Beam-powered propulsion - keeping the beam focused

[Cutter Ketch]

Am I using the evolving beam width equation correctly? I have no idea if it's reasonable to assume something close to an ideal Gaussian beam in this case. If the math is right, then it suggests to me that you would need to focus the laser through a telescope with a 713.65-meter aperture to produce a beam with those characteristics.

Real beams can with work be forced to behave very similar to the ideal Gaussian. How closely a real beam follows Gaussian behavior is beam quality and is parameterized as M^2. This is a simple scaling of the Gaussian behavior. For a given M^2 if the waist is made M times bigger than an ideal Gaussian then the divergence will also be M times bigger. The beam quality product of waist size times beam divergence is therefore M^2 bigger and that's the reason that the parameter is M^2 and not just M.

 

[schplade]

I've been wondering lately about the challenge of building a giant laser-projecting telescope, like the one mentioned earlier. How does it compare to the challenge of building a giant light-gathering telescope of the same size?

At our current level of technology it's a little ridiculous to talk about building a 700-meter telescope, but the 100-meter Overwhelmingly Large Telescope is something that has already been proposed. The projected cost was about 1.5 billion Euros.

https://en.wikipedia.org/wiki/Overwhelmingly_Large_Telescope


Segmented mirror technology is allowing us to build telescopes bigger and bigger. Could a segmented mirror telescope used "in reverse" produce something close to an ideal Gaussian laser beam?

Without knowing much about the subject, I'm guessing that there would be certain advantages to building a telescope like this vs. building a light-gathering one. As mfb said, the mirrors could be made much more reflective if you're only dealing with a single wavelength. Also, I assume you would never have to refocus it. You could adjust it to the beam divergence angle that works best and leave it at that. You'd still have to aim it at the spacecraft , of course, but at these distances the spacecraft would have to be capable of keeping itself on the beam no matter what.

One complication that I could imagine is the "jitter" problem. At extreme distances, every tiny vibration at the laser's source is probably going to have a big effect when you're hundreds of millions of kilometers away. That's why I've wondered if it wouldn't be best to have multiple space-based lasers along the spacecraft 's flight path, but I don't think I've seen any beamed propulsion concepts where the beam isn't directly in line with the acceleration path of the spacecraft .

 

[sophiecentaur]

Segmented mirror technology is allowing us to build telescopes bigger and bigger. Could a segmented mirror telescope used "in reverse" produce something close to an ideal Gaussian laser beam?

Such a telescope would need to be in space and it would need to be pretty massive to maintain such a stable optical performance. (I do mean massive and not just a large area) That would be a very high cost construction.

 

[mfb]

Such a telescope would need to be in space

Why?
Breakthrough Starshot plans ground-based lasers, for example.

OWL was canceled with estimated project costs of € 1.5 billions, but that was too low. E-ELT with a 42 m mirror (instead of 100) has construction costs of about € 1 billion. Increasing the mirror area by a factor 7 would certainly raise the price tag by more than 50%.

Also, I assume you would never have to refocus it. You could adjust it to the beam divergence angle that works best and leave it at that.

You would need adaptive optics, correcting atmospheric turbulence. You need active optics to position the mirrors precisely as well. You have to aim precisely at the spacecraft and constantly change the orientation of the telescope (as the Earth rotates).

 

[schplade]

You would need adaptive optics, correcting atmospheric turbulence. You need active optics to position the mirrors precisely as well. You have to aim precisely at the spacecraft and constantly change the orientation of the telescope (as the Earth rotates).

That's part of why I think it might actually be easier to build the telescope in space. This would definitely NOT be a short-term project. What if the beam needs to be aimed at the spacecraft for days, or even weeks at a time? It's just hard for me to imagine how you could ever aim a laser so precisely that it hits a departing spacecraft light seconds, minutes, or hours away. How could you make the necessary adjustments in time? Even at the distance of the moon, any signal from the spacecraft would take more than a second to reach the earth.

The only way that I can conceive of this working is if the spacecraft has the capability of keeping itself centered on the beam. As far as the laser source itself is concerned, the goal would be to keep it stable and to keep it aimed at the destination star system at all times. That's obviously not going to be possible on a rotating body like the earth. In addition to that, it's kind of scary to think of the dangers that would be posed by such a powerful laser being reflected back at the earth.

I agree that you would need some sort of active optics system to position the mirrors so precisely, so in that sense you would be constantly refocusing the laser. I don't see why you would necessarily have to change the distance to the focus though. That could definitely be my ignorance talking.

How would you keep the beam stable, and aimed at the destination star system? I have no idea. Maybe you could build it into a large asteroid, and correct the rotation of the asteroid somehow so that one side is constantly facing the destination. That's just guesswork, of course. I'm still not convinced that something on this scale would ever be feasible.

 

[GTOM]

Is there any estimation, what if they used particle beams instead of lasers? Any plausible ways to have neutralized particle beams don't diverge for a long time?
(Maybe speed particles very near to c, so length contraction applies?)

 

[mfb]

It's just hard for me to imagine how you could ever aim a laser so precisely that it hits a departing spacecraft light seconds, minutes, or hours away.

You don't. You have to accelerate it quickly to keep the distance reasonable.

The JWST has a primary mirror with 6.5 meters, and estimated costs of $8 billions. For 1/8 of that price you get the E-ELT with 42 meters diameter, 40 times the area. Or 10-15 E-ELTs for the same price, with hundreds of times the mirror area. Ground-based telescopes are so much easier to build. They also have easier access to electricity, and you don't have to place a weapon of mass destruction in orbit.

Aiming telescopes is necessary both in space and on the ground, and it is easier on the ground (where you have a solid surface as reference). It is not an issue at all, every telescope does that routinely for operation. Forget spacecraft steering - the telescope has to aim. You need several laser "telescopes" anyway: Even a magic spacecraft with perfect steering from nowhere would still need multiple telescope beams that aim at the same spot.

In addition to that, it's kind of scary to think of the dangers that would be posed by such a powerful laser being reflected back at the earth.

The reflection would be scattered and spread out enough to be not dangerous.

Is there any estimation, what if they used particle beams instead of lasers? Any plausible ways to have neutralized particle beams don't diverge for a long time?

You cannot reflect a particle beam with >99.99% efficiency, not even with 10% efficiency. The sail would evaporate.

 

[GTOM]

You cannot reflect a particle beam with >99.99% efficiency, not even with 10% efficiency. The sail would evaporate.

How about harnessing the particle beams power with a magnetic field instead of a sail?

 

[mfb]

Any plausible ideas how to generate a strong magnetic field with a gram-sized spacecraft ?

~30 microrad beam divergence has been demonstrated at the LHC, although that is not optimized for that parameter. That is still orders of magnitude more than an optical beam. And you cannot shoot it through the atmosphere.

 

[GTOM]

~30 microrad beam divergence has been demonstrated at the LHC, although that is not optimized for that parameter. That is still orders of magnitude more than an optical beam. And you cannot shoot it through the atmosphere.

Is it an all same charge, or a neutralized beam?

 

[sophiecentaur]

Why? (not Earth based)
Breakthrough Starshot plans ground-based lasers, for example.

If the source is on Earth, you have only a very few hours per day to fire it in the direction you want. Half the time, it's the other side of the Earth and low elevation beams are very much disturbed by the atmosphere. That would have the effect of reducing the propulsive power by a factor of around 25%, maximum. Also, the Earth is a very wobbly platform to operate from. Initial development and proof of concept work would make sense on Earth - and the Starshot project is described as such. When you get down to it, it's swings and roundabouts and the optimum choice can only be made after a lot of work. A space platform would be a major investment and that could only be justified way down the line. Atmospheric absorption will always be a factor in any link budget and you can't avoid clouds in most parts of the Earth.
Has anyone considered the problem of backscatter from a high power Earthbound laser source? I would have to be significant. Passing satellites and planes would also be pretty vulnerable if this beam were turned on continuously. I'm sold on an eventual location in Space.

 

[mfb]

Is it an all same charge, or a neutralized beam?

It is a charged beam, but neutralizing it will at best stop a further increase in divergence, it won't reduce the existing divergence.

If the source is on Earth, you have only a very few hours per day to fire it in the direction you want.

That is more than sufficient with a launch process shorter than an hour. You can't launch that many probes into the same direction, but I expect power consumption to limit that number anyway. If it is not, just launch probes in different directions to different stars. The line of sight requirement does not reduce the power to launch a spacecraft because the lasers can be close together.

There are places with a very low probability of clouds. If the lasers are powerful enough, you might even be able to evaporate the water droplets in the clouds.

Has anyone considered the problem of backscatter from a high power Earthbound laser source?

100 GW over millions of square kilometers? Completely irrelevant.
You don't want a satellite to fly through the beam, sure, but the beams are narrow. It would rarely be an issue, and you can switch off a laser for a second if necessary.

 

[sophiecentaur]

100 GW over millions of square kilometers? Completely irrelevant.

I was thinking that the 'spot size' through the atmosphere would be small. Coming back from a single cloud, that spot would surely appear pretty bright but at least the reflection wouldn't be specular.
Edit - but not all that small, I guess as it's still converging.

 

[mfb]

Well, let's assume we build OWL and use it in reverse with a 1 GW laser. That is an area density of 150 times the solar irradiation, with a nearly constant cross section across the whole atmosphere. It will burn every bird flying directly through the beam - but the beam will be visible, birds can avoid them.
If you scatter 10% of that at 3 km height uniformly in all directions, that leads to a peak ground irradiation of 0.9 W/m². Clearly visible, but nothing compared to sunlight.

 

[sophiecentaur]

I was reading more about this star shot project. Something I don't understand is what sort of link budget is working for getting a useful signal back from the remote star from a 'nano-sized' transmitter. The (reversed) receiving dish would have loads of gain but power circuits need to be a reasonable size in order to produce significant transmit power. Power dissipation doesn't scale and circuits producing tens (hundreds / millions) of Watts need to be pretty big or they have to operate at impractical temperatures. I know you can use narrower bandwidths to make up for this but eventually you can't carry any worthwhile amount of data . I could only find journalist-type articles on the project and they were very thin on technical details.

 

[mfb]

There is a technical article somewhere, but it is hard to find.

The data rate would be extremely low, but a single bit once in a while can be sufficient to get a picture over time.

 

[sophiecentaur]

over time.

For a hi res picture, you could end up having to wait as long as the actual journey took. But that would be planned and a decision made as to whether it's worth it.
There will be a finite beam width achievable and it strikes me that a vehicle with a sail that's of a comparable size, you get the best value. Why would they use such small craft, rather than a craft that's thousands of metres across?

 

[mfb]

A larger spacecraft will need more powerful lasers, and it is harder to align the spacecraft properly. Sending more smaller spacecraft is easier.

Even with a single bit per hour you get an interesting picture over 1 year (10,000 hours).

 

[sophiecentaur]

Sending more smaller spacecraft is easier.

Why? The same power is needed for one big one or many small ones (it's total mass in the end.)
If the many small craft could get close together at the end, they could make a larger transmitting system.

Even with a single bit per hour you get an interesting picture over 1 year (10,000 hours).

There we are then. To get a number of worthwhile images would take something like the total journey time. I think your 1bit per hour was a fair enough guess.

 

[mfb]

Why? The same power is needed for one big one or many small ones (it's total mass in the end.)

You can distribute the energy over a longer time, which means you need a lower peak power: A smaller amount of lasers and telescopes.

There we are then. To get a number of worthwhile images would take something like the total journey time. I think your 1bit per hour was a fair enough guess.

1 year is significantly shorter than the journey time.

 

[sophiecentaur]

1 year is significantly shorter than the journey time.

But you would want several images, wouldn't you?

You can distribute the energy over a longer time, which means you need a lower peak power: A smaller amount of lasers and telescopes.

When you put it that way, it's true but where is the problem with operating the beam for many days? (Particularly if the beam is produced out in space.) It strikes me that they want to do it that way and are making a virtue out of necessity.

 

[schplade]

Aiming telescopes is necessary both in space and on the ground, and it is easier on the ground (where you have a solid surface as reference). It is not an issue at all, every telescope does that routinely for operation. Forget spacecraft steering - the telescope has to aim. You need several laser "telescopes" anyway: Even a magic spacecraft with perfect steering from nowhere would still need multiple telescope beams that aim at the same spot.

Obviously you would need to aim the laser at the spacecraft to get things started, but from that point forward, couldn't the spacecraft make adjustments of its own to stay on the beam? It should be relatively easy for the spacecraft to detect when it's moving off the center of the beam. Perhaps it could use thrusters, or even adjust the sail somehow, to stay centered? It would need to make corrections fast, for sure. I'm not saying it would be easy by any means, but wafer-sized space probes present massive challenges too, especially when it comes to communication.

 

[GTOM]

Thanks for your information mfb.

Otherwise I'm skeptical about that grams weight probes idea. How their delicate electronics supposed to withstand cosmic rays?

A GW laser focused by 100m Overly Large Telescope. Any price estimation for a GW laser? I guess it has to produced by a really large facility.

 

[sophiecentaur]

Obviously you would need to aim the laser at the spacecraft to get things started, but from that point forward, couldn't the spacecraft make adjustments of its own to stay on the beam?

This worries me a lot. If the craft goes off course (due to some asymmetry in its construction or even just some dirt on one side, firstly it would be hard / impossible to see the tiny craft and several years(?) delay in the control loop would mean that the beam couldn't correct for it. Could you really work 'open loop' for twenty years?

You can distribute the energy over a longer time, which means you need a lower peak power: A smaller amount of lasers and telescopes.

I think I must be working with a model in my head that's not what's been published but the journalese has probably got in the way of clarity.
Is it assumed that the whole of the beam power would hit one tiny craft? How long would the beam be turned on for? You seemed, at one point, to suggest that the boost would be given in just one day. Radiation pressure needs to act for a long time to transfer any significant momentum which means months / years worth of drive.

 

[mfb]

But you would want several images, wouldn't you?

The 1 bit/hour was a random number, don't treat it like a proper estimate. It will depend on the spacecraft design, the available receivers and so on.

When you put it that way, it's true but where is the problem with operating the beam for many days? (Particularly if the beam is produced out in space.) It strikes me that they want to do it that way and are making a virtue out of necessity.

Acceleration goes down with increasing distance of the spacecraft as your beam spreads out over time. A longer, slower acceleration leads to a lower final velocity. Even if you can point the beam at the spacecraft for days (which would need space-based lasers, or way too many ground-based ones), it is not a practical solution to achieve high speeds. It would also reduce the launch rate a lot.

Obviously you would need to aim the laser at the spacecraft to get things started, but from that point forward, couldn't the spacecraft make adjustments of its own to stay on the beam? It should be relatively easy for the spacecraft to detect when it's moving off the center of the beam. Perhaps it could use thrusters, or even adjust the sail somehow, to stay centered? It would need to make corrections fast, for sure. I'm not saying it would be easy by any means, but wafer-sized space probes present massive challenges too, especially when it comes to communication.

The smallest current systems that can stabilize itself with thrusters are of the order of a kilogram. A gram-sized spacecraft , where most of the mass is the sail? No way.
It is trivial to adjust the direction of telescopes. It has to be done anyway. There is no point in adding complexity to the spacecraft that doesn't save anything elsewhere.

How their delicate electronics supposed to withstand cosmic rays?

There are electronic components with a very high radiation tolerance (>1 gigarad). Something that can survive the conditions in the LHC detectors can also survive interstellar space. It won't survive collisions with dust particles, but those are rare, and many probes can be sent to have some of them surviving.

A GW laser focused by 100m Overly Large Telescope. Any price estimation for a GW laser? I guess it has to produced by a really large facility.

We don't have such a laser yet, hard to estimate.

This worries me a lot. If the craft goes off course (due to some asymmetry in its construction or even just some dirt on one side, firstly it would be hard / impossible to see the tiny craft and several years(?) delay in the control loop would mean that the beam couldn't correct for it. Could you really work 'open loop' for twenty years?

You can easily see it during the acceleration phase (you illuminate it with a very bright laser...) and steer it with the beam, afterwards there is no way to steer it - it will just fly in a straight line.

I think I must be working with a model in my head that's not what's been published but the journalese has probably got in the way of clarity.
Is it assumed that the whole of the beam power would hit one tiny craft? How long would the beam be turned on for? You seemed, at one point, to suggest that the boost would be given in just one day. Radiation pressure needs to act for a long time to transfer any significant momentum which means months / years worth of drive.

The Breakthrough program uses 20 minutes of acceleration time as baseline. You cannot drive it for months or even years, there is no way to give it any relevant acceleration over the corresponding large distances. OWL can keep a beam nicely focused over 20 million kilometers, after that the intensity drops with 1/r². 20 minutes acceleration to 0.2 c lead to a final distance of 36 million km - that works. 1 day of acceleration would lead to a final distance of hundreds of millions of kilometers - that does not work. Months, years? No way.

 

[schplade]

The smallest current systems that can stabilize itself with thrusters are of the order of a kilogram. A gram-sized spacecraft , where most of the mass is the sail? No way.
It is trivial to adjust the direction of telescopes. It has to be done anyway. There is no point in adding complexity to the spacecraft that doesn't save anything elsewhere.

You're still thinking in terms of wafer-sized spacecraft . I was thinking of something that's more comparable in mass to our current space probes, pushed by an extremely powerful laser. Something that would take a long time to accelerate. We obviously can't aim a laser to hit a departing object that's several light-minutes away, unless we can use an amazingly accurate predictive algorithm. So I'm wondering if it's possible to build a sailcraft that could make its own adjustments and keep itself centered on a beam, riding that beam deep into space. If not, then blasting gram-sized spacecraft up to their top speed at 50,000 G's is probably the best we can ever do.

 

[sophiecentaur]

You can easily see it during the acceleration phase (you illuminate it with a very bright laser...) and steer it with the beam, afterwards there is no way to steer it - it will just fly in a straight line.

OK, that's clear now. But, unless there's some restriction on running time of the lasers, why not use them for a lot longer (even in daily doses) and give the same impulse to a much bigger craft? It would not be hard to pick up where you left off overnight. What a waste of a massive light source, if it only operates for 20 minutes per vehicle. Is there another possible use for it when it's not being used for propulsion?
I haven't grasped the advantage of using small craft rather than something 100 (10,000) times bigger. It sounds like standing up in a hammock to me.

 

[mfb]

You're still thinking in terms of wafer-sized spacecraft . I was thinking of something that's more comparable in mass to our current space probes, pushed by an extremely powerful laser. Something that would take a long time to accelerate. We obviously can't aim a laser to hit a departing object that's several light-minutes away, unless we can use an amazingly accurate predictive algorithm. So I'm wondering if it's possible to build a sailcraft that could make its own adjustments and keep itself centered on a beam, riding that beam deep into space. If not, then blasting gram-sized spacecraft up to their top speed at 50,000 G's is probably the best we can ever do.

We can easily aim at it. Aiming does not get notably harder with distance, as you are diffraction-limited anyway: The necessary angular accuracy does not increase. And it is within the capabilities of current telescopes.

OK, that's clear now. But, unless there's some restriction on running time of the lasers, why not use them for a lot longer (even in daily doses) and give the same impulse to a much bigger craft? It would not be hard to pick up where you left off overnight. What a waste of a massive light source, if it only operates for 20 minutes per vehicle. Is there another possible use for it when it's not being used for propulsion?

See the previous post. At the end of the 20 minutes the efficiency is down to ~1/4 the maximal value already, after an hour it would be less than 1%, after 10 hours it is below 0.01%. Yes you can keep shooting the laser at it, but the effect becomes completely negligible.

In terms of using the lasers: The project proposes to send thousands of small spacecraft , that takes a good fraction of a year, and you can target more than one star. You can burn space debris with the lasers. And you can use the telescopes as telescopes.

 

[GTOM]

We can easily aim at it. Aiming does not get notably harder with distance, as you are diffraction-limited anyway: The necessary angular accuracy does not increase. And it is within the capabilities of current telescopes.See the previous post. At the end of the 20 minutes the efficiency is down to ~1/4 the maximal value already, after an hour it would be less than 1%, after 10 hours it is below 0.01%. Yes you can keep shooting the laser at it, but the effect becomes completely negligible.

Sophiecentaur talked about a much bigger craft, could that laser be much more useful if pushing a ship with a km sail?
(It could deliver cargo inside solar system, or eject lots of tiny probes, and hope they withstand conditions better than the ones sent to Fukushima.)

 

[mfb]

A 1 square kilometer sail will keep the full efficiency longer - something like 200 million kilometers. At the same target velocity you can decrease acceleration by a factor 10 and increase time by the same factor.

Delivering cargo within the solar system is much easier as the velocities are orders of magnitude lower. You could probably just use sunlight with such a light sail.

 

[sophiecentaur]

At the end of the 20 minutes the efficiency is down to ~1/4 the maximal value already,

That will be because of spillage, I guess. The area of the nano sails will be a small part of the beam area so why not actually use an area that's equal to the beam area at the start (or even further out along the beam?). I seems that this project all hangs on an explosive (almost) start to the trip with nano sails, which is not what's usually expected with light sails. It is a bit hard to get rid of preconceived ideas about this method of propulsion. The acceleration involved must be phenomenal (around 10⁵ms^-2) to get to a speed approaching light speed in just 1200s. The whole idea seems to require some seriously 'adventurous' concepts, which are highly counter intuitive.

 

[sophiecentaur]

A 1 square kilometer sail will keep the full efficiency longer - something like 200 million kilometers. At the same target velocity you can decrease acceleration by a factor 10 and increase time by the same factor.

. . . . and increase the payload to something that approaches a credible value.

 

[mfb]

You need the high acceleration. If we fix the target speed and don't want to build telescopes beyond 100 meters in diameter (and tricky adaptive optics), the main acceleration process cannot take longer than 20 minutes. You can keep shooting the laser at it later, but it won't change the overall speed notably. Could be interesting for steering, but for the discussion here we can neglect it.

A smaller sail leads to a higher acceleration. The 1/4 is actually not very accurate, because it assumes a 100 meter beam diameter until the envelope has to increase. We can start with a much smaller cross section and therefore a higher acceleration value. But that only works with a smaller sail.

. . . . and increase the payload to something that approaches a credible value.

Only if you increase beam power by a similar factor.

 

[mheslep]

...What a waste of a massive light source, if it only operates for 20 minutes per vehicle. Is there another possible use for it when it's not being used for propulsion?

A similar comment could be made about the current chemical boosters and their mega-Newton thrust. The beam would necessarily be highly inefficient. I don't think it would valuable to use for anything except a launch (assuming viability, and I don't see a path to viability)

 

[GTOM]

A similar comment could be made about the current chemical boosters and their mega-Newton thrust. The beam would necessarily be highly inefficient. I don't think it would valuable to use for anything except a launch (assuming viability, and I don't see a path to viability)

It might could be used to power a lunar colony, without take a nuclear reactor there.