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THE HISTORY OF SOLAR SAILING |
| Date Added: December 21, 2008 11:35:26 PM |
| Author: planetary.org |
| Category: Applications: Vehicles |
| by Louis Friedman Excerpted from Starsailing: Solar Sails and Interstellar Travel, John Wiley & Sons,1988 Twelve years after writing a book on solar sailing, of which this is the first chapter, and 25 years after leading the first space agency attempt to develop a solar sail mission I was given the chance to lead a project to attempt the first solar sail flight. This is Cosmos 1. Solar sailing is a romantic idea and an exciting technology, but it is missions that excite me. In the case of the NASA mission attempt in the 1970s, it was the idea of rendezvous with Halley's comet. More generally solar sails enable traveling to and from the planets without fuel. But, most of all, sailing on light beams is the only technology that we know today that leads to practical interstellar flight. Robert Forward, the enormously creative and brilliant physicist and author, developed the idea of a sail for interstellar flight. He came out with interstellar sail concept based on discussions with Freeman Dyson at a Caltech workshop on interstellar flight chaired by Planetary Society president, Bruce Murray. He then published technical articles and a science fiction book about it, and extended the concept from solar sailing, to laser sailing, to microwave sailing. We hope to conduct not only the first application of photon sailing for space propulsion, but perhaps to conduct the first interstellar flight experiment with a microwave experiment on our mission. Sadly, as this mission was being developed, we learned of Dr. Forward's terminal illness due to cancer. We are pleased that we can include his paper on the CD on our spacecraft, along with other pioneering works in the history of solar sailing. In some small way we dedicate ourselves to his motto: Ad Astra. LDF Sketch by Orban from an early publication on solar sailing, "Clipper Ships of Space," by Carl Wiley, 1951. (Reprinted from Astounding Science Fiction) At a meeting of the Advanced Projects Group at the Jet Propulsion Laboratory, Chauncey Uphoff and Phillip Roberts told me of some work on solar sailing being done by Jerome Wright, an engineer at the Battelle Memorial Institute. The actual concept of solar sailing was pretty new to me at that time, and I didn’t know quite what they were talking about—”propelling” a spacecraft by means of the force from sunlight pressure. Of course, I knew about sunlight pressure and, in fact, had done some work analyzing it in my thesis years earlier. Sunlight pressure is one of the important perturbation forces that affect spacecraft motion. Very early in the space age, the National Aeronautics and Space Administration (NASA) had launched small metal needles in the ionosphere to study communication and the way signals behave in the ionosphere. A major controversy had greeted the prediction that the sunlight pressure would push these needles in such a way that their orbit would be lowered and they would harmlessly burn up in Earth’s atmosphere. When they did exactly that, fulfilling the prediction, the force of solar pressure was confirmed as was the magnitude of the solar constant. I also knew that sunlight pressure had been a significant force in controlling the attitude, or orientation, of the Mariner 10 spacecraft that went to Venus and Mercury. Aboard that spacecraft were some small panels that acted as vanes when they were tilted at different angles to the sun. As a result of the sunlight pressure hitting those vanes, the spacecraft could be made to turn on its axis. This is called an attitude force, and it causes a rotational motion-in other words, it turns the spacecraft but does not put it on a different trajectory. A translational force changes the trajectory, and trajectory calculations for interplanetary spacecraft must take into account the perturbation from sunlight pressure on the motion of the spacecraft. The actual use of sunlight pressure as a means of propulsion was a novel idea, although the theory that it could be so used had been around for many years, having been suggested back in the 1920s. Still, it didn’t sound very practical. We already had a means of low-thrust propulsion—solar-electric propulsion—on the drawing boards, and there were many proponents of an advanced low-thrust system using nuclear-electric propulsion. These two methods appeared to be adequate to all the propulsion requirements we could think of for interplanetary travel. Why get interested in another academic technique- Then, at the group meeting, Chauncey Uphoff and Phillip Roberts dropped the bombshell. “Jerry Wright has found a possible way to rendezvous with Halley’s Comet,” Chauncey announced. “You mean fly-by, not rendezvous,” I said. “No, I mean rendezvous.” “With a trip time of ten years?” “Would you believe four years?” Prior to this discussion, as far as we knew, the only known way to rendezvous with Halley’s Comet was a rather theoretical design. This design called for a very advanced solar- or nuclear-electric propulsion system that could find trajectories that could result in a rendezvous with the comet about ten years after launch. Rendezvous, by the way, means not just passing by the target but matching its speed and actually spending some time traveling with it. In other words, to rendezvous with another celestial body, the spacecraft must first get to the target and then match its orbit perfectly and fly in formation with it. Any rendezvous in space is tricky, but a meeting with Halley’s Comet is especially difficult, because it requires us to “stop the world and get off.” Here’s why: Earth goes around the sun at a speed of approximately 30 kilometers per second. Halley’s speed around the sun is, on the average, slower, since it is on a much larger ellipse. But when the comet gets near the inner solar system it is on the fast part of the ellipse. Kepler’s laws of elliptical motion state that equal areas are swept out in equal times. For an elongated ellipse this requires the speed near the sun to be much faster than those in the outer solar system. As a result, the speed of the comet as it nears Earth’s orbit is more like 40 kilometers per second. But the comet goes backward through the solar system-that is, it moves clockwise wound the sun while Earth and the other planets move counterclockwise. And because the comet is going the other way, its relative speed is about 70 kilometers per second. This means that a spacecraft that wants to rendezvous with the comet has to lose all of its motion around the sun and then begin going in the other direction. Such a rendezvous had seemed too challenging—until Chauncey Uphoff spoke up at the meeting, and suddenly it was possible. We wanted to hear more. Chauncey Uphoff’s first step was to write a memo in which he explained the principles of solar sailing. The mission designers at the Jet Propulsion Laboratory (JPL) were so interested in the subject that in the spring of 1975 we invited Jerome Wright to come out from Battelle and conduct a seminar on the concept. He gave the seminar in May. This proved to be such a success that we persuaded Wright to come to JPL to work on the solar sailing idea and study the practical possibilities for a rendezvous with Halley’s Comet. Wright arrived in December 1975; NASA approved a small study at JPL; and a team began to study the design of a solar sailing vehicle and the possibilities of a sail mission. Although the idea of the solar sail had come as something of a surprise to us, the concept has a rich technical history dating back to the 1920s. The JPL study team, especially Uphoff and Wright, ferreted out most of the early literature on solar sailing. The first scientists to mention the use of solar pressure as a propulsive force were Russians. One was the great space pioneer Konstantin Tsiolkovsky, and the other was an engineer, Fridrickh Arturovich Tsander, who in 1924 wrote, “For flight in interplanetary space I am working on the idea of flying, using tremendous mirrors of very thin sheets, capable of achieving favorable results.” It is noteworthy that in the same article Tsander proposed Earth-orbiting space stations. Scientists at that time knew that light exerted pressure because the theory of electromagnetism, developed in the 1860s by James Clerk Maxwell, had been proved in various physical experiments in the nineteenth century. Maxwell himself provided the basic description of light as a “packet” of energy acting as if it were made up of tiny atomlike particles. These particles are called photons, and like other particles, they obey physical laws of motion. Photons have energy and momentum when they move, and the “sailing” is made possible by the transfer of momentum when a photon bounces off the reflective sail. The first serious technical paper about solar sailing for spacecraft propulsion was Carl Wiley’s “Clipper Ships of Space,” a nonfiction article published in the May 1951 issue of Astounding Science Fiction. It is not surprising that solar sailing has often been mentioned in science fiction. Various twentieth-century stories include suggestions for interplanetary and interstellar travel on solar sails. Wiley, an aeronautical engineer, published the article under the pseudonym “Russell Saunders” because he didn’t want to lose scientific credibility by suggesting “way-out” ideas in a science-fiction magazine. Many years later, however, during the JPL study—when the subject was no longer considered “way out”—Wiley, then an engineer at Rockwell, came to some of the technical presentations. Even in 1951, in fact, the topic was taken seriously enough that, in the same issue of the magazine, the space writer Willy Ley commented on Wiley’s article. Ley liked the idea of solar sailing but didn’t think it practical at that time. He thought it couldn’t happen until “after rockets opened up space and enabled us to build artificial satellites” because too much time would be spent on slow trajectories spiraling away from Earth and to another planet. The first article in a professional publication was written in 1958 by Richard Gamin, a Defense Department consultant with IBM. Dr. Gamin’s paper was published in Jet Propulsion and included preliminary calculations of sail-vehicle performance. Following that, a number of technical papers were presented in the engineering literature and in NASA and university publications. Later, in the mid-1970s, Gamin was partly responsible for NASA’s renewed interest in solar sailing. NASA administrator James Fletcher commissioned new studies after communicating with Gamin about the concept. Those studies were then assigned to Jerome Wright at Battelle Memorial Institute in Ohio. Also in 1958, Ted Cotter, at the Los Alamos Scientific Laboratory, came up with the notion of spinning the sail. Cotter also wrote a short article describing the technical aspects of sailing in the early 1960s. Time magazine picked up on Cotter’s work in a 1958 editorial, “Trade Winds in Space.” FIGURE 1.1 Spinning sails were considered first because the spin provides a means of stabilizing the sail without a structure. The mechanisms for controlling the vehicle are, however, rather complicated. This is a concept for a small Mars vehicle. (Philippe Villers, MIT, American Rocket Society preprint, December, 1960) In 1960, Philippe Villers wrote his master’s thesis on the subject at Massachusetts Institute of Technology (MIT). A meeting on solar sail design was held that same year at NASA’s Langley Research Center, and a short course on solar sailing was offered at the University of California at Los Angeles (UCLA) the following year. At first, scientists concentrated on spinning sails (an example is shown in Figure 1.1), although they also considered rigid sails, especially for spacecraft stabilization. Between 1965 and 1967, Richard MacNeal and John Hedgepath invented the heliogyro, a spinning helicopterlike vehicle. NASA began technology studies in the mid-1960s. These studies examined various designs and technology requirements for solar sailing vehicles without reference to specific missions. As the space program began to shrink after the Apollo missions, however, NASA dropped this work. By the mid-1970s, no research at all was going on in solar sailing except for the small study by Jerome Wright at Battelle. Wright did his work under a contract to NASA. This contract had as its primary purpose the calculation of a launch vehicle and the propulsion requirements for various missions that were then under consideration. Because he wanted to be thorough, he also did a cursory analysis of the possibility of using solar sail capabilities for traveling to other planets in the solar system. Then Wright found the Halley rendezvous opportunity. By the time the JPL study team began its task, two major developments had occurred. First, NASA was developing the space shuttle, which promised to carry large-volume payloads into orbit. Second, there had been great advancements in the technology of deploying huge structures in space. The shuttle also made it possible for scientists to test space concepts, and the JPL study team hoped to test the solar sail from a shuttle in orbit. The positive results of the 1976 and early 1977 JPL studies captured the imagination of the new director of the Jet Propulsion Laboratory, Dr. Bruce Murray. With his approval, the study team made a major effort to put together a project plan for a rendezvous with the comet. This work, however, had to be done rapidly. In order to launch in late 1981, the project would have had to start moving by the end of 1978. 1 was put in charge of the study and we quickly wrote a proposal for a one-year study and gave it to NASA. NASA accepted the proposal, and in 1977-78, the JPL team conducted a solar sail design study for the mission, with the help of a half-dozen industrial contractors and support from NASA’s Ames and Langley research centers, The year-long work on preliminary design demonstrated that, indeed, solar sailing was a feasible spacecraft-propulsion technique. Despite the confidence of the technical team and the completion of a valid preliminary design, however, the NASA management was conservative. They felt the design and implementation could not be accomplished in time for a 1981 launch to Halley’s Comet. NASA also thought that the technology for solar sailing was not sufficiently “mature” to be implemented on a near-term space project. Indeed, the Halley mission requirements were severe—and even our willingness to incur great risk for great gain was insufficient to overcome management’s skepticism. And as it turned out, the conservatives were right, we could not have done it. It was a self-fulfilling prophecy. In general, our space abilities were slowing down. NASA assumed that the space shuttle would be operating and ready for interplanetary flight by 1981. This assumption was even more optimistic than the ones we had made about solar sailing. Also, the sail proposal had become caught up in a competition with a proposal by solar-electric propulsion advocates for a complex advanced low-thrust ion-drive system. (Ion drive is a propellant-driven system that provides rocket propulsion by expelling ions continuously from on-board thrusters. The ions are made by converting solar- or nuclear-electrical power, in high-energy vacuum chambers, a process that strips away electrons from a gas such as mercury or argon.) Although solar-electric propulsion won the competition, it lost the war: it was quickly rejected for the Halley mission, and the United States ended up with no comet mission at all. At this point, NASA withdrew its support for work on solar sailing, although some smaller research programs continue. The solar sailing idea, however, had captured the imagination of workers involved in space mission design and planning around the world. A group in France had begun work on a design for la voile solaire. Another private group in France now carries on some work in solar sailing. After the NASA program was terminated, a group of engineers, principally from JPL, set up a private organization partially to seek public funding in order to carry on solar sail development. This group, the World Space Foundation, has actually fabricated a prototype sail and conducted a test deployment demonstrating how it would work. The private efforts to develop solar sailing are discussed in Chapter 10. |
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