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High Altitude Airship: Detailed Project Report

RESTRICTED DOCUMENT 132

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



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



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



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



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”



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].



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].



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.



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.



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.



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].



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



• 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



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)



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:



Table 5.13: Various Properties of Tedlar Film [30]



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.



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]



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



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



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



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



• 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



(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].



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.



Table 5.18: Standard Testing Methods for Airship



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)

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