Laser drive

[RoboSpy]

This is odd, but...

A solar sail is driven by the pressure of photons making contact with the sail, right? The reflection generates a pressure. The problem with a solar sail, though, is that one can only go away from Sol (or whatever other star you might happen to be traveling near.)

So I was thinking, what if a spacecraft were built with extremely massive solar arrays with really high efficiency, like exceeding 50% (I know our best technology only gets us to about 20% right now, but bear with me.) The power generated by these solar panels is then routed to a really, really powerful laser which can be pointed in any direction. If the laser is strong enough, will the photons streaming from it act as a rocket exhaust to give the spacecraft a thrust comparable to a solar sail? If this works, and the solar panels exceed 50% efficiency, I think it could be used to fly towards Sol, thought quite slowly since the solar panels would reflect some photons.

Or do I just misunderstand solar sails?

[JohnD]

Robbie,
If all that power were available, then a very small amount of mass would work far better as a reaction drive than pure light. Light has no mass, but look at ion drives.

And solar sails: they are supposed to work by reflection, but there is an old Victorian scientific instrument, now an executive toy, called a Crooke's radiometer. I have one. It is a vacuum filled (!) globe, with a four small foil squares on a light rotor. Each square is silvered on one side and soot black on the other. If the solar sail theory was right then it would rotate with the silvered sides trailing. In fact it goes the other way as if the light was 'pushing' at the BLACK squares.

So how do solar sails work?

John

[swansont]

Robbie,
If all that power were available, then a very small amount of mass would work far better as a reaction drive than pure light. Light has no mass, but look at ion drives.

And solar sails: they are supposed to work by reflection, but there is an old Victorian scientific instrument, now an executive toy, called a Crooke's radiometer. I have one. It is a vacuum filled (!) globe, with a four small foil squares on a light rotor. Each square is silvered on one side and soot black on the other. If the solar sail theory was right then it would rotate with the silvered sides trailing. In fact it goes the other way as if the light was 'pushing' at the BLACK squares.

So how do solar sails work?

John

It's not a very good vacuum.

Radiation pressure is used in laser cooling. There's nothing wrong with the theory. But the momentum of a photon is small - the effect is easily swamped with a poor vacuum.

[RoboSpy]

The reason I would want to use only light would be to eliminate the necessity for fuel, is all. Ion rockets are extremely fuel efficient, its true, but they still require some fuel. It would be good if there were a way to eliminate that necessity completely for extremely long-term spaceflights.

As for this Crooke's radiometer: Are you sure the globe is a vacuum? If it weren't, I could see perhaps the black sides of the foil absorbing light and heating the air directly in front of them, generating a slight force by the air's expansion. Otherwise, if it IS a vacuum... :-? ...I have no idea how it works! I doubt it has anything to do with photon pressure, though, since even a massive solar sail is only capable of accelerating a lightweight spacecraft at just a tiny fraction of g. I doubt the photon pressure on four foil squares is sufficient to spin the rotor.

[RoboSpy]

Oh, so it's just a poor vacuum. Thanks swansont. I guess you posted while I was writing my last reply.

[daver]

This is odd, but...

A solar sail is driven by the pressure of photons making contact with the sail, right? The reflection generates a pressure. The problem with a solar sail, though, is that one can only go away from Sol (or whatever other star you might happen to be traveling near.)

That's not quite correct. For giggles, assume the mirror is perfectly reflective. Then the reflected photon also imparts some momentum, and the thrust is along a line halfway between the line normal to the sail and the line to the sun.

The component of the sail's thrust that is away from the sun doesn't do an awful lot of good (assuming the sail is in a circular orbit initially)--it might move the sail a smidgeon further from the sun, but there it stays. What helps is the component of the thrust along the sail's orbit--that will increase (or decrease) the sail's orbital velocity, and allow it to spiral in or out.

There's a bit of a balancing act involved--the more the sail is tilted, the higher the percentage of its thrust goes in a useful direction, but the lower the overall thrust.

The laser sail you describe has several disadvantages--you have the overall efficiency of the solar cells, the overall efficiency of the laser, the weight of the solar cells, and the weight of the laser. As described, it's pretty much a non-starter. However, there's really not much point in using a laser--it's not a particularly efficient light source--what you'd like is a way to gather the light from several square km and be able to direct it in a fairly parallel beam in a fairly arbitrary direction at a fairly low weight penalty. I don't know if there's some configuration that would end up being any better than a straight solar sail.

[JohnD]

Daver,
Spiral in?
How can any vector on a sail cause it to spiral in?
If the sail is turned more than directly radial to the direction of the star, the photons hit it on the other side forcing it out again.

Sailing into the wind on earth is a resultant of the LIFT from aerodynamic forces on the down wind side of the sail and the resistance of the boat's keel. Irrelevant to solar sailing. You can't tack against the solar wind.

Robbo's megalasers could be used from Earthside to power a sail away from the planet in either diecrtion, but that's old hat. See Larry Niven's ideas, The Mote in God's Eye, and the other stories.
John


PS Sorry, slowing it down would spiral it in. Still, not nexactly Lord of All He Surveys, is it?

[daver]

Daver,
Spiral in?
How can any vector on a sail cause it to spiral in?
If the sail is turned more than directly radial to the direction of the star, the photons hit it on the other side forcing it out again.

Orienting your sail perpendicular to the sun (so the photons are pushing you directly away from the sun) essentially reduces the sun's gravitational pull on you by whatever the photon pressure on the sail is (the photon pressure also obeys an inverse square law). For most sails, the photon push is going to be less than the sun's pull, so you end up in an elliptical orbit.

Now, tilt the sail. Assuming you keep the sail at the same angle to the sun (you wouldn't, but we're simplifying here), you're going to have a component towards the sun that essentially cancels a fraction of the sun's pull, and a component perpendicular to that that increases or decreases your orbital velocity. It's this component that does something useful--allowing you to spiral in or out.

If your sail was perfectly non-reflective, you'd only get the radial component of the thrust, and the sail would be essentially useless.

Sailing into the wind on earth is a resultant of the LIFT from aerodynamic forces on the down wind side of the sail and the resistance of the boat's keel. Irrelevant to solar sailing. You can't tack against the solar wind.

And again, yes, you can tack against the solar wind. The thrust vector is not radial, you've got a component that can be directed along or against your velocity vector.

PS Sorry, slowing it down would spiral it in. Still, not nexactly Lord of All He Surveys, is it?

Hmm, not sure what you're getting at here. It's hard to imagine a scenario in which solar sails are actually economical, unless you go to a laser-augmented sail such as Forward or Niven/Pournelle described.

[RoboSpy]

And again, yes, you can tack against the solar wind.

You CAN tack aginst the sun? I presume here you're still talking about slowing down an orbit to spiral in, and not, I presume, about tacking against the wind the way a ship does. If the latter is possible, and I was under the distinct impression that it was not, then the whole laser drive question becomes irrelevant.

As for redirecting light instead of using a laser, that was my initial idea some time ago, but I realized that the only way to effectively redirect the light would be with mirrors, which, of course, recieve momentum from the light which they reflect, thus countering much of the thrust which might have been provided by the redirection. On the other hand, there are prisms, though I would imagine that prisms must also have momentum imparted to them from photons passing through as well...oh...of course, its just in internal reflection, just like the mirrors anyway.

Ah, but I just thought - would it be possible to effectively redirect light using bundles of fiber optic cables? Or are fiber optic cables just really long prisms, using internal relfections that will impart momentum?

[Morrolan]

i was reading this just now and what struck me was the analogy of propelling a sailboat by using a large fan that blows wind in the sails: it won't work because the back thrust of the fan negates the effect of the wind blown into the sail.

wouldn't the effect of using a laser on a solar sail (assuming the laser is carried onboard) be more or less the same (in a vacuum)?

also a solar sail, IIRC, is an extremely slow method of acceleration, useful only for a long time accelerations of objects that do not have the capacity to carry loads of fuel.

or am i missing something here?

[Maksutov]

Daver,
Spiral in?
How can any vector on a sail cause it to spiral in?
If the sail is turned more than directly radial to the direction of the star, the photons hit it on the other side forcing it out again.

Orienting your sail perpendicular to the sun (so the photons are pushing you directly away from the sun) essentially reduces the sun's gravitational pull on you by whatever the photon pressure on the sail is (the photon pressure also obeys an inverse square law). For most sails, the photon push is going to be less than the sun's pull, so you end up in an elliptical orbit.

Now, tilt the sail. Assuming you keep the sail at the same angle to the sun (you wouldn't, but we're simplifying here), you're going to have a component towards the sun that essentially cancels a fraction of the sun's pull, and a component perpendicular to that that increases or decreases your orbital velocity. It's this component that does something useful--allowing you to spiral in or out.

If your sail was perfectly non-reflective, you'd only get the radial component of the thrust, and the sail would be essentially useless.

Sailing into the wind on earth is a resultant of the LIFT from aerodynamic forces on the down wind side of the sail and the resistance of the boat's keel. Irrelevant to solar sailing. You can't tack against the solar wind.

And again, yes, you can tack against the solar wind. The thrust vector is not radial, you've got a component that can be directed along or against your velocity vector.

PS Sorry, slowing it down would spiral it in. Still, not nexactly Lord of All He Surveys, is it?

Hmm, not sure what you're getting at here. It's hard to imagine a scenario in which solar sails are actually economical, unless you go to a laser-augmented sail such as Forward or Niven/Pournelle described.

Huh, I wonder what the maximum "close hauled" point of solar sailing would be then? On Earth it's 45 degrees either side of the direction of the wind. With nothing to act the way the bow does in the water, etc., would it less than 45 degrees? Or would it also be 45 degrees as the components of the vector become equal and cancel each other out?

[RoboSpy]

i was reading this just now and what struck me was the analogy of propelling a sailboat by using a large fan that blows wind in the sails: it won't work because the back thrust of the fan negates the effect of the wind blown into the sail.

wouldn't the effect of using a laser on a solar sail (assuming the laser is carried onboard) be more or less the same (in a vacuum)?

I hope I'm not being misunderstood by everyone here in this way. I'll clarify; I mean to fire the laser out into space, not onto another solar sail. The idea is to use the photons from a laser almost like a reaction engine, utilizing them the same way a chemical rocket utilizes exhaust gases. It doesn't even really have to be a laser, just some device capable of generating large numbers of photons and directing them in one direction.

[Morrolan]

i was reading this just now and what struck me was the analogy of propelling a sailboat by using a large fan that blows wind in the sails: it won't work because the back thrust of the fan negates the effect of the wind blown into the sail.

wouldn't the effect of using a laser on a solar sail (assuming the laser is carried onboard) be more or less the same (in a vacuum)?

I hope I'm not being misunderstood by everyone here in this way. I'll clarify; I mean to fire the laser out into space, not onto another solar sail. The idea is to use the photons from a laser almost like a reaction engine, utilizing them the same way a chemical rocket utilizes exhaust gases. It doesn't even really have to be a laser, just some device capable of generating large numbers of photons and directing them in one direction.

oops... ops: sorry about that.

[Wally]

Huh, I wonder what the maximum "close hauled" point of solar sailing would be then? On Earth it's 45 degrees either side of the direction of the wind. With nothing to act the way the bow does in the water, etc., would it less than 45 degrees? Or would it also be 45 degrees as the components of the vector become equal and cancel each other out?

[-X

45 degrees may be max for your big, beamy cruisers, but a well designed racing boat normally points closer to 35 degrees (give or take a couple) to the wind.

[Bozola]

And solar sails: they are supposed to work by reflection, but there is an old Victorian scientific instrument, now an executive toy, called a Crooke's radiometer. I have one. It is a vacuum filled (!) globe, with a four small foil squares on a light rotor. Each square is silvered on one side and soot black on the other. If the solar sail theory was right then it would rotate with the silvered sides trailing. In fact it goes the other way as if the light was 'pushing' at the BLACK squares.

So how do solar sails work?

John

A Crooke's is not a "good" vacuum at all.

A Crooke's radiometer works by heating up one side of the vane. Gas molecules collide with greater momentum transfer on the heated side, thus directionally spinning the vanes. This has nothing to do with "photon pressure".

[daver]

And again, yes, you can tack against the solar wind.

You CAN tack aginst the sun? I presume here you're still talking about slowing down an orbit to spiral in

Yes, that's effectively tacking--it allows you to move "upwind".

> As for redirecting light instead of using a laser, that was my initial idea some time ago, but I realized that the only way to effectively redirect the light would be with mirrors, which, of course, recieve momentum from the light which they reflect, thus countering much of the thrust which might have been provided by the redirection. <

I'm not to sure where you're going with this. Your solar arrays absorb the momentum of the light that impinges on them, so you get p units of momentum radially outwards, regardless. You get something less than that that can be directed in any other direction. That's the same situation that you get with lenses and mirrors.

[Maksutov]

Huh, I wonder what the maximum "close hauled" point of solar sailing would be then? On Earth it's 45 degrees either side of the direction of the wind. With nothing to act the way the bow does in the water, etc., would it less than 45 degrees? Or would it also be 45 degrees as the components of the vector become equal and cancel each other out?

[-X

45 degrees may be max for your big, beamy cruisers, but a well designed racing boat normally points closer to 35 degrees (give or take a couple) to the wind.

OK, so I prefer comfort over speed. Hoist me on yer Cunningham! 8)

[daver]

Huh, I wonder what the maximum "close hauled" point of solar sailing would be then?

That probably depends on the performance of the solar sail (say, its mass per unit area). The lighter the sail the better (up to a point--making it lighter may also reduce the reflectivity, which lowers performance. Making it lighter may also limit how close to the sun it can operate, which reduces its utility.


Here is a NASA site on solar sails.

[Emspak]

Daver,

would sola sails be useful in braking operations? I was thinking that they might be really good for slowing large masses down ansd getting them into useful orbits with a minimum of fuss. (It would take a while but this would be for long-term projects).

Seems to me it would be a pretty efficient way to do it, since the "fuel" is essentially free and you aren't wasting massive amounts of propellant, nuclear fuel or whatever just to slow down slightly. (I was thinking the configuration you describe would work for a body that is far away, and you want to give it a wee nudge sunward, and then use the sail to control the acceleration).

[daver]

Daver,

would sola sails be useful in braking operations? I was thinking that they might be really good for slowing large masses down ansd getting them into useful orbits with a minimum of fuss. (It would take a while but this would be for long-term projects).

The solar flux at Earth orbit is about 10 micro-newton/sq meter. If you pretend that the mass of your craft is dominated by the sail and that the sail plus control mechanisms plus payload averages 20 grams/sq meter, you get an acceleration of roughly 40 m/sec/day, or maybe 1 km/sec/month. That, of course, is at Earth's distance from the sun.

According to the NASA site, the most likely solar sail material we currently have averages around 12 grams/sq meter; that doesn't count control mechanisms. So getting to 20 grams/sq meter counting the payload doesn't seem particularly likely; if instead we had a total of 40 grams/sq meter (half payload, half structure) our acceleration would drop to 20 m/sec/day; one kg would require 50 sq meters of sail, one ton 50k sq meters, twenty tons, a square kilometer.

Note also that twenty tons of payload requires twenty tons of solar sail/rigging. Assuming this sail ends up at 1 au from the sun, the same NASA site says the solar sail can dump a bit more than 8.5 km/sec.

Seems to me it would be a pretty efficient way to do it, since the "fuel" is essentially free and you aren't wasting massive amounts of propellant, nuclear fuel or whatever just to slow down slightly. (I was thinking the configuration you describe would work for a body that is far away, and you want to give it a wee nudge sunward, and then use the sail to control the acceleration).

This is not all that clear. If you're coming from an outer planet, the solar sail isn't all that useful for much of the journey. The solar sail is tied up for decades at a time, and presumably requires periodic repairs and patches. If you have to do significant repair/refurbish on the solar sail each trip, it may end up cheaper to have just use a nuclear reactor and send up new fuel each leg.

[TrAI]

Hmmm.. Perhaps one could make a solar pumped laser; instead of converting the sunlight to electricity and then back to light, just use mirrors to focus the light into the lasing medium... It may be more efficient than using a multistep approach...

[Wally]

Huh, I wonder what the maximum "close hauled" point of solar sailing would be then? On Earth it's 45 degrees either side of the direction of the wind. With nothing to act the way the bow does in the water, etc., would it less than 45 degrees? Or would it also be 45 degrees as the components of the vector become equal and cancel each other out?

[-X

45 degrees may be max for your big, beamy cruisers, but a well designed racing boat normally points closer to 35 degrees (give or take a couple) to the wind.

OK, so I prefer comfort over speed. Hoist me on yer Cunningham! 8)

I tend to feel the same way every time I happen to step aboard a Catalina! Talk about comfort!!! Gheeeesh! Makes me wonder why I bother with racing!!!