Focus on the Sail

by Paul Gilster on July 3, 2013

The British geneticist and biologist J.B.S. Haldane has left us with one of the more memorable lines about scientific inquiry, one that draws on the richest of all of Shakespeare’s plays for its punch. Hamlet tells Horatio that there are more things in heaven and Earth than are dreamt of in his philosophy (Act 1, Sc. 5), a thought Haldane adapts in the service of intellectual surprise. In his collection Possible Worlds and Other Essays (1927), he writes:

I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose … I suspect that there are more things in heaven and earth than are dreamed of, or can be dreamed of, in any philosophy.

HaldaneJBS

We can imagine Olaf Stapledon nodding as he read those lines. Haldane sketched out a human history covering the coming 40 million years in his essay “The Last Judgement,” one that Stapledon drew on in creating his own Last and First Men (1930). We can also imagine the reader surprise that probably met Haldane’s use of solar sails as a way of getting around the Solar System, all part of the same essay, in which Venusian colonists (after the demise of the Earth) contemplate crossing to a passing star on beams of the Sun’s light.

Image: John Burdon Sanderson Haldane (1892–1964), who gives us one of our earlier references to solar sails as a way of reaching the stars.

In Defense of the Sail

These days we have seen solar sailing in action through the Japanese IKAROS mission and the smaller NanoSail-D project from NASA, even as larger sails have been constructed for ground tests and we look forward to a late 2014 launch of NASA’s Sunjammer, which will deploy a sail 38 meters on a side (the sail’s name comes from an Arthur C. Clarke story that envisioned a race to the Moon using solar sails, one that ends with the abandoned ship being targeted for interstellar space). Solar sails, in other words, are not — like fusion, like antimatter — propulsion systems of the future. They’ve reached flight status for deployment and testing in space.

These sails work because light, although it has no mass, can impart momentum. If we can make the necessary upgrades in materials to enable a close pass by the Sun (perhaps with the aid of an occulter to shield the craft at perihelion), we can imagine driving a sail deep into the Solar System at speeds of several hundred kilometers per second. But as Jim Benford shows in his essay “Sailships” in Starship Century, we need to go much faster for an interstellar mission, and that involves beaming energy that will produce force at great distances.

From the essay:

I say this because sailships have a singular advantage: they leave the engine behind. So we can build a spacecraft that consists of only payload and structure — no fuel at all. The propellant is light itself, so sails reflect light waves, whether visible or microwave or laser produced, from a beam generated elsewhere. Sails can be made both light and smart, in the sense that control systems, sensors and computational ability can be embedded in the structure of the sail itself — a smart sail, with dispersed circuitry, and therefore far harder to damage by meteors or accident.

All this makes a solid case for sail technologies, and so does the fact that in the year 2000, Benford and his brother Gregory produced laboratory data showing that carbon sails could be driven by microwave beams to produce accelerations of several g’s. When I wrote about this work in Centauri Dreams (the book), I was struck by the fact that this laboratory work demonstrated beam-riding, in which the pressure of the beam and the concave shape of the sail work together to produce a sideways restoring force. The sail can also be stabilized against yaw and drift because the beam being directed at it can carry angular momentum that can be imparted to the spacecraft, thus spinning it.

Near-Term Work on Beamed Power

Benford looks at issues of stability, deployment and large-scale space construction for sails and the beam sources that drive them, all of which demand a stable infrastructure to build upon. But the fact that we already have sails capable of space deployment points toward building experience with sail engineering and in particular materials as we learn to optimize our work with carbon nanotubes and carbon micro-truss structures. It’s an intriguing thought that, sails being large structures we will learn to work with in space, their growth should also help in the development of large transmitting antennae of the kind we will eventually need to build.

A stair-step series of beamed power applications builds the groundwork for the sail infrastructure, one that grows out of engaging commercial interests:

In today’s frugal climate, it is important for technology development to be coupled to commercial applications. Several of the missions we’ve described are potentially commercial matters. Starting with orbital debris mapping, one can see an incremental commercial development leading first to satellite power recharging. Eventually, as the space market and business confidence grows and capital becomes more available, this development plan leads to the repowering of satellites in GEO and ultimately to launch services. Investment costs are minimized because the research program leads to applications, which feed capital back into research, leading to new applications.

Ultimately, we’d like to build a true beamer — the source of the laser or microwave beam we’re sending to the sail — that would be assembled in space from materials mined from the moon or from asteroids. Placing the beamer in a close solar orbit would maximize the power available. The beauty of the beamed energy concept is obvious: Early fusion designs like Daedalus could carry a payload that was less than one percent of its initial mass, while sailship payload masses can be considerably higher. After all, we’re leaving the propellant behind us in the Solar System. Moreover, a single beamer, once established, could be used for many sailship missions.

Imagine, then, future sails many kilometers in diameter that are deployed by spinning up the initial, folded package and, in close proximity to the beamer, are pushed out by laser or microwave beam. The acceleration quickly increases as the beam stays fixed on the sail for hours, then days. As the sailship reaches the outer Solar System, the beam switches off and the spacecraft is launched on its interstellar journey, perhaps stowing the sail for cruise. The fact that electromagnetic waves can transfer power over long range makes this scenario possible.

beam-driven-sail_1

Image: An interstellar sail pushed by laser or microwave. Credit: Michael Carroll.

Fusion, Benford notes, is still struggling with physics and engineering issues, so that cost estimates for continued research and development are wide open. If the idea is to solve the physics, then tackle the engineering questions and finally look at the economic feasibility, then sails have the edge. We know the basic physics and have an engineering requirement that demands large antenna and optical arrays, along with assembly of our photon sources. Usefully, we have considerable experience in the sub-systems that are a foundation of this work.

We often think of beamed sail concepts in terms of gigantic structures like the Fresnel lens that Robert Forward wrote about in the outer Solar System, thousands of kilometers in diameter and massing half a million tons, or his 75,000 ton staged laser sails destined for Epsilon Eridani. But researchers like Geoffrey Landis have gone to work on Forward’s concepts using high-temperature materials like boron and carbon that would allow better acceleration, and proposing a string of lenses that would drastically reduce the size of the Fresnel lens.

The theoretical work continues as we press on with our early sail deployments. J.B.S. Haldane had no doubt that the future would surprise him, and doubtless the fiction of Olaf Stapledon took his thoughts in directions he could never have anticipated. We will learn in the coming century whether the audacious idea of a sail being pushed between the stars is another Haldane whim that nudges our philosophy, pointing to a workable approach to crossing the interstellar gulf.

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CatharSeamus July 3, 2013 at 12:46

I’ll be talking on Starship Congress about another interesting idea in this space, which is the long-term stabilisation of the string of lenses that Geoffrey Landis proposed. The idea can be taken a step forward as a full-blown laser interstellar highway that allows acceleration and deceleration in both ways, from and to the destination star

The abstract of my talk “The Laser Star Way: A Light Bridge to the closest Stars” is:
“Beamed laser propulsion for interstellar flight is promising but it is severely limited by beam divergence over interstellar distances. In the current work, a system of power relays is proposed to deliver and route light power to accelerate and deccelerate ships flying in both directions between two stars, as well to keep the relays in their equilibrium positions. The system, once deployed, creates a natural radiation roadway where interstellar dust grains tend to evaporate, addressing the problem of relativistic impacts with high-speed ships.”

Alex Tolley July 3, 2013 at 14:54

The potential for sails, beamed or otherwise is exciting. Looking at current sail technology vs potential technology (nanotubes, graphene, etc), the sail loading performance can improve by 2 to 3 orders of magnitude, giving realistic accelerations with light payloads of 1/100 t0 1/10 g. That sort of performance gets you to orbit Saturn in a year or so (if my BOE calcs are correct).

Add in beaming and the flight times are further reduced. It was almost incidental that Benford talked about circular polarizing a microwave beam to spin a sail for stability. If that can be used to deploy a large sail, that would be good as the forces might be very evenly distributed across such a delicate surface.

As for costs, I see that the Planetary Society is spending less than $1.5m a year on their LightSail-1. This isn’t even a rounding error for a space organization. It seems to me that the near term potential of this propulsion system is very large, and the uncertainties much lower than some of the more exotic options.

If sails can be conformed to the concave shape for beam riding, that means they could potentially act as solar concentrators, allowing solar thermal/electric propulsion using readily available resources. This would allow fast, high thrust acceleration when needed.

Why is this technology taking so long to gestate, when rockets have reached their peak for nearly half a century? “Project Solar Sail” was published in 1990 with the thought that private organizations could launch a sail. Nearly a quarter of a century later and sails are still not taken very seriously as a viable propulsion system, despite their obvious promise.

andyet July 3, 2013 at 15:19

So how does the sail decelerate on approach to its destination star in the absence of an orbiting laser/maser array beaming energy to slow it down?

Perhaps the first mission profile would consist of initial acceleration (my old copy of “The Starflight Handbook” says a beamed sail can achieve 0.1c), followed by continuous deceleration from the light of the destination sun until arrival (or could a large magnetic field be used for deceleration?).

Then the probe and its crew could construct an orbiting laser/maser system at the destination star that would allow faster subsequent missions? This seems analogous to the laying or rail road tracks – tracks of laser light! – between the stars.

Also, instead of an orbiting laser/maser array, why not cover the surfae of Mercury with photovoltaic arrays and laser cannons at intervals around the equator?

David Cummings July 3, 2013 at 16:51

What if right now, in our galaxy, there is a civilization with a network of laser starways linking 50 or a hundred or more stars together? Should we be able to see that? Should that be detectable from thousands of light years away?

SM July 3, 2013 at 17:44

The future put forward imagined “future sails many kilometers in diameter”. However; a recent proposal developing via Pocket Spacecraft through thier Kickstarter, aims at developing and sending thousands of small solar sails (weighing much less than a gram each) towards the disired celestial object. The benefit lies within the low cost associated with launch, and the ability to land on a planet (as atmosphere wouldn’t have a negative effect on an object of this weight and size). Maby the technology could be adapted to an interstellar mission.

http://www.gizmag.com/pocket-spacecraft/28130/
http://www.kickstarter.com/projects/1677943140/send-your-own-pocket-spacecraft-on-a-mission-to-th
http://pocketspacecraft.com/

Alex Tolley July 3, 2013 at 18:07

@CatharSeamus – I’d like to hear more about how the relay works. Will you be publishing the talk?

CatharSeamus July 3, 2013 at 18:13

@David Cummings, I discussed this briefly with Adam Crowl, and If they are serious about keeping one of these connecting directly to our solar system, at 100 AU they just need an ‘umbrella’ of about 5km wide to hide the angular disk of a close star like Alpha Centauri. Such structures can be thermally and spectroscopically hidden from passive observation.

The only way we could think of to detect that kind of nearby structures is with Hydrogen nukes that would send flashes of UV radiation, and reflect off from nearby objects on the solar system.

CatharSeamus July 3, 2013 at 18:15

I meant, if they are serious about keeping it hidden from our knowledge

CatharSeamus July 3, 2013 at 18:17

Ah the other thing we didn’t consider is looking for the lasers, which should be more obvious, and the best clue we have that none of those structures are located nearby.

Seeing these structures from a few light years should be really difficult, since each node doesn’t need to be larger than about 2 or 3 kilometres

Joy July 3, 2013 at 18:37

Thumbs up for sails! The “extraordinary evidence” required to validate fission/fusion/antimatter engine concepts as flight hardware has yet to be seen. Pulsed nuclear propulsion (Orion) might work but is a nonstarter due to the fact that it could only be done on a massive scale. On the other hand, sails have been demonstrated to work, and are scalable. Sails have an immediate application for interplanetary probes, especially to the inner planets.

Starlight powered sails could carry artilects and frozen biological materials on grand voyages around the (bright) stars with multiple ports of call, long travel time irrelevant for machines. Energy rich societies could use power beaming to enable fast interstellar travel. IMO, what small influence we have should be targeted towards pushing sail development.

Paul Gilster July 3, 2013 at 19:27

SM writes:

a recent proposal developing via Pocket Spacecraft through thier Kickstarter, aims at developing and sending thousands of small solar sails (weighing much less than a gram each) towards the disired celestial object.

Haven’t looked at this yet, but the idea seems reminiscent of Jordin Kare’s SailBeam idea, as discussed briefly here:

http://www.centauri-dreams.org/?p=25626

CatharSeamus July 3, 2013 at 19:53

@Alex Tolley. The paper will be published initially on JBIS, so a freely available version will take a couple of months, but I’m allowed to send preprints to individuals. Actually, I should re-read the JBIS terms of publishing, I think I’m allowed to distribute preprints, but will have to check with the journal editor

Eniac July 3, 2013 at 22:52

Paul:

But researchers like Geoffrey Landis have gone to work on Forward’s concepts using high-temperature materials like boron and carbon that would allow better acceleration, and proposing a string of lenses that would drastically reduce the size of the Fresnel lens.

The large aperture is required by the diffraction limit. I do not see how it could ever be done away with. The best you can do is have a synthetic aperture made of many smaller elements, but the area still has to add up to the same size. All the elements have to be coherently phase-locked, which is not something we can currently do. I there a reference to the Landis work?

Early fusion designs like Daedalus could carry a payload that was less than one percent of its initial mass, while sailship payload masses can be considerably higher. After all, we’re leaving the propellant behind us in the Solar System.

This is true. However, sails provide a new constraint that may turn out to be no better than the rocket equation: You are limited to very little mass per area. VERY little. A solar sail at 1 AU generates a force of at most 10 micronewton per square meter. To get any useful acceleration at all, your craft will need to make do with a gram or so per square meter, and that includes the payload. Beams could improve that, but not drastically, as you also need to keep the sail from evaporating.

A square kilometer of sail (that is huge) could propel one ton of total weight, but much of that will have to be the sail and the rigging. I imagine it will not be easy to come up with a design that actually has a better payload ratio (payload/sail+rigging) than Daedalus (payload/engine+fuel).

And when it comes to high velocities, things look really troublesome. At a certain velocity (I think it is only a few tens of km/s), incoming atoms will cause sputtering, i.e. each will knock one or more atoms out of the sail. It turns out that the interstellar gas between here and Alpha Centauri is about as thick as a sail would be, in terms of atoms per square meter. So, as the velocity exceeds the sputtering threshold, the sail would be completely gone by the time it gets there. Likely much earlier, and much more so as you raise the velocity above a few tens of km/s. I may be wrong about this, but as far as I know it has not been seriously considered in existing proposals. Or has it?

As for beams clearing a path, that is not going to happen. There will be close to no effect of beams on neutral hydrogen gas, and what little effect there might be will be quickly blown away by the “interstellar crosswind” (in the order of several tens km/s, same as relative motion of stars).

Michael July 4, 2013 at 1:02

Mag or light sails could be used to good effect on a Icarus type starship in dropping slower moving probes off in target system. Simply allow the final stage of the craft to start firing the engine again near the system and drop off sail probes into the exhaust stream (softened with He from the storage tanks) that will slow the probes down real fast.

Mephane July 4, 2013 at 3:14

“So how does the sail decelerate on approach to its destination star in the absence of an orbiting laser/maser array beaming energy to slow it down?”

Orbital mechanics allows for some maneuvres that appear counter-intuitive at first glance, but are nontheless real. Any spacecraft approaching another star system would not fly in a straight line, but due to gravity in a curved trajectory. Deceleration is never performed in the sense of pointing the vector of your propulsion towards the destination object, but retrograde to the craft’s trajectory, preferrably at the periapsis (closest approach). It takes but miniscule adjustments early on to get a trajectory around the target star where the craft’s retrograde direction at periapsis would point sideways (don’t forget how angles of reflection work!), not away from us, and then the sail has only to be angled correctly and the very laser beam that inititally accelerated the spacecraft, could now decelerate it into a solar orbit in the destination system.

David Cummings July 4, 2013 at 6:32

Thanks, CatharSeamus.

By the way, since this article started with a J.B.S. Haldane quote, I’d like to add another here, even if off topic.

He famously said (and supposedly after doing some math on a napkin will sitting at a bar), that he would willingly die for two brothers or eight cousins… which led to a formalized set of equations — Hamilton’s Rule — and the mathematics of kin selection.

Paul Gilster July 4, 2013 at 9:20

Eniac writes:

Is there a reference to the Landis work?

One Landis reference is the one Adam used for his article. It’s “Advanced Solar- and Laser-pushed Lightsail Concepts,” which is available as a NIAC final report (1999) in the NIAC archive at http://www.niac.usra.edu.

But also useful is “Optics and Materials Considerations for a Laser-Propelled Lightsail,” Paper IAA-89-664, 40th IAF Congress, Torremolinos, Spain (1989).

Landis wrote up some of his concepts in “Small Laser-propelled Interstellar Probe,” JBIS Vol. 50 (1997), pp. 149-154.

I can’t recall which of these goes into the multiple lens issue at greatest depth, but I think it’s the first. It’s been a long time since I’ve read them, so you’ve suggested a future article for me.

andyet July 4, 2013 at 11:23

Could sails be built from single atom thick sheets of carbon material (such as the new water desalination and filter sheet, Perferene)?

Would this be the theoretically lightest possible material to make sails from?

How many kW of laser light per square km of sail would be needed to accelerate a payload say, twice the size of the ISS (almost 1,000 metric tons) at 1g to 0.10c?

And again, in the absence of a receiving laser array in orbit around its target star, how exactly does a laser propelled sail decelerate once it arrives at its destination ?

Alex Tolley July 4, 2013 at 11:39

@Eniac “I imagine it will not be easy to come up with a design that actually has a better payload ratio (payload/sail+rigging) than Daedalus (payload/engine+fuel). “

A 1 tonne payload with a square km sail is your extra 1g/m^2 sail loading. Therefore you can set your payload ratios accordingly. Wright “Space Sailing” p57 has a nice chart on acceleration vs payload ratios for different unleaded sail loading. For e.g. a 20% payload ratio on an unleaded sail mass of 1 gm/m^2 gives you about 6 mm/s^2 (6 x 10^-4 g). Vulpetti et al “Solar Sails” suggests the ultimate performance using fast sailing trajectories could give you velocities of 1030 km/s (0.0034 c) exiting the solar system, with a reverse trajectory to slow down at the destination (no payload given). This is just with sunlight. Beaming would improve that performance considerably, especially with the short term boosts of >g that can be attained with coating the sail with an evaporative paint.

As noted by Joy, sails are highly scalable. You do not need 50k tonne Daedalus class vehicles as a minimum size (with associated costs). A 1 kg sail (or less) may be adequate for some missions.

As regards the problem with the interstellar medium. If the sail ship is mostly cruising, then it could stow its sail during much of the flight, obviating the damage issue. If it is a beamed sail using a magsail to decelerate, then loss of the sail is not so important, although I would still think that an operational sail would be most useful for exploration at the target system.

Alex Tolley July 4, 2013 at 18:37

@andyet. This paper by Spieth & Zubrin has some example accelerations using the sun for advanced theoretical sails. Note that they have some accelerations of unloaded sails at 5 m/s^2 = 0.5 g

As this is with the sun at 1 AU, we can use the solar constant and scale, for 1 g acceleration and 1 km^2 sizes. That gets us into the ballpark of 5 GW/km^2 laser power.

To throw a vehicle at 2000 tonnes, even with a sail loading of 0.1 g/m^2 (probably way too high for the accelerations needed), requires around 20,000 km^2 sail area (140 km on a side). Big. probably needs sail sizes orders of magnitude larger. Very big :)

At 1 g you would reach 0.1c in about a month.

I estimate you would need 10^22 joules, or about 2×10^9 GWh. Let’s put that in perspective. Global energy consumption is about 100 PWh, = 10^8 GWh. So your power requirement is more than 100x global power consumption for the month the beam is turned on. Not going to be doable anytime soon.
Obviously we would need solar power satellites on an enormous scale to generate the power needed to accelerate the sail and payload. You will need to shed a lot of energy at the target star. It would make a nice light show if you hit anything on the way in.

Reach for stars in our lifetime July 4, 2013 at 18:48

Why not a holographic lens to focus a beam on a sail?

vincent teofilo July 4, 2013 at 21:48

This discussion of solar sails as an interstellar method of propulsion reminds me of the course I developed for the Space Power Systems class at Lockheed Marin Space Systems Co. staring in 2002. one of the 12 two hr lectures I developed and still present Advanced Space Power Systems at SCU. The problem I give the students to comprehend the issue of interstellar flight is to calculate the mass of propellant and time for a STS [100MT] vehicle to be propelled to Alpha Centura and back at constant v and then at 1 g acceleration. Using the kinetic energy equation the first part is easily calculated to be 10^6 kg. The 2nd part requires an integral table which usually stump most students to get the correct answer of ~ 10^13 kg of matter 100% which is converted to energy. Solar sails are never considered since they would require inconceivable deployed structures for sails and beamed photonic sources the radiators of which maybe larger then the sails. What is concluded is the only known current physics to achieve such travel is must allow the development of warp drive or transversable worm holes . [see; http://mech372.engr.scu.edu/Lectures/Wk3/Advanced%20Space%20Power%20Systems-Teofilo-2013.pdf ]

Aaron Johnston July 5, 2013 at 0:45

What about developing like a magnify glass or multiple ones to be focused into a smaller sail to propel a vessel?

andyet July 5, 2013 at 9:15

Would not the “impact” of massive amounts of light energy on the sail create a lot of waste heat?

If so, could the sail be designed to act as its own radiator?

Also (one more time), in the absence of a receiving laser array in orbit around its target star, how exactly does a laser propelled sail decelerate once it arrives at its destination ?

Mark Wakely July 5, 2013 at 9:44

Is it safe to assume that any sail would have to have self-healing abilities since the longer the voyage, the more interstellar dust and debris it would encounter head on? The overall square footage of sail lost to micro impacts might be significant in the long run, reducing speed and efficiency. And any major impact- while not diminishing sail square footage to any large degree- might cause its own problem with steering unless the overall structure was rigid enough not to fold or distort in such a way as to send the craft off course, or unable to make any mid-course corrections.

Just some considerations.

Michael July 5, 2013 at 14:07

@Mark

Even a micro impact would have little effect on the sail, the wavelength of light is the most important. Think of why microwave ovens don’t fry someone who is cooking their food (see all the holes and no effect outside).

Mark Wakely July 5, 2013 at 20:13

@Michael

Thanks for your answer, but I’m not convinced that a tattered, twisted sail has much hope of completing its journey should the interstellar medium be “dirtier” than projected or expected. The unique “True Beamer” sail proposed in the article above does mitigate the problem by stowing the sail for the cruise, but otherwise I have to agree with Eniac that sail decay is inevitable in those designs where the sails remained deployed. The larger the sail, the faster the speed and the longer the journey only makes the potential for disaster worse, perhaps even inevitable. Virtually undetectable filaments of cold, wispy interstellar gas could be a sail’s undoing at near relativistic speeds, as could the thinnest of dust clouds that are beyond our ability to detect.

But hey, I’m not trying to shatter anyone’s “Centauri Dreams,” just suggesting a worse case scenario that might be unfounded but should be considered. My guess is that it’s the luck of the draw- the path to one star might be “clear sailing” while another turns out to be an interstellar minefield, better suited for a hale and hearty Bussard ramjet-style vehicle than an delicate sail with gossamer wings.

Eniac July 5, 2013 at 23:07

As Michael says, dust and debris are not the problem. It is the gas that is the problem. At sufficient velocity, gas becomes very abrasive. 99% of the ISM is hydrogen gas. A hull even just a mm thick could probably withstand all the gas between here and Alpha Centauri. A light sail a few microns thick would not.

@Alex Tolley: I am guessing that neither Wright nor Vulpetti are presenting actual engineering designs. I know you can make it work quite well on the back of an envelope, but an actual design with all the rigging, stability, resistance to tearing, etc taken care of would be much more difficult while still getting a good mass ratio.

Yes, you can stow or jettison the sail early. The question is, though: How far along do you get in the acceleration before the sail is shredded? My fear is it will not be very far, and my concern is that no-one is addressing the problem.

@Mephane: Orbital mechanics is useless for the velocities we are talking about. Gravity would produce only a minuscule change in direction, even if you passed directly over the surface of the target star. It would certainly not keep the craft from escaping just as fast as it arrived.

@andyet: Carbon is black, so it will probably not be suitable material for a single layer light sail. It may work as a backing for a composite sail, though, on the dark side of a reflective metal layer for both strength and improved radiative cooling.

Being its own radiator is the only way a light sail could ever work, of course. There just isn’t any room in the mass/area constraint for other means of cooling. With heat-stable materials, you could go a few orders of magnitude up from 1 AU insolation before the sail evaporates. Beryllium has been suggested as an optimum compromise between heat stability and weight. A two-layer composite sail reflective on one side and absorptive on the other may allow raising the power further, and there is also talk about “dielectric” sails which would absorb less power than reflective ones, but as far as I know those only exist in the imagination at this point.

@Mark Wakely: Self healing would not be useful here. A few holes will not affect operation too much, and how do you accomplish self-healing without adding mass, anyway? The material loss from ISM gas sputtering is unavoidable and final, no “healing” in the world can bring back the escaped atoms. You can take more material to begin with, but that would cost you dearly in terms of reduced acceleration, and would only delay eventual disintegration.

coolstar July 6, 2013 at 2:23

Beamed power to sails also gives one other “benefits”: very effective weapons come quickly to mind. With close-in solar power satellites of enormous size to power them, and the rest of your civilization (which I’ve suggested here before might be detectable and apparently Marcy et al. have a Templeton grant to search for them in Kepler data) one could, perhaps, easily control all, or nearly all, access to space within a few AU of the parent star.

Brett Bellmore July 6, 2013 at 10:35

I think that the best use of sails for interstellar travel is probably as a form of smart propellant, as has been discussed in several places. It allows your actual craft to be quite compact, while still benefiting from momentum picked up by many square kilometers of sail area. A “beam” of small sails is continuously manufactured in the home system, is accelerated to considerably higher speed than the craft, and homes in on some sort of momentum gathering system on the craft.

Some portion of the beam could skirt around the craft, and then close up after it, and form a continuously renewed shield punching a hole in the interstellar media for the craft to go through.

This requires the sails to be not only accelerated near the launching system, but to be provided with enough illumination for course corrections on their way to the craft, and some sort of beacon to home in on. And it still doesn’t resolve the question of how to decelerate at the target system.

Lionel July 6, 2013 at 12:28

@ Eniac
Sail loss due to ISM sputtering is clearly a big factor, I absolutely agree that if it hasn’t been looked into so far then it deserves attention.

However in beamed power concepts it seems that all the accelerating is proposed within a short portion of distance – such as the example Paul presented above, where the craft reaches maximum velocity while in the outer solar system, and then is stowed. In this were possible, the sail would only have met a small percentage of all the ISM between us and the target system

Of course the beam power is limited by the rate at which the sail can radiate away heat whilst remaining at a safe temperature…

Lionel July 6, 2013 at 12:29

@ Eniac
Sail loss due to ISM sputtering is clearly a big factor, I absolutely agree that if it hasn’t been looked into so far then it deserves attention.

However in beamed power concepts it seems that all the accelerating is proposed within a short portion of the distance – such as the example Paul presented above, where the craft reaches maximum velocity while in the outer solar system, and then the sail is stowed. In this were possible, the sail would only have met a small percentage of all the ISM between us and the target system

Of course the beam power is limited by the rate at which the sail can radiate away heat whilst remaining at a safe temperature…

Alex Tolley July 6, 2013 at 13:28

@Eniac
If sail erosion is your big worry, then mesh sails powered by microwave beams would be your preferred sail solution, I think.

You are correct that no-one has detailed engineering plans for an interstellar sail (or even for a high performance solar system sail, for that matter), but neither have we the engineering designs for any class of starship. Everything is conceptual at this stage until we actually start to design a real craft. There are many possible designs for sails that include rigging and control. Getting from concept to vehicle shouldn’t be that hard with some decent funding and experimentation.

Max Schönfisch July 6, 2013 at 18:26

Why bother with huge apertures and sails when you can use powerful lasers with much smaller apertures to rapidly accelerate many tiny sails with some form of steering mechanism to significant fractions of c? Their propulsion systems would allow the stream to stay collimated and “home in” on the starship. Upon arrival, they would be blasted into plasma by onboard lasers and deflected by a magnetic sail, thus transfering their momentum to the spacecraft. Such “mass beam riders” as desribed by Singer (macroscopic pellets), Nordley (self-steering nano or mesoparticles) and Kare (aforementioned laser-accelerated microsails – probably the most near-term option) offer a way around the huge engineering challenges associated with megameter sail manufacturing, sail deployment, beam spread and pointing accuracy over distances measured in significant fractions of a light-year.

The huge power arrays necessary to collect the energy required to accelerate large masses to relativistic velocities could only be constructed by dedicated self-replicating machines. Designing a useful self-repliacting robotic mining and construction system for use in space is THE key technology for achieving interstellar flight. Once such a system exists, it will only be a matter of decades before the first starships leave the solar system, probably riding on streams of self-steering matter.

Michael July 7, 2013 at 2:02

With using a large sail we could after the acceleration phase roll it up around the probe cigar fashion and orientate it pointing towards the star. It would give a much small area target for oncoming material and great impact protection. Once near the target system unwind the cigar and use it as a brake to slow the probe down, it won’t slow it to stand still but will give the probe more viewing time.

william collins July 7, 2013 at 17:00

I have continued to find the concept of solar sails as a way to the outer planets and/or to the stars to be a source of fascination. It is more plausible than warp drives fueled by mythical exotic energy sources or Non-existant wormholes “tunnels from noplace to nowhere”. Harsh tones, I know.

Eniac July 7, 2013 at 22:45

@Max Schönfisch: I agree completely. Kare’s sailbeams are the most realistic external energy propulsion concept yet presented.

I do think, though, that they are far less developed than, say, Daedalus, in terms of thinking through and envisioning a solution for every hurdle in the concept. There are big holes left, such as the extremely low energy absorbance needed, stability under high acceleration, the feasibility of the micro or nano-scale guidance and propulsion systems, and many more.

william f collins July 9, 2013 at 9:12

What would be the most feasible and efficient propulsion system to carry human crews to the outer planets and the surrounding moons?

Max Schönfisch July 9, 2013 at 9:24

I think with an effort similar in scale to the Daedalus and Icarus studies, a feasible beamed interstellar propulsion system could be designed, with a greater chance of being built in this century than a massive one-shot fusion rocket.

Another option is the beam-blown magsail. The problem is beam spread due to random thermal velocity in a neutral particle beam. Could this be overcome by using low-energy laser light emitted from the beam source and “wrapped around” the particle beam to nudge errant particles back into the beam and thus keep it collimated?

Eniac July 13, 2013 at 10:57

What would be the most feasible and efficient propulsion system to carry human crews to the outer planets and the surrounding moons?

Nuclear powered ion drives, no doubt about it.

Mariusz July 13, 2013 at 11:46

I wonder if anyone took under consideration the fact that the best lasers we currently have are only about 10-20% efficient.

Paul D. July 17, 2013 at 17:11

Would this be the theoretically lightest possible material to make sails from?

I suspect the lightest would be a plasma sail. Construct a plasma of partially ionized atoms and shine a laser on it tuned to a resonance of the ions. Singly ionized alkali earth elements would be sodium-like, and have very high cross sections near resonance. If the wavelength is properly tuned it can preferentially scatter off ions of higher kinetic energy and cool the plasma! More generally, lasers can be used to refrigerate objects (laser cooling of cystals has been demonstrated); it might be possible to use this to actively cool a more conventional thin film sail.

Andrew Palfreyman August 29, 2013 at 18:22

Does anyone have a ballpark figure for the max tolerable flux [W/m^2] of (say) sunlight onto a graphene monolayer? I know that James Benford has already quoted 10 MW/m^2 here for his carbon mesh. I suspect the monolayer (or better, bilayer with lambda/4 spacing for reflectivity) fares better.

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