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m74z00219
2009-Jan-14, 12:03 AM
I have a question...and I have feeling that it is a dumb one.
I'm trying to understand what is flawed about the concept of a powerful laser drive propelling a starship.

When I look at the equation for the energy of light, E = pc, i note that i can rearrange it as such, E/c = p.

Then, taking the time derivative of both sides...

[dE/dt]/c = dp/dt = Force

Okay, with a "simple" 2GW laser, this tells me that 6.66N of thrust would be created. If it were this simple, it would have already been down. So...could anyone explain to me where I've gone wrong?

Thanks,
m74

alainprice
2009-Jan-14, 12:53 AM
Looks good to me.

Are you asking why a 2 gigawatt laser hasn't been used to produce roughly a pound of thrust? I can think of a few reasons why. Firstly, that's a lot of juice for such little thrust.

p.s. If you work thru an example where a laser is inside a shoebox to one side and firing to the opposite side, we determine that momentum from the photons leaving the laser cause the box to recoil until the photons hit the opposite wall. A little algebra and voila, E=mc2

Grashtel
2009-Jan-14, 01:08 AM
The reason why its not used is that at speeds lower than a large fraction of light speed laser rockets are vastly out performed by more conventional systems. Work out the mass of a 2GW power source and laser (using as favorable assumptions as you like) and you will see that for the same power usage you can get much better performance from things like ion engines than from a laser.

m74z00219
2009-Jan-14, 04:24 AM
The reason why its not used is that at speeds lower than a large fraction of light speed laser rockets are vastly out performed by more conventional systems. Work out the mass of a 2GW power source and laser (using as favorable assumptions as you like) and you will see that for the same power usage you can get much better performance from things like ion engines than from a laser.

Thanks Grashtel. You know, I think that although I was working with gigawatts, a part of me was still thinking megawatts. However, when I consider the enormous infrastructure required for a few tens of MW nuke plant, I realize that it would not be practical for in-system efforts. However, it seems that large powerplants could be built to run very powerful lasers to propel ships on longer voyages. Perhaps, to the stars.

While of extreme practical relevance, my main concern was whether or not there was anything fundamentally unsound about the idea.

WayneFrancis
2009-Jan-14, 04:29 AM
But again you have to think what is better for the distance we want to travel. Ion Drives are very efficient too.

Are you talking about a ground based laser beaming up to a space ship to provide power? If you are then I have to ask what you propose to do the collecting of the laser energy on the ship. 2GW laser would burn through anything I can think of.

alainprice
2009-Jan-14, 01:02 PM
Laser beams diffract. Figure about 1mm spread for every meter of travel.

A laser from the earth to space would be a great way to power a solar sail.

Bonus: You get double the push if the light is reflected off the sail.

mugaliens
2009-Jan-14, 05:35 PM
Well, for some video of laser propulsion at work (http://www.youtube.com/watch?v=LAdj6vpYppA)...

It's even at a missile range in White Sands, New Mexico. Shows you how it's done (at least one approach that seems to work well).

swansont
2009-Jan-15, 12:07 AM
XKCD addressed this topic. The numbers are right - 1.21 GW will levitate a squirrel (with reflection)

http://blag.xkcd.com/2008/02/15/the-laser-elevator/

swansont
2009-Jan-15, 12:15 AM
Well, for some video of laser propulsion at work (http://www.youtube.com/watch?v=LAdj6vpYppA)...

It's even at a missile range in White Sands, New Mexico. Shows you how it's done (at least one approach that seems to work well).

That's not propulsion from radiation pressure, though, which is described in the OP.

swansont
2009-Jan-15, 12:17 AM
But again you have to think what is better for the distance we want to travel. Ion Drives are very efficient too.

The one advantage radiation pressure has over ion drives is that you never run out of fuel. You can make as many photons as you want.

mugaliens
2009-Jan-15, 05:36 PM
The one advantage radiation pressure has over ion drives is that you never run out of fuel. You can make as many photons as you want.

One disadvantage is that it is blitheringly inefficient, orders of magnitude lower than even chemical rockets.

Another is dispersion, which means that it's useless beyond immediate, localised use.

astromark
2009-Jan-15, 06:35 PM
Let there be no dought... we will find a better way of propelling space vehicles. We need to. To become real explorers of just this solar system we need to be able to get across it much quicker that our current Flintstones technology allows us. We have a want. There will be progress. At this time photon streams like laser beams may not be powerful enough in there conversion of energy into thrust. You have done the math correctly. Its the energy available that is at issue.

Jens
2009-Jan-16, 07:32 AM
The one advantage radiation pressure has over ion drives is that you never run out of fuel. You can make as many photons as you want.

I don't understand this. I thought the OP was talking about a laser, which would require fuel to operate. How does one generate radiation pressure without a source of power? Do you mean just using ambient radiation?

m74z00219
2009-Jan-16, 08:06 AM
Is it even theoretically possible to use uranium to create gigawatts of power for long periods of time?

astromark
2009-Jan-16, 09:58 AM
Yes Jens; we have wondered off OP.. and m74.;Yes. Many space craft are powered by this sort of exchangers. Do a search and all will be clear.

And to solve this question... Go stand on a skate board. Turn on a torch. Try using a lazer... If you get any movement. I will be very surprised.
Light alone is not an efficient source of thrust., but its very good for seeing with.:)

swansont
2009-Jan-16, 04:59 PM
I don't understand this. I thought the OP was talking about a laser, which would require fuel to operate. How does one generate radiation pressure without a source of power? Do you mean just using ambient radiation?

I meant to say propellant rather than fuel. Sorry for the confusion.

swansont
2009-Jan-16, 05:01 PM
One disadvantage is that it is blitheringly inefficient, orders of magnitude lower than even chemical rockets.

Another is dispersion, which means that it's useless beyond immediate, localised use.

Not a problem if you carry it with you.

mugaliens
2009-Jan-16, 06:22 PM
Not a problem if you carry it with you.

What? Sit on the end of the boat with a fan aimed at the sails?

What?

swansont
2009-Jan-16, 09:39 PM
What? Sit on the end of the boat with a fan aimed at the sails?

What?

Sit in space and shine a laser out the back of your craft. You accelerate, admittedly very slowly, from the recoil, just like any other rocket.

m74z00219
2009-Jan-16, 11:48 PM
Sit in space and shine a laser out the back of your craft. You accelerate, admittedly very slowly, from the recoil, just like any other rocket.

swansont is right. It's newton's third law (action/reaction). The light shoots out of your ship and in doing so, it repels your ship. Or you can think of it as simple momentum conservation.

Newton's 3rd applies to what mugaliens said as well, but not in the same way. Pointing a fan a part of your ship that is physically attached will null the two forces (action/reaction) in such a way that you won't move. It would be like trying to pick up your chair while your sitting on it. total momentum and total force are zero.

The light out the back of the ship would be like you throwing a rock while standing on a frictionless surface. Total momentum and force are zero, but you're going somewhere.

astromark
2009-Jan-17, 12:40 AM
You seem to be deliberately missing a very important point. Are we being deliberately obscure or misled at all ? Can we use a laser to propel a space ship. No. Not if you want it to go some place quickly. The energy available is insufficient. Just doing the mathematics soon shows that light photons as a propulsion force are not efficient. Bang for bucks... Not even in the ball park. A solid fuel burner will out perform it...easy. This is not to say this will always be the case. As our ability and understanding of sub atomic and positrons and quarks. Anti mater and all... we might yet be on the edge of discovery.

eburacum45
2009-Jan-17, 08:13 AM
This wiki page on Beamed Propulsion is a good place to start;
http://en.wikipedia.org/wiki/Beamed_propulsion
Note that the concepts on that page all assume a non-moving power source, propelling a moving spacecraft.

It is possible in theory to propel a spacecraft using a laser or other beam projector on board the ship; this concept is sometimes called a torchship
http://en.wikipedia.org/wiki/Torchship
although the wiki article on the 'photonic drive' gives a bit more information
http://en.wikipedia.org/wiki/Antimatter_photonic_drive

The advantage of such a photonic drive is that the exhaust travels at the speed of light, so given an inexhaustible source of energy, a spacecraft could approach arbitrarily close to light speed.

The disadvantage is that the momentum of a light beam is very small, so you would get almost no thrust from the photons coming out of the rear of the ship.

Winchell Chung makes the point very well here.

http://www.projectrho.com/rocket/rocket3c2.html#photon
As he says, one lousy Newton of thrust requires three hundred megawatts of power to the laser drive.

m74z00219
2009-Jan-17, 08:18 AM
You seem to be deliberately missing a very important point. Are we being deliberately obscure or misled at all ? Can we use a laser to propel a space ship. No. Not if you want it to go some place quickly. The energy available is insufficient. Just doing the mathematics soon shows that light photons as a propulsion force are not efficient. Bang for bucks... Not even in the ball park. A solid fuel burner will out perform it...easy. This is not to say this will always be the case. As our ability and understanding of sub atomic and positrons and quarks. Anti mater and all... we might yet be on the edge of discovery.

I'm not sure what I'm deliberately missing... I agree that there is probably no practical source of energy that could be used to run a laser powerful enough. But I suppose the point of my question was should there be some hypothetical energy source to stick in a starship, that would allow 2GW lasers, then you would create 6.6N of thrust "just" from light. All in all, I see that it might make no PRACTICAL sense, but I wanted to understand if it makes PHYSICAL sense.

eburacum45, thanks for the links, i'll check these out!

Jens
2009-Jan-17, 11:49 AM
Sit in space and shine a laser out the back of your craft. You accelerate, admittedly very slowly, from the recoil, just like any other rocket.

Yes, but the problem that I've been asking is: how do you power the laser? You need some source of energy for the laser, and you'll eventually run out of whatever it is that you are using to power it. So you haven't gained anything. You might as well use the fuel to power the craft rather than using the fuel to power a laser to power the craft.

swansont
2009-Jan-17, 12:54 PM
Yes, but the problem that I've been asking is: how do you power the laser? You need some source of energy for the laser, and you'll eventually run out of whatever it is that you are using to power it. So you haven't gained anything. You might as well use the fuel to power the craft rather than using the fuel to power a laser to power the craft.

There are two separate issues, and my unfortunate misuse of "fuel" confuses things a bit. You need an energy source and you need propellent to expel. An advantage of ion engines, as mentioned by WayneFrancis, is that they are pretty efficient — you accelerate particles to high speed and get a high specific impulse. But once you run out of those ions, you're done. You may have lots of fuel (i.e. energy) left, but nothing to shoot out of your rocket to accelerate it. If you bring more propellant, that's more mass to haul into space, and it reduces your acceleration.

If you were running a reactor to power your ion drive and you run out of the atoms that were your ion source, your power won't propel you, unless you chuck the reactor out the back of the spaceship.

So there are two issues here, not one. Energy source, and propellant. Laser propulsion solves one of them — you will not run out of propellant — and does so at a cost of efficiency.

swansont
2009-Jan-17, 12:59 PM
You seem to be deliberately missing a very important point. Are we being deliberately obscure or misled at all ? Can we use a laser to propel a space ship. No. Not if you want it to go some place quickly. The energy available is insufficient. Just doing the mathematics soon shows that light photons as a propulsion force are not efficient. Bang for bucks... Not even in the ball park. A solid fuel burner will out perform it...easy. This is not to say this will always be the case. As our ability and understanding of sub atomic and positrons and quarks. Anti mater and all... we might yet be on the edge of discovery.

The OP asked if the concept was flawed. The answer to that is "no." If the OP had asked of it were practical/optimal/efficient, the answer to that would also be "no." "Can we use a laser to propel a space ship" is not the same question as "Can we use a laser to efficiently propel a space ship"

mugaliens
2009-Jan-17, 01:05 PM
I agree that there is probably no practical source of energy that could be used to run a laser powerful enough. But I suppose the point of my question was should there be some hypothetical energy source to stick in a starship, that would allow 2GW lasers, then you would create 6.6N of thrust "just" from light. All in all, I see that it might make no PRACTICAL sense, but I wanted to understand if it makes PHYSICAL sense.

It makes about as much sense as using a mouse to power a locomotive.

The point others have been trying to make is that is about the least efficient or effecive use of energy for propulsion. With all that energy, using it to heat a reactant mass is far, far more efficient and effective. Alternatively, if that energy is in the form of electricity, hook up some ion drives. Again, you're millions, if time billions of factors ahead than if you simply waste (and it is a MASSIVE waste) that energy in the form of a beam of light out the backside.

The reason the approach given in the link I provided works is because the laser isn't propelling the spacecraft by the light beam's momentum. Rather, it's being used to heat air (reactant mass) which expands, and in so doing propels the spacecraft upward.

Please quit trying to advocate using the momentum of light as a means of propulsion! You're off by millions of times... That won't work. It's broken. It's ill-conceived. It's...

fill in the blank.

cjameshuff
2009-Jan-17, 01:54 PM
Please quit trying to advocate using the momentum of light as a means of propulsion! You're off by millions of times... That won't work. It's broken. It's ill-conceived. It's...

None of the above.
The advantage everybody seems to be tripping over, and moving on like nothing happened, is that the power source is essentially stationary, and does not accelerate with the craft. Yes, accelerations are small, and it's not likely to be useful for manned craft for the foreseeable future, but all of that thrust is applied toward useful payload rather than accelerating heavy reactors and reaction mass. However, for achieving high delta-vs for long trips, it's not broken or ill-conceived at all, it in fact works quite well. Roughly, 5 kilowatts per kg per km/s/year. A 5 megawatt laser station could accelerate a 100 kg probe by 10 km/s over the course of a year, and it could dedicate many times that mass of solar panels, radiators, etc to doing so, and apply them to accelerating another payload once the first is out of range.

For shorter, higher acceleration craft, possibly manned ones, you could use the same laser to power ion drives. You're now hauling reaction mass, but you can get many times the power out of the same mass of photovoltaic panels, and it simplifies the matter of accelerating across or into the beam.

eburacum45
2009-Jan-17, 03:05 PM
The links I gave to various beam-propulsion methods in my earlier link were to (at least) two quite separate concepts. The first, beamed power (including lightsails), is a perfectly sensible method of propulsion and may offer the only feasible method of reaching other stars.

The second, the 'torchship' or Photonic drive, is much less feasible, as it carries the laser on board the ship, so all the fuel for the power source also has to be carried.

Don't get confused between the two concepts; the first one is okay, given a few reasonable advances in technology; the other is piffle.

swansont
2009-Jan-17, 05:08 PM
Please quit trying to advocate using the momentum of light as a means of propulsion! You're off by millions of times... That won't work. It's broken. It's ill-conceived. It's...

fill in the blank.

"Not very effective" is different than "the physics is wrong." You're mixing the two up. Light has momentum and can exert a force. The problem is that the momentum is incredibly small. Manipulating atoms, sure. Propelling spaceships, not so much. There's nothing incorrect about what's stated in the OP.

m74z00219
2009-Jan-17, 07:42 PM
Ok, I'm done with this question. Thanks to swansont for phrasing my question more elegantly. Thanks to cjameshuff, but I did mean for the power source to be on the ship. And no thanks to mugaliens for being a pompous jerk.

astromark
2009-Jan-17, 11:44 PM
You have a right to your point of view...
Only that I see it as wrong.
But thats acceptable. Lets talk through this...

If we could use the entire output of the Hoover Dam. Its impressive. It powers Los Vegas. Could it power a space craft. Yes. but its to heavy to take along. Can we get all that energy into a laser beam and push a vehicle into space. And the answer has to be yes... but. Can you see the problem yet?
It would seem that the idea itself of laser power is not at issue. The energy to run it is.

mugaliens
2009-Jan-18, 01:04 PM
The idea of a laser-powered spacecraft is viable, but not as a means to capture or use the momentum of light. Rather, it's beauty is the ground-based location of power that's easily transmitted significant distances to be used elsewhere - as a means of heating a reactant mass that is carried.

The problem with this is that reactant mass itself can contain the chemical means to achieve the same result.

Alternatively, the laser can be used to power ion thrusters. But wait.... There's old Sol, right over there, which already provides the same capability, and does so in such incredible quantities that it can afford to sent it out in all directions at once.

m74z00219, I do not appreciate your comment. This forum is for discussing many different things. Viewpoints not supportive of one's personal perspective are quite common. It's about discussing things from a rational, fact-based, scientific perspective. cjameshuff brings up some very good points in his post, here (http://www.bautforum.com/questions-answers/83500-spaceship-laser-propulsion-question.html#post1412575). I am considering his comments to that end, and they deserve consideration as they're mentioned from that fact-based, scientific perspective. While it may be technically possible, we must also consider whether it's economically viable as compared to other technologies available to achieve the mission objective. I maintain that using our Sun remains a better solution, either directly via photovoltaics (for ion thrusters) or solar sail, aluminized mylar-reflected energy onto either photovoltaics or a solar-heated engine that uses mass.

Perhaps in the dim reaches of space, we can use lasers to propel spacecraft, but there are some technical limitations with lasers that have yet to be solved, dispersion being the principle issue. The second issue is one of atomspheric turbulence. Indeed, the Starfire optical range (http://en.wikipedia.org/wiki/Starfire_Optical_Range), in Albuquerque, is engaged in using adaptive optics to do just that. Even if that's successful, however, there's a limit as to how much that effect can be minimized, and there will always be some dispersion inherent in the beam.

To put this into perspective, using our best optical instruments, we're unable to resolve the lunar module sitting on the Moon. How are we expecting to improve things so that we can maintain pinpoint accuracy all the way to Mars, thousands of times more distant?

Why bother, when other technologies are readily available and at acceptible costs?

tusenfem
2009-Jan-18, 01:59 PM
And no thanks to mugaliens for being ************.

m74z00219, there is no place for namecalling here on the board and I don't see any reason for it in the messages. You can attack the ideas someone has, but no personal "attack" of this kind are allowed. Please refrain from doing so. This is an official warning.

cjameshuff
2009-Jan-18, 06:49 PM
Alternatively, the laser can be used to power ion thrusters. But wait.... There's old Sol, right over there, which already provides the same capability, and does so in such incredible quantities that it can afford to sent it out in all directions at once.

Unfortunately, it's sent out in all directions at once, rather than in the direction you need it.

Perhaps in the dim reaches of space, we can use lasers to propel spacecraft, but there are some technical limitations with lasers that have yet to be solved, dispersion being the principle issue. The second issue is one of atomspheric turbulence. Indeed, the Starfire optical range (http://en.wikipedia.org/wiki/Starfire_Optical_Range), in Albuquerque, is engaged in using adaptive optics to do just that. Even if that's successful, however, there's a limit as to how much that effect can be minimized, and there will always be some dispersion inherent in the beam.

Diffraction, not dispersion. Putting the laser station on the surface of a planet is an absurd idea for several reasons beyond the atmosphere...any planet will be facing the wrong way half the time, and it'll be impossible to erect a mirror large enough in the gravitational field and wind. It has to be in orbit.

The spot radius is proportional to distance, so its area is proportional to the square of distance. However, the spot area is inversely proportional to the square of the radius of the laser aperture. That is, doubling the mirror radius doubles the distance at which a given amount of power can be delivered. The mirrors required are big, but they are not beyond our capability to put up in segments in such a way that we can expand them over time.

Some numbers for an extreme case, supplying power to a probe 40 AU from the sun, from a base near the sun: solar irradiance at 40 AU is about 0.85 W/m^2. This can be exceeded at 40 AU by a diffraction-limited 1 GW 351 nm laser with a 67 meter (diameter) aperture, a 450 MW laser with a 100 m aperture, or a 50 MW laser with a 300 m aperture. (The 351 nm wavelength is that used by the NIF laser-triggered inertial fusion project, there's probably other, better-suited lasers with similar wavelengths.)

As a sailcraft, that only means more thrust than a solar sail, which doesn't mean a whole lot at that distance. It means more for beamed power, however, since the photovoltaics can be tuned for the laser wavelength...the same wattage of highly monochromatic light at the wavelength of peak conversion efficiency could double or triple the amount of electrical power available for the same photovoltaic panel+concentrator mirror mass.

For a mission to Pluto or other distant objects, the vehicle might operate in sail mode while close to the sun and laser, and use an ion drive to brake at the destination. For a mission to Jupiter or asteroids, it might mean panels sufficient to run instruments off sunlight, and beam power for maneuvers. Or take advantage of the inverse square law, and build local stations for the gas giant systems...a 30 meter reflector with a 2 MW 351 nm laser in the Jovian system could out-shine the sun out to a distance of 10 times Callisto's orbital radius in terms of pure wattage, ignoring the increased conversion efficiency.

For something much closer to home, it may even mean enough power for an ion engine upper stage to take payloads from a suborbital trajectory the rest of the way to orbit, something that would ordinarily take far too much power to be practical.

m74z00219
2009-Jan-18, 09:18 PM
You have a right to your point of view...
Only that I see it as wrong.
But thats acceptable. Lets talk through this...

If we could use the entire output of the Hoover Dam. Its impressive. It powers Los Vegas. Could it power a space craft. Yes. but its to heavy to take along. Can we get all that energy into a laser beam and push a vehicle into space. And the answer has to be yes... but. Can you see the problem yet?
It would seem that the idea itself of laser power is not at issue. The energy to run it is.

There is no point of view and there's nothing personal about my question. I see the problem very clearly astromark. The mounting mass of an increasingly strong energy source in exchange for pitiful acceleration. But, my question, at the heart of it, never was "can we find a power source to do this" or "would this produce meaningful acceleration (the vehicle would have to be sufficiently low in mass for this), but my question was and always haas been "would you generate real thrust from photons."

to tusenfem, the moderator, I appreciate that we should not be calling one another names. However, mugaliens, you should feel no affront from me when you placed a personal attack on me. "Please quit trying to advocate using the momentum of light as a means of propulsion! You're off by millions of times... That won't work. It's broken. It's ill-conceived. It's...
fill in the blank."

I am not part of some advocacy group; I am not an associate of the American Laser-Propulsion Association; I am simply a physics student who wanted to confirm that a 2GW laser would indeed exert 6.6N of thrust. Not from an in-system laser setup, but from a laser on-board. I do not "care" about how pitiful the acceleration would be; merely, I want to know that it is real physics that would allow a ship to thrust from the unidirectional (dispersion aside) output of an on board laser.

I am highly intrigued by other ideas that are more practical (both what mugaliens and cjameshuff had to say), but to call me misguided or insinuate that I am stupid for wanting to discuss the physics of an idea if unfair.

Personally, I thought this website would be a place to openly discuss ideas in physics without "fear" of being ridiculed.

mugaliens
2009-Jan-19, 02:40 PM
to tusenfem, the moderator, I appreciate that we should not be calling one another names. However, mugaliens, you should feel no affront from me when you placed a personal attack on me. "Please quit trying to advocate using the momentum of light as a means of propulsion! You're off by millions of times... That won't work. It's broken. It's ill-conceived. It's...
fill in the blank."

That was not a personal attack, as was your calling me a "pompous jerk." That was an attack on the idea, which is not only allowed here, but encouraged.

Huge difference.

I am simply a physics student who wanted to confirm that a 2GW laser would indeed exert 6.6N of thrust.

Here's the equation (http://en.wikipedia.org/wiki/Light#Quantum_theory)for calculating the momentum of a photon. Reflected photons impart twice the momentum as absorbed or emitted photons. Thus, the question of whether or not a laser imparts a thrust is answered - yes, it does. As to whether or not a 2GW laser imparts 6.6N of thrust I'll leave to you, the physics student, to peruse these links and calculate it for yourself:

Finally, your efforts should include the history of measuring light pressure, which began more than a hundred years ago, at Dartmought (pdf (http://dujs.dartmouth.edu/2002S/pressureoflight.pdf)).

Personally, I thought this website would be a place to openly discuss ideas in physics without "fear" of being ridiculed.

You were never ridiculed. Nor was anything of a personal nature insinuated. I'm sorry you reached that conclusion, but it was an errant conclusion to have been reached.

We most certainly attacked the idea itself, and for good reason. Light pressure works - no doubt about it. Using lasers when you're next door to the Sun, however, makes little sense.

Solar radiation at Earth's distance from the sun is about 1,366 W/m2. Given a solar sail that's just one acre in size (about 90% the area of a football field), that comes to 5.5 MW. Upscale that to a sail that's 1 km on a side, and you have 1.4 GW. To reach your 2 GW requirement would require a modest increase.

The best material for a solar sail is currently thought aluminized 2 micrometer Kapton film. Other, more exotic designs are on the drawing boards, including the use of carbon fiber "sponge" and nanotube mesh weaves, with promises of weights as little as 0.1g/m2, which would meam the sailcloth for our 1 km sail would weigh in at 100 kg, or about 220 lbs. Currently achievable designs weigh closer to 10,000 lbs, so there's much room for improvement!

By contrast, m74z00219, I have to ask you what would be the weight of a 2 GW, continuously-firing laser, along with it's power supply?

You see, we didn't address you, as a person. We merely addressed the idea of using a laser (on-board, no less), to provide impulse power for a spacecraft given the fact that far more viable alternatives exist.

It was never personal.

mugaliens
2009-Jan-19, 03:53 PM
Unfortunately, it's sent out in all directions at once, rather than in the direction you need it.

While that is true, so much is sent out in all directions it requires but a relatively small sail to what what you do need. :)

Diffraction, not dispersion. .... It has to be in orbit.

Agreed - I was hoping you'd go there... The momentum you'd be imparting to the spacecraft, you'd also be imparting to the space-based lasing platform. Are you planning on firing the laser in two opposing directions? Or will you be using another means of propulsion? If so, which? What competitive advantage do you have using that to offset the momentum imparted to your lasing platform as opposed to simply using it on the spacecraft itself?

The spot radius is proportional to distance, so its area is proportional to the square of distance. However, the spot area is inversely proportional to the square of the radius of the laser aperture. That is, doubling the mirror radius doubles the distance at which a given amount of power can be delivered. The mirrors required are big, but they are not beyond our capability to put up in segments in such a way that we can expand them over time.

Understand - much like firing a laser backwards through a large-mirror telescope. Of course, it would have to be a special telescope to handle the energy densities of the beam

Some numbers for an extreme case, supplying power to a probe 40 AU from the sun, from a base near the sun: solar irradiance at 40 AU is about 0.85 W/m^2. This can be exceeded at 40 AU by a diffraction-limited 1 GW 351 nm laser with a 67 meter (diameter) aperture, a 450 MW laser with a 100 m aperture, or a 50 MW laser with a 300 m aperture. (The 351 nm wavelength is that used by the NIF laser-triggered inertial fusion project, there's probably other, better-suited lasers with similar wavelengths.)

And what of a sailcraft's greater efficiency from close-in to out there? Even though at 40 AU the laser is better, wouldn't a sailcraft be well beyond a laser propelled craft by the time it gets to 40 AU?

As a sailcraft, that only means more thrust than a solar sail, which doesn't mean a whole lot at that distance. It means more for beamed power, however, since the photovoltaics can be tuned for the laser wavelength...the same wattage of highly monochromatic light at the wavelength of peak conversion efficiency could double or triple the amount of electrical power available for the same photovoltaic panel+concentrator mirror mass.

Integrate the thrust/velocity/distance from Sol curves, noting that the first manuver for a solar-sail spaceship is to perform a flyby of the Sun (http://en.wikipedia.org/wiki/Solar_sail#H-reversal_sun_flyby_trajectory)so as to gain the most thrust at the earliest part of it's journey. In fact, it reaches escape velocity while still very close to the Sun, and continues to accelerate as distance is increased.

As this optimal strategy (http://en.wikipedia.org/wiki/Solar_sail#Optimal_Strategy)claims, it sweeps past the sun (0.2 AU) with the velocity increasing as it recedes from the sun, then decreasing, but reaching a nearly constant velocity of 70 km/s at 2.7 AU that continues all the way to 100 AU.

This would would require just 2.7 years to reach your 40 AU mark - how does that compare with your laser-propelled craft?

For a mission to Pluto or other distant objects, the vehicle might operate in sail mode while close to the sun and laser, and use an ion drive to brake at the destination.

Ahh... Hybrid propulsion. Now we're talking. If we're already sporting a solar sail, adding a boost via laser when the sun's pressure drops below designed max wouldn't hurt and would only augment.

The rest of your post is well put.

I envision a solar sail that's also a concentrating mirror for photovoltaics powering ion thrusters, for additional thrust during the boost phase, and for braking later on.

cjameshuff
2009-Jan-19, 11:27 PM
While that is true, so much is sent out in all directions it requires but a relatively small sail to what what you do need. :)

This is part of why I'm interested in the possibilities for Mercury and Venus. Further out, however, beamed power starts to look more and more attractive.

Agreed - I was hoping you'd go there... The momentum you'd be imparting to the spacecraft, you'd also be imparting to the space-based lasing platform. Are you planning on firing the laser in two opposing directions? Or will you be using another means of propulsion? If so, which? What competitive advantage do you have using that to offset the momentum imparted to your lasing platform as opposed to simply using it on the spacecraft itself?

The laser station would be many orders of magnitude heavier than the probes, though, and it might be obsolete before it accumulates enough delta-v from operating as a photon drive to be a problem. If not, maybe continuously running ion engines, using far less power to achieve the same thrust...it's a bit easier to resupply the nearly-stationary laser station with stationkeeping fuel than it is to do so with the distant probe. It could also cancel some or all of it with solar sails.

Understand - much like firing a laser backwards through a large-mirror telescope. Of course, it would have to be a special telescope to handle the energy densities of the beam

Actually, even the smaller and higher-power (and thus less efficient) options only have a beam intensity on the large mirror a few times that of Earth-orbit sunlight. The bigger mirror versions with those laser powers actually are more dimly illuminated by the laser they focus than by the sunlight shining on them. There are smaller optics that need to withstand higher intensities, of course...fortunately, they're smaller and easier to make with more exotic materials and processes.

And what of a sailcraft's greater efficiency from close-in to out there? Even though at 40 AU the laser is better, wouldn't a sailcraft be well beyond a laser propelled craft by the time it gets to 40 AU?

If you're doing fast flybys of outer solar system objects, the solar sail craft would be the winner. It's not a very useful approach for sending stuff inward from Mars, asteroids, or gas giant moons, though.

There's the potential for using laser stations to augment the fast solar sailing technique to achieve even higher velocities, and perform it during flybys of Jupiter or other gas giants. Consider a powerful laser station with a relatively short range mirror in Jovian orbit boosting a sailcraft or beam-powered ion craft during a close flyby.

This would would require just 2.7 years to reach your 40 AU mark - how does that compare with your laser-propelled craft?

Aside from just augmenting the thrust from that maneuver, laser propulsion could apply it toward sending payloads in other directions. It would be a good way of achieving the delta-v required for the initial maneuver of dropping in toward the sun, reducing the time required to do it using the sails with the more limited sunlight out past Earth orbit.

I envision a solar sail that's also a concentrating mirror for photovoltaics powering ion thrusters, for additional thrust during the boost phase, and for braking later on.

My gut feeling is that it would involve mass penalties that would make a separate sail and pure concentrator mirror (though one built with the same techniques as the sail) more effective. Combining the two is something to try, though.