Stephen E. Scarborough

INFLATABLE RIGIDIZABLE ISOGRID BOOM DEVELOPMENT

Ultra-lightweight inflatable rigidizable space structures have been identified as an enabling technology for many large-scale and Gossamer type spacecraft planned by NASA and DoD for future space missions. The unique benefits of this class of structure, such as low packing volume, reduced complexity, and reduced mass, enable the development of large antennas, solar sails, and sunshields that would have been otherwise unrealizable. Through the maturation of this technology, many applications such as space based radar would benefit directly, and other applications such as mechanical actuation devices would benefit indirectly but significantly from material advancements. ILC Dover, under contract to the Jet Propulsion Laboratory (JPL) and NASA Langley Research Center (LaRC), has developed an ultra-lightweight inflatable rigidizable boom structure for use on gossamer spacecraft. The wall of the boom structural component is comprised of a grid-work of equilateral triangles that provide isotropic performance properties. This type of construction is termed an isogrid boom (See Figure 1). The grid-work is encased in two tubular polymeric films that act as gas containment vessels to enable inflation for deployment of the structure, and prevent the material from adhering to itself when packed. The exterior film also provides insulation to control the thermal conditions that govern the deployment process and structural performance in space. This structural concept was selected for study because of its high structural efficiency and simplicity in design. The material used in the fabrication of the isogrid structure is a composite, which consists of graphite and a shape memory polymer (GR/SMP). This GR/SMP material acts as a thermoplastic material and is able to be repeatedly heated and cooled to alter the structural shape. This allows the gossamer structure flight hardware to be packed and deployed for evaluation several times during ground test for checkout prior to launch and deployment in space. Shape memory polymers exhibit the unique property of returning to the originally formed shape when heated in a packed condition. In the case of structures with considerable deployment loads, inflation is used in conjunction with the weak SMP restoring force for deployment. This type of material was selected for study because of its structural performance properties, ability to be deployed and rigidized several times prior to launch, and storage life. The overriding goal was to develop a structural rigidizable beam technology that was close to flight readiness. This goal was met by performing numerous materials development tests on several candidate composite materials, manufacturing and testing numerous subcomponents and tube sections in simulated space and lab environments, and performing a deployment and rigidization test of a long boom section in a simulated space environment. The summation of the data indicated that the GR/SMP Isogrid was at a NASA Technology Readiness Level TRL of 5 to 6, but further study is required before the technology can be transitioned to a flight program. INTRODUCTION AIAA-2002-1297 Figure 1. 24K Tow Inflatable Isogrid Boom without Membranes * ILC Dover, Inc., Frederica, DE † Member AIAA ‡ Associate Fellow AIAA Copyright

Light Curing Rigidizable Inflatable Wing

The objective of this study was to prove the feasibility of using light-curing resins to rigidize an inflatable wing for terrestrial and space applications. Current inflatable wings rely on the continuous presence of an inflation gas to maintain their shape. Rigidization of inflatable wings provides several potential advantages, including reducing the vulnerability to punctures, increasing stiffness and load-carrying capability, allowing a higher aspect ratio for high altitude efficiency and longer missions, and reducing weight by eliminating the make up pressurization supply. This study was a multifaceted approach that included defining operating environments for Mars survey craft and military UAVs; analyzing wing loads during deployment and rigidization as a function of internal pressure and leak rate to determine needed rigidization times; developing rapid cure resin formulations with long shelf lives; fabricating, deploying, and rigidizing a wing half-span; and testing and characterizing the rigidized wing. Results show that the wings must deploy and cure rapidly at low temperatures for some missions. The maximum time allowed for the resin to rigidize is the range in time that the inflated and unrigidized wing maintains structural integrity to fly and provide lift for the vehicle while the wing is undergoing rigidization. A series of epoxy acrylate-based resin formulations were developed that cure in 10 seconds or less at 0qC. These resins also exhibited greater than 10 year storage lifetimes in accelerated aging studies and showed mechanical properties close to thermally cured aerospace epoxies. A half-span demonstration Eppler 398 airfoil was fabricated from E-glass fabric/ATI-ROCTME37X1 resin and a polyurethane bladder. After fabrication, the wing was packed and deployed two times. The unrigidized prepreg material was very compliant and was able to be packed tightly. After the packing and deployment trials were completed, the wing was inflated to 7 psig and given a 30-minute solar cure. The rigidized wing exhibited the desired high stiffness without inflation pressure.

Development of a Finite Element Model of Warping Inflatable Wings

This paper presents development of a structural model of warping inflatable wings. In order to quickly and effectively analyze the effects of aerodynamic loading and to consider design options for warping actuation, a detailed finite element model is required. The methodology used to develop and initially validate a finite element model of an inflatable wing is described in this paper. The wing is a complex structural system including intricate geometry, internal pressurization and a woven fabric shell structure. This effort includes determination of material properties via laboratory tests. These are verified with finite element simulations of simple inflatable cylinders constructed of similar material. The process used to create a finite element simulation of the wing is presented, including modeling assumptions and nonlinear analysis stages. Results for various loadings of interest are included. Finally, the paper presents initial validation of the finite element model using results of static cantilever wing bending and twisting loads applied at the inflatable wing tip. Although the model represents important deformation characteristics of the inflatable wing, it is generally too stiff compared to experimental results. Future modifications to the model and further validation are also discussed.

UV RIGIDIZABLE CARBON-REINFORCED ISOGRID INFLATABLE BOOMS

The objective of this study was to demonstrate sunlight cure (UV) of a carbon fiber-reinforced open isogrid tube for Gossamer-type spacecraft. An epoxybased resin was developed and characterized that cures in sunlight at low temperatures (10°C) on carbon and carbon/glass hybrid tows. 1.5-m-long open isogrid tubes were fabricated using wet filament winding techniques. The tubes were sunlight cured and tested for degree of cure and mechanical properties. The demonstration hardware had a 99 percent cure and showed peak buckling loads equivalent to thermally cured tubes. This technology will allow fabrication of large, lightweight and low cost inflatable Gossamer structures that have significantly improved compliant packing efficiency without degradation of deployed precision and mechanical performance.

A Novel Concept for Stratospheric Communications and Surveillance: The StarLight

StarLight is a first-ever, persistent, maneuverable, high-altitude, hybrid, lighter-thanair (LTA) vehicle designed to provide continuous communications and surveillance capabilities over a wide geographical area. StarLight will operate at an altitude between 70,000 and 100,000 feet mean sea level, for a minimum duration of 6 weeks, giving its payloads an operational area of coverage exceeding 160,000 sq miles at maximum altitude. In addition to its LTA capabilities, StarLight incorporates an innovative flight control system to provide a maneuverable vehicle capable of station keeping and/or flying a specified ground track. Control innovations include a mechanically -driven rotating lower stage to easily change the direction of thrust (0-360°), and an actuator control that changes the pitch/roll attitude of the upper stage balloon envelope to accommodate vertical maneuvering above neutral buoyancy. The associated concept of operations allows for remote operations with minimum logistics and infrastructure. The total system design provides multifunctionality to maximize platform utility and easily support defense, security, communications, intelligence, earth sciences and other federal and commercial applications.

Recent Development and Test of Inflatable Wings

In recent years the Departments of Defense and Homeland Security have had a desire to tightly pack small Unmanned Aerial Vehicles (UAVs) in order to allow them to be launched via gun, by hand, or air dropped for reconnaissance or munitions delivery. Inflatable components enable compact packaging and rapid deployment on the ground or in flight, while minimizing system mass and complexity. Inflatable structures also provide UAVs with a significant amount of robustness as they can sustain hard landings without damage due to their inherent inflatable nature, in essence functioning as airbags. The combination of these two factors, compact packaging and damage tolerance, can be combined to provide UAVs that are easily transportable and cost effective. Numerous laboratory and flight tests have been performed to demonstrate the damage tolerance of inflatable wings. The survivability rate has remained at 100% beyond one hundred flight test impacts, and has been verified by similar laboratory testing. The resilience of the inflatable components manufactured from engineered materials is outstanding and tracks well with related inflatable structures such as the Pathfinder and MER airbags, which landed on the rocky surface of Mars. Inflatable wings have also demonstrated two aspects of morphing for UAVs or other flight platforms (such as airships). These are high aspect ratio changes via the deployment of inflatable tip extensions, and camber morphing for aerodynamic control. Inflatable wings with embedded actuation systems have been developed that are deployable and can easily be shape morphed to provide the required aerodynamic control for small UAVs. The flexible composite materials used in inflatable wings also allow for the inclusion of multi-functional elements to augment performance. Multi-functional elements for deployable wings include those that perform structural or aerodynamic functions, but are also used for functions such as aerodynamic control, power generation, power storage, and communication. Key tests conducted during this research and discussed herein include: rapid simultaneous wing deployment, gust and impact loading survivability tests, and wing shape vs. inflation pressure as characterized through wind tunnel testing. This paper discusses the various morphing concepts in detail and the subsequent development and testing of various components for UAVs. The design and fabrication of a small UAV with embedded actuation technology on the inflatable components is also detailed along with flight-test data.

A High -Altitude Test of Inflatable Wings for Low -Density Flight Applications

Recent projects involving unfolding rigid -wing aircraft have demonstrated high -altitude, low -density flight capabilities, moving the concept of an unmanned aircraft exploring Mars or Venus nearer to reality. Motivated by the requirement fo r a minimal packed -volume -to - weight ratio, an alternate approach for the wing design is an inflatable wing. Two previous balloon -launched high -altitude experiments have successfully demonstrated the feasibility of deploying and curi ng inflatable/rigidizabl e wings . Rugged inflatable wings constructed of materials used for the Mars Lander airbags with maintained internal pressurization ar e also a viable alternative for both high -density and low -density flight applications. Low -altitude flight tests have demon strated high reliability, al ong with their unique ability for wing shaping to expand flight capabilities . This paper presents the development and results of a successful high -altitude test to demonstrate the feasibility of rugged inflatable wings for plane tary exploration. Flexible solar cells were mounted on the wing surface to illustrate the potential for multifunctional inflatable structures with power generation capabilities as well. The balloon -launched experiment concept is described, along with detai ls of test article design, fabrication and ground testing. The flight test was conducted on April 30, 2005. The wings were deployed at approximately 96,000 ft, reaching a maximum altitude of 97,987 ft . The test article descended under parachute to recovery . F light results, including onboard images, temperature s, pressure s and solar cell power are included .

UV rigidized carbon-reinforced isogrid boom for Gossamer applications

This work examined the feasibility of curing carbon fiber-reinforced open isogrid structures using sunlight. An orbital thermal analysis was conducted for these Gossamer structures with no insulation to determine the temperature profiles during the cure process. An epoxy-based resin was developed that showed near complete cure on carbon and hybrid carbon/glass tows and also cured at low temperatures. Demonstration hardware cured in sunlight and tested in compression to failure performed as well as similar thermally cured isogrid composites.

Resin and Manufacturing Development for Light Curing Inflatable Composite Booms

†† ‡‡ Inflatable structures that become rigid after reaching the required shape are a promising approach for fabricating large space structures. A need exists for a controlled, clean rigidization technology to harden inflatable spacecraft after they have achieved the required shape. This program is addressing that need through the development of a family of radiation (ultraviolet [UV] and visible light) curable resins for structural composite matrices termed Ridigization on Commandi (ROC). These resins are being formulated to cure in low-temperature conditions with varying kinetics at low power inputs and at various wavelengths. This program is investigating cure using internal light sources under a blanket of multi-layer insulation (MLI). A study of using visible light emitting diodes (LEDs) for the internal light sources is presented in this paper. Topics covered include selection of LEDs and resin sensitizers that are active at those wavelengths, modeling of resin cure kinetics, measurement of resin mechanical properties after curing with LEDs, modeling of LED placement in isogrid booms, and manufacturing of isogrid booms using internal LEDs. Results show that the use of internal cure with LEDs is a viable approach for rigidizing inflatable space structures with low power in cold conditions. When optimized, the ROC technology will provide a versatile rigidization technology for the inflatables community.

Elevated Temperature Mechanical Characterization of Isogrid Booms

Structurally efficient isogrid booms, manufactured from rigidizable composite materials, are becoming an enabling technology for spacecraft structures because of their high packing efficiency. Selection of the materials used in the construction of rigidizable space structures is commonly driven by mechanical performance properties at elevated temperatures. Mechanical properties testing was performed on composite tow samples and on an isogrid boom at various temperatures. To characterize elevated temperature behavior, the isogrid booms, and its subelement composite tows were manufactured from ILC’s TP283E shape memory polymer (SMP) matrix resin and a carbon reinforcement. Both the flexural modulus and the tensile modulus of the composite tow samples were determined as a function of temperature. These values were compared to the calculated values for the composite based on rule of mixtures analysis. The predicted rule of mixtures composite modulus is used in ILC’s isogrid analytical code to predict the structural properties of the isogrid boom. A number of composite tow samples were fabricated by ILC and mechanically characterized by the Aerospace Corporation to gather independent performance data. An isogrid boom was fabricated by ILC and mechanically characterized at elevated temperatures by James Madison University (JMU). JMU tested this boom in tension, compression, and also performed preliminary creep testing at various temperatures. A similar isogrid boom was fabricated by ILC and tested by The Aerospace Corporation for composite CTE performance. This paper discusses the results of both the composite tow testing and the isogrid boom testing in preand post-packing conditions. A discussion of the correlation between the predicted values and the actual test values is also presented. Introduction NASA and DoD space missions in the near future will require much larger satellites, the sizes of which will be beyond the capabilities of current technologies. The types of Gossamer spacecraft that will be needed include antennas, solar arrays, sunshields, solar sails, and telescopes (Figs. 1-2). Some systems being considered are hundreds of meters in size to accomplish mission goals. Due to the increase in payload size required, innovative support structures, which can be packed into the faring of available launch vehicles, must be developed. In recent years, research and development work has been performed in this area. Of the available options, one of the most promising technological advancements is the rigidizable inflatable structure. A rigidizable inflatable structure is one that is fabricated on Earth, packed into the launch container, and inflated for deployment once on orbit. After deployment, the material is rigidized, or hardened, to form a stiff composite structure that no longer needs the inflation gas for support. This class of structures has unique benefits such as low packing volume, reduced mass, and in most cases, very high deployed structural efficiency. Several types of construction can be used in a rigidizable inflatable including monocoque, isogrid, IsoTruss, and truss-frame booms. Each composite structure can be fabricated into a varying geometric shapes utilizing any number of resin and fiber types. The fibrous reinforcement can be in tow or woven fabric form. In order to optimize the structure, the sizes of the tows and the weave styles of the fabrics can be varied. It is also possible to manufacture near-zero coefficient of thermal expansion (CTE) booms through the fiber and resin selection and by optimizing the volume fractions of each. However, key to all mechanical performance properties is the ability to fold and tightly pack the material. Member AIAA † Associate Fellow AIAA Undergraduate Research Assistant, Dept. of Int. Science and Tech. Associate Professor, Dept. of Int. Science and Tech. Senior Scientist, Materials Sciences Dept. Distinguished Scientist, Space Materials Lab Figure 2. ILC 3.2m Diameter TSU Hexapod Testbed Figure 1. 1⁄2 Scale Next Generation Space Telescope Sunshield