CHAPTER 10 Gossamer spacecraft A. B. Chmielewski1 & C. H. M. Jenkins2 1Jet Propulsion Laboratory, Pasadena, CA, USA. 2Compliant Structures Laboratory, Mechanical Engineering Department South Dakota School of Mines and Technology, USA. Abstract Modern trends are driving spacecraft to the extremes of length scales. On the one hand, the ubiquitous trend toward miniaturization is pushing spacecraft ever smaller; one the other hand, many commercial, military, and science applications can only be done with very large systems. In either case, ultra-lightweight or gossamer technology will enable a future generation of spacecraft that have remarkable capability to accomplish missions beyond the reach of current systems. In this chapter we discuss the technology, and the challenges and opportunities, of gossamer spacecraft. 1 Technology background 1.1 Overview of gossamer spacecraft technology Recent technological advancements in structural analysis materials, fabrication, and testing, have presented the space community with a low-cost, lightweight alternative to mechanically deployed space structures. These gossamer spacecraft have many benefits and advantages over current mechanical systems. They are low in mass and can be packaged into small volumes, which can potentially reduce the overall program cost by reducing the launch-vehicle size. Reduction in total system mass and deployment complexity can also increase system reliability. They are of particular advantage for mission architectures at the extreme ends of the size spectrum—from the very small to the very large. To be clear, gossamer spacecraft are typically realized as membrane structures. We mean by membrane structures those structures (load-carrying artifacts or devices) comprised of highly flexible (compliant) plate or shell-like elements. This usually implies thin, low-modulus materials, such as polymer films. Membrane structures have very little inherent stiffness, and do not lend themselves well to carrying compressive loads. Thus, they are often found either in WIT Transactions on State of the Art in Science and Eng ineering, Vol 20, © 2005 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) doi:10.2495/978-1-85312-941-4/10 204 Compliant Structures in Nature and Engineering tensioned-planar or inflated-curved configurations. In what follows, we will use the terms membrane, inflatable, and membrane/inflatable interchangeably. There is great interest in compliant structures by near-term space missions, and a possibility of enabling several breakthrough missions in the more distant future [1–20]. Membrane structures are envisioned for such applications as deep space antennas, earth radiometers, radars, concentrators, telescopes, sun shields, solar sails, solar arrays, and spacecraft booms. In 1997 NASA embarked on a technology development program to advance gossamer spacecraft technology and make it available to 21st century space missions. NASA has concentrated on such areas as deployment techniques, membrane and rigidizable materials, and analytical tool development. It is hoped this will accelerate technology development in the areas of large ultra-light apertures and solar sails, for example, which are among the most challenging applications of membrane structures. The analytical simulation of mechanical performance of compliant space structures is absolutely essential to support the development of technology and projection of performance for this new class of space structures. There are a number of elements of this technology that must be understood analytically to develop and apply this technology. In addition to the modeling needed to characterize the structural dynamics of the on-orbit configurations, highly specialized approaches are needed to simulate other specific mechanical events and structural relationships. Such specialized capability should include simulation of a) specific control deployment techniques for structural members, b) generic spatially organized and controlled deployment processes, c) identification and application of scaling laws to enable the use of limited test results to project full-scale mechanical performance, and d) deflection characteristics of thin membranes under tension and loaded by differential pressure, among many others. There are currently a number of different techniques for control of the deployment of gossamer structural elements. The most promising techniques currently under development have been simulated and will be verified experimentally with scaled hardware. New and unique techniques for spatially controlling deployment through the management of a number of discrete masses have been developed. The potential benefits of this process have been identified. Scaling laws for this new class of space structures have been developed. They have been applied to the test results of scaled hardware to show the relationship with full-scale structures. The basic equations have been developed to determine the deflection characteristics of a tensioned membrane under differential pressure loading. This characterization will be modified to include segmented membranes and the stiffness effects of their seams. This will enable the development of an error budget for precision inflatable reflector structures. And what is the future of membrane structures in the new millennium? It can be summarized with just two words – gossamer spacecraft. “Technology to find it and then shake its hand” is a long-term vision. This somewhat humorous sounding motto appears even more outrageous when translated into scientific terms. NASA will develop technology to enable imaging of extra-solar planets to detect the ones that carry life. Once we find such planets, we will not be satisfied with “just” creating somewhat blurred pictures of these worlds or finding spectroscopic “smoking guns” of life’s existence - we will want to travel there and take a closer look at our cosmic friends. We do not know how long this search will take; we do not know how successful we will be. One thing we do know is that, to make this vision more than just science fiction, we will need to develop radically different observatories and spacecraft. A gossamer telescope, which will produce the first detailed image of an extrasolar planet, will bear little resemblance to any current space observatories. The gossamer telescope will be about 150 times lighter than the Hubble Space Telescope and its diameter will be 10–20 times greater. When such a telescope, or an interferometer consisting of an array of such telescopes, WIT Transactions on State of the Art in Science and Eng ineering, Vol 20, © 2005 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) Gossamer spacecraft 205 will finally produce an image of “the other world,” there will be strong public summon to take an even closer look by means of interstellar travel. In this day and age we envision three major ways that offer the possibility of interstellar travel. They are: matter–antimatter propulsion, nuclear propulsion, and solar sailing – the latter method recently getting much attention. The first-generation interstellar sail missions are envisioned to use gravitational boost from the sun in addition to the solar pressure. In the more distant future, lasers will propel the sails. Flexible structures will eventually make an interstellar sailing possible. The “vision missions” for flexible space structures are the Interstellar Probe and the Extrasolar Planet Imager. Both of these missions can be considered the pinnacles of the technology-development roadmap. Before the technology is ready to enable either of these vision missions, there will be a series of technology-development products benefiting several types of space endeavors. In general, gossamer spacecraft technology will enable very large, ultra-lightweight systems for bold missions of discovery such as: x Very large telescopes for imaging extra-solar planets, studying formation of large-scale structure in the early universe, and continuously monitoring the Earth from distant vantage points. x Large deployable and inflatable antennas for space-based radio astronomy, high- bandwidth communications from deep space, and Earth remote sensing with radar and radiometers. x Solar sails for low-cost propulsion, station keeping in unstable orbits, and precursor interstellar exploration missions. x Large solar power collection and transmission systems for human and robotic exploration missions, and for the commercial development of space. The overarching goal of gossamer spacecraft technology development is to achieve breakthroughs in mission capability and cost, primarily through revolutionary advances in structures, materials, optics, and adaptive and multifunctional systems all described briefly in the following sections. (For a more complete discussion of these topics, the reader should refer to the monograph Gossamer Spacecraft: Membrane/Inflatable Structure Technology for Space Applications [11].) 1.2 History of gossamer aerospace structures There has been interest in inflatable deployable space structures since the 1950s due to their potential for low-cost flight hardware, exceptionally high mechanical packaging efficiency, deployment reliability, and low weight. The earliest gossamer aerospace structures were the kite and the balloon. A number of significant technology developments, focused on the demonstration of such potential, include the Good Year antennas in the early 1960s, the Echo Balloon series from the late 1950s to the early 1960s, the Contraves antennas