≡ Menu

A Microwave-Beamed Sail for Deep Space

It’s been some time since Centauri Dreams looked at the work Gregory Benford (University of California at Irvine) and his brother James (Microwave Sciences) are doing with solar sail concepts. But I just noted, in paging through a back issue of the Journal of the British Interplanetary Society, that their proposal for a microwave-beamed sail was written up there, based on a talk at the 2005 IAA symposium in Aosta, Italy. And because I want to keep sail concepts visible in a time when funding constraints have all but driven them from the news, let’s revisit that work.

What got the Benfords headlines not so long ago was the speeds they were proposing. Five years or less to Pluto? That’s almost a halving of New Horizons’ travel time, and it makes for some intriguing conjecture indeed. The Benfords learned from earlier laboratory experiments that heating up ultralight carbon sail materials causes accelerations greater than would be expected from the pressure of photons alone. Apparently causing the effect are molecules evaporating from the hot side of the sail.

So here’s a solar sail scenario that a science fiction writer (especially one named Benford) could put to good use: ‘Paint’ various compounds onto a sail to take advantage of these effects, which vary depending on how they’re managed. The duo envisage a ‘Sundiver’ mission that uses microwave beaming, a close Solar pass and a second boost from the Earth-orbiting microwave transmitter to achieve maximum speed.

The solar sail could be deployed in low Earth orbit by conventional rocket. It would then be launched by microwave beam, the heat of which would cause a polymer layer to desorp from the sail (think of desorption as the opposite of absorption — some of the ‘paint’ material is released to provide propulsion).

The microwave beaming cancels most of the sail’s orbital velocity around the Sun, causing it to fall toward it. The craft approaches edge-on but at perihelion, a few solar radii out, it rotates to face the Sun. Now a second layer of polymers desorps away under the intense sunlight, and the craft gets a 50 kilometer per second boost, departing the area as a conventional reflecting solar sail with its final layer of aluminum now exposed. Mission speed is approximately 10 AU per year. A parting boost from a microwave transmitter in Earth orbit could add still more delta-V.

The numbers in this paper are quite encouraging. The authors explore, for example, Mars travel times of roughly one month (New Scientist liked that headline) though they pass on the question of how to slow down once the vehicle arrives. And it’s useful indeed to know that beamed power from Earth can heat sail temperatures enough to simulate the conditions of a near Solar pass, allowing a nearby ‘laboratory’ to pursue materials research on a space-based sail.

The paper is Gregory Benford and James Benford, “Power-Beaming Concepts for Future Deep Space Exploration,” in the Journal of the British Interplanetary Society Vol. 59 No. 3/4 (March/April 2006), pp. 104-107. It outlines a hybrid concept that draws on the best features of solar and laser-driven sails, adapting the beaming strategy to the microwave region for maximum efficiency. And it’s the kind of thinking that could push sails out past the heliopause.

Comments on this entry are closed.

  • Adam December 8, 2006, 19:48

    Hi Paul

    Could be a good way to combine a Matloff style star-sail and beamed power – use the desorption process to send it sunwards and to remove the substrate that an ultra-thin sail has been ‘deposited’ or ‘sprayed’ on to.


  • Eric James December 9, 2006, 0:58

    Reactive sail concepts are intersting, but wouldn’t it be more efficient (power to weight) to just install a small ion thruster? It seems the thrust achieved by even a quick evaporation of the special paint (propellant) couldn’t possibly match the efficiency fo a more reactive rocket or ion engine. If the propellant coating is too efficient, it seems likely to damage the sail. Is this correct?

  • bigdan201 December 9, 2006, 4:45

    cool idea..

    i posted about this in the fermi paradox entry, but since thats off the front page, im going to repost here..

    The best way that i can see for interstellar travel is Foldspace. Using gravity to bend and fold the fabric of space itself. That way, we could jump to alpha centauri or wherever we want to go. We would only be limited in how much gravitational power we could harness.

    Until then, we’ll stay in this solar system, maybe sail around it.

  • Adam December 9, 2006, 5:38

    Hi Eric

    The problem of all self-contained electrical drives is power sources – they’re always too heavy. A sail leaves the power at home where it’s most useful. You could beam power to a huge collector that powers an ion drive – Geoff Landis has a JBIS paper on the idea – but above a certain speed it’s less efficient than a pure microwave or laser pushed sail. And the collector is dead mass – unless it’s also reaction mass, just like the Benford twins describe.

    However a thermal drive that doesn’t ionise its target will only produce an Isp of at best 2,000 seconds – it’s not a star-drive. But it might be a good way to launch a star-sail and to give it a perihelion kick too. The Sun does most of the work via gravity and light-pressure in that case.


  • Administrator December 9, 2006, 8:34

    Eric, Adam answered that question far better than I could have. But as to sail damage, that’s an interesting point, and one I’d like to run by the Benfords. Let me do that and we’ll see how carbon-carbon stacks up in handling desorption.

  • Eric James December 9, 2006, 17:27

    Adam and Paul,

    Instead of a pure solar sail and/or a pure ion drive, why not use a portion of the solar sail as a parabolic reflector to reflect thermal energy into a central reactive propulsion mass (kind of a rocket engine designed to disintegrate away as it’s used)? That way, you could get a really strong kick up to speed around the sun – then continue sailing without having to worry about residual coatings that might lessen the efficiency of the sail or potentially uneven propulsive effects. It seems to me that this would be lighter, easier to control, and more efficient.

    I fear that a reactive sail might be prone to uneven propulsive characteristics over the vast acreage of sail material, possibly causing it to turn away from the sun, or even lose control altogether. And again, with the propulsive mass attached to the central structure I think you could achieve a higher thrust to mass ratio without risking damage to the delicate sail material.

  • Adam December 10, 2006, 15:39

    Hi Eric

    I’m sure a suitable substrate could be used for the sail. Any star-sail is going to be made of tough stuff anyway as it has to dive to within 0.01 AU of the Sun for a maximum boost. Personally I’d like to see a two stage system that uses a big reflective concentrator to push a lighter sail harder and for longer than just natural sun-light.

  • Ron S December 10, 2006, 16:12


    Pardon my ignorance on solar sails, but as I follow along this thread (and the most recently posted article) I can’t help but think up possible issues with this.

    0.01 AU is a very hostile environment. That’s in the outer reaches of the corona and also with light heating of 10,000x at Earth’s surface. It would take very little absorption to cause thermal problems, both with melting and impairing sensitive electromechanical devices such as for sail unfurling. I would guess you’d want to unfurl the sail near perihelion where conditions are worst, not before, to maximize outbound speed.

    Coronal, solar wind, and photonic ablation of the sail this close in must be a serious problem. At 0.008 kg/m^2 (per the later article) that would be (if I calculated right) about 0.006 mm thickness of mylar, and less for carbon or metal. Multiple punctures seem likely, which could compromise the sail before it recedes to a safer distance.

    I am also unclear on how a reflective concentrator helps. All area contributes to thrust so it would seem best to expose it perpendicularly to the incoming radiation rather than angling it to reflect to another part of the sail.

    Am I out to lunch on my guesses? Sorry if I’m going over well-understood matters.

  • Eric James December 11, 2006, 3:51

    I think Adam meant that a reflector would be left behind in a close solar orbit (short lived orbit), used to concentrate photons on the receding craft.

    I think this would be very difficult to achieve technically (nigh impossible). Achieving the constant focusing and alignment required under these conditions would be an engineering miracle the likes of which Montgomery “Scotty” Scott (of Star Trek) would be proud.

  • Adam December 11, 2006, 5:54

    Hi Guys

    Of course it’s all difficult to do – that’s why it’s FUN!

    Seriously tho, all those problems have been tackled before by star-sail researchers – usually they propose unfurling the sail behind an occulting disk at close to perihelion etc. etc. High-temperature technology is assumed in all this sort of discussion. And naturally the sail material has to have a high strength at operating temperature. Matloff calls star-sails ‘diamond’ – any high-strength carbon allotrope really – which they’d probably have to be to take the punishment.

    The Benford twins’ approach makes outgassing at perihelion a virtue, giving the sail an extra-kick where it will do the most good.

    The reflector idea has a few problems, but anything worth doing is never easy. Star-sailing isn’t really near term, as the Matloff paper in a different post after this one makes quite clear. But it’s fun!


  • Administrator December 11, 2006, 8:03

    I asked the Benfords for a comment re Eric’s original questions (see above). Here’s Gregory Benford’s response:

    “1. Adding another thruster adds so much mass, a ton or so, as to destroy the ~ 200 kg sail’s entire low mass advantage.

    “2. The “paint” to be blown off has to be studied in detail. Once it’s all gone, at perihelion, the reflecting metallic substrate reflects sunlight quite well. Properly designed, none of the radiation, whether solar or microwave, should damage the sail structure.

    “Designing all this demands far more lab work than Jim or I have done, and then trials in low Earth orbit. But then we’ll have a way to get really large velocities for the outer solar system exploration that should (because of probable lifetimes and failure rates) be done on mass, with many sails flown for redundancy.”

  • Eric James December 12, 2006, 2:05



    That’s why I like my idea of a disolving propellant motor. The mass is only there when you need it.

    Another problem I perceive with the “painted” sails: As the propellant leaves the surface of the sail, it seems likely to block incoming photons. Therefore, might it actually reduce the overall efficiency to paint the whole sail’s surface?

  • roid December 12, 2006, 2:16

    This really isn’t making sense to me. Isn’t this essentially just painting solid rocket fuel onto the entire surface of the sail? Why would you do this when it’s more efficient to just have it in a detatchable solid rocket booster?

    And this paint has a lower Isp than solid rocket fuel, why is it better than solid rocket fuel?

    > [i]“1. Adding another thruster adds so much mass, a ton or so, as to destroy the ~ 200 kg sail’s entire low mass advantage.[/i]

    Weight is weight. Adding another detatchable/disposable thruster will add LESS weight than painting that same solid rocket fuel onto the surface of a solar sail, because the detatchable thruster has a higher Isp – it uses it’s weight more efficiently than any paint fuel would. If this solar sail needs a 1ton solid fuel thruster to get it to where it needs to go – less than 1ton of lower Isp paint layering is not going to be better, it’s going to be WORSE.

    Can someone please clue me in to what i’m missing?

  • roid December 12, 2006, 5:08

    nevermind, i tracked down Benford’s paper which clued me in

    seems the Isp of it is not only greater than a solid fuel rocket, but even greater than that of a bipropellant rocket. Exhaust velocity of over 5km/s

  • Administrator December 12, 2006, 8:59

    Yes, it’s quite a promising concept, and thanks for posting the link to that paper, which I hadn’t realized was online. I want to post (probably next week) a backgrounder on the Benfords original work that created their interest in desorption. You get quite a bang for the buck with their methods.

  • Ron S December 12, 2006, 23:40

    I read through the Benford paper roid linked to (thank you) and it answered some of the questions I posted earlier. It also raised a few more in my mind, that I won’t bother with here. I also realized during my reading of the paper that my assumption the sail temperature could rise to a destructive level was not necessarily true. A safe, albeit high, equilibrium temperature could be reached.

    I still wonder about how the physical solar wind, and turbulence, in the outer corona will affect the sail’s temperature and stability, but I don’t have the ability to do more than state a qualitative concern about it.

    The shield Adam mentions could both protect the sail during unfurling and perhaps prevent premature adsorption during solar approach. Of course the shield itself adds payload mass and would have to be safely ejected at some point, and the whole assembly would likely need to oriented nearly edge-on to avoid some amount of deceleration.


  • tmayes1999 August 1, 2007, 1:32

    The viking long boats of the 9th and 10th centuries provides a source of inspiration for a future star ship of our time. A starship must be well adapted
    to live off of its enviroment, and it should propell itself by interacting with
    the natural enviroment of interstellar space . A viking ship used its sail to propell itself by interacting with the natural winds of the marine enviroment .
    It used its oars to propell itself by interacting with the water, and it rode
    the natural currents of the ocean. Likewise a starship should combine the use
    a light sail, with the the use of nuclear powered rocket or ramjet engines
    to propell it self . A starship should be propelled by the combination of a light sail and an interstellar ramjet ..It will ride the currents of gravity between gravitating bodies, and it will use its light sail to propell itself through interplanetary space by reflecting the sun light off of its light sail to produce
    photonic thrust. Likewise it should use electric fields or combined electric and
    magnetic fields to gather its propellant from the thin clouds of of gas that floats in interstellar space through out the galaxy . It should then accelerate this propellant inside nuclear electric ion or plasma propulsion engines, and expell it to generate thrust. The recoil from the ramjet engines exchaust will
    then indefinitely,gradually, & incrementally accelerate the star ship closer and closer to light velocity in interstellar space