In sailing, the keel of the ship keeps it from blowing off course by using resistance against the water to stabilize the direction of the ship despite the wind direction. Could an underpowered Keel Rocket provide the same stability for a ship that is powered by the solar wind, and thus, allow it to tack back and forth on its return journey upwind?
Graeme,
Let's start with the basics.
A non-reflecting solar sail absorbs momentum from photons and uniformly re-radiates them. The sail is driven in the direction of the incoming light. The sail is blown radially from the Sun.
A 100% reflective sail is driven in a direction normal to the sail's plane. If the sail is face on to the Sun, it too is driven radially outward. If the sail is angled at almost 90° to the Sun's rays, it is driven *transverse* to the rays - and the sail's orbital velocity is changed.
Note, the sail can direct its thrust in the prograde or retrograde sense. So if a sail, in orbit around the Sun, is angled to be almost edge on to the Sun's rays, but directing its face in the prograde direction, then a deorbiting force will be produced and its orbital velocity will be reduced and the sail will spiral *in* to the Sun.
Hope that this answers your question.
@James
Well not really, the physics of a mirror and a sail are very similar in that the sail operates in much the same manner, however the difference between the Latern sail, and those used in today's sailboats is that the ones used in todays sailboats can direct energy at a much steeper angle to the wind, and when combined with a keel, can accellerate across the wind, allowing them to build up momentum that allows them to not only decellerate but accellerate against the wind. Part of the fun of thinking about sunjammers is the idea that they might make a rather tame racing ship, that like sailboats does not require large amounts of money to operate.
Half the fun, is to figure out a way of "Tacking" against the solar wind to allow the ship to arrive back home faster than simply deorbiting or allowing gravity to accellerate the ship back to earth. If we can get the accelleration up close to 1/3 g, and the payload up to one human plus their supply needs, people could sail to mars much cheaper than doing the "Rocket thing" and their productivity could arrive at earth much cheaper as well. It is probably too early to tell if the economics would work for this, because we do not yet have a prototype sunjammer to work from.
We can however do the math and simulations and get a feel for how expensive the craft would be to make, which would go a long way to telling us how economic it would be.
Graeme,
May I suggest the rather excellent text on solar sail physics and missions by Colin McInnes?
The fundamental problem is that, unlike an aerodynamic wing/sail, the reaction force on a reflective sail cannot subtend an angle greater than 90° to the inbound ray. So a sail cannot push itself closer to the Sun. One has to either deploy a secondary mirror (as Starwisp does) or use conventional reaction engines. Feel free to take this to email as we're drifting (not tacking) off topic I suspect.
With tacking, you sail across the wind, and then use your momentum to turn into the wind. I admit that without the Bernouli effect you can't get as much lateral effect, but even the old multi-masted ships were able to tack across the wind and beat to the windward, and some of them used the Latern sail. What is the difference between using a second sail, and using a number of sails to get the best effect?
The trick as I understand it is to come-about into the wind, fast enough that you do not lose your way before you are on the opposite tack. The keel was an important part of that type of sailing. But in space there is no friction with water for the keel to act on, which is why I was asking about a rocket engine as being an alternative.