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PRESSURE VESSEL TECHNOLOGY DEVELOPMENTS
4th INTERNATIONAL PLANETARY PROBE WORKSHOP
27 June – 30 June, 2006 Pasadena, California, USA
Michael Pauken(1), Elizabeth Kolawa(1), Ram Manvi(1), Witold Sokolowski(1), Joseph Lewis(1)
(1) Jet Propulsion Laboratory, 4800 Oak Grove Dr. M/S 125-109, Pasadena, CA 91109, USA, Email:
michael.t.pauken@jpl.nasa.gov
ABSTRACT
Historically, titanium and aluminum have been used as
structural shells or pressure vessels for extreme
environment planetary probes and landers.
Improvements in the state-of-the-art of pressure vessel
materials are sought to reduce the mass of such
components by 20 to 50% over titanium shells. The
pressure vessel represents the single largest mass
element for deep atmospheric probes at Venus, Jupiter
and the other Gas and Ice Giants, or landers to the
surface of Venus. The high loads on the spacecraft due
to atmospheric entry, landing and external atmospheric
pressure require high strength structures and new
fabrication techniques. Significant improvements to the
overall spacecraft design can be realized by reducing
the overall mass of the pressure vessel to allow
additional payload mass. New structural shell materials
exhibit high strength and stiffness at elevated
temperatures and are resistant to creep and buckling
under high external pressures.
1. INTRODUCTION
The Decadal Survey identified missions to the surface
of Venus and the Jupiter Deep Atmospheric Probe as 2
of the 6 highest priority science missions. The present
state-of-the-art pressure vessel material, titanium,
represents one of single largest mass elements for a
Venus Lander or Deep Atmospheric Probe. Given that
this material has been used since the early 1970s as the
structural shell for these kinds of missions, it is worth
examining material improvements over the last 3
decades to see if significant mass reductions can be
realized with different shell materials. The pressure
vessel for a Venus Lander mission about 1 m diameter
would have a mass around 200 kg if it were made of
titanium. Using new materials and manufacturing
methods, it appears that mass reductions on the order of
50% can be realized over a monolithic (solid metal)
titanium shell. However, the Technology Readiness
Level (TRL) for these new materials and manufacturing
methods as they would apply to a spherical shaped
structural shell are typically around TRL 3 even though
they may have significant heritage in other applications.
Lighter weight pressure vessels impact the whole flight
system by reducing the loads carried by the structures
within the aeroshell, carrier spacecraft bus and the
launch vehicle itself. This is especially significant
when designing structures for handling the atmospheric
entry loads encountered for missions to Venus or
Jupiter which can have decelerations as high as 200 to
300 Gs. This paper describes an investigation into new
materials and their associated manufacturing processes
for fabricating a spherical shaped pressure vessel (or
structural shell) that has potential for use in a space
flight mission to the surface of Venus or through the
atmosphere of any of the Gas Giant Planets.
2. HISTORICAL PERSPECTIVE
In 1961 the Soviet space agency started an extensive
Venus exploration program that eventually included
orbiters, atmospheric probes, landers and balloon
missions. The program was very successful resulting in
many completed missions but it took Soviets many
years to learn how to survive and conduct science
investigations in Venus environment. In the late
seventies NASA conducted a multiprobe mission,
Pioneer Venus, aimed at understanding Venus
atmosphere.
The Soviet program lasted more than two decades from
their first attempt to send a spacecraft to Venus –
Venera 1 launched in 1961 to their last mission,
VEGA1 and 2, in 1984 which included both a lander
and a high altitude balloon on each of two vehicles.
The two spacecraft continued to an encounter with
Halley’s Comet after deployment of their payloads at
Venus.
The first spacecraft, Venera 1, had no provision for
surviving entry. At that time, Venus was believed to
have a much thinner atmosphere and benign
temperatures than those we know today. As successive
missions were launched they had increasing levels of
capability and were equipped to deal with the more
severe environmental conditions. In 1965, Venera 3
was the first successful spacecraft to land (by impact)
on another planet. However, it was designed to
withstand 5 bars external pressure and 80ºC
temperature. From 1967 through 1969, Venera 4, 5 and
6 were sent to Venus and were designed to withstand
300ºC and 25 bar. When Venera 5 and 6 were sent, it
was known that the surface temperature of Venus was
427ºC and the surface pressure was at least 75 bar.
However, it was too late to change the design of those
spacecraft. A drawing of the Venera 5 descent module
is shown in Fig. 1.
Fig. 1. Venera 5 Descent Module Layout showing all
systems were protected by a pressure vessel with
passive thermal control.
By 1970, Venera 7 was designed to survive 150 bar and
540ºC and used a titanium spherical pressure vessel.
The earlier Venera landers used a hemispherical
capsule. This spacecraft successfully survived on the
Venus surface for 23 minutes becoming the first
spacecraft to transmit from another planet. When
Venera 8 was launched in 1972, scientists had accurate
estimates of the temperature and pressure environment
on Venus. The titanium pressure vessel was designed
for surviving 490ºC and 100 bar. It transmitted data
from the Venus surface for 50 minutes. A drawing of
the Venera 8 descent module layout from [1] is shown
in Fig. 2.
The remaining Venera missions 9-14 continued to use
the same spherical pressure vessel design criteria as
Venera 8, however, they were larger in diameter and
were mostly successful missions. Venera 13 and 14
were launched in 1981. The limiting factor in the later
missions was communication time with the flyby
spacecraft. Fig. 3 shows a photograph of the Venera 13
Lander.
Fig. 2. Venera 8 Descent Module Layout showing the
systems packed in a spherical pressure vessel.
Fig. 3. Venera 13 external configuration photograph.
The only NASA mission to the Venus surface was
Pioneer Venus which was launched in 1978. It
consisted of one large probe and three small proves.
The large probe had a 78 cm diameter titanium pressure
vessel while the small probes at 47 cm diameter
pressure vessels. Only one of the small proves survived
on the surface; none of them were specifically designed
to survive landing. Fig. 4 as in [2] and [3] shows an
inside view of the Pioneer Large probe.
Fig. 4. Pioneer Venus Large Probe interior layout.
A summary of the general development trend in the
pressure vessel ratings for missions to the Venus
surface is shown in Table 1.
Table 1. Historical Summary of Pressure Vessel Ratings
for Venus Landers
Mission Launch Pressure Rating
Venera 3 1965 5 bar
Venera 4 1967 20 bar
Venera 5,6 1969 25 bar
Venera 7 1970 150 bar Titanium
Venera 8-14 1972-1981 100 bar Titanium
Pioneer Large Probe 1978 100 bar Titanium
Pioneer Small Probe 1978 100 bar Titanium
PRESSURE VESSEL DESIGN GUIDELINES
A standard set of guidelines was established to compare
different pressure vessel materials to one another.
Three basic mechanical parameters were used to
estimate pressure vessel mass for a given shell diameter.
The shell must satisfy these criteria at a temperature of
500ºC: (1) No buckling at the ultimate load of 150 atm
pressure using standard NASA knockdown factor of
0.14 for pressure vessels. The common industry
standard knockdown factor is 0.30. Knockdown factors
account for imperfections in the material and the
manufacturing process which deviate from the ideal
case. The elastic modulus determines the bucking limit.
(2) No yielding at the proof load of 125 atm pressure.
The yield strength determines the yield limit. (3) Total
allowable creep in 10 hours under 100 atm external load
must be less than 0.5%.
These criteria are evaluated based on compressive yield
strength, compressive modulus, and creep strain rates.
Additional necessary requirements for the pressure
vessel material include impermeable to gases and
compatibility with the Venus chemical environment. It
is also desirable to have low conductivity, however, this
requirement can be mitigated against through better
insulation. Other factors to be considered in selecting
shell materials include: fracture toughness, heat
capacity, and thermal expansion coefficient.
Several material candidates were examined to determine
their suitability for a spacecraft pressure vessel
operating in a Venus environment. These materials
were compared to the current state-of-the-art material
titanium-6Al-4V. Materials were classified as metallics
or composites and are listed below:
Metallic Materials
- Titanium Beta S
- Nickel-chromium alloys: Inconel 718, Inconel
X and Haynes 230
- Nickel-chromium-cobalt alloys: Haynes 188
- PH stainless steels: 17-7 PH or 15-5 PH
- Beryllium I-220H
Advanced Composite Materials
- Silicon carbide fiber reinforced titanium matrix
- Boron fiber reinforced titanium matrix
- Inorganic Sialyte based composite
- Aluminum-sapphire carbide metal matrix
- Aluminum-silicon carbide metal matrix
- Epoxy Polymer matrix composite
MATERIAL EVALUATIONS
Inconel 718 showed the best performance in both creep
and tensile property comparisons and is the best
metallic candidate for a pressure shell using a
honeycomb sandwich construction. The high density of
nickel alloys prohibits them from being considered for
monolithic shell designs. Ti-6Al-4V was the second
best performer in the creep and tensile comparisons at
temperature. This is the traditional Venus lander
spacecraft pressure vessel material and is fabricated in a
monolithic shell.
Haynes 188 was originally selected as a candidate
because of its superior creep properties at high
temperature. However the operating temperature of the
pressure shell (500ºC) is not high enough to utilize the
creep resistance of Haynes 188. The Haynes 188 alloy
(cobalt base) was designed to perform in the 900ºC to
1100ºC range where it is clearly superior to the other
materials selected. At 500ºC it is no better than Inconel
718. Haynes 188 was not retained for further
consideration.
15-5 PH showed reasonable creep properties at 500ºC,
but the creep resistance falls very rapidly in this
material above 500ºC leaving little margin. 15-5 PH
was not retained for further consideration. Creep data
was not available for 17-7 PH. However it is not
expected to perform significantly better than 15-5 PH
and was not be retained for further consideration.
Beryllium is lightweight and has high elastic modulus,
high thermal conductivity and high specific heat but
low creep resistance in tension at temperature. Toxicity
issues raise concerns regarding fabrication; however
established vendors are available to fabricate beryllium
products.
In all the metal candidates, bucking was the limiting
criteria except for beryllium because of its high elastic
modulus. A comparison of elastic modulus as a
function of material density at room temperature is
shown in Fig. 5 for the metallic candidates. At 500ºC,
the magnesium and aluminum alloys drop out of
consideration. Beryllium clearly has the highest
modulus per unit mass of all candidates even at 500ºC.
It is limited by yield and creep.
Fig. 5. Modulus comparisons of various metals at room
temperature.
SiC/Ti matrix composite has superior strength/density
performance compared to other materials. It is creep
resistant at 500ºC. It is suitable for fabricating a
monolithic shell configuration. Boron fiber titanium
matrix composite has good strength to density
performance but the boron fibers degrade significantly
above 400ºC. It could possibly be used with an external
insulation system that would keep the shell temperature
below the degradation temperature limit; however it
was decided not to pursue this option.
Sialyte is a trademarked inorganic resin product
developed by Cornerstone Research Group. The resin
is used to fabricate lightweight fiber-reinforced
composite structural components. It has a low
coefficient of thermal expansion, and relatively high
compressive strengths offering consistent performance
up to 900ºC. However, it did not have sufficient
strength to be considered as a viable candidate for a
Venus Lander pressure vessel.
The aluminum-sapphire carbide metal matrix and
aluminum-silicon carbide metal matrix are fabricated by
passing sapphire-carbide fibers or silicon carbide fibers
through a bath of molten aluminum and then wrapped
around a mandrel. The process is similar to the method
used to make composite pressure cylinders. This allows
lightweight tanks to be made without aluminum liners
for gas retention. These materials/processes are worth
consideration for a Venus Lander pressure vessel but
they have not been thoroughly evaluated. The
composite material properties are dependent upon the
manufacturing technique which needs to be developed
for a hemispherical geometry featuring flanges, feed-
throughs, ports etc.
An epoxy polymer matrix composite material using the
trademarked name Kiboko has been developed by
Composite Technology Development Inc. to fabricate
lightweight linerless composite wound pressure vessels.
It has been primarily developed for storing cryogenic
materials or for gases around room temperature. A
novel toughened epoxy resin provides the sealing
necessary to eliminate the need for a tank liner. Two
issues with this material are similar to those above with
the aluminum-silicon carbide material: (1)
manufacturing a wound product into hemispherical
shapes that incorporate many complicating features and
(2) performance at high temperatures has not been
demonstrated so it may require an exterior insulation
system to prevent premature failure.
MANUFACTURING METHODS
Three different pressure vessel configurations have been
identified based on the different materials that were
considered in this study. Monolithic shells can be
fabricated from titanium or beryllium, which has been
the traditional manufacturing process for spacecraft
landing on Venus’ surface. Composite wrapped shells
are commonly seen in pressure cylinders and the
technology is well developed. This manufacturing
technique would be used for aluminum/sapphire or
aluminum/silicon carbide or Polymer Matrix Composite
materials. Honeycomb sandwich shells are often
formed into curved geometries for aircraft engine
cowlings for example. This is an appropriate fabrication
technique for Beta S titanium or Inconel 718.
Fabricating a monolithic shell for a pressure vessel uses
fairly common manufacturing processes. A titanium
hemisphere can be shaped using spin forming. Flanges,
windows, feed-throughs, brackets etc. can be welded
onto the shell to create the spacecraft pressure vessel.
An example of a three piece sphere is shown in Fig. 6.
A three piece sphere allows two equipment shelves to
be mounted to a central ring, while the forward and aft
sections of the sphere serve as caps mounted to the
center section.
Fig. 6. Cut-Away sectional view of a 3 piece monolithic
shell.
Fabricating a monolithic shell out of beryllium can be
more difficult than out of titanium. Beryllium is a
brittle material and cannot be shaped by spin forming.
The spherical sections would have to be machined from
solid billets. Flanges and windows etc. cannot be
welded to a beryllium shell so these features would
have to be machined as part of the shell from the parent
billet material. A three piece spherical shell is preferred
for beryllium because the billets for each section would
be smaller than if it were made into hemispheres. It
also would reduce rework costs if mistakes were made
during the fabrication process by having to scrap say
only 1/3 of the sphere instead of ½ of the sphere. Issues
regarding toxicity of beryllium during fabrication
processing are of concern and must be dealt with. Also,
the vendors qualified to work with beryllium are
limited.
Composite wrapped tanks are now commonplace and
the latest innovations involve linerless tanks. The
impermeable aluminum liner has been replaced by
using resins that form a gas-tight barrier which resists
microcracking as the pressure cylinder is loaded and
unloaded over its lifetime. The manufacturing process
consists of passing the wrapping fibers through wet
adhesive such as molten aluminum or epoxy. The
wetted matrix is then wrapped around a mandrel to form
the tank shape. The composite wound tank is then
cured at an elevated temperature to set the tank. A
picture of composite wound linerless tanks is shown in
Fig. 7. While this process is conducive to fabricating
pressure cylinders or composite tubes, it has not be used
to fabricate hemispherical sections. Thus the
manufacturing process for creating a structural shell for
a spacecraft with flanges, windows, feed-throughs etc
still needs to be developed.
Fig. 7. Linerless composite tanks developed by
Composite Technology Development Inc.
Honeycomb sandwich construction produces strong
lightweight panels for many applications. While a large
majority of honeycomb structures are flat panels, many
curved components are fabricated with a honeycomb
sandwich construction. To manufacture a spherical
shaped segement using honeycomb requires forming the
inner and outer facesheets into the desired shape using a
bulge-form technique. A picture of a bulge forming
tool is shown in Fig. 8. The honeycomb core is made
by diffusion bonding thin corrugated sheet (ribbon)
sliced to the desired web thickness. The core is then
bulge-formed to match the inner and out facesheets.
The core is assembled to the facesheets in a special
toolset and a braze alloy is added to bond the facesheets
to the core. For a titanium structure, TiCuNi braze alloy
would be used, while for Inconel the braze alloy would
be BNi-8. There are several methods of completing the
brazing process, but typically the assembly would be
placed in a vacuum braze furnace. A vacuum braze
furnace is shown in Fig. 9. When the brazing process is
complete, the tool set is removed and the part is ready
for attaching features such as windows, flanges,
brackets, etc. Adding these components would require
cutting the shell for openings and brazing in window
ports for example. Flanges and brackets would be
brazed on to the shell.
Fig. 8. A typical bulge forming tool for fabricating
honeycomb facesheets.
Fig. 9. A vacuum braze furnace for bonding
honeycomb structures.
CONCLUSIONS
Development of improve materials and manufacturing
methods for fabricating space qualified pressure vessels
or structural shells is far from complete. Some of the
remaining technology development tasks to improve the
current state-of-the-art include: (1) Develop more
detailed manufacturing engineering plans for the
leading candidate materials. There are many issues
involved in fabricating a simple hemispherical shape
that can be sealed together with a mating part. Adding
features such as optical windows, electrical
feedthroughs, flanges, brackets etc. makes the
manufacturability of a spacecraft shell even more
challenging. (2) Estimate comparative fabrication costs
for the different manufacturing technologies. This can
help select which technologies are financially feasible
to pursue further development. (3) Obtain
samples/prototypes of shells from leading candidate
materials to demonstrate that the technology is practical.
And (4) Perform testing on subscale prototypes under
Venus-like environmental conditions for temperature
and pressure survivability. The materials and the
manufacturing methods examined in this study have the
potential for reducing the mass of titanium baseline
pressure vessel for a mission to a high
pressure/temperature environment by 30 to 50%.
ACKNOWLEDGEMENTS
This work was performed through an internal research
and development effort at the Jet Propulsion
Laboratory, California Institute of Technology under a
contract with the National Aeronautics and Space
Administration.
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Analysis and Science, Lisbon Portugal, 2003.
3. Hennis, L.A. and Varon, M.N., Thermal Design and
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