LCD 'Engine' For Spacecraft Attitude Control 95
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."
Re:I have a CRT (Score:4, Informative)
so.... ion propulsion then?
Re:Next up... (Score:3, Informative)
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.
Re:It's the Loony Tunes sail! (Score:4, Informative)
Haven't read TFA but I don't think a solar sail ship could propel itself by shining a light into its own sails. Equal-and-opposite reaction and all that; the light source would try to propel the ship backwards and what photons hit the sail would propel it forward. Imagine trying to propel a fan boat by directing the fan into a parasail -- the sail would just be a drag. You can't lift yourself up by your own bootstraps. You'd do better shining the light out into space.
Given that the light would bounce off the sail it would not cancel (as in the fan/sail case) but serve as a thrust when the light reflects backward off the sail. The forward thrust on the sail would be about twice that of the backward thrust on the craft.
While you'd get essentially the same thrust firing the laser toward the rear, it would all be thrust on the craft, none on the sail. So there might be times when it makes sense to shoot the sail. Like the one below...
I believe what is being described in the summary is using LCDs to reflect photons hitting the sides of the ship into the sails at an angle, to generate torque. The LCDs are adjustable reflectors in this case.
It sounds to me like they're using it to switch areas of the sail to diffuse reflection. This reduces the thrust by scattering the reflected light in a range of directions (some of them partially canceling others) rather than reflecting it essentially straight back. By having, say, the right side of the sail develop less thrust than the left, you turn the sail to the right. It's not "on the edge" as in right ON the edge. But it's an area of the sail adjacent to the edge in order to get the most leverage from a given area of LCD material.
You could achieve the same effect by bouncing a laser (or other light source) off a patch near one side of the sail. But that would take kilowatts per square meter to get thrust equivalent to full sun at earth's orbital distance. Why burn such amounts of power when you can just modulate the sunlight you've already got hitting the sail?
Re:Next up... (Score:2, Informative)
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).
Crookes Radiometer is NOT turned by light pressure (Score:5, Informative)
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].
All the torque that's needed (Score:4, Informative)
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.
Re:Useful for stationkeeping? (Score:3, Informative)
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 on the satellite model, this can be done either by setting each solar power panel at a slightly different angle or by having small auxiliary reflective panels that can be turned to the specific angle needed to apply the needed torque to the satellite.
Actually, the needed correction is small, because satellites are designed to be more or less symmetrical to begin with.