exploration Analysis Group
Venus Materials Test Facility
I. Introduction
O
UR celestial neighbor Venus has long been a source of fascination for astronomers, stargazers, and science
fiction buffs. While Venus is sometimes called "Earth's sister planet", conditions on it are very un-earthlike. It
is a frustrating planet for astronomers because its surface is perpetually obscured by layers of clouds which disrupt
their view. With a temperature higher than the inside of an oven (over 460°C / 860°F ) and atmospheric pressure
equal to that of a kilometer under the ocean (~9.3MPa), the surface of Venus is one of the most hostile environments
in the solar system. This leads to a challenge in developing technology for the Venus exploration1. In the past, radar
probed the planet's surface. In addition, more than 20 robotic spacecraft have visited Venus since 19612. In recent
years, NASA has developed a growing interest in exploring Venus, prompting the Venus Exploration Analysis
Group (VEXAG; http//www.lpi.usra.edu/vexag/) to be formed in 2005 to develop strategies and technologies for
exploring Venus.
Due to the extreme environment on Venus, exploration at its surface requires that the mission complete its tasks
rapidly. So far, none of the ten missions from the Soviet Union or the United States that landed successfully have
survived for more than two hours at the surface. This is partially due to the thermal control architecture for the
Venus Landers which were designed to survive for the mission life time of the Landers. Additionally, Bugby3 (2008)
developed a thermal control architecture for a longer lifetime so that a potential Venus Lander could sustain surface
exploration for up to three to four hours. This corresponds to the optimal operating window for surface exploration
based upon maximizing data transmission volume from a potential Venus Lander to a Flyby Carrier that works as a
telecom relay to Earth. The thermal architecture needs thermal resistance to heat flow entering a potential Lander
vessel and uses heat capacity from phase change materials to extend the operating time of electronics within the
Lander4.
There are many thermal and mechanical characteristics for a Venus Lander insulation system that need to be
addressed prior to recommending and using a material for a Lander spacecraft. Some of these characteristics include
thermal conductivity in the Venus environment, mechanical strength in compression, shear and tension and bonding
methods. The thermal conductivity of several insulation candidates was measured in both Earth ambient pressure air
and in carbon dioxide in a Venus-like environment. The durability of the thermal insulation was tested in a vertical
acceleration drop test and in a centrifuge acceleration test. Mechanical properties of bonded insulation coupons were
made. These property measurements will help in designing and implementing the insulation system into a possible
future Venus Lander mission.
II. Thermal Conductivity Measurements
We evaluated five different external insulation candidate materials for a conceptual Venus Lander spacecraft.
Three of these materials were comprised of porous silica formulations from three different vendors. The trade names
of the materials are Microtherm Super-G, Min-K and Zircal-18. The 4th candidate was a JPL produced TiO2 filled
aerogel. The 5th candidate material was Q-fiber blanket, a material that is used on the upper aft sections of the Space
Shuttle. We looked into Shuttle tile materials but they were too expensive for our budget and had long lead times.
Thermal conductivity was measured for each material in an oven to confirm manufacturer’s data and to validate
the method of measurement. This data served as a baseline for the insulation materials. A separate data set was
collected in the Venus Materials Test Facility (VMTF) to estimate the conductivity under Venus-like atmospheric
conditions. A standard ASTM thermal conductivity test was not possible with the VMTF because there was no way
to install a cold sink inside the chamber that fit within our budget. Instead of using a cold sink and test under steady
state conditions, a transient method for calculating thermal conductivity was developed.
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Figure 1 shows one of the test articles used for
measuring the insulation thermal conductivity. The test
article was an aluminum bar, 5 cm x 5 cm x 25 cm
enclosed by 2.5 cm thick insulation. The aluminum bar
temperature was measured by a thermocouple. During
the oven test, the oven was brought to a high
temperature and dwelled for a period of time. The
temperature rise of the aluminum bar was used to
determine the heat flow through the insulation. A similar
approach was taken with the VMTF by raising the
temperature with a dwell period. Using the geometry of
the insulation and the heat flow enabled us to calculate
the thermal conductivity.
The heat flow through the insulation equals the
energy absorbed by the aluminum bar within the
insulation over time. The heat transfer rate Q into the
Figure 1. Insulation block test article for thermal
aluminum bar is computed using the transient energy
conductivity testing. Block was also used in
equation for a material.
centrifuge testing.
T
Q mc
t
The mass of the aluminum bar is m, the specific heat of the aluminum is c, the change in temperature of the
aluminum bar is T and the time interval is t. Heat conduction through the insulation required accounting for the
rectangular geometry surrounding the aluminum bar. The heat conduction equation for variable conduction area was
used.
To Ti
Q kA x
L
The heat conduction area A(x) changes with position
within the insulation. The value of the area function was
computed using the inner and outer surface area of the
insulation block. The thickness of the insulation
surrounding the aluminum bar is L, the external
insulation temperature is To and the internal insulation
temperature is Ti. The internal insulation temperature
was taken to be the temperature of the aluminum block
since it was difficult to actually insert a thermocouple
between the aluminum block and the insulation. The
thermal conductivity k, is the data to be determined by
the testing.
The VMTF is shown in Figure 2, without external
insulation. During a test the VMTF is insulated to reduce
heat loss to the room. This facility has an internal
working space that is approximately 18 cm diameter by
45 cm deep. The facility can be filled with carbon
dioxide gas and heated to 500 C. The gas pressure can
reach 100 bar. These temperature and pressure limits
permit testing articles at Venus-like conditions. The
facility has a back-pressure regulator to control and limit
the gas pressure as the temperature rises to the operating
conditions. It is equipped with several feedthrough ports
Figure 2. Venus Materials Test Facility –
that permit passing thermocouples into the chamber to
chamber for testing materials under Venus-like
record the temperature of test articles, gas temperature
atmospheric conditions.
and internal chamber all temperatures.
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Thermal Conductivity, W/mK
Figure 3 shows the results of the conductivity measurements. The conductivity values were calculated during the
dwell period after the oven and VTMF reached the operating temperature. The dwell period ranged from 1 to 4
hours for the tests. A general trend of increasing thermal conductivity is observed as the pressure of the environment
increases. This trend confirms observations by others5,6 that the conductivity of porous materials increases in the
presence of a high pressure fluid. The thermal conductivity measurements made in the oven at ambient pressure are
shown on the left hand side of the plot in Fig. 3 at a gas pressure of 1 bar. The thermal conductivity of the insulation
samples in the hot oven environment was found to be between 0.02 and 0.025 W/mK, except for the aerogel which
had a conductivity value of 0.06 W/mK. The aerogel was somewhat defective in that numerous cracks were in the
material. Several attempts were made to formulate the aerogel insulation without cracking but it required much more
development effort than we could afford at the time.
Tests in the VTMF were performed at several different temperatures and pressures for many insulation materials.
These intermediate values were taken to confirm trends in conductivity as a function of gas pressure. One
manufacturer (Microtherm) provided test data for their insulation at pressures up to 40 bar. This data is plotted in
Fig. 3 along with the test data we collected. Our measurements for conductivity of Microtherm in carbon dioxide
show lower thermal conductivity than in air measurements. This is to be expected since the thermal conductivity of
carbon dioxide is much lower than that of air. Under high pressure, high temperature conditions simulating the
Venus surface atmosphere, the thermal conductivity for most materials reached an asymptotic limit around 0.06
W/mK.
The aerogel insulation performed
Microtherm Air, Mfr
0.14
relatively poorly compared to the
Microtherm CO2
porous silica materials because of the
0.12
Zircal-18 CO2
cracks in the material. Without cracks it
Min-K CO2
may have performed as well as the
0.10
Aerogel #2
porous silica insulation. The Q-fiber
blanket had an unusual response to
Q-Fiber
0.08
pressure. At atmospheric pressure its
0.06
conductivity is much higher than the
porous silica materials and the aerogel.
0.04
But at high pressure it has better
performance and is nearly as good as
0.02
the other materials. The physical
structure of the Q-fiber blanket is
0.00
different than the porous silica so the
effects of pressure are different. The
0
20
40
60
80
100
energy transport mechanism for the
Gas Pressure, Bar
increase in conductivity with pressure is
Figure 3. Summary of insulation thermal conductivity test results.
not well understood, since the thermal
conductivity of a gas is not dependent
on pressure.
III. Insulation Bonding Development
Bonding insulation to the structural shell for a potential Venus Lander is critical to survival to ensure the
insulation protects the pressure vessel from the hot environmental conditions after experiencing launch vibrations,
nearby pyro-shock impulses, atmospheric entry deceleration and impact loading from the landing. Six candidate
adhesives were evaluated for bonding insulation: (1) Ceramabond 571, (2) Ceramabond 569 (3) Resbond 940, (4)
Resbond 989, (5) Durabond 952 and (6) Fibroclad 490. The adhesives were first evaluated using lap shears to
determine the basic metal-to-metal shear strength of the adhesives. The Durabond 952 had the highest shear
strength. The shear strength of all the adhesives exceeded the needs for handling the acceleration body forces on the
insulation. Flat-wise tensile test coupons were made by bonding 5 cm square Zircal-18 blocks to titanium surfaces.
The adhesives used in the flat-wise tensile tests were Resbond 989, Durabond 952, Ceramabond 571 and
Ceramabond 569. There was considerable variation in the bond strength, all breaking at less than 70 kPa. The
Resbond 989 had the best performance of the group. The surface of the insulation is dusty and many attempts were
made to remove the dust. However, a new dust layer is always formed during dust removal. The insulation
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manufacturers recommend wetting the surface with water prior to bonding but this didn’t help improve the bond
strength. The inability to get the adhesive to penetrate beyond the dust layer on one side of the insulation prevented
us from obtaining higher strength. Further development on this area is recommended for developing a more robust
bonding method.
Shear coupons were made with several 2.5 cm thick by 5 cm long Zircal insulation samples using Resbond 989
adhesive. Insulation coupons were bonded to each side of an aluminum tongue to give a balanced shear load on the
tongue during testing in the Instron machine. The shear strength of the adhesive joint exceeded the tensile strength
of the insulation coupons. The insulation failed before the adhesive joint failed.
Tensile strength data on insulation materials are generally not available since no insulation product is expected to
handle tensile loading. However, the acceleration body
forces will put the aft side material in tension on a potential
Venus Lander. We had 6 cone shaped insulation specimens
made form Zircal-18. The cone shape is per ASTM spec
for friable materials like insulation. We used cone cups for
pulling the samples in an Instron machine. The minimum
tensile load observed in the 6 samples was 500 kPa. This
means the insulation can handle 750-g acceleration forces
for 2.5 cm thickness. Figure 4 shows a tensile test coupon.
Compression data of the porous silica materials is available
from the manufacturers. All the porous silica products are
capable of handling 150-g loads in compression. The Qfiber blanket material is so light that there is little concern
Figure 4. Insulation coupon for tensile testing.
for it handling 150-g acceleration loads in any orientation.
It could be bonded to a structural shell using metal Velcro.
IV. Insulation Acceleration Testing
The insulation must be able to handle 150-g deceleration loads from atmospheric entry and landing impact. We
selected this level of acceleration based on the most probable requirements of future Venus mission scenarios. Two
kinds of acceleration tests were used to evaluate the performance of the bonding and restraining system for the
insulation. Impact acceleration testing was accomplished using attest fixture shown in Fig. 5. Three insulation
coupons, each measuring 5 cm x 5 cm x 2.5 cm. were bonded to an aluminum substrate with the Resbond 989
adhesive. The coupons were covered and restrained with a 2-mil thick stainless steel foil. The test apparatus allowed
shear, compression and tension loads to be covered in a single impact. In Fig. 5 the shear coupon is mounted on the
vertical leg, the compression coupon is mounted on the top of the horizontal leg and the tension coupon is mounted
on the bottom of the horizontal leg.
The acceleration test fixture had a 22-kg weight
that was rapidly brought down upon a foam cushion
base. Eight Latex rubber bungee cords were used to
augment the acceleration due to gravity. For each
impact test the text fixture was raised approximately
5-m above the foam cushion base. The bungee cords
were stretched about 300 percent at this elevation and
provided an initial force around 1000 N on the test
fixture. When the test fixture reached the foam
cushion base the bungee cords were slack. Gravity
provided a constant 230 N force on the test fixture. A
three-axis accelerometer was securely attached to the
test fixture to record acceleration data at a rate of 512
Hz. This instrument was a stand-alone unit that
recorded data after a 2-g threshold acceleration was
reached. The acceleration data was downloaded to a
Figure 5. Insulation coupons mounted on vertical
computer after each impact test.
acceleration drop fixture.
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The foam cushion base used six different
compression layers to achieve 150+ g-load
impacts. The first layer (relative to receiving the
impact) was a 2-cm thick polyethylene foam with
a 25 percent compression stroke at 60 kPa. The
first layer allowed the impact area to spread into a
second layer of 10-cm thick foam that had a 25
percent compression stroke at 2 kPa. The third
layer was another 2-cm thick (60 kPa) foam.
Layer number three spread the impact stroke
from the soft layer number two into a stiffer layer
number four. The fourth layer was a 10-cm thick
foam with a 25 percent compression stroke at 7
kPa. The fifth layer was another 2-cm thick (60
kPa) foam. The sixth layer was a 10-cm thick
piece of polystyrene which was used as the last
resort protection to prevent damage to the test
article or the floor. This particular arrangement of
foam was selected after several different test
arrangements were tried. The purpose of this
arrangement was to provide not only a peak gload above 150-g but to broaden the peak force as
much as possible.
Figure 6 shows the acceleration data of two
typical impact tests. Several impact tests were
made with the bonded insulation coupons both
with and without the stainless steel foil covers.
Tests were done without the foil covers to see if
the insulation was able to handle the acceleration
loads on their own. The foil covers protect the
insulation from damage in handling and storage.
Since the insulation tends to generate dust
particles, the foil helps contain the dust so it
would not collect on other surfaces. It also
restrains the insulation to keep it intact during
atmospheric entry and impact upon landing.
Stainless steel was chosen because it provides
good strength as a foil cover. After each impact
test the insulation coupons were visually
inspected for cracks or breakage. All insulation
coupons passed visual inspection after the impact
acceleration test, both with and without the 2-mil
foil restraint.
The centrifuge acceleration tests used the
insulation blocks from the thermal conductivity
testing. This allowed testing of the insulation on a
larger dimensional scale. Figure 7 shows the
aluminum blocks were modified to hold two 3/8
inch threaded rods for connecting to the vendor’s
centrifuge mounting plate. The insulation was
bonded on the tensile side of the block to the
aluminum bar. This is the side opposite the
threaded rods. The insulation was wrapped with
stainless steel foil skin for surface protection.
Figure 6. Vertical drop test impact acceleration curve of
insulation coupons.
Figure 7. Centrifuge test article, aluminum block
enclosed by insulation.
Figure 8. Two Insulation blocks mounted into
centrifuge.
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Acceleration, Z-dir, g
Three straps were wrapped around the insulation block to ensure it would hold together during the acceleration test.
One block of Microtherm insulation and another block of Min-K insulation were used in the centrifuge acceleration
tests. Both blocks were mounted to the centrifuge as shown in Fig. 8. An accelerometer was mounted to the
centrifuge interface plate. Since the location of the accelerometer was about 10 cm closer to the centrifuge rotational
axis, the acceleration measured by the instrument was about 2-g less than experienced by the aluminum block.
The centrifuge acceleration tests were performed at three different levels: 50-g, 100-g and 150-g. The centrifuge
was spun-up until the target g-level was reached. The centrifuge then dwelled at that load for about 6 seconds then it
was spun-down. Figure 9 shows the accelerometer data for the 150-g test. The acceleration force on the insulation
can be determined from the rotational speed of the
Venus Insulation Acceleration Test Profile Detail
centrifuge. The rotational speed confirmed the
160
acceleration levels measured by the accelerometer.
The advantage of the accelerometer is that it
155
records the vibration levels on the centrifuge. Since
the centrifuge was not finely balanced the vibration
150
on the machine was about +/- 1-g as shown by the
amplitude variation in the accelerometer data.
145
After each test the insulation specimens were
visually inspected. Although the surface of the
140
insulation could not be seen because of the
stainless steel foil covering, each block was
135
physically checked to see if chunks of insulation
separated. After the 150-g test, the insulation
130
blocks were disassembled from the centrifuge and
5.8
5.9
6
6.1
6.2
6.3
6.4
the foil skins were removed for visual inspection of
Time, minutes
the insulation. There were no visible cracks or
Figure 9. Centrifuge acceleration curve for insulation
other defects in the insulation by this inspection. In
blocks.
conclusion, the insulation blocks passed the
acceleration test in this configuration.
V. Conclusion
Through this project we developed the thermal and mechanical properties of several candidate insulation
materials for use on a possible future Venus Lander mission in a relevant environment. As a result of this effort we
have been able to determine the thermal conductivity of several insulation materials with the actual Venus
environment, which is essential to the success of potential Venus missions. We demonstrated the insulation can be
bonded to a titanium substrate, which is the material used for previous Venus Lander pressure vessels on Venera and
Pioneer Venus missions. Furthermore, we demonstrated that the insulation can handle both impact and gradual
acceleration loads in excess of 150-g.
Acknowledgments
This publication was prepared by the Jet Propulsion Laboratory, California Institute of Technology, under a
contract with the National Aeronautics and Space Administration.
We thank Phil Stevens for providing the oven for the thermal conductivity testing, Al Owens for helping with
assembly and refurbishment of the Venus Materials Test Facility and Randy Williams and Robert Williams for
providing technician support in plumbing and assembling the VMTF for several thermal conductivity tests. Dannah
Almasco was responsible for the conductivity testing and data analysis. Eric Oakes bonded the insulation samples.
Dominic Aldi performed the tensile testing of the insulation coupons. Lin Li was responsible for the impact and
centrifuge acceleration testing.
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References
1
Laub, B. and Venkatapathy, E., "Thermal Protection System Technology and Facility Needs for Demanding Future
Planetary Missions," Proceedings of the International Workshop: Planetary Probe Atmospheric Entry and Descent Trajectory
Analysis and Science, Eur Space Agency Sped Publ ESA SP, 2004.
2
Basilevsky, A., Ivanov, M., Head, J., Aittola, M., Raitala, J., "Landing on Venus: Past and future," Planetary and Space
Science vol 55, issue 14, Nov 2007, pp 2097-2112.
3
Bugby, D., Seghi, S., Kroliczek E., Pauken, M.,. "Novel Architecture for a Long-Life, Lightweight Venus Lander," Space,
Propulsion & Energy Sciences International Forum: SPESIF-2009 Huntsville AL (AIP Conference Proceedings Series Volume
1103). Vol. 1103, 2009, pp. 39-50.
4
Pauken, M., Emis, N, Van Luvender, M., Polk, J., Del Castillo, L. “Thermal Control Technology Developments for a Venus
Lander” Space, Propulsion & Energy Sciences International Forum: SPESIF-2010 AIP Conference Proceedings Series Volume
1208). Vol. 1208, Baltimore MD, Feb 2010, pp 68-75.
5
Litovsky, E,. Shaprio, M., Shavit, A., “Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous
Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%”, J. Am. Ceram. Soc. 79 (5), 1366-76,
(1996).
6
Zheng, G., et al., “An Update on Heat Transfer in a Porous Insulation Medium in a Subsea Bundled Pipeline”, Journal of
Energy Resources Technology, ASME, vol 123, pp 285-290, December (2001).
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