David P. Cadogan

Morphing Inflatable Wing Development for Compact Package Unmanned Aerial Vehicles

any military and commercial applications for Unmanned Aerial Vehicles (UAVs) have been identified and numerous vehicles are under development. Many of these vehicles have a need to stow their wings and control surfaces into very small volumes to permit gun launch or packaging into aircraft mounted aerial drop assemblies. One technology that has shown promise in achieving this goal is the inflatable wing. Coincidentally, aircraft developers and researchers have identified a need for aircraft components that can morph to provide performance enhancements over traditional wing and tail assemblies, through the elimination of mechanical actuation system complexity and improved aerodynamics. The combination of the inflatable and morphing system technologies has lead to a unique approach for small UAV platforms with deployable, controllable wings that may also facilitate transition through multiple flight regimes. Inflatable wings have been in existence for decades and have found application in manned aircraft, UAVs, munitions control surfaces, and Lighter Than Air (LTA) vehicles. Recent system design challenges have ushered advances in the areas of materials, manufacturing, and configuration that have advanced this technology into a practical form for near term application. Inflatable wings can be packed into volumes tens of times smaller than their deployed volume without damaging the structural integrity of the wing. Deployment can occur on the ground or in flight in less than one second depending on the size of the wing and the type of inflation system used. The focus of this paper is to discuss efforts in reshaping, or morphing, the inflatable wing to provide roll control through wing warping, i.e. actuation of the aft end of the wing to achieve changes in section camber. Several approaches have been developed that lend themselves to camber control via locally altering the geometry of the wing. Apart from use as a stand-alone aerodynamic surface on a small UAV, the inflatable assemblies can also be used as an aspect ratio increasing device on a larger aircraft to enable a more radical change in wing configuration. This approach serves to improve system efficiencies across changing flight regimes, allowing transitions from highspeed target approach to low speed loitering. Several actuation methods that are applicable to flexible structures have been studied and traded-off. Actuators with strong force generation capability (i.e. high blocked stress) can be added to inflatable structures to alter the length of the load bearing textile components of inflatable wings, thus altering overall shape. Performance requirements for such actuators were derived from a consideration of useful roll rate in a representative aircraft. Other requirements were also compiled and include such items as high frequency response, ability to be folded and packed, low mass, low power consumption, and high cycle life. Some of the actuator types considered include piezoelectric actuators, electro-active polymers, shape memory alloys, pneumatic chambers, nastic cells, and distributed motor-actuator assemblies. * Manager, Research & Technology, AIAA Associate Fellow. Cadogan@ilcdover.com. † Principal Investigator, AIAA member. ‡ Project Engineer. § Design Engineer, AIAA member. AIAA 2004-1807 SDM Adaptive Structures Forum M

AN INFLATABLE MICROSTRIP REFLECTARRAY CONCEPT FOR KA-BAND APPLICATIONS

An Inflatable Ka-Band Microstrip Reflectarray system for use in spacecraft application has been identified as an enabling technology for the challenging requirements of deep-space communication. To meet the challenge of such application, the National Aeronautics & Space Administration (NASA) has embarked in an effort to develop lightweight, large aperture radar and communication antenna. JPL and ILC Dover, Inc. have successfully developed a 3-meter inflatable Ka-band Microstrip Reflectarray prototype. The general concept of this Reflectarray consists of an inflatable rigidizable support structure bridged by rigid composite structures, an inflatable rigidizable feed support, a RF reflective membrane assembly, and membrane tensioning and adjustment hardware. The feasibility and efficacy of this concept were demonstrated by fabricating and testing of a full size model. The stringent requirements of flatness, 0.5 mm RMS and 0.8 mm peak, were met with significant margin. The membrane flatness was measured 0.118 mm RMS with a maximum deviation of ±0.3 mm. Additionally, RF tests performed at the indoor compact range of Composite Optics Incorporated (COI) in San Diego, California demonstrated excellent radiation characteristics, including good main-beam shape, excellent sidelobe level, and low cross- pol radiation. A detailed description of the mechanical and structural features of the 3m inflatable Reflectarray and discussion of some of the RF characteristics and results obtained during the testing of the antenna at the indoor compact range are presented in this paper. In addition, comments on continuing structural and RF design work and configuration studies are presented.

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.

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 Hybrid Inflatable Dish Antenna System for Spacecraft

Inflatable communication antennas are the subject of current space research because of their potential for enabling high-bit-rates. However, a significant problem associated with inflatable technology is the all-or-nothing scenario, where success of the mission depends on successful deployment of the antenna. For this reason, few satellite programs are willing to take the risk of using an inflatable unless it is mission enabling. The Hybrid Inflatable Antenna, a concept developed by the Johns Hopkins Applied Physics Laboratory and ILC Dover, addresses the risk by providing a backup capability within the inflatable dish. This system combines a fixed parabolic dish with an inflatable reflector annulus that greatly increases antenna area. For example a 1-meter diameter dish can be increased to 4-meter resulting in a 16X improvement in reflector surface. A dual feed ensures operation of the smaller fixed dish throughout the mission providing a risk buffer to the inflated dish. The inflatable annulus is stowed compactly under the fixed dish prior to launch to fit a variety of spacecraft and launch vehicle envelopes. Moderate gas pressure deploys the annulus and support tubes. A prototype Hybrid Inflatable Antenna has been successfully fabricated and tested. This scale model demonstrates that a highly accurate inflatable Ka band reflector surface can be achieved and that large inflatable surface distortions can be minimized.

A Novel Concept for Stratospheric Communications and Surveillance: The StarLight

Staruf0b2Light 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. Staruf0b2Light 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, Staruf0b2Light 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.

The Development of Inflatable Space Radar Reflectarrays

The need for advanced spaceborne radar antenna systems has been identified by the National Aeronautics & Space Administration (NASA) and the user community. The NASA Jet Propulsion Laboratory (JPL) has been selected as the NASA center of excellence for inflatable structures, and has identified these structures as a possible way to advance radar array technology. JPL has combined this technology with some development work that is being conducted on flat passive radar arrays to create ultra-lightweight reflectarrays. To this end, JPL has teamed with ILC Dover, Inc., to design, manufacture, and test prototype arrays to demonstrate the validity of the approach. Inflatable, rigidizable structures offer several

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, IsoTrussuf8ea, 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