How Solar Sails Work

Gary writes "You can also learn more about How Solar Sails work. In mid-April 2001, The Planetary Society, in collaboration with several Russian space organizations, will test launch the deployment mechanism for the first solar-sail mission, Cosmos-1, in a sub-orbital flight. It is a rounded solar sail that is divided into eight triangular blades with inflatable booms for support. Maneuvering a solar-sail spacecraft requires balancing two factors: the direction of the solar sail relative to the sun and the orbital speed of the spacecraft."

Comments

Oopsie

[Bearpaw]

The sail must be thin and have a minimal mass, because the more mass, the less force that sunlight imparts to the sail.

Um. I think the force imparted by the sunlight is the same. The force imparted just results in a lower acceleration if the mass of the sailcraft is higher.

Re:Sailing Issues

[markmoss]

The sails are reflective (mirror-like), so by angling them you can get sideways thrust -- a combination of the incoming momentum of the sunlight and the reaction from the momentum of the reflected light. So angling the sails at 45 degrees gives you thrust at 45 degrees to the sun, but it cuts the thrust by half. (sin(45) squared -- once for the reduction in light intercepted by angling the sail, once for the off-axis thrust.)

You can't tack like a ship tacking into the wind (where the vector sum of forces on the sails, keel, and rudder gives a net force partially into the wind) because you don't have any equivalent to the keel. But you can use an angled sail to reduce the orbital velocity, so the ship drops sunward. And the best way to go outwards is to use sideways thrust to increase the orbital velocity.

The real problem with solar sails is the very tiny force per square meter that is possible. It should be enough for minor orbital adjustments. If you plan way ahead, it might be possible to use light sails to slowly spiral in or out (like weeks or months to the moon, years or decades to Mars). Or you find a way to augment the thrust -- like building a giant battery of lasers on the moon to provide much more intense light. In a couple of Larry Niven stories, they used lasers to launch an interstellar lightsailer, but to brake at the destination, it had to almost dive into the sun...

Non-issues

[Spamalamadingdong]

Addressing the points in order:

1. The further you are from the sun, the less force is exerted on the sails, until the forces from other stars/suns outweighs the force of our sun.

True. However, this occurs at such an absurdly vast distance from Sol that it is not a factor for any near-term mission.

2. Travelling towards the sun is difficult and requires some sort of "tacking" process.

False. All you have to do is reflect light in the direction of orbital motion, which causes a braking thrust. This causes the orbit to move closer to the Sun.

3. Travelling in any direction other than away from the sun [a] requires a centerboard and rudder for maintaining a direction [b]. Both these systems rely on resistance [c]. In space there is no resistance [d]. There sill need to be something to exert force away from the sun to keep the correct heading [e].

[a]False, [b] true, [c] irrelevant, [d] no shit, Sherlock, [e] false.

You can generate thrust in any direction in the half-sphere centered on the Sun-spacecraft line, with the thrust falling off according to cos^2 theta. So long as your trajectory changes can be performed without a thrust vector aiming away from the Sun, you can (theoretically) perform them with a solar sail. In space, gravity (especially from a third body, like Earth) and inertia allow tack-like maneuvers.

4. Sun-spots change the intensity of the solar winds, creating 'gusts'. Gusts will be difficult to control.

Solar sails use the pressure of the photons, not the plasma wind. There may be some small influence from plasma impingement, but it is just that: small.

From what I can see, there is only one direction the vessel can travel, and that will be determined by the release time from the earth, and the velocity at releas.

You should look up the "heliogyro" concept for a Comet Halley rendezvous probe (it was never built, of course). The details will show you where your thinking is faulty.
--
Give a man a fish and he eats for a day.

This is nothing new

[Magumbo]

The scientologists used a similar technology when they came to earth 65,000,000,000 billion years ago. Everybody knows that.

--