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s Spacecraft Structures and Materials BACKGROUND AND STATUS Spacecraft structures- small or large must be made of materials that resist, without failure or excessive distortion, the static, dynamic, and thermal stresses that occur during launch, deployment, and service. Payloads and ancillary equipment also must be protected from undesirable distortion, vibration, and temperature changes. Appendages such as antennas and reflectors that are too big to fit into the spacecraft in their operational configurations have to be packaged in collapsed states during launch and subsequently deployed. These design requirements should be met within guidelines for weight, cost, and reliability conditions that are always inextricably coupled and have to be reassessed in the context of the small spacecraft philosophy. Structural weight of spacecraft has historically been only about 20 percent of the total dry weight. However, structural weight saving may assume accentuated importance for many small spacecraft missions, where each kilogram shaved from the structure is precious, and may provide increased capacity for additional payload, autonomous control devices, or auxiliary equipment. However, this emphasis on low weight may be tempered in some small spacecraft applications that involve demands for low cost, easy adaptability, and growth capability. Although the spacecraft structure and the material of which it is composed are inextricably linked entities in their influences on cost, strength, stiffness, weight, reliability, and adaptability to change, it is nevertheless convenient to discuss separately issues that may be regarded as being predominately in either the structures or materials category. STRUCTURES Currently, in most small spacecraft, a simple truss structure provides the primary resistance to static and dynamic loads, en c! flat panels (often of sandwich construction) support the payload and associated spacecraft contents. While it does not appear that much attention has been paid to optimizing the spacecraft structural configuration, future missions will require more efficient design of the central bus structure. Fortunately, past 42
Spacecraft Structures and Materials research and flight application in airplanes and large space buses have made available proven, high-efficiency configurations such as stiffened shell structures and skin-stiffener panels. In addition to conventional bus structures, there is a need for deployable and special-pu~pose structures on most spacecraft, whatever the size. The status of these enhanced spacecraft structures is discussed below. Deployable Structures In order to accomplish its mission, a small spacecraft may require an appendage, such as a boom or a surface, that is very large relative to the size of the spacecraft. Such appendages must be packaged in collapsed states during launch and subsequently deployed prior to operation. Past and present spacecraft have used a variety of articulated deployable structures as booms supporting instruments or solar cell blankets or as area structures forming antennas or solar arrays. Some of these deployable structures were developed during the 1960s and early 1970s for use on the small spacecraft of that time, but during the past two decades, advanced development at NASA and DoD in the area of deployable structures has been directed almost entirely toward large antennas and platforms, particularly those for which precision is a dominant requirement. Nevertheless, the technologies developed may be useful for small spacecraft, particularly if high accuracy is required. Most existing deployable structures are deemed reliable only by virtue of being thoroughly tested by repeated ground-based deployments, which is complicated and expensive because of the need to counteract the effects of gravity on configurations that are designed to operate in the gravity-free space environment. Even so, recent flight experience has involved a distressing number of deployment hangups. Inexpensive small spacecraft may require new and simpler reliable deployable designs. One of the present thrusts of development efforts involves the use of inflatables, which are possibly cheaper and more dependable than articulated structures. Control-Structures Interaction and Smart Structures The age of control-structures interactions is well underway, and that of its offspring, smart stnuctures,2 has dawned. These technologies have particular relevance to small spacecraft designs. Counteracting the dynamic load environment during launch by the provision of sufficiently stiff structural packaging alone may not make sense in a small spacecraft if active vibration suppression could achieve the required isolation 43 ~ Control-structures interaction refers to the coupling between the displacements of deformable structures and the performance of control systems. 2 A smart structure has sensors and actuators as integral parts along with a control computer that is required to actively control vibrations and shape.
44 Technology for Small Spacecraft (from dynamic stress and acceleration) with lower mass. In addition, after launch, control-structures interaction and smart-structure design play an important part in the suppression of jittery The jitter problem may actually be accentuated in small spacecraft by the effects of scale. Although most current small spacecraft are being designed without the use of control-structures interaction and smart structures, these advanced techniques will become essential as scientific and other payloads become more sensitive and as pointing requirements and dimensional precision constraints become more severe. Experimental smart structures developed by NASA, by DoD, and elsewhere consist of composite material plies containing piezoelectric4 sensors and actuators to control mechanical behavior. Other possible actuator technologies are based on shape-memory materials (e.g., Nitinol), electrostrictiveS and magnetostrictive effects, 6 and micromotors. The U.S. Air Force Phillips Laboratory has demonstrated an increase in spacecraft structural damping by two orders of magnitude and has provided on-orbit demonstrations of the use of embedded sensors and actuators for both active and passive vibration suppression. MATERIALS Aluminum is the conventional material for flight structures of all types. In addition, graphite-fiber/polymer-matnx composite materials having much higher strength to density ratios and stiffness to density ratios are finding substantial use in aircraft and spacecraft, more in commercial satellites than in NASA spacecraft, and even less in military spacecraft. For early small spacecraft, the tendency has been to use aluminum solely and to avoid the perceived extra costs of more advanced materials. Future small spacecraft with requirements for higher performance and lighter weight will necessarily use the advanced materials. The status of these candidate advanced materials is discussed below. 3 litter is the unacceptable disturbance-induced vibrations during critical performance time windows. 4 A piezoelectric device undergoes reversible change in dimension when an electric force is applied. The change in dimension is dependent on the polarity of the Delhi. ~ l, ~ s An electrostrictive effect is a reversible dimensional change in a material when the material is subjected to an electric field. The direction of dimensional change is independent of electric field polarity. 6 A magnetostrictive effect is a reversible dimensional change in a material when the material is subjected to electric or magnetic fields.
Spacecraft Structures and Materials Aluminum-Lithium Alloys A weight-saving alternative to the use of conventional aluminum alloys in spacecraft design could be the use of aluminum-lithium alloys. The lower density of aluminum-lithium alloys, coupled with their somewhat increased stiffness and, in specific alloys, higher strength, could provide immediate weight savings of 7 to 20 percent with few required changes in fabrication and design. Moreover, specific aluminum-lithium and magnesium-aluminum-lithium alloys show markedly increased toughness at cryogenic temperatures, an important property for liquid oxygen and liquid hydrogen fuel tanks. With respect to space structures, these characteristics can be particularly important, as the failure of most structures will be associated with buckling or stress fractures. Based on buckling and yield strength, an increase in the elastic modulus and yield strength or tensile strength should produce a corresponding decrease in the structural weight. Aluminum-lithium alloys can provide up to 12 percent higher elastic stiffness and, in the case of Alcoa alloy 2090, an increase of almost 20 percent in tensile strength over conventional aluminum alloys such as 2219 and 2014. Moreover, processing and fabrication techniques (e.g., machining, chemical milling, gas tungsten arc welding, shot peen forming, etc.) similar to those employed for conventional alloys can be utilized for aluminum-lithium alloys. in addition, studies (e.g., at General Dynamics and NASA) suggest that techniques for low-cost, near net-shape processing7 of aluminum-lithium alloys that are under development may lead to cost savings of 20 to 30 percent compared with integral machined structures. However, although substitution of aluminum-lithium alloys for conventional alloys can essentially be achieved with no redesign, and several alloys are becoming "flight tested" as commercial aircraft components, care must be exercised with the use of forgings of certain aluminum-lithium alloys due to their low through-thickness (short-transverse) toughness. The following aluminum-lithium alloys are currently available. . . . Wel~a~ite_ is an aluminum-lithium alloy developed by Martin Marietta, which has excellent welding characteristics, strength, comparable toughness to aluminum, and stress corrosion resistance. Two variants of Weldalite are Reynolds Metals alloys 2195 and MD345. Alloy 2090 was developed by Alcoa to replace the conventional alloy 7075-T6, and for some applications, to replace alloy 2024-T3. Alloy 2090 has the highest strength of all aluminum-lithium alloys. Alloy 8090 was developed by Alcan, with approximately 15 percent to 20 percent lower strength than alloy 2090, but improved damage tolerance and short-transverse toughness (Venkateswara Rao and Ritchie, 19921. product. 45 7 Near net-shape processing produces a part that requires little machining of the finished
46 Technology for Small Spacecraft To date, aluminum-lithium alloys have not been used in small spacecraft structures, although they have appeared in launch vehicle designs. Polymer-Matrix Composites In currently planned small spacecraft programs, there is a trend toward considerable exploitation of organic-matrix composites in structural truss members, in propellant tanks (or as overwraps on metal tanks), and in flat pane! components. Very significant weight savings (perhaps 25 to SO percent) could be achieved in the spacecraft structure through use of polymer-matrix composites. However, the question of the cost of such composites cannot be divorced from the engineering effort needed to establish confidence in their use, which varies as a function of the expertise available to individual agencies and companies. Nevertheless, the overall level of accumulated experience in design with composites in the United States, especially in the aircraft industry and large . . a. · , , ~ ~ ~ ~ ~ · ~ ~ , , , · ~ at, ~ spacecraft prime contractors, should be high enough to counteract residual tendencies to accept the weight penalties associated with designs based on the exclusive use of conventional aluminum alloys. Further, industry estimates suggest that the costs of graphite epoxy or similar composite materials may actually, in the long run, be less than those of monolithic metals in the same application. Although polymer-matrix composites are subject to space environment degradation effects that must be considered, there are no indications so far that their structural performance would be seriously threatened by the three-to-five year exposures currently contemplated for most small spacecraft missions. Several contractors and government laboratories including Space Systems/Loral, Lockheed Missiles and Space Company, Martin Marietta Astro Space, and Lawrence Livermore National Laboratory are developing techniques for the economical production of composite structures for spacecraft. The most commonly used polymer-matrix composite for primary spacecraft structures is graphite epoxy. Structural forms, such as tubes, can be obtained at varying cost from several commercial suppliers, which range from fabricators of golf club shafts to the aerospace prime contractors. Other well-used polymer-matrix composite fibers are glass and Keviar,_ which are processed similarly to graphite fibers. Fiberglass, particularly the S-glass variant, can be subjected to 3 percent strain without harm and is useful for applications requiring large strain capability, but its strength and stiffness is unremarkable. Keviar fiber, on the other hand, has high specific tensile strength and stiffness and is useful where electrical or dielectric properties are of concern. KevIar, however. has a relatively low compressive crushing strength. Metal-MairLx Composites Metal-matrix composites are becoming available with possible applications to spacecraft frames and components. As spacecraft frame materials, aluminum alloys reinforced with silicon carbide, alumina, or boron particulates or fibers may offer
Spacecraft Structures and Materials advantages of increased stiffness and strength; however, these materials may be an order of magnitude more expensive than conventional aluminum alloys and have certain mechanical property disadvantages (e.g., the particulate-reinforced alloys have, until recently, shown poor ductility and toughness properties). In addition, specific metal- matrix composites, such as graphite-reinforced magnesium alloys, can offer increased stiffness at coefficients of thermal expansion (for dimensional stability) comparable with those of graphite-resin composites. Such metal-matrix composites can be designed with tailored physical and mechanical properties and do not have the outgassing characteristic of graphite epoxy. NASA is considering boron-aluminum me~-matrix composites for selected applications in primary structures for its space transfer vehicles and silicon-carbide particulate-aluminum alloys for cryogenic tanks. Titanium and titanium-matrix composites are generally applicable for higher-temperature environments. For example, the silicon- carbide reinforced Timetal 2IS alloy is useful at temperatures up to 800°C and has excellent resistance to corrosion and oxidation in elevated temperatures. Metal-matrix composites have also found application as lightweight, strong, and highly conductive materials for hiah-temnerature thermal management systems. For ~ ~ "7 , example, Rockwell has developed copper-matrix composites with fiber reinforcements of graphite, molybdenum, or tungsten for actively cooled structures in hypersonic aircraft and rocket nozzles and in radiator fins for space power systems. These composites are stable in high heat flux and in thermal cycling applications, and they offer improved creep resistance compared with conventional conductive alloys. Fairchild Space and Defense Corporation is working on electro-emissive panels for thermal management of small spacecraft. Carbon-Carbon Composites Carbon-carbon composites are generally used in applications requiring extreme temperatures, typically up to about 1650°C. In fact, combined with active cooling, they can be used for the leading edges of nose, wings, and tails of airframes exposed to temperatures as high as 3300°C. For the National Aerospace Plane vehicle, for example, carbon-carbon composites were being used as thin panels mechanically attached to the underlying titanium-matr~x composite structure over parts of the fuselage. However, despite their very high thermal resistance, carbon-carbon composites are highly susceptible to oxidation; on the National Aerospace Plane, they needed to be protected by thin multilayer coatings of silicon carbide. For spacecraft, carbon-carbon composites may offer significantly reduced time and cost for fabricating structures through rapid densification processes. NASA is developing continuous and batch processing techniques for carbon-carbon spacecraft tubular frames and precision reflector, antenna, radiator, and aerobrake panels with appropriate thermal, reflective, and radiator coatings. 47
48 Technology for Small Spacecraft STRUCTURE/MATERIALS SYSTEMS The challenge to imaginative designers in the age of small spacecraft will be to meld the technologies of advanced materials, structures, deployable appendages, and control-structures interaction into small and inexpensive configurations. There exists a large body of structures and materials technology pertinent to aircraft and large spacecraft (and the small spacecraft of the early space decades) that can provide a serviceable springboard for the design of present and future small spacecraft, but, in various technical areas and their synthesis, there is a wide range of needs for further research and development. The aforementioned substitution of aluminum-lithium alloys for aluminum in traditional structural metal designs would provicle immediate, if modest, weight savings. But the current knowledge base for the production of, and design with, composite materials polymer-matrix composites in particular has to be not only thoroughly absorbed but may have to be substantially enhancer! by the emerging small spacecraft community in order to meet demands of Tow cost as well as the promise of low weight. Composite materials and components explicitly configured to fulfill multiple requirements (such as those of strength and thermal conductivity) clearly offer scope for weight savings. The design of simple, reliable, and cheap joints and attachments in composite structures is a structure/materials systems problem that never goes away, as is the related requirement for easy design and fabrication modification to accommodate unforeseen (but inevitable) changes in payloac! configurations. Although some existing concepts and technologies for the compact storage and reliable cleployment of appendages may find continued applicability to small spacecraft, there is considerable potential for new invention and development in this area, given the inevitable conflict between the smallness of the structure and the desirability of large appendages. Finally, against a background of considerable existing theoretical and laboratory research, but with little established flight experience available, small spacecraft engineers will have to be heavily involved with the nascent technologies of control-structures interaction and smart structures and their exciting promise, including their integration into the overall spacecraft system as cost-cutting and weight-saving elements. FINDINGS AND PRIORITIZED RECOMMENDATIONS NASA has potentially important roles to play in the creation, enhancement, and application of structures and materials technology for small spacecraft, both in its traditional capacity as an agency for frontier. generic enaineerina-science research ~. .. . . , O . . ~ O rocu sea on particular relevant topics and as a leader In joint projects with industry intended to demonstrate the design, fabrication, and deployment of high-performance, reliable, and adaptable small spacecraft in accordance with the central guidelines of low cost and low weight. As always, vigilance is essential to ensure that these activities nourish each other.
Spacecraft Structures and Materials The following explicit recommendations for NASA action are listed in a priority order that reflects the integrated judgment of the Pane} on Small Spacecraft Technology, after considering the state of development of new technology and potential payoffs that can reasonably be expected. I. Research on simple, low-cost deployable booms and surfaces should be emphasized. The objectives should include high deployment reliability, compact stowage, and adequate precision. Ground-test proof of successful deployment in space is essential. 2. A joint NASA-industry program should be initiated to demonstrate developments of advanced small spacecraft designs that are based on polymer-composite components, exploiting available as well as novel technology as appropriate to meet the paramount demands of low cost, low weight, reliability, and adaptability. The NASA Small Spacecraft Technology Initiative may fulfill this objective. 3. In coordination with ongoing research at universities and other government agencies, research efforts should be intensified in the area of smart structures and control-structures interaction. Research should be generic in character as well as focused on specific needs for small spacecraft. 4. A short-term demonstration program with industry should be undertaken to design, construct, and qualify a small spacecraft structure based primarily on current structural design configurations that exploit aluminum-lithium alloys in lieu of aluminum in order to determine the feasibility of rapid weight savings with minimal effort and cost. 5. Sufficient expertise in polymer-matrix composite technology should be maintained within NASA to identify and pursue opportunities for research aimed at improving strength, stiffness, thermal properties, and economy of fabrication, with explicit attention to the possibilities of multiple-use components and the engineering of modular attachments and joints. 49