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September 11, 2011 / 74

Solar Sailing and the Idiots Who Don’t Understand It

There’s a nice little article in the September 10 issue of “Science News” titled “Flying on sunshine.” My problem  with the article is the same problem I had with one of the leaders of the solar sail projects – a guy who arrogantly told me he’s been sailing boats for decades and he KNOWS what he’s talking about.

Apparently not. He seems to be under the very mistaken impression that sailing on the sea is little different than sailing in space. On the sea, you have the wind for your motive force. In space, you have the light of the Sun. Actually, that’s the ONLY similarity.

What he continues to miss is that there is a HUGE and critical difference between sailing on the sea and sailing in space. In a word the difference is – FRICTION. On the sea, the boat’s hull is in contact with the water – which contact creates friction (resistance). Space, on the other hand, is a frictionless environment.

The effect of this difference is meaningless if you are “sailing” away from the Sun – it would be the same as sailing before the wind. The difference becomes critical when you want to sail INTO the wind (tacking), or toward the Sun. Sailing into the wind, you use the water/hull friction to keep the sailboat from being pushed directly downwind – you angle across the eye of the wind toward your base course at an angle, then reverse your angle and sail into the wind on the “opposite tack”. (Also known as “beating into the wind”.) And it works because the friction between the boat’s hull and the water keeps the boat from just sliding directly down wind.

If the boat’s hull was a shallow, near frictionless saucer, you can bet you wouldn’t be doing much if any “tacking.” So sailing into the wind requires at least TWO independent forces – the wind (motive force) and an analog source of friction (resistance).

The solar sail vessel that is currently orbiting the Earth (NanoSail-D) has three forces to use. It has photons from the Sun for motive force, and it has the Earth’s gravity and atmospheric drag for the resistance. But this little satellite only orbits the Earth.

The Japanese have a SailSat (IKAROS) that is flying toward Venus. I don’t know if it’s taking an angular approach (intercepting orbit), or if they are trying to fly it directly toward the Sun or an a small angle. But in either case, once the SailSat leaves the Earth’s gravity, it will probably be using the Sun’s gravity as it’s resistance force.

The folks who are pioneering this technology seem to think that they will be able to turn these little “boats” once they reach areas of little to no gravity – as in the outer solar system. But I contend that once the SailSat reaches an area where the light force exceeds the resistance by a significant amount – the Sat will stop being able to change it’s vector, and will just be pushed outward from the light source.  ie once you effectively lose resistance, you lose maneuverability.

My problem with the sail boaters is that the guy I emailed with seems to think that you can angle the light receptors/reflectors and that will change the angle of flight (vector) because the light will hit the receptor at an angle. (He even drew some cute little force vector diagrams!) But he didn’t seem to realize that without that second and counter force, you’ll only change the angle of the vessel’s orientation to the motive force/sun, and the Sat will just blissfully continue in a straight line AWAY from the light source. ie your motive force vector WILL NOT CHANGE!

What it will take to educate these folks is a satellite that sails away from the Sun AND the Sun’s and the planet’s gravity – such as launch a SailSat at right angles to the plane of the elliptic. Let the sat get out beyond the point where the Sun’s gravity is significantly less than the motive force of the photons hitting the drive surface, (we could call it the “no friction line”?) and then try to turn it.

I’m betting that they won’t be able to change the Sats course. Oh, the Sat will turn – but it will only pivot. The satellite will be pointing a different direction from the Sun, but the sail will still be at maximum exposure to the photon stream.

The article also says that the Japanese (JAXA) want to combine novel propulsion systems – combining ion engines and solar sails. Now – THAT would work because you could use the ion engine as either your vector change force (like retro-rockets), or as a resistance force in areas of little to no gravity.

I propose an inexpensive experiment. Build a small boat with a near frictionless saucer shaped hull. Then put a remote controlled sail on it and see if you can tack into the wind. Bet you can’t!

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6 Comments

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  1. Dag Digler / Jan 31 2013 17:20

    http://www.ugcs.caltech.edu/~diedrich/solarsails/intro/tacking.html

    • Michael E Picray / Jan 31 2013 22:08

      There would be no “force vector” from “reflected sunlight” because there would be no force. Photons impart twice their momentum at the moment of contact with the sail, and thus the force is away from the Sun, NOT at an angle to the light coming from the Sun. Again – without a counter force to provide a “friction analog” for the angled sail to push against, the solar sail vehicle could go but one direction – away from the light source. The only force vector provided by the Sun is DIRECTLY AWAY from the Sun. Any force vector line that goes from the craft must originate from a power source ON the craft. Your reflected force vectors are the equivalent of putting a fan on the fantail of a sail boat for motive power… only possible in cartoons.

      • jh / Aug 29 2013 04:22

        This is incorrect. A photon strikes the reflector with angle of incidence 45°. It bounces off the reflector with angle of reflection 45°. The momentum change will be normal to the plane of the reflector. The reaction force will be anti-normal to the plane. The force imparted on the reflector will be equal in magnitude and opposite in direction to the force imparted on the photon This is consistent with Newton’s third law of motion. Example: http://en.wikipedia.org/wiki/Radiation_pressure

        In addition, the craft will be in orbit around the sun. In according with the orbital energy conservation equation, an increase in orbital velocity will result in an increase of the orbital radius. Similarly, a decrease in orbital velocity results in a decrease of the radius.

        In the case of our tacking solar sail, our reflector is pointed halfway between the prograde vector and the radial-in vector. The photon hits it, bounces off, and the reaction force is imparted in the opposite direction. We can simplify the resulting change in motion by looking at the components of the force vector.

        There will be a radial-out component. This force will not change the angular velocity, but the orbital energy will increase, increasing the eccentricity, as well as slightly raising the radius.

        The other component will be a retrograde component. The orbital velocity will decrease, and as a result, the radius will decrease. While the radial-out and retrograde components have opposite effects on the semi-major axis, the retrograde component will have a much more powerful effect due to it decreasing the angular velocity.

        Bear in mind, this is nothing like the “tacking” on a sailboat, which relies on the force of the keel to keep the boat running straight, and zig-zagging to back in forth. The boat analogy breaks down here, because the boat moves in a straight line, not a circular orbit. The solar sail is performing orbital maneuvers to adjust its distance to the sun.

        Just to perform a quick reality check, I tested this out in an orbital simulator. I established a 300km circular orbit around a body, with 1.981 km/s orbital speed. I oriented my craft to 45° between the retrograde and radial-out vector, and very slowly accelerated. I slowly began to spiral toward the body. I stopped the simulation at an altitude of 225km, with a perigee of 217.17km, an apogee of 254.80km, and an eccentricity of 0.023. I am fully convinced this is possible: all you would need is a gyroscope or reaction wheel to keep the sail at the same angle toward the sun.

  2. Anonymous / Dec 21 2012 19:33

    Finally, someone who gets it.

  3. Andrew Riley / Sep 11 2011 12:11

    That was really interesting. I must be a bigger nerd than I though. 🙂

    • Michael E Picray / Sep 11 2011 18:07

      It is the nerds who power the world these days – they own the i-pads – and actually know how to use them!!! That lets ME out!
      ;-D

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