Beam-powered propulsion - keeping the beam focused

[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.

 

[mheslep]

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

No, like chemical boosters, the beam system would be necessarily be designed to deliver very high bursts of power for a few minutes, not supply continuous power.

 

[GTOM]

No, like chemical boosters, the beam system would be necessarily be designed to deliver very high bursts of power for a few minutes, not supply continuous power.

Valid point. Still if it could power Earth-Moon vessels for example, than it could be frequently used.

 

[sophiecentaur]

A smaller sail leads to a higher acceleration.

I puzzled over this but I have to assume that you are assuming the proportions would need to be the same so area : mass = l²:l³
But why would that be necessary? You see, I cannot see how you can ever get over the spillage with a sail of just a few cm³ area. Surely a sail area that's comparable with the beam area has to get most momentum transfer ( = acceleration). The only reason that small sail = higher acceleration might be that mechanics don't scale as you'd think and small structures are stronger for a given shape. But the pressure over a large sail would / could be fairly uniform so stresses would still be low. This hasn't been justified in anything I have read.

 

[sophiecentaur]

A similar comment could be made about the current chemical boosters and their mega-Newton thrust.

A different comparison. The cost of a rocket is much less than a once in a lifetime telescope. The 30m telescope has a budget of £$1200M so bigger would be considerably more (third power law perhaps) and a rocket launch is upward of £$100M (figures trawled from all over) If the telescope were doing this job in its spare time then things would be different as it's highly re-usable.

 

[mfb]

I puzzled over this but I have to assume that you are assuming the proportions would need to be the same so area : mass = l²:l³
But why would that be necessary? You see, I cannot see how you can ever get over the spillage with a sail of just a few cm³ area. Surely a sail area that's comparable with the beam area has to get most momentum transfer ( = acceleration). The only reason that small sail = higher acceleration might be that mechanics don't scale as you'd think and small structures are stronger for a given shape. But the pressure over a large sail would / could be fairly uniform so stresses would still be low. This hasn't been justified in anything I have read.

A smaller sail means you can start with a more focused beam - a higher intensity. At the same mass per area ratio (=most of the mass is the sail), this leads to a higher acceleration.
You probably want the beam to be very uniform over the sail area to have a uniform acceleration. Which means the sail has to be smaller than the total beam width.

The cost of a rocket is much less than a once in a lifetime telescope.

It is not a once in a lifetime telescope if you use it for thousands of small spacecraft , and use it as regular telescope.

 

[schplade]

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.

I might be misunderstanding something still. Let's say the spacecraft is 5 light-minutes away from the telescope/laser. You might be able to see it, but you won't see it at its current location. You'll see where it was 5 minutes ago, right? You can't know with certainty what its current location is, so if you want to aim a laser at it, you have to predict where it will be with great accuracy. Maybe it's possible to do that; it just seems outrageously difficult.

 

[GTOM]

I might be misunderstanding something still. Let's say the spacecraft is 5 light-minutes away from the telescope/laser. You might be able to see it, but you won't see it at its current location. You'll see where it was 5 minutes ago, right? You can't know with certainty what its current location is, so if you want to aim a laser at it, you have to predict where it will be with great accuracy. Maybe it's possible to do that; it just seems outrageously difficult.

If it don't make fast random movements, aiming will be still pretty accurate.

 

[sophiecentaur]

It is not a once in a lifetime telescope if you use it for thousands of small spacecraft , and use it as regular telescope.

Which is why I asked, further up, about other uses for such a telescope. Unfortunately, the word "telescope" is not really accurate because the transmitter would not be a good receiving system without serious modifications. I can't imagine it could be operated with a simple TX/RX switch. The use as a cargo propellor to the Moon would make sense, though.
But I still can't see the advantage of using such small craft when the beam could not be focussed on just one sail. Why not, at least, fill the beam with a single sail? One single craft would be much more use than many smaller ones (there can be no argument about that, surely). I can understand how a short sharp burst of acceleration makes sense, due to beam spreading at a great distance but spillage around a small sail would mean a huge wastage of hard-won propulsive energy. The Numbers Game is very relevant here and I haven't seen any actual numbers to dispel my intuitive objections. The project could be based on "We want to do it this way", and it wouldn't be the first such project.
Comms would be needlessly complicated in a case like that. Each craft would need its own channel and a dedicated receiver because of the necessary low bandwidth and the very long duration of a useful signal.

 

[sophiecentaur]

If it don't make fast random movements, aiming will be still pretty accurate.

How can you be sure that there would be no perturbations from, as yet unknown, sources. Just because the star appears at a fixed place in the sky, doesn't tell you about mechanical forces that could be operating way out there and a very tiny disturbance (a large, undiscovered Kuyper Belt object coming close, for instance) could deflect the craft. Open loop (ballistic) operation is a very risky way to go, imho. We have absolutely no way of being sure about this.

 

[mfb]

@sophiecentaur: Please read the proposal of the Breakthrough Starshot project. You keep proposing things that are completely impractical, things discussed in detail (why they don't work) by the Breakthrough project.

doesn't tell you about mechanical forces that could be operating way out there and a very tiny disturbance (a large, undiscovered Kuyper Belt object coming close, for instance) could deflect the craft

The spacecraft is fast. A near miss at Pluto would alter the velocity by something like 5 cm/s, or 30,000 km over 20 years. Completely negligible.
The probability of such a near miss (or impact) for a random object at a random location in the sky at the distance of Pluto is less than 10^-14. It is even smaller for objects further out. Negligible as well.
You could check the numbers before you invent issues that do not exist.

I might be misunderstanding something still. Let's say the spacecraft is 5 light-minutes away from the telescope/laser. You might be able to see it, but you won't see it at its current location. You'll see where it was 5 minutes ago, right? You can't know with certainty what its current location is, so if you want to aim a laser at it, you have to predict where it will be with great accuracy. Maybe it's possible to do that; it just seems outrageously difficult.

At the time it is 5 light minutes away, the laser won't be focused well any more and the acceleration (if present at all) is negligible.
The spacecraft has to have some control over its orientation in order to transmit data back to Earth once it reaches its destination, but that has to work with regular sunlight only, not in the intense acceleration phase.

 

[sophiecentaur]

@sophiecentaur: Please read the proposal of the Breakthrough Starshot project. You keep proposing things that are completely impractical, things discussed in detail (why they don't work) by the Breakthrough project.

It might have been helpful to post a link to the document if you have found it already. Pretty much all I have found is Pop Science with generalisations. I have seen an animation which shows a big array of Hollywood / James Bond style microwave dishes, which rather put me off reading much further. There are numbers quoted, such as that the nano craft could get to Mars in 2m. What could it do when it got there, apart from to keep on going. Likewise for Pluto etc. That sort of popular blurb proves nothing either way but it worries me that the drivers of the project are happy for it to go out with the publicity material. I guess they want backers and don't expect many potential backers to have much of an idea about the practicalities.
Meanwhile, if you could let me have a link to the large amount of information you are able to draw on, I should be very grateful.

You could check the numbers

Likewise, I would be grateful for some basics for working that out without starting from scratch. Or you could just show your workings?

 

[mfb]

I found it when the announcement was new, and read it, but didn't save the link. I don't think it is my task to find it again.

I guess they want backers and don't expect many potential backers to have much of an idea about the practicalities.

They have enough funding from billionaires to study the concept in more detail.

Going to Mars in 2 minutes (?) won't work, not even light is that fast.

Likewise, I would be grateful for some basics for working that out without starting from scratch. Or you could just show your workings?

Standard orbital mechanics, with some rough approximations. Pluto has a surface gravitational acceleration of 0.6 m/s² and a radius of 1200 km. The spacecraft will fly in a nearly straight line. As conservative estimate (leading to a larger deflection), assume the surface gravitational acceleration acts as perpendicular force over 5000 km of the spacecraft trajectory. At 20% the speed of light, or 60,000 km/s, that gives an acceleration over 1/12 s, for a speed change of 5 cm/s. Over 20 years this leads to a deflection of 30000 km.

The fraction of solid angle Pluto occupies in the sky is pi (radius of pluto)^2 / (4 pi (distance to pluto)^2).

Basic high school physics.

 

[sophiecentaur]

I don't think it is my task to find it again.

You told me I should read it so it is the least you could do for me.

Going to Mars in 2 minutes (?) won't work, not even light is that fast.

You are right - but I got it from one of those easily losable Google hits about eat topic.

for a speed change of 5 cm/s. Over 20 years this leads to a deflection of 30000 km.

Thanks.
I was also considering gas or dust clouds but I guess, if the star group is easily visible, there can't be much in the way.

 

[GTOM]

Too bad i can't found the comments of a phys.org article, but there was a discussion about the focusation problems of high-powered lasers, due to amplification process (multi mode lasers) or heat stress of lenses/mirrors or something like that.
Although in that case the laser equipment dotn have to be compact unlike a militaty application, so there can be a forest of single mode low powered lasers.