Solar Sails

[Raider5678]

Hi guys, I was wondering if someone could give me some specifics on how Solar Sails work. I understand them, I was just wondering if there is anything I should note when explaining these things. Also, would it be possible to build this? I mean, not counting the response of the public if we were to start funding a giant mirror....

[Enthalpy]

An important message about solar sails is that they exist already. Some have been flown as a propulsion method

http://en.wikipedia.org/wiki/Ikaros

and many geosynchronous telecom satellites use the radiation pressure to keep their orientation passively and save propellants to extend their life.

 

Explaining... Perhaps interesting is that you can compute the thrust from the photon's momentum (F=P/c where P is the received and the reemitted power) or, as Maxwell did, from the electric currents induced in the mirror and their interaction with the electromagnetic field. Fortunately, both results match - well, sometimes, when there is no mistake, but it already happened, permitting us to tell that theories are consistent.

 

One funny history, very useful for people still learning to doubt, is that ol' James Clerk was so pleased with explaining Crooke's radiometer that way that he completely botched the explanation. Apart from the fully unreasonable orders of magnitude, the explanation would have wanted more push on the clear sides, while in the radiometer, the dark sides trail in the movement. Nevertheless, this frenzy was published in the best peer-reviewed journal and wasn't questioned as it should have been, and still now, you find people who repeat this explanation. Also, other meaningless (but by known people) explanation attempts from early 1900's are still repeated today as plausible possibilities (see Wiki) despite the proper explanation is simple and well established. That's a very nice example showing that even scientists believe an author rather than a theory, that is, they don't behave like scientists.

 

So solar sails are not only possible to build but already in action. Alas, they are small up to now, like 10m*10m, hence they bring only an observable effect, still not a main propulsion method. I suggest there how to build them about 100m*100m big and test them on Earth:

http://www.scienceforums.net/topic/78265-solar-sails-bits-and-pieces/#entry763027

http://www.scienceforums.net/topic/78265-solar-sails-bits-and-pieces/#entry857978

that would already provide interesting propulsion, like a few tons to our Moon or to Mercury, and is a step towards real size. The real game changer is rather 1km*1km big, and I have no idea for it. My hope is that, by building one 100m*100m big, we can observe what the difficulties are and find new answers.

 

I really believe this is an enabling technology, to be developed. It's by far not as difficult as it first looks: mechanical engineering, no rare knowledge nor fundamental research needed, just inventions. If I could find good looking ideas for 100m*100m, other people can go on to bigger sizes. Presently, mainly associations put hard work in it, and have very interesting concepts. I feel agencies should develop sails - rather not internally, but rather over some means to tap and help the advance at the associations.

 

Giant mirror... Do you mean, the sail itself, or a giant mirror to concentrate some light on a smaller sail? Because at astronomical distances, concentrated light isn't inside human technology. Sunlight sails at slightly outside the fringe of what we do up to now, but sending 1kW/m² to just one Sun-Earth distance is very far from anything we do now, and since much effort has already been done, further progress will be slower than on sunlight sails. I know that one project wants to push a tiny sail to a nearby star by concentrating something like 1GW on it, emitted over 1km². That's a perfect example of a project much exaggerated over many aspects at the same time, expected to cost horribly much for a doubtful result. While its goal is seducing, I radically prefer to develop sunlight sails, something cheaper than most space projects, and which shows a clear usefulness.

[Raider5678]

Giant mirror... Do you mean, the sail itself, or a giant mirror to concentrate some light on a smaller sail? Because at astronomical distances, concentrated light isn't inside human technology. Sunlight sails at slightly outside the fringe of what we do up to now, but sending 1kW/m² to just one Sun-Earth distance is very far from anything we do now, and since much effort has already been done, further progress will be slower than on sunlight sails. I know that one project wants to push a tiny sail to a nearby star by concentrating something like 1GW on it, emitted over 1km². That's a perfect example of a project much exaggerated over many aspects at the same time, expected to cost horribly much for a doubtful result. While its goal is seducing, I radically prefer to develop sunlight sails, something cheaper than most space projects, and which shows a clear usefulness.

Delightful response!! Thank you soooooo much! As for giant mirrors, I was referring to the sails, which I assumed reflected the pressurized radiation, or something like that.

[Enthalpy]

The ones designed up to now do reflect sunlight, to

• Direct the thrust in the desired direction, most often not the one of incoming light
• Limit the heat, especially if the sail must go nearer to the Sun
• Increase the thrust when possible

Absorbing the incoming light would push always towards the Sun's opposite direction. Reflecting it creates a second push that can be directed, for instance forward on the orbit to brake the craft and let it lose altitude, say to go nearer to the Sun.

[Raider5678]

The ones designed up to now do reflect sunlight, to

• Direct the thrust in the desired direction, most often not the one of incoming light
• Limit the heat, especially if the sail must go nearer to the Sun
• Increase the thrust when possible

Absorbing the incoming light would push always towards the Sun's opposite direction. Reflecting it creates a second push that can be directed, for instance forward on the orbit to brake the craft and let it lose altitude, say to go nearer to the Sun.

Newtons 2nd law right? Every action has a opposite reaction? Or something like that? I just know that if the light is bounced up it will exert force downward.

Edited May 20, 2016 by Raider5678

[Strange]

If the light is absorbed it will exert a force. If it is reflected it will exert twice the force (conservation of momentum).

[Raider5678]

If the light is absorbed it will exert a force. If it is reflected it will exert twice the force (conservation of momentum).

Thanks +1 simple and helpful.