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5.4 MATERIALS:
This detailed project report (DPR) introduces the knowledge base required for the development
of materials for high altitude airship (HAA). This report basically deals with the materials and the
related technology and testing facilities required for the development of high altitude airship. The
report particularly describes the challenges in the development of materials currently used as
well as the potential materials for future HAA development. The required testing facility and their
availability in India and abroad have also been discussed. The report also covers the list of
leading R & D labs as well as organizations in the world, actively involved in development of
HAA.
Material development suitable for high altitude airship application, presents many a challenges
to the material designer. The strength-to-weight ratio significantly affects HAA system size and
altitude. The challenge is to develop a very lightweight as well as strong material that is capable
of containing lifting gas and is resistant to the environment. The stratosphere extends from
approximately 17 km to 50 km above the Earth’s surface. The stratosphere is also called the
“ozone layer” because 90% of the earth's ozone is concentrated in this region. At this altitude,
the nominal temperature is -70ºF which can cause the material to become brittle with a resultant
loss of flexibility. The high ozone concentration and intense UV radiation can also deteriorate
LTA material, resulting in a loss of strength and permeability
The high strength-to-weight ratio, low creep, low moisture regain, and improved hydrolysis
resistance makes polyester fiber a good choice for lighter than air (LTA) applications. A typical
conventional airship envelope is made up of polyurethane-coated polyester fabric laminated
with a gas-retention layer. The strength and weight for these laminates samples was typically
400 N/cm and 400 gram/m2 respectively [7, 8]. However, using these conventional envelopes,
we cannot realize a high altitude airship which can operate in the harsh environment of
stratosphere. Development of high performance fibers converted the concept of HAA into a
reality. Currently, Korea, Japan and United States are the three major countries that have real
activities in the research and development in this area. Recently, Korea and Japan flew
prototype 50–60 m unmanned airships as a technical demonstration [7]. Zylon® and Vectran®
are the two important materials which are being used intensively for LTA application for the
strength layer. Fluoropolymers, Tedlar® and Teflon® films are being used as a protective layer
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in LTA hull material applications due to their excellent weather resistant properties [7]. There are
other materials under research which can be potential materials for HAA development.
5.4.1 Hull: external skin of HAA
Hull, the external envelope of the HAA system, remains exposed to harsh outer atmosphere
throughout the working period. It does not only provide the structural strength for the system but
also acts as the primary barrier between the outside air and lifting gas. For high altitude lighter-
than-air (LTA) applications, the hull material must exhibit low gas permeability, high
environmental resistance, high strength-to-weight ratio, and excellent tear resistance. To serve
these specific purposes as an outer skin of the HAA system it should have the following
properties:
• Light in weight to minimize Airship size and weight
• High strength
• UV and ozone resistance
• Weatherproof outer layer to protect the system from environmental degradation
• Lower gas permeability to minimize lift loss and maximize on station time
• Flexibility at wide range of temperature
• Longer Durability or service life
A typical hull material should be a multi-layered flexible laminate as a single layer may not be
able to meet all the requirements. The material generally consists of components, which permit
tailoring of the various material properties to optimize the resulting balance between tensile
strength, service life, weight, gas retention, and
flexibility.
Fig 5.13 shows a typical layout of hull material which
consists of multiple layers having different
properties.
This multiple layered structure is composed of
mainly three important layers namely:
1. Strength Layer
2. Protective Layer
3. Adhesive layer Fig 5.13: Typical Hull material layout
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Strength layer provides the required strength to sustain the super pressure experienced by the
system while the protective layer protects the system from harsh environmental conditions such
as Intense UV, Ozone, and weathering. Protective layer also serves the purpose of gas barrier
to retain the “He-gas” for a longer duration which increases system’s service life. Adhesives are
used to bond the various components layers together to give a final multilayered structure [7].
5.4.2 Strength layer:
In a typical LTA hull material, the strength layer is woven fabric, either single or multi layered. It
should be able to provide structural strength for the system and should be lighter in weight at
the same time. So that it can sustain the super high pressure created in the system as well as it
should be capable of taking higher pay loads. Hence the important properties required for the
strength layer are:
• High strength
• Light weight
• Service at in a wider range of Temperature
• Good bond-ability / sealability
A. Protective / barrier holding layer
The airship is normally of a huge size and has to fly at harsh environments of stratosphere
(Wide range of extreme temperature, low pressure and high UV and cosmic radiation) and also
for a long endurance time [20]. At higher altitude, the nominal temperature is as low as -56ºC
which can cause the material to become brittle with a resultant loss of flexibility. The high ozone
concentration and intense UV radiation can also deteriorate high altitude airship (HAA) material,
resulting in a loss of strength and permeability [7].
Hence, a protective/barrier layer is necessary to provide weather-ability and gas retention
characteristics. The material chosen for this layer(s) must also have good shear stiffness, bond-
ability, and thermal reflective/emissive properties. So, the properties required for a material
suitable as a protective layer are:
• weatherability
• gas retention
• Low temperature flexibility
• thermal reflective / emissive properties
• bondability
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B. Adhesive layer
Adhesive is a type of material often used in multilayered composite material to bond the various
component layers together. In the case of HAA system, adhesive layer binds strength layer and
protective together. A thin adhesive layer is critical to minimize material weight. Flexibility and
good bondability to each component (to prevent delamination) are essential requirements for
the adhesive.
The intermediate adhesive layers can be either thermoset or thermoplastic depending
upon the material and technology used. However, if welding technology is used, the
inner layer must be thermoplastic. The adhesive properties are mainly controlled by the
bulk properties of the polymer selected. Various formulating agents, like plasticizers and fillers,
are used to modify the base polymer. In addition to the adhesive, substrate surface properties,
treatments, and application processes also significantly affect the finished material weight,
flexibility, bond strength and other physical properties. Selecting the adhesive, the substrate
surface treatment, and the proper application technique are key parameters in achieving a well
engineered material.
The property required for the adhesive materials are:
� compatibility with other materials used in different layers
� Good Bondability
� Low temperature flexibility
C. Present scenario of material development
The most challenging aspect in designing the strength layer is the identification of a structural
fiber which makes it suitable for its purpose. With the development of synthetic fibers, the
textiles used in the LTA industry have undergone many changes. Synthetic fibers provide the
highest strength-to-weight ratios, and scientists continue to work on design and development of
new synthetic polymers and fibers.
Fig. 5.14 shows the history of modern textile fiber development. Nylon and polyester fiber are
synthetic fibers that were commercialized in the 1940's and 1950’s. The combination of
moderately high strength and moderately high extension gives a very high energy to break or
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work of rupture. Nylon and polyester fiber have broad range of applications including clothing,
furnishings, and the industrial textile market [7]. However, the high strength-to-weight ratio, low
creep, low moisture regain, and improved hydrolysis resistance makes polyester fiber a good
choice for LTA applications. Almost all current LTA hull materials are made from a high tenacity
polyester fiber composite.
Fig 5.14: Textile fibre development [7]
The high performance fibers became available in the late twentieth century. High performance
fibers are defined as high-modulus (over 300 g/den) and high tenacity (over 20 g/den). The
commercially available strongest fibers are Zylon® and Spectra®. Other high performance
fibers include Vectran® and Kevlar®. M5® fiber is a new fiber in development. It is a “designer”
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fiber that is targeted to be used both as a ballistic fabric and a composites material – a dual-use
status that previously has not been successfully achieved by any polymer fiber.
High performance fibers are attracting considerable attention in the LTA industry. Zylon® and
Vectran® are the two important materials which are being used intensively for LTA application
as strength layer. NAL and JAXA, both the Japanese organization are using Zylon® for this
purpose while NASA and Lockheed Martin, both from USA are using Vectran®. Korea and UK
are also working in the area of LTA system and they are also using Vectran®.
However, concerns such as stress concentration due to low extension, creep characteristics (for
Spectra®), moisture and UV resistance (for Zylon) remain for these fibers. Another important
consideration is that most high performance fibers are developed for special rather than for
commercial applications. This means that the cost for high performance fiber is much higher,
resulting in limited availability, research, and technical information.
D. Potential materials
Potential material for strength layer
With the inventions of new high performance fibres the capability of airships has also increased.
Now the duration as well as the altitude of floating for airship is increasing. People are looking
for lighter weight material with higher strength so that it can sustain higher pressure developed
inside at high altitude and also to carry high pay load. Different materials which can be a
potential material for strength layer are discussed below.
High performance fibre for strength layer
• M5 Fibre
M5 fiber is a high performance fiber originally developed by Akzo Nobel (Brew et al, 1999; Van
der Jagt and Beukers, 1999; Sikkema, 1999; Lammers et al, 1998; Klop and Lammers, 1998;
and Hageman et al, 1999) and currently produced by Magellan Systems International
(Magellan) which is a majority owned subsidiary of DuPont [21-23].
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Fiber properties are continually increasing with advances in processing of the fiber. Recently,
M5 fibers with tensile strength of up to 7.2 GPa and tensile modulus of 344 GPa have been
observed. It has been reported that M5 fibers were observed to be stable after exposure to
visible and ultraviolet light. After exposure to Xenon lamp for up to 100 hours, the M5 fibers
retained essentially all of the virgin fiber strength; by comparison, Zylon fibers lost over 35% of
the original fiber strength at this exposure time. The M5 yarns were similarly stable after
exposure to elevated temperature and humidity, as illustrated in Fig. 5.15. After exposure to 180
oF and 85% relative humidity (RH) for up to 11 weeks, the M5 yarns retained essentially all
of the virgin fiber strength; Zylon yarns lost over 20% of the virgin strength at this
exposure time [25].
Fig 5.15: Strength loss in M5 and Zylon after exposure to 180F, 85% R.H [25]
• Dyneema® fibre
Dyneema® fibre possesses extremely high strength and modulus, low density, high impact
strength, high durability, and excellent light and chemical stability which make it a highly
potential candidate for the strength layer.
DYNEEMA® is manufactured by “Gel spinning technology” in Nippon Dyneema Co., Ltd
(Osaka,Japan), a joint venture established by DSM Dyneema (Urmond, Holland) and Toyobo
(Japan). It is nothing but a super strong polyethylene fiber that offers maximum strength
combined with minimum weight [26].
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DYNEEMA® SK60 has the highest level value of the specific strength and modulus among
commercialized organic fibers. Specifically, the brand-new DYNEEMA® NKS achieves strength
of 40cN/dtex (c.a.4GPa) or more [26].
The specific gravity of DYNEEMA® SK60 is 0.97, which is the lowest value among super fibers
and is lighter than water. DYNEEMA® has excellent energy absorption which is the most
important property for protective clothing.
Fig 5.16 Strength and Density comparison of some High Performance Fibres [26]
DYNEEMA® has excellent abrasion as well as fatigue resistance. Due to its ability to be
processed easily during weaving, knitting etc., this leads to wide applications for industrial use.
DYNEEMA® shows good light stability due to its chemical and highly crystallized structure.
DYNEEMA® can be used without special covering or coatings.
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Fig 5.17: Good Light and Excellent Chemical Stability (treat with 23 X 2000 hrs) [26]
DYNEEMA® shows excellent chemical stability in a wide pH range, which prompts the
application for chemical industries. Because there is no degradation from sea water absorption,
DYNEEMA® is an ideal material for marine and off-shore use [26].
• Spectra fibre
Spectra® fiber, a polyethylene fiber is produced using a patented gel-spinning process. Now it is
commercially available in 3 forms: Spectra® fiber 900, Spectra® fiber 1000 and Spectra® fiber
2000. It is commercially being produced by Honeywell [27].
Spectra® fiber 900 was the first commercially available extended-chain, high-strength
polyethylene fiber and the first in a series of Spectra® fibers.
Spectra®fiber has one of the highest strength-to-weight ratios of any man-made fiber. Its high
tenacity makes it, 15 times stronger than steel, more durable than polyester and gives it a
specific strength that is up to 40 percent greater than that of aramid fiber. Specific performance
is dependent upon denier and filament counts.
Physical properties of Spectra® fiber 900 are given in the table 5.8 below.
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Table 5.8: Physical properties of Spectra fibre 900 [27]
Spectra fiber 1000, the second in a series of Spectra fibers, was developed to meet customers’
needs for increased performance. It is available in a multitude of deniers for use in a wide range
of applications. This extended-chain polyethylene fiber has one of the highest strength-to-weight
ratios of any man-made fiber. Spectra fiber 1000 has tenacity 15 to 20 percent higher than that
of Spectra fiber 900 [27].
The latest generation of Spectra® fiber was developed to provide super-fine, super-strong and
ultra-lightweight fibers for armor, aerospace and high-performance sporting goods applications.
Spectra® 2000 fiber, Honeywell’s premier ballistic fiber, is stronger and lighter than most
commercial high-modulus fiber and has the highest strength to weight of any manmade fiber.
Physical properties of Spectra® fiber 2000 are given in the table 5.9 below.
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Table 5.9: Physical properties of Spectra fibre 2000 [27]
• Vectran™ fibre
Vectran, aromatic polyester, is an example of successful thermotropic liquid crystalline
polyester, and it was commercialized in 1985. Vectran was developed by Kuraray in cooperation
with Celanese with full-scale production started in early 1990. Copolymers of 4-hydroxybenzoic
acid (HBA) and 2-hydroxy-6-naphthoic (HNA) are the basis for the commercial products sold by
Hoechst Celanese as Vectran films and fibers. At high temperatures these thermoplastic
materials become liquid crystalline melts. The elongational flow generated during the fiber
spinning and film extrusion can result in unusually high molecular orientation. This high
orientation greatly influences the tensile properties of these materials [24, 28].
Various attractive properties of Vectran include excellent mechanical properties, properties
retention over a wide range of temperature, excellent chemical resistance, and low moisture
pick-up. The tensile strength, modulus, and decomposition temperature of this fiber are 2.85
and 65 GPa, and 400°C, respectively [28-29].
Vectran fibre is a polyester-polyarylate fibre. Vectran fibre is thermotropic, it is melt-spun, and it
melts at a high temperature. [28-29].
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The physical properties of the Vectran fibre in comparison to the others are shown in the table
5.10 below.
Table 5.10: Physical properties of Vectran fibre in comparison with others [24]
• ZYLON® (PBO fiber) Research and development of PBO fibre occurred at Stanford Research Institute (Menlo Park,
CA) and Dow Chemical Co. (Midland, MI), and the ZYLON® fiber was ultimately commercialized
by Toyobo Co. (Japan) in 1998[24]
ZYLON® (PBO fiber) is the next generation super fiber with strength and modulus almost double
that of p-Aramid fiber. ZYLON® also shows 100°C higher decomposition temperature than p-
Aramid fiber. The limiting oxygen index (LOI) is 68, which is the highest among organic super
fibers.
Further details on the properties can be readily referred to in the report from Dr. Mangala Joshi
of IIT-Delhi. Such as-
• Creep Properties
• Thermal properties
o Decomposition Temperature
o Strength retention after thermal treatment
o Strength retention at high temperature with humidity
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• Flame resistance
• Light resistance
Table 5.11 shows the comparison of properties of various fibres. From the table we can see that
Zylon, Kevlar, PIPD (M5), Spectra and Vectran have the higher tensile strength value.
Material Tensile strength (GPa)
Tensile Modulus
(GPa)
Density (g/cm3)
Elongation %
Moisture Regain %
Decomposition Temperature (
oC ) ZYLON®AS 5.8 180 1.54 3.5 2.0 650 ZYLON®HM 5.8 270 1.56 2.5 0.6 650
Kevlar 49 3.6 130 1.44 2.8 4.5 550 Kevlar 149 3.4 185 1.47 2 3 550 PIPD (M5) 3.96 271 1.7 2.5 4.5 500
M5 Conservative
8.5 300 1.7 2.5
Dyneema 3.4 111 0.97 Spectra 900 2.4 70 0.97 4 - 150
Spectra 1000 3.1 105 0.97 2.5 - 150 Spectra 2000 3.25 113 0.97 2.8 - 150
Vectran 2.85 65 1.4 3.3 0.1 400 Polyester 1 15 1.38 20 0.4 260
Steel 2.8 200 7.8 1.4 - - Table 5.11 Properties of various Fibres
5.4.3 POTENTIAL MATERIALS FOR PROTECTIVE LAYER:
Floropolymer, Tedlar® film due to excellent properties, is already being used for most large LTA
hull material applications. Teflon® film can be heat-sealed, thermoformed, welded, metalized,
and laminated to many other materials. The low permeability to gases, and low temperature
toughness (service temperature range from -240 to 205 ºC) make Teflon® film a good candidate
for high altitude LTA applications. Polyurethane coating on textiles gives a wide range of
properties to meet diverse end uses like apparel, artificial leather, fuel and water storage tanks,
inflatable rafts, containment liners, etc. Polyurethane is available in many formulations and
possesses an excellent balance of properties. It has outstanding overall toughness, high tensile
strength, tear strength, abrasion resistance requiring much less coating weight, low temperature
flexibility, fair gas permeability, good handling properties, crease resistance, and good
weatherability and ozone resistance. Thermoplastic polyurethane can be heat sealed,
adhesively bonded, and laminated to other substrates. Silicone rubber has the best low
temperature flexibility of all polymeric materials. However, its high gas permeability, low
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toughness, and low abrasion resistance are problematic. Plasticized polyvinyl chloride (PVC) is
commonly used in commercially coated fabrics. It has good low temperature flexibility, exhibits
good weathering (5 years) and ozone resistance, is heat sealable, and inexpensive. Low density
polyethylene (LDPE) is a very flexible and heat sealable polymer. It is normally coextruded with
other films to improve its gas permeation. It is reported in the literature that a lightweight
LDPE/Mylar®/polyester fabric laminate has been used in a super-pressure balloon applications.
Vinylidene chloride/vinyl chloride copolymers (Saran®) are excellent barrier materials and
extensively used in packaging applications. However, they are not recommended for use in a
composite LTA fabric because of their poor flex life, especially at low temperatures.
a. PVF Tedlar
Tedlar® polyvinyl fluoride (PVF) film was commercialized by DuPont in 1961 [30]. Its unique
balance of weather resistance, inertness, non-staining properties, and chemical and graffiti
resistance, as well as its ability to maintain toughness and flexibility over a wide temperature
range, has made it the ideal choice for demanding indoor and outdoor applications. Initially
developed for residential siding, Tedlar® film has expanded into many markets, including
aircraft interiors, wall coverings, truck/trailer sides, graphics, highway sound barriers,
architectural building panels, air supported structures, and flexible signs and awnings.
The fluorocarbon nature of Tedlar® film is the basis for the film’s outstanding durability and
resistance to a variety of solvents and harsh chemicals. It is impermeable to greases and oils
and has excellent resistance to sunlight degradation. Tedlar® film stands up well to atmospheric
Table 5.12: Materials for Protective/barrier layer (ref 2)
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pollutants and resists acid rain attack and mildew. Most airborne dirt does not adhere to Tedlar®
film. Tedlar® film has high tensile strength, good abrasion resistance, and is flexible.
Tedlar® film is available as a pigmented film or a near-colorless, transparent film. The
pigmented films offer the highest level of protection from ultraviolet (UV) light degradation, as
the pigments block nearly all UV and visible light from passing through the Tedlar® film. This
means that the materials underneath the film will not be exposed to high-energy, destructive
light. The transparent films are available in an enhanced UV-screening formula that initially
blocks nearly all of the UV light up to 350 nm. These UV-absorbing films screen out
progressively less UV light at the less harmful, lower energy end of the UV spectrum (350 to
400 nm) and block very little visible light [30].
The various properties of Tedlar film supplied by DuPont are shown in the table 5.13 and 5.14 below:
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Table 5.13: Various Properties of Tedlar Film [30]
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Table 5.14: Typical Properties of Tedlar Film [30]
Tedlar® SP is supplied with different surface characteristics. Films are available as one-side
adherable (A), two-side adherable (B), or strippable (S). Adherable surfaces are used with
adhesives for bonding to a wide variety of substrates. These surfaces have excellent
compatibility with many classes of adhesives, including acrylics, polyesters, epoxies, rubbers,
and pressure-sensitive masses. The strippable surface has excellent release properties for use
as a mold release agent for epoxies, phenolics, rubbers, and other plastic resins.
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b. PTFE® TEFLON DuPont™ Teflon® FEP film is a transparent, thermoplastic film that can be heat sealed,
thermoformed, vacuum formed, heat bonded, welded, metalized, laminated-combined with
dozens of other materials, and can also be used as an excellent hot-melt adhesive .
Table 5.15: Properties of Teflon Film [95]
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Table 5.16: Available DuPont Teflon Film [95]
Superior anti stick properties makes it an ideal release film for many applications. A cementable
type with an invisible surface treatment is available for bonding to none or both sides with
adhesives. This versatility is augmented by the superior properties of a true melt-processible
fluorocarbon and by the wide choice of product dimensions available from DuPont.
c. Mylar® polyester Film Since Mylar® polyester film was invented in the early 1950s; it has been used in a variety of
applications that add value to products in virtually all segments of the world. During the
1960s cellophane gave way steadily to Mylar® with its superior strength, heat resistance, and
excellent insulating properties. The unique qualities of Mylar® made new consumer markets in
magnetic audio and video tape, capacitor dielectrics, packaging and batteries possible. By the
1970s, Mylar® had become DuPont’s best-selling film, despite mounting competition [31].
Mylar® is now a product of a joint venture, DuPont Teijin Films. After nearly 50 years, the future
still holds great promise for Mylar®. It offers the unique advantage of chemical and thermal
resistance, durability and unique optical properties. Its excellent balance of properties and
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extraordinary range of performance capabilities make Mylar® ideal for a broad array of
applications in the electrical, electronics, magnetic media, industrial specialty, imaging and
graphics, and packaging markets.
Mylar® polyester film, only by DuPont Teijin Films, is available uncoated or coated in a broad
variety of thickness and width.
5.4.4 BALLONET MATERIAL:
Ballonet material is another important component of a Lighter-Than-Air system. The ballonet is
an internal barrier that separates the air and helium compartments inside of the hull. The
ballonet allows the lifting gas in the hull to expand and contract during altitude and temperature
changes and is continually flexed as it is inflated and deflated. Because the ballonet is required
to inflate/deflate during operation, the ballonet must remain flexible throughout the most extreme
temperature conditions the system will experience. The ballonet material must also be
lightweight, have good abrasion resistance, and exhibit low gas permeability to minimize lifting
gas loss and purity decay.
Requirement
Flexible film and fabric supported materials have been used as ballonet material in LTA
systems. However, under any flight condition that puts the ballonet membrane under stress, the
film will stretch to relieve the stress. If the stress is below the yield point of the film, the film will
fully recover when the load is removed. A reinforced material, such as a coated lightweight
woven material is more commonly used. For large ballonets, both film and coated fabric may be
used in combination. The coated fabric is used as the ballonet "skirt material" to join the main
body of the ballonet to the hull. This skirt material is intended to support the ballonet body
weight and resist the attachment forces while an unsupported flexible film is used for the main
ballonet body material to provide better flexibility and save weight. Desirable properties for high
altitude LTA ballonet material include
• Low temperature flexibility
• Light weight
• Low gas permeability
• Ozone resistant
• Abrasion resistance
• Long service time
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Potential Material
The most promising polymeric materials were found to be polyolefin, polyurethane, ethylene
propylene diene monomer (EPDM) rubber, and silicone rubber. Table 5.17 compares these
polymeric materials along with some other materials used in LTA industry. Some candidate
ballonet materials were developed and evaluated for high altitude LTA application. Polyurethane
is a good polymeric material for moderately low temperature ballonet applications. It has been
successfully used for years by the LTA industry. This material exhibits excellent material
properties above approximately -20°C, including excellent flexibility, good abrasion resistance,
and fair helium/air permeability, with minimal coating thicknesses. However, the low
temperature flexibility is not satisfactory. Silicone rubber exhibits the best low temperature
flexibility of all the candidate polymeric materials. Unfortunately, however, silicone rubber has
the worst gas permeability of all the candidate polymeric materials. Considerably more work
needs to be done to improve the permeability of silicone rubber. Investigation of multi-
layer structures, gas barrier additives, plasma coating, and polymer blending are
approaches that may improve the material performance. Table 5.17 gives a
comparative idea of different properties for different potential material.
Table 5.17: Potential candidate for Ballonet material
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5.4.5 Sealing technology: adhesives / laminates:
Sealed Seam formation for inflatable airship
Inflatable structures: Options for creating air sealed seams between fabric panels
Sheet material typically used for inflatable structures especially the airships for
meteorological/defense applications are composite fabrics consisting of different layers with
each layer having a specific application.
One of the layers in the composite sheet is invariably a woven textile fabric to provide high
strength to the structure. This is required as at high altitudes, the helium gas contained in the
airship expands due to highly rarified atmosphere and puts the structure under a lot of pressure.
Other layers have different functions: for example the outermost layer should provide adequate
protection from highly degrading environment at high altitudes and an inner layer may provide
good gas barrier properties to retain the gas in the airship.
As the surface area of an airship is quite large as compared to the width of the fabric that can be
prepared on the widest looms, the starting material for airships are fabric panels of limited
length and width. These fabric panels are first made into multilayered composites by
coating/lamination and then joined together to create the final shape of the airship.
To create the final shape of the airship the panels must be joined at the edges resulting in the
formation of seams. The seams can be created by either stitching the panels together, by use of
adhesives or by welding.
In this particular application, seams can not be created by stitching the panels together as the
holes created by needles will affect the gas barrier properties of the structure adversely. The
seams need to be created in such a way that these are airtight.
Hence for inflatable structures, seams are created by either welding the edges of the panels or
by applying an adhesive.
• Welding
• Radio Frequency Welding (RF)
• Hot Air/wedge Welding
• Ultrasonic Welding
• .Dielectric or High frequency welding
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• Vibration or friction welding
• Laser welding
• Induction Welding
• Hotplate (Heated Tool or Thermal) Welding
5.4.6 Envelope specifications:
At the beginning of development, it is necessary to specify all requirements. Form, Fit and
Function require detailed investigation and analysis to provide the basis for the materials
specification. Additionally, airworthiness regulations must be considered, as these will provide a
guideline for the designer.
The FAA - ADC (Airship Design Criteria) or the German LFLS (Lufttüchtigkeitsforderungen für
Luftschiffe) provide the minimum requirements for non-rigid and semi-rigid airships.
The following is a list of paragraphs taken from the LFLS, which specifically need to be
addressed when establishing the means of compliance for the airship envelope.
General
The suitability of each questionable design detail and part having an important bearing on safety
must be established by tests.
Materials and workmanship
(a) The suitability and durability of materials used for parts, the failure of which could adversely
affect safety must be properly checked.
(b) Workmanship must be of a high standard.
Fabrication methods
(a) The methods of fabrication used must produce a consistently sound structure. If a fabrication
process requires close control to reach this objective, the process must be performed in
accordance with an approved process specification.
(b) Each new aircraft fabrication method must be substantiated by a test program
Protection of structure
Each part of the airship must
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(a) Be suitably protected against deterioration or loss of strength in service due to weathering,
corrosion, abrasion, or other causes;
(b) Have adequate provisions for ventilation and drainage.
Material strength properties and design values
(a) Material strength properties must be based on enough tests of material meeting
specifications to establish design values on a statistical basis.
Fatigue strength
The structure must be designed, as far as practicable, to avoid points of stress concentration
where variable stresses above the fatigue limit are likely to occur in normal service.
Envelope design
(a) The envelope must be designed to be pressurized........ While supporting the limit design
loads for all flight conditions and ground conditions...... The effects of all local aerodynamic
pressures ...... must be included in the determination of stresses to arrive at the limit-strength
requirements for the envelope fabric.
(b) The envelope fabric must have an ultimate strength not less than four times the limit load
determined by the maximum design internal pressure combined with the maximum load
resulting from any of the requirements specified herein.
(d) It must be demonstrated by test in accordance with the section Tearing Strength of the
appendix that the envelope fabric (in both the warp and woof (fill) directions) can withstand limit
design loads without further tearing.
(h) Internal and/or external suspension systems for supporting components such as the car
must be designed to transmit and distribute the resulting loads to the envelope in a uniform
manner for all flight conditions. The fabric parts of such systems and their connection with the
envelope must be designed and constructed in such a manner that the bonds are not subjected
to peeling loads [85].
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5.4.7 TESTING METHODS/FACILITIES: Testing is the most important and critical part in any product development exercise. At the end
of the day only after getting test results we can say whether our product is meeting desired
specification or not. To get the reproducible results standard testing methods are used
worldwide.
To test a material for an airship and its parts is a daunting task. It requires several
environmental regimes (hot, cold, humid, high UV), several test directions (warp/fill/bias), and
obviously for a numbers of test specimens of the same material. Hence the amount of testing for
qualification of an airship material becomes a complicated task.
The important tests are listed below. The details can be found in the report from
Dr. Mangala Joshi of IIT Delhi:
• Tensile Testing
• Tear Propagation Testing
• FLEX TESTING
• UV + AO Test
• He Permeability Test
Table 5.18 provides a partial test matrix of required testing done by Zeppelin for airship
qualification.
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Table 5.18: Standard Testing Methods for Airship
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5.4.8 INTEGRATED SENSOR IS STRUCTURE (ISIS):
The Integrated Sensor Is Structure (ISIS) developed by Lockheed Martin Aeronautics will
provide a new model for persistent, autonomous ISR platform. As the name suggests the
sensors (radars) are an integral part of the structure i.e. the sensors are embedded in to the
envelope. If successful it’ll considerably reduce the size and weight of the airship and thus
facilitating more space for payloads on the airship.
Lockheed Martin intends to use this technology for the stationary stratospheric airship. The
airship structure will be integrated with lightweight, phased-array radar antennas, providing
highly accurate and sensitive sensor capable of detecting ground-bound and airborne moving
targets under all weather conditions. ISIS will provide a persistent early warning sensor able to
detect cruise missiles at distances of 600 kilometers or dismounted enemy combatants at a
range of 300 km. exploiting its huge size, ISIS will utilize its large aperture instead of high power
to meet the radar performance requirements.
Fig 5.18: ISIS application in sea, land and air (source: DARPA)