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Space Science

LCD 'Engine' For Spacecraft Attitude Control 95

Posted by kdawson
from the little-more-to-the-left dept.
Bruce Perens writes "Japan's IKAROS satellite, which earlier performed the first successful demonstration of a solar sail, has broken more new ground. Liquid-crystal displays — yes, like in your video monitor — were fabricated into strips on the edges of the solar sail. By energizing some of the LCDs and changing the reflective characteristics of parts of the sail from specular to diffuse, JAXA scientists successfully generated attitude control torque in the sail, changing the spacecraft's orientation."
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LCD 'Engine' For Spacecraft Attitude Control

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  • I have a CRT (Score:1, Insightful)

    by Anonymous Coward

    "yes, like in your video monitor"
    No, I have an old fashioned Sony CRT monitor.

  • by Anonymous Coward

    My wife needs that just about every month.

  • It's neat to see this phenomenon being used for a spacecraft.

    • Crookes believed that his radiometer was turned by light pressure, but he was wrong! It's actually a phenomenon of low-pressure gas moving around a temperature differential. If you pump your radiometer down to a really good vaccumm, it stops working! The light pressure is not sufficient to conquer the bearing friction.

      There's a good explanation in Wikipedia [wikipedia.org].

      • Interesting! I didn't know a radiometer needed a partial instead of hard vacuum to work properly. I bought the "light pressure theory" as well. It makes sense—solar wind couldn't get through a glass bulb.

        It seems to me the effect of the LCDs interacting with the solar wind would be pretty small, but it's a neat idea since there are no moving parts.

        Ya learn something new every day.

  • Come on, you have to admit that's a pretty clever design element.

    Imagine if the entire sail surface could be selectively modulated in this way.
    • Come on, you have to admit that's a pretty clever design element.

      I agree. It's brilliant. (No pun intended, though one is available.)

      (Downsides: You need to keep 'em from freezing (or design 'em to survive it) and you probably need to build the actuators as a large number of independent units, so a meteorite puncture or other damage doesn't take too much of the control area out of service (or "stick" it in an undesired mode).)

      Imagine if the entire sail surface could be selectively modulated in this way.

      T

    • by Gilmoure (18428)

      And if you could form it in to a dish...

      • And focus it.

        Blink and it's a weapon, blink again and it's a communication system, blink once more and it's a star drive.

        I think that's from A Mote in God's Eye or The Gripping Hand.

        • by Gilmoure (18428)

          I was thinking of using it to deflect space debris. By modulating it, you could induce many fanciful beams and radiations to the surrounding area.

          • Holography (Score:3, Interesting)

            by Bruce Perens (3872)
            How about using computed holography driving embedded LCDs to make a light sail act as a sort of synthetic-aperture device? You could have multiple steerable beams, receive with multiple steerable reflections, etc.
  • by metiscus (1270822) on Thursday July 29, 2010 @01:43PM (#33072368)

    I wonder what amount of torque they were able to develop with this? It seems like it was pretty effective.

    • by JustOK (667959) on Thursday July 29, 2010 @01:46PM (#33072430) Journal

      just some light torque, i would think.

    • by mangu (126918) on Thursday July 29, 2010 @04:52PM (#33075868)

      I wonder what amount of torque they were able to develop with this? It seems like it was pretty effective.

      IAARS (I Am A Rocket Scientist). If there are no fluid leaks anywhere, as there shouldn't be in a properly functioning spacecraft, then *all* of the torque that changes the attitude of a spacecraft comes from solar radiation pressure alone. Therefore there should be not much problem in controlling attitude by modulating solar radiation pressure.

      As a matter of fact, this effect is already being used today in commercial satellites. Some of them have adjustable panels that can be turned so that the solar radiation torque is zeroed. The new idea here isn't using solar radiation for attitude control but using LCD panels to modulate the radiation pressure.

      The problem in understanding how such a small pressure as solar radiation can cause a spacecraft to rotate is that we are used to thinking about things here on the earth surface, where there are many other forces around us. In orbit, the spacecraft is in free fall in a vacuum, there's no friction and no wind, it will move to the slightest impulse applied. A typical commercial geostationary satellite may need attitude maneuvers a few times a week.

      • by metiscus (1270822)

        My original query was more in relation to what degree of control authority that they could exert based on differential pressure. If one side was turned off completely and the other completely on, what moment would be generated?

        Also, I'm guessing they use the on-board supply of hydrazine for delta-v only then?

        • Re: (Score:3, Interesting)

          by mangu (126918)

          As I understood from the press release, the purpose of the test was to find a viable way to control the attitude of the sail itself. Being so thin, it would flutter and probably be ripped apart if handled roughly. An LCD is an interesting idea in this context, although I believe the LCD would be orders of magnitude thicker and heavier than the solar sail.

          As for the momentum needed, it would be very small, because the disturbing momentum itself is very small. Since all the perturbation comes from radiation p

  • Next up... (Score:4, Interesting)

    by peacefinder (469349) <alan...dewitt@@@gmail...com> on Thursday July 29, 2010 @01:46PM (#33072420) Journal

    Downwind faster than the solar wind!

    • Re: (Score:3, Informative)

      by MozeeToby (1163751)

      I may be wrong, but going downwind faster than the wind is only possible because sailboats have a keel which transfers some of the sideways force into a forward force. Not possible in space I'm afraid so unless the light pressure is higher than the solar wind pressure I don't think you're gonna be able to do it.

      • by gurudyne (126096)

        Well, you are wrong as far as land cars are concerned.

        Check out http://www.fasterthanthewind.org/

        They have gone over 2.5X faster than the wind, DIRECTLY DOWNWIND.

        No side forces on the wheels. Straight downwind

        • Re: (Score:2, Informative)

          by Anonymous Coward

          Rather than using a keel to convert sideways forces forward they are using wheels to convert torques forward. Therefore this isn't applicable either.

          The analogy here would need to be a wind powered plane traveling faster than the wind (and not using gravity).

        • Re: (Score:3, Insightful)

          "...faster than the wind, DIRECTLY DOWNWIND."

          There's a bit of a cheat in the directly downwind assertion.

          While it true that the vehicle is going directly downwind, its propeller is rotating in the wind. This causes to blade to experience the wind at an angle, just like a sailboat tacking into the wind. And in addition to the "lift" force perpendicular to the blade forcing the car forward, its rotation is used to drive the wheels.

          Very clever nonetheless.
      • Or, if we could set the gravity keel to provide that sideways force diversion in relation to the solar flux.

        Oh yea, wrong century... resetting my temporal resonator to a more reasonable timeline.
      • Re: (Score:2, Insightful)

        by osu-neko (2604)
        This is essentially true. However, it's probably simpler to note that solar sails are pushed along by light pressure, generated by the photons hitting the sail. The photons, being light, tend to travel at light speed (by definition). There are other considerations besides the lack of a keel for why a spacecraft won't be exceeding that speed.
        • For what it's worth, I wasn't actually serious. :-)

          (Especially given that - if I understand my relativity rightly - photons would still be traveling at C in the frame of reference of the vehicle.)

      • may be wrong, but going downwind faster than the wind is only possible because sailboats have a keel which transfers some of the sideways force into a forward force

        You are close.

        Sails generally act as a vertical airfoil. You point it into the wind, and the force is created due to a pressure differential. This is just a guess, as I'm not a sailor, but wouldn't the sail(s)'s surface area be the major factor in allowing a larger force to be extracted? (The keel plays a factor, but it isn't the reason why a

    • by w0mprat (1317953)
      Perfectly possible. Solar wind is a stream of charged particles. You could easily get 'traction' against this by accelerating this medium as you pass through it. See http://en.wikipedia.org/wiki/Magnetic_sail [wikipedia.org] - but to go down wind faster than the solar wind (600km/s), you would need an external energy source such as solar panels, and a way to accelerate the interplanetary medium.

      In short your magnetic sail could also double as a kind of ion engine. An arrangement of electrically charged hoops would accel
  • Neat. Anyone have an order-of-magnitude idea if this could be used for stationkeeping on sats in Earth orbit or for attitude control in deep space missions? Just wondering if it produces enough torque to control a real spacecraft. IIRC, for most spacecraft fuel for attitude control is the limiting factor on mission duration, and I think in some cases (e.g., Kepler) it's the only expendable. Could a spacecraft using this technique have virtually unlimited life? If you're solar powered and don't burn fuel
    • Radiation damage!
    • by gstoddart (321705) on Thursday July 29, 2010 @01:55PM (#33072616) Homepage

      Just wondering if it produces enough torque to control a real spacecraft.

      What, exactly, do you mean by a 'real spacecraft'.

      IKAROS is real. It's in space. It's actually using this.

      Have I missed something? From what I can tell, this is about as real as you can get.

      • Re: (Score:2, Interesting)

        by GreenTom (1352587)
        Fair enough, and didn't mean to imply that IKAROS is not real. But, IKAROS is a technology demonstrator. As far as I can tell, the solar sail is the payload, and it's performance requirements are based around testing the solar sail. I was wondering about the amount of torque this kind of setup produces, and if this technology is a potential alternative to thrusters for bleeding the reaction wheels on future spacecraft.
        • by gstoddart (321705)

          But, IKAROS is a technology demonstrator. As far as I can tell, the solar sail is the payload, and it's performance requirements are based around testing the solar sail.

          Well, if it's got a working solar sail, and using LCD technology to generate torque ... wow, what a hell of a demonstrator.

          But, yes, I see your point. Of course, now that they've shown both of those technologies, I'm sure someone will see what practical use they can put it to.

        • No offense, but I'd think it is pretty obvious that this is a proof-of-concept and any interpretation that this is in any way billed as "ready for prime time" shows a complete lack of understanding of the real accomplishment here. It also devalues what's been accomplished.

          This is a MAJOR accomplishment. Like many other early stage technologies it might not be practical but will most certainly pave the way for very practical applications. You do realize that this essentially represents free energy for b

        • That's a good point regarding momentum dumping. I am also curious about what kind of pointing accuracy they could acquire with this method as the primary attitude control system. Apparently, it works well enough to steer the spacecraft over long, gradual thrust maneuvers, but I would be very surprised (pleasantly so) if it could achieve the kind of pointing accuracy that reaction wheels give...or hell, even spin stabilization. I suppose we'll have to wait and see.
        • Re: (Score:3, Interesting)

          by mangu (126918)

          if this technology is a potential alternative to thrusters for bleeding the reaction wheels on future spacecraft.

          I suppose that the idea is to make momentum dumping unnecessary. If the torque is always perfectly zero there will be no momentum accumulating on the wheels.

    • Magnetic torquers [wikipedia.org] are already in use and would have the same benefits/limitations as these LCD thingies.

      Magnetorquers are lightweight, reliable, and energy-efficient. Unlike thrusters, they do not require expendable propellant either, so they could in theory work indefinitely as long as a sufficient power source is available to match the resistive load of the coils.

      • by osu-neko (2604)

        Magnetic torquers [wikipedia.org] are already in use and would have the same benefits/limitations as these LCD thingies.

        Not exactly. The OP asked about satellite station keeping AND deep space missions. The magnetic torquers may work fine for the former but not for the latter. They don't have the same benefits/limitions, they are in fact much more limited, being only useful while in orbit around planets with significant magnetic fields. The LCD thingies work whereever the sun shines, which is a much larger volume of space (although still essentially limited to the inner solar system).

      • Magnetorquers need an ambient magnetic field. The terrestrial one is close and reliable. The solar varies between a zero-crossing with polarity reversal and a maximum on an 11-year cycle (a complete two-reversal cycle every 22 years). I haven't found a figure for its RMS power at earth orbit but I guess the inverse square law applies.

    • Neat. Anyone have an order-of-magnitude idea if this could be used for stationkeeping on sats in Earth orbit or for attitude control in deep space missions? Just wondering if it produces enough torque to control a real spacecraft. IIRC, for most spacecraft fuel for attitude control is the limiting factor on mission duration, and I think in some cases (e.g., Kepler) it's the only expendable.

      You are correct that fuel for attitude control is generally the limiting factor for spacecraft (useable) lifetime. Using solar sails for attitude control would be possible, I think, for spacecraft operating far enough away from a planetary atmosphere. Otherwise, drag from the sail would certainly overwhelm solar pressure. So, though it may be possible, I'm not sure how economical it would be to use for stationkeeping. I would be interested in seeing a trade study between electric propulsion (another low thr

    • by wagnerrp (1305589)
      Solar pressure at Earth's orbital distance is around 4.6 Pa for absorption, or double for reflection. While the force will certainly build up over time, you're not going to be using it for direct attitude control. You would use momentum wheels or CMGs, and then use solar pressure to bleed off energy and prevent saturation.
    • Re: (Score:3, Informative)

      by mangu (126918)

      for most spacecraft fuel for attitude control is the limiting factor on mission duration

      Not for geostationary satellites. For those, inclination control consumes about 90% of the fuel. Drift control depends on the longitude where the satellite is, but it typically consumes 90% of the rest, so attitude control consumes only a few percent at most of a geostationary satellite fuel budget.

      There are already some commercial geostationary satellites that use solar radiation pressure for attitude control. Depending

    • The sail and spacecraft will receive an electrostatic charge transfer from the solar wind, I think. It might get really large over time. I guess this would tend to drive away dust.
  • LCD? (Score:2, Funny)

    by r00tyroot (536356)
    Now if only they could equip the spacecraft with some sort of LCD Soundsystem.
  • ... and changing the reflective characteristics of parts of the sail from specular to diffuse...

    I knew it. They photoshopped it. [xkcd.com]
  • LCD Engine not so much. LCD Rudder... yep that fits. Nevertheless it is darn impressive. It is great to see someone working with this technology.
  • The whole concept (Score:1, Flamebait)

    by Dunbal (464142) *

    Of a solar sail is rather neat. Problem is they can never accelerate outside of our solar system. Once they hit the termination shock that's it, no more power. I wonder if someone has done the math to see what the max theoretical speed they could reach is. Of course they could probably do more if they put the sail away, slingshot around Jupiter back close to the sun and deploy the sail again once they pass the sun.

    The problem however is that the "sail" only works in one direction - "away from the sun". Unli

    • You can tack using reflection. What you say would be true if it only worked by absorption.
      • Oh, the reason you didn't know about reflection was that you were confusing solar wind and light pressure. Easy to do. I did it too.
  • "Eat at Joe's"

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