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Sailcraft: Concepts, Design, Lab Work

Although we can trace the growth of research into interstellar flight all the way back to the days of Konstantin Tsiolkovsky, the effort has often operated outside of government channels. Scientists and engineers whose day job might take in aspects of rocketry were hard pressed to find time for studying trips to the stars when the proximate needs were better communications satellites or improved designs for reaching low Earth orbit. Nonetheless, work continued, marked by the enthusiasm of the practitioners for what was clearly the ultimate mission. Official or unofficial, small groups hammering on ideas have continued to debate the core concepts.

When the Jet Propulsion Laboratory in Pasadena turned Aden and Marjorie Meinel loose on a mission concept aimed at reaching 1000 AU back in the 1970s, the duo looked at two propulsion options. As the new edition of Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2014) points out, the first of these was a nuclear-electric ion drive using xenon. But the Meinels also evaluated the use of a solar sail unfurled near the Sun. It’s interesting to read here that Chauncey Uphoff, senior analyst on the propulsion phase, was unable to publish the results of the sail study, which wound up circulating only as an internal memo within NASA. [Note: The link above goes to the first edition of this book. The new edition is scheduled for publication within the next few weeks. I advise waiting for it.]

Uphoff’s memo considered the solar sail as an alternative propulsion method, and even if this particular deep space sail concept remained out of view, the efforts of Gregory Matloff and Eugene Mallove soon brought interstellar missions using solar sails to the attention of the community. One of the three authors of Solar Sails, Matloff went to work on what might be done with a close solar pass and sail deployment (partial or complete) at perihelion. He and Mallove evaluated space-manufactured metal sails closing to within 0.04 AU of the Sun’s center, with cables approximating the tensile strength of industrial diamond.

Much of this work was published in the 1980s in the Journal of the British Interplanetary Society, focusing not on beamed laser sails (Robert Forward’s theme) but solar sails using solely the momentum imparted by solar photons. A sail like this, once outbound, could wind cable and sail around the habitat section to provide shielding against cosmic rays. Even a large payload might be accelerated to speeds allowing a trip to Alpha Centauri in 1000 years. Matloff’s 1984 paper “Solar Sail Starships – The Clipper Ships of the Galaxy” (JBIS 34, 371-380) is a classic that he and Mallove would soon update not only in optimization studies in JBIS but also in popular texts like The Starflight Handbook (1989).

The Italian Sail Effort

But back to the theme of small-scale collaborations, from which the interstellar community has for so long benefited. It was back in 1993 at the International Astronautical Congress in Graz, Austria that a small group of solar sail enthusiasts gathered to organize a study of the technology. The study group that emerged was dubbed the Aurora Collaboration, a nod to Greek mythology, in which Aurora was the younger sister of Helios, the god of the Sun. Matloff was one of the core seven behind this collaboration, as was Giovanni Vulpetti, who became team coordinator. Other names familiar to Centauri Dreams readers will be FOCAL mission advocate Claudio Maccone and engineer and author Giancarlo Genta.

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Image: Artist’s conception of a solar sail in space. Credit: Rick Sternbach.

Excellent work can come out of highly motivated small groups like these, and I think the Aurora effort deserves greater attention than it has received. Fifteen published papers emerged from its labors, with three presentations to European space agencies and a workshop held at the University of Rome. Computer code for optimizing sail trajectories, experimental work on layered sail construction (a plastic substrate that can be detached once the sail has been constructed in space), and a number of deployment concepts resulted. The collaboration also studied telecommunications systems, analyzed aluminum sail optical properties, and optimized trajectories for potential missions to near interstellar space and the Sun’s gravitational lens.

The latter deserves a note: The gravitational well created by the Sun’s mass causes light to curve as it grazes the Sun from an object directly behind it (as seen by the observer). The resulting lensing effect is promising for observations at various wavelengths, which is where we get the idea of a FOCAL mission to the focus beginning at 550 AU. What the Aurora team did — with results presented at a meeting of the International Academy of Astronautics in Turin, Italy in 1996 — was to produce preliminary results for a less demanding mission, a sail to the heliopause. The team members presented a thin-film 250-meter square sail and analyzed ways of reducing the sail areal mass thickness, as well as exploring communications options and offering a structural analysis.

The Aurora team’s sail would act as a bridge between the Voyager probes ( the Aurora spacecraft would exit the system about three times faster than Voyager) and later deep space designs. Massing 150 kg, it would use a close solar pass for acceleration and its target was a more manageable (in the near term) 50 to 100 AU. Without benefit of press coverage or large amounts of funding, the Aurora Collaboration moved the ball forward through serious volunteer efforts of the kind the interstellar community has always relied on.

Beamed Sail Experiments

As we go through the papers groups like these create, it’s easy to think of the interstellar effort as being almost entirely theoretical. But laboratory work on some of these technologies goes back a long way, and we can trace early sail studies in the lab to the work of Russian physicist Peter Lebedev in 1899, who experimented with metal sheets of differing levels of reflectivity to measure the effect of the exchange of momentum from photons. I mentioned above the Aurora Collaboration’s experimental work on layered sail construction, and in the early years of the 21st Century, Gregory and James Benford studied beamed sail technologies in a JPL lab.

Their findings are important as we move from straightforward solar sailing to the beamed variant that Robert Forward studied both for microwave and laser designs. As president of Microwave Sciences in Lafayette, CA, James Benford’s experience with microwaves led him to join with brother Gregory, a physicist and well-known science fiction writer, to use advances in materials technologies to attempt these laboratory experiments. Temperature was a key here: Working on Earth’s surface, a sail would have to overcome gravity, and to do that, the sail materials would need to be heated to temperatures higher than 1500 degrees Celsius.

benford_jpl

Aluminum can’t handle that kind of punishment (its melting temperature is 660 degrees Celsius), and the problem persists with many potential metal sails. But carbon undergoes sublimation at temperatures above 3000 degrees Celsius, making the emergence of lightweight carbon structures as potential sail experiments the key. The Benfords used a 10-square centimeter sail in a vacuum chamber, demonstrating acceleration under a 10-kilowatt, 7 GHz microwave beam. Their sails remained intact after experiencing temperatures up to 1725 degrees Celsius.

The carbon microtruss used in the JPL work, developed by San Diego’s Energy Science Research Laboratories and ten times thinner than a human hair, handled the heat requirement with ease. In fact, the Benfords were able to observe accelerations of several gravities in their tests. They also saw a phenomenon known as ‘desorption,’ in which the rapid heating from the microwave beam evaporates molecules — CO2, hydrocarbons and hydrogen — incorporated into the sail during the manufacturing process, adding an interesting second source of acceleration to a sail. A carefully applied layer of compounds painted onto a sail thus creates a propulsive layer of its own.

Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.

Solar Sails notes the desorption findings, but the Benfords produced a result that I consider far more valuable. Their experiments have demonstrated that the pressure of a microwave beam will keep a concave-shaped sail in tension. The beam is itself producing a sideways restoring force. The terminology here is ‘beam-riding,’ and in the case of future sail designs, it means that a properly shaped sail will be stable under the intense beam that drives it.

Moreover, it becomes clear from this laboratory work that the beam can carry angular momentum which it can communicate to the sail. We have, then, a mechanism for allowing ground-based controllers to stabilize a beamed sail against yaw and drift. This important finding grows out of comparatively inexpensive experiment and meshes with ongoing efforts to study the deployment and control of conventional solar sails. What we are seeing is a technology track that holds the promise for space missions on both an interplanetary and interstellar scale.

Tomorrow I’ll continue this series on solar sail and beamed sailcraft with a look at near-term sail concepts discussed in Solar Sails and the mission needs that drive them.

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Comments on this entry are closed.

  • NS November 4, 2014, 14:13

    Re the advanced material needed for the Benfords’ beamed sail tests, is it right that a sail operating in space wouldn’t necessarily require it though? If the radiation from the beam never exceeded (say) the natural intensity of solar photons in Earth orbit you could do beamed propulsion with an aluminum sail? Of course if the advanced material is readily available you might as well use it…

  • william November 4, 2014, 16:21

    I gotta say, Paul you sure are really taken by the solar sail concept; you have perhaps 35% of your articles concerning that subject. Why such extreme interest in this particular subject? In some ways its plausibility must be question because of the enormous energy requirements and construction behind it.

  • Andrew Palfreyman November 4, 2014, 16:30

    I had never fully appreciated how far out is 550 AU – five times the distance to the heliopause!

  • Paul Gilster November 4, 2014, 16:55

    william writes:

    Why such extreme interest in this particular subject? In some ways its plausibility must be question because of the enormous energy requirements and construction behind it.

    I do find sails fascinating and extraordinarily promising, but this week I’m also using the opportunity of a recent reading of the new Solar Sails edition to dig into aspects of the tech that I haven’t explored as much before. If I get my hands on, say, a new text on antimatter or fusion, I will be treating it the same way.

  • Paul Gilster November 4, 2014, 16:57

    NS writes:

    Re the advanced material needed for the Benfords’ beamed sail tests, is it right that a sail operating in space wouldn’t necessarily require it though? If the radiation from the beam never exceeded (say) the natural intensity of solar photons in Earth orbit you could do beamed propulsion with an aluminum sail? Of course if the advanced material is readily available you might as well use it…

    NS, I don’t want to put words into Jim Benford’s mouth, but my take on this is that the closer you can get the sail to the beam — i.e., the higher the temperature you’re going to expose it to — the better the acceleration for these deep space missions. Also depends on what kind of distance you can keep the sail under the beam.

  • Michael November 4, 2014, 17:23

    The close solar approach design I was working on would use tungsten in a compound form which when sprayed out towards the sun would breakdown into a fine metal particle mist which would then be blown back onto the rapidly deployed sail coating it to atomic thickness and stiffening it. Tungsten has a factor of ten thousand less vapour pressure at 3600 degrees Celsius than carbon which would go a long way to offset its much higher density factor of ten -you would need a lot less of it.

  • NS November 4, 2014, 17:51

    “enormous energy requirements and construction behind [solar sails]”

    We can actually build solar sails now (admittedly small ones) and the energy for the pure solar (not beamed) type is free. Potentially their speed is greater than ion rockets. Until we have fission/fusion propulsion (which may be decades) solar sails offer the best chance of reaching at least near-interstellar space in a reasonable amount of time.

  • Eniac November 4, 2014, 17:52

    The desorption thing confuses me, a little. It seems of little use for spacecraft, as it brings back the rocket equation, avoiding which is the main reason for using sails in the first place.

    However that may be, the real and critical question about the Benford experiments is this: Can the (quite incredible) acceleration of several gravities be maintained without desorption?

  • ljk November 4, 2014, 23:06

    May the worst thing that happens in our days be that we get multiple Centauri Dreams articles on solar sails by Paul Gilster.

  • Alex Tolley November 4, 2014, 23:07

    @Eniac

    Can the (quite incredible) acceleration of several gravities be maintained without desorption?

    I don’t believe so.
    However, this is exactly the “hybrid” approach that is being addressed in this series. A short, high acceleration phase might be just what you want to quickly escape a gravity well, rather than a slow outward spiral. I could see this as a viable space business too. The “space marina” offers a “spray on” desorbtion paint layer that offers a quick boost for the beamed sail carrying payloads, reducing transit time. (I’ve never read this idea in SF stories, and it is freely offered here).

    Another result of the Benford’s experiment was to show that the sail can be spun. This offers a way to stabilize the sail structure. The old Nasa heliogyro design also used rotation to deploy and stabilize the sails. If the beam method scales up, it offers a nice way to support large sail deployment and maintain structure. This might be very useful for extremely large, ultra lightweight sails.

  • Rob Henry November 7, 2014, 18:20

    Eniac, I also had problems understanding interest in deabsorption for just the same reasons as you gave then the answer struck me…

    If the theoretical closest approach to the sun without deabsorption is X, then it will be less than that with it. By the time the craft reaches X, all that excess material may have already gone. The main advantage gained is through the Obeth effect (the sum of the acceleration along its path may even prove lower in extreme circumstances).

    The rocket-type boost helps mitigate the excess weight carried, but I doubt it would be advantageous to carry much of that material beyond point X. It could certainly never be a significant factor in beamed propulsion – at least not one with interstellar aspirations.

  • Eniac November 9, 2014, 19:02

    Rob: It is hard to see how adding reaction mass can be efficient for a, say 100 km/s solar escape mission. Say you double the mass of the sail by adding propellant to it: This can only provide a paltry few km/s in extra delta-v, seeing that it’s desorption happens at thermal velocities. If the same mass could be added on as extra sail, it could in principle double the delta-v (or, the payload). The latter seems almost always better, but, of course, a definite evaluation needs to await more detailed treatment. If the Oberth effect were really as beneficial as you claim, should it then not be better to bring a loaded rocket close to the sun and not bother with sails? A proper rocket engine would produce thrust much more efficiently than more or less uncontrolled desorption, one would think.

  • Rob Henry November 11, 2014, 16:39

    In answer to your last question Eniac, if we take a normal chemical rocket that can achieve a delta v of 10 km/s,but whose size is negligible in proportion to the deabsorption paint that allows it to skim the sun, then we would get 110 km/s. 20 would get 160. Sure I have seen figures for a sundiver attaining a theoretical maximum of 500 km/s, but that assumes that that it isn’t weighed down with deabsorption paint, and that the supporting wires are so very thick that they can hold the higher acceleration that would otherwise be impossible to utilise near the sun. I think the true theoretical maximum is around half that, and so, perhaps, a bit better than our rocket.

  • Eniac November 12, 2014, 23:37

    Rob, It sounds like you are right about the numbers, except I am not clear what deabsorption paint you are referring to in the case of the rocket-only approach. Any thin foil of shiny, refractory material should be able to shade the rocket from the sun completely. The surface of such a shield would be very much less than that of a sail, and its mass should be negligible compared to rocket or payload.