Can a space ship with a solar sail tack "upwind"?

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Alan Whitmore

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Last April I was visiting a Special Projects lab at the NASA facility in Huntsville, where research was being done on a solar sail, which would use the "photon wind" from the sun to power a spacecraft. I asked whether such a craft was limited to moving away from the sun, or whether it might tack "upwind", back towards the sun. I got no answer that satisfied me, I got the impression that such a question had never occurred to them.
My own opinion, totally without evidence to back it up, was that without a keel, tacking would not be possible. A sailboat moves simultaneously in two fluids with very different viscosities, and that with a keel digging into the more viscous fluid (water) and providing some leverage, the boat would be limited to sailing away from the wind, like dust motes floating through the air.
Anybody else have any opinions on this, especially those of you who are experienced sailors?
 
I agree 100%. No keel, no surface, no tacking.

In addition, I don't think photons act on the sail the same as air does. In sailing, you get the greatest speed sailing abeam of the wind — a whole lot faster than running downwind. This is because the sail is basically an upright wing, using aerodynamic lift to pull the vessel forward. I doubt that photons create lift the same way.
 
Nope. Sailing into the wind only works because of the force exerted by the keel (as the wind tries to forces the hull sideways through the water). A bit like squeezing a watermelon seed and having it shoot out orthogonal to the force of your fingers.

In space, there is no keel, and nothing to act against it. That means there is nothing to resolve a component of the force of solar wind to act in any direction but down wind. You could use a propulsion system to alter your direction of travel., but the force of a solar sail can't help you in any vector other than "downwind". You could cant the sail to the wind, but with nothing exerting a force normal to the sail (like a keel, kite string, or the like), that would simply reduce the area of the sail and the downwind force it provides.

If the sail were an ideal specular reflector (of both photons and particles), then a canted sail would generate sideways force as solar radiation carried momentum away opposite the angle of incidence. Unlikely to very work well with a real reflector, and in any case this could yield a crosswind component to the sail's force, but never an upwind component.

Now, if you could make a keel that reacted only to dark matter, and if . . .
 
Nope. Sailing into the wind only works because of the force exerted by the keel (as the wind tries to forces the hull sideways through the water). A bit like squeezing a watermelon seed and having it shoot out orthogonal to the force of your fingers.

In space, there is no keel, and nothing to act against it. That means there is nothing to resolve a component of the force of solar wind to act in any direction but down wind. You could use a propulsion system to alter your direction of travel., but the force of a solar sail can't help you in any vector other than "downwind". You could cant the sail to the wind, but with nothing exerting a force normal to the sail (like a keel, kite string, or the like), that would simply reduce the area of the sail and the downwind force it provides.

If the sail were an ideal specular reflector (of both photons and particles), then a canted sail would generate sideways force as solar radiation carried momentum away opposite the angle of incidence. Unlikely to very work well with a real reflector, and in any case this could yield a crosswind component to the sail's force, but never an upwind component.

Now, if you could make a keel that reacted only to dark matter, and if . . .
There is also the Bernoulli effect that allows a sail to go upwind. The shape of the sail makes an airfoil that generates lift up to 30 degrees off the wind direction. You don't need a keel for this but you do need a rudder. I am not sure photons are governed by Bernoulli.
 
If you can re-emit the photons in a chosen direction, you can gain some momentum in the opposite direction. I don't think you could quite go "upwind" since the photon's momentum comes from its energy. So going upwind would require emitting photons with higher energy than the ones you're catching. But you wouldn't be stuck going only straight downwind.
 
There was a concept mission called the mini magnetosphere explorer that would use a plasma bubble to sail the solar -wind- rather than solar photons. The modeling suggested they should be able to shape the bubble to off-spherical and generate steering lift in the wind.
 
Can a space ship with a solar sail tack 'upwind'?
No. Next question?

If it seemed like "such a question had never occurred to them," I guess it never occurred to the tour guide. Anyone working on the project directly would have told you "No."
 
It’s unanimous — NO.

I think referring to the technology as a solar “sail” that uses the solar “wind” is fine, but maybe it suggests an analogy to sailing on the ocean that really doesn’t hold up very well. And I also think that unless a person has ever actually sailied a boat, they might not even know about tacking. “Wait… wut? U can sail upwind?”
 
In a book detailing one of his cases concerning the murder of a yachting couple at Palmyra Island in mid-Pacific, Vincent Bugliosi (noted attorney, who prosecuted the Manson/Tate/LaBianca murders) highlighted a pivotal moment - when he found out, to his surprise, that a sailboat could travel upwind. Apparently not obvious to everyone, including some with extensive education (but still doesn't work in space).
 
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