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A document describes a 5.5-m-diameter, helium-filled balloon designed for carrying a scientific payload having a mass of 44 kg for at least six days at an altitude of about 55 km in the atmosphere of Venus. The requirement for floating at nearly constant altitude dictates the choice of a mass-efficient spherical super-pressure balloon that tracks a constant atmospheric density. Therefore, the balloon is of a conventional spherical super-pressure type, except that it is made of materials chosen to minimize solar radiant heating and withstand the corrosive sulfuric acid aerosol of the Venusian atmosphere. The shell consists of 16 gores of a multilayer composite material. The outer layer, made of polytetrafluoroethylene, protects against sulfuric acid aerosol. Next is an aluminum layer that reflects sunlight to minimize heating, followed by an aluminized polyethylene terephthalate layer that resists permeation by helium, followed by an aromatic polyester fabric that imparts strength to withstand deployment forces and steady super-pressure. A polyurethane coat on the inner surface of the fabric facilitates sealing at gore-to-gore seams. End fittings and seals, and a tether connecting the end fittings to a gondola, are all made of sulfuric-acid-resistant materials.

Science and technology development from balloon-borne telescopes and experiments is a rich return on a relatively modest involvement of NASA resources. For the past three decades, the development of increasingly competitive and complex science payloads and observational programs from high altitude balloon-borne platforms has yielded significant scientific discoveries. The success and capabilities of scientific balloons are closely related to advancements in the textile and plastic industries. This paper will present an overview of scientific balloons as a viable and economical platform for transporting large telescopes and scientific instruments to the upper atmosphere to conduct scientific missions. Additionally, the paper sheds the light on the problems associated with UV degradation of high performance textile components that are used to support the payload of the balloon and proposes future research to reduce/eliminate Ultra Violet (UV) degradation in order to conduct long-term scientific missions.

The development and characterization of materials suitable for ultra long duration balloon flights has recently been the focus of the materials R&D efforts for the NASA balloon program. Although basic materials selection criteria is similar to those used for conventional balloon missions, additional considerations related to balloon design, fabrication, durability, environmental effects, and cost must also be considered. Among these, the highest impact on material selection is, related to the design shape of the balloon. Work done by independent researchers indicate that for pumpkin type balloons, the load-carrying member is the tendon rather than the structural envelope. This in turn lowers the strength requirements on the envelope material to a large extent. Several materials and material combinations were explored for both design systems. This paper will present the progress made to date in the selection and characterization of these materials and the technical challenges remaining to be overcome

Inflatable structures are gaining wide support in planetary scientific missions as well as commercial applications. For such applications a new class of fabric/film laminates is being considered for use as a structural gas envelope. The emerging composite materials are a result of recent advances in the manufacturing of lightweight, high strength fibers, fabrics and scrims. The lamination of these load-carrying members with the proper gas barriers results in a wide range of materials suitable for various loading and environmental conditions. Polyester-based woven fabrics laminated to thin homogenous film of polyester are an example of this class. This fabric/film laminate is being considered for the development of a material suitable for building large gas envelopes for use in the NASA ultra long duration balloon program (ULDB). Compared to commercial homogenous films, the material provides relatively high strength to weight ratio as well as better resistance to crack and tear propagation, The mechanical, creep and viscoelastic properties of these fabric film laminates have been studied to form a material model. Preliminary analysis indicates that the material is highly viscoelastic. The mechanical properties of this class of materials will be discussed in some details.

Inflatable structures are gaining wide support in planetary scientific missions as well as commercial applications. For such applications a new class of materials made of laminating thin homogenous films to lightweight fabrics are being considered us structura1 gas envelops. The emerging composite materials are a result of recent advances in the manufacturing cf 1ightweight, high strength fibers, fabrics and scrims. The lamination of these load-carrying members with the proper gas barrier film results in wide range of materials suitable for various loading and environmental conditions. Polyester - based woven fabrics laminated to thin homogeneus film of polyester (Maylar) is an example of this class. This fabric/ film laminate is being considered for the development a material suitable for building large gas envelopes for use in the NASA Ultra Long Duration Balloon Program (ULDB). Compared to commercial homogeneus films, the material provides relatively high strength to weight ratio as well as better resistance to crack and tear propagation. The purpose of this papers is to introduce the mechanical behavior of this class of multi-layers composite and to highlight some of the concerns observed during the characterization of these laminate composites.

Films reinforced with woven fabrics are being considered for the development of a material suitable for long duration scientific balloons under a program managed by the National Aeronautics and Space Administration (NASA). Recently developed woven fabrics provide a relatively high strength to weight ratio compared to standard homogenous films. Woven fabrics also have better crack propagation resistance and rip stop capabilities when compared to homogenous lightweight, high strength polymeric films such as polyester and nylon. If joining is required, such as in the case of scientific balloons, woven fabrics have the advantage over polymeric thin films to utilize traditional textile methods as well as other techniques including hot sealing, adhesion, and ultrasonic means. Woven fabrics, however, lack the barrier properties required for helium filled scientific balloons, therefore lamination with homogenous films is required to provide the gas barrier capabilities required in these applications.

Morphological characterization of several polyethylene balloon films have been studied using various techniques. The objective is to determine, if any, differentiating structural or morphological features that can be related to the performance of these balloon film materials. The results of the study indicate that the films are composed of either linear low denstiy polyethylene (LLDPE) or low density polyethylene (LDPE). A selective examination of these data imply that films limited degree of branching and larger crystallites size (same % crystallinity) showed good mechanical properties that appear to correlate with their high level of success in balloon flights.

The effect of stress and temperature path on the mechanical properties of a balloon film material (Astrofilm E2) have been studied using two techniques. The first technique is based on uniaxial tensile measurements of prestrained strips of the film as a function of temperature. Data are analyzed in terms of two temperature coefficients, defined here as a stiffness coefficient and a strength coefficient; and by comparing the areas under the stress-strain curves. The second technique investigated the effect of stress and temperature path on inflated cylinders of the same material. The changes in the mechanical properties due to preconditioning in the biaxial stress state were evaluated using the ball burst test. Preliminary findings indicate that the material is stress and temperature path dependent. Changes in stiffness coefficient, draw strength and toughness were measured and discussed. The above techniques may be suitable to document path dependent changes, and to discriminate among films of different manufacture.

Uniaxial and ball burst toughness for eight commercial polyethylene balloon films have been investigated and compared. Uniaxial toughness was measured in both machine and transverse directions using a rectangular sample 25.4 mm wide by 50.8 mm gage length with an average nominal thickness of 20.32 microns (0.8 mils). Ball burst toughness was measured using a ball burst test fixture with 127 mm hole diameter and 50.8 mm ball diameter resulting in a 0.4 ball to hole diameter ratio. Both the size and shape of the deformed area were observed and the ball burst toughness was calculated based on the deformed area of the film instead of the full area. Specimens were tested at 23 C and -80 C and cross head speed was maintained at 508 mm per minute. The uniaxial toughness were in general, higher than the ball burst toughness. The toughness of the films varied from 12 to 365 MPa when measured uniaxially and from 24 to 73 MPa when measured in the ball burst test. Further investigation showed an apparent correlation between film performance in flight as measured by its stress index and its toughness.