Author Topic: A not laser boosted solar sail  (Read 2308 times)

aero

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A not laser boosted solar sail
« on: 08/29/2010 04:13 AM »
From reading Scienceguy's post about 0.01 c from a solar sail, I got to thinking about methods to boost the radiation pressure on the sail. Of course, augmenting the solar radiation with a laser is one technique, but consider this.

Use the solar sail vehicle with one square km sail and mass of 10^5 kg. Then on the moon, or in the desert on Earth for that mater, put about 10^5 flat mirrors each 10 x 10 meters and angled to reflect sun light from the sun to the solar sail. Using the moon as a platform avoids wind and weather induced problems and gives about a week each lunar month during which the reflected beam could be pointed at the solar sail. Of course each 10x10 mirror would need its own pointing control, and each beam would spread, but the solar sail would be 10 AU distance before the individual beams spread to wider than the solar sail's intercept area.

There are obviously problems to be worked but roughly speaking this system offers the thrust of a 1 x 10^7 m^2 solar sail for the mass of a 1 x 10^6 m^2 sail, and 10 times more thrust from the solar sail gives an appreciable amount of acceleration.
« Last Edit: 08/29/2010 04:14 AM by aero »
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alexterrell

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Re: A not laser boosted solar sail
« Reply #1 on: 08/29/2010 01:11 PM »
When reflecting from a mirror, the mirror cannot be brighter per unit of perceived area than the sun. With a shaped mirror, you can burn an object, but only because, to the object, the mirror appears bigger than the sun.

Your 10^7m2 mirror, is about 3.2 km wide. Once you're 500km away from it, it will have less effect than the sun.

aero

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Re: A not laser boosted solar sail
« Reply #2 on: 08/29/2010 03:49 PM »
Quote
Your 10^7m2 mirror, is about 3.2 km wide. Once you're 500km away from it, it will have less effect than the sun.
I think you are saying that beyond 500 km, the light reaching the 1 km^2 solar sail from the mirrors will exert less radiation pressure than the light reaching it directly from the sun (equivalent geometric orientation of course). I don't understand why this is. Could you please explain in a little more detail? But note that I do not propose a single 3.2 km wide mirror, rather 100,000 mirrors of 10 m wide. So yes, it could be considered one big focusing mirror.
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Proponent

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Re: A not laser boosted solar sail
« Reply #3 on: 08/30/2010 02:53 AM »
The mirrors focus the sun's light on the sail, right?  Using basic geometric optics, calculate the size of the the sun's image as a function of distance from the mirror.  You'll find that it's about 0.01 times the distance from the mirror (i.e., it is the angular diameter of the sun, expressed in radians, times the distance from the mirror to the sail).  Once the sail is more than 100 km away, the efficiency drops off quickly as the sail intercepts smaller and smaller fractions of the reflected light.

Lampyridae

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Re: A not laser boosted solar sail
« Reply #4 on: 08/30/2010 11:54 AM »
Sounds like you need a lens rather than a mirror... perhaps a fresnel lens would do, but there is still the brightness problem.
« Last Edit: 08/30/2010 12:00 PM by Lampyridae »
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Proponent

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Re: A not laser boosted solar sail
« Reply #5 on: 08/30/2010 12:12 PM »
Sounds like you need a lens rather than a mirror...

A lens will suffer from exactly the same problem.  Just do the ray tracing and you'll see what I mean.

I am assuming that the optical system (lens or mirror) images the sun onto the sail.  There are ways of getting higher intensities using non-imaging optics, but to my knowledge these do not work over long distances.

All of this illustrates why lasers are proposed for propelling light sails and for beaming power from solar power sats.

Proponent

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Re: A not laser boosted solar sail
« Reply #6 on: 08/30/2010 12:20 PM »
When reflecting from a mirror, the mirror cannot be brighter per unit of perceived area than the sun.

According to the Wikipedia article that I linked to in the immediately preceding post, BTW, non-imaging optics actually can corral the sun's rays in such a way as to produce a higher brightness than the sun.  In accordance with the second law of thermodynamics, however, it remains impossible to heat an object to a temperature higher than the surface of the sun (i.e., there must be a limit as to the angular size of the reflector as seen from the object being heated).
« Last Edit: 08/30/2010 12:20 PM by Proponent »

aero

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Re: A not laser boosted solar sail
« Reply #7 on: 08/30/2010 03:11 PM »
Well shucks. I was hoping that, along with the high speed interstellar probes, we could come up with a high speed transit system to Mars and back, using solar sails. But I do remember the physics class now. A reflection of the sun from the Earth's surface will spread to the  size of the moon by the time it reaches the moon. That's why you can't make a light  spot on the moon with  mirrors on earth. (The sun and the  moon have the  same apparent diameter, viewed from Earth.) The reflected sunbeam spreads to cover the whole moon by the time it reaches the moon.

So it seems its back to lasers. Is it reasonable to consider a laser driven light sail ship as an Earth - Mars commuter? This should probably be a new topic, since it is no longer "not laser" as the subject reads.
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Lampyridae

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Re: A not laser boosted solar sail
« Reply #8 on: 09/06/2010 03:26 PM »
Problematic. The problem with solar radiation is that for all intents and purposes, it's already collimated. What we're trying to do is collimate collimated light...
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kalish

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Re: A not laser boosted solar sail
« Reply #9 on: 09/07/2011 10:02 AM »
Hello I just registered to answer this topic. First of all I am french, so I don't speak/write a perfect english/american, and sometimes I hardly understand what you say. I have a degree in physics. Now presentations are done,  so I would like to say the proposition of aero doesn't look stupid, or useless to me at all. As radiative pressure is just a question of energy flow through a surface, oriented mirror could focus the energy of a big big surface onto a small one, that's the advantage. We don't want to collimate collimated light, we want to focus collimated light, that's what I understood of his propsition, am I right?
« Last Edit: 09/07/2011 10:04 AM by kalish »

Tass

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Re: A not laser boosted solar sail
« Reply #10 on: 09/07/2011 11:52 AM »
Problematic. The problem with solar radiation is that for all intents and purposes, it's already collimated. What we're trying to do is collimate collimated light...

No the problem is that it is not. It does not come from a point source but from an object extending over 0.01 radians.

aero

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Re: A not laser boosted solar sail
« Reply #11 on: 09/07/2011 07:45 PM »
Hello I just registered to answer this topic. First of all I am french, so I don't speak/write a perfect english/american, and sometimes I hardly understand what you say. I have a degree in physics. Now presentations are done,  so I would like to say the proposition of aero doesn't look stupid, or useless to me at all. As radiative pressure is just a question of energy flow through a surface, oriented mirror could focus the energy of a big big surface onto a small one, that's the advantage. We don't want to collimate collimated light, we want to focus collimated light, that's what I understood of his propsition, am I right?
to kalish -- Thanks for the support but I am now convinced that apparent diameter of the sun as seen from any reasonable distance will cause the reflected light to difuse, not focus. Think of any point on the mirror. That point will reflect light emanating from the center of the sun's disk as well as light emanating from the limb of the sun. I don't see any way to collimate those two rays of sun light. Of course it is more than a point or two, it is every point on the mirror reflecting light from every point on the solar disk.
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kalish

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Re: A not laser boosted solar sail
« Reply #12 on: 09/16/2011 10:37 AM »
Hello, sorry but I can really say that you are wrong. wavefronts are flat or parallel, so you can focus them to a point like any collimated light. Don't you know how works the concentrated solar power ?:
http://en.wikipedia.org/wiki/Concentrated_solar_power
the same principle as a lens which focus energy, so how a lens focus the energy that is the question? The radiation pressure is the effect of the energy going through a surface, as the energy flow is conserved if you don't have energergy outside an area, you have it elsewhere, so you actually can concentrate it, it seems obvious to me but I will tell you why it is wrong, you say you see the sun just as a tiny point from a distant point of view, but that s the case of a laser also, and you see it tiner and tiner if you leave it away. The fact is when you are on the moon, you see a point from a place and another point from another place on the moon, so it is possible to add all those energies and focus them, it is not hard, you need curved mirror, wuth a curved mirror you can see the sun as a BIG point on one place, or a very shing point. The advantage would be to get all the energy from a huge surface and to focus it on a small one. I have another parallel for you. Imagine you have the same area full of photovoltaic pannels, then you get all this energy from this area which is quite big, then you put all this energy in a laser, you have now the same pressure.

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