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Post by shiolle on Jun 23, 2017 14:07:47 GMT
99.9% Reflectivity is more than sufficient for 1000x bounces. Not even close to sufficient. There is no magic here, wherever you get your light, either directly from the laser or by bouncing it between mirrors, the irradiance of the sail is the same. Your configuration requires 19.6 kN of thrust, thus at 150MW per newton that means it receives 2.94 TW of laser energy. Bouncing only allows you to have less powerful laser source. 1 percent of that power is still 29.4GW per square meter. Even if you increase your area to 10 m 2, you still have 2.94 GW/m 2. So no, you need a few orders of magnitude better mirror. To drop it to 50MW/m 2, your mirror should be 58840 m 2. 1) Photon pressure = 20kPA, the yield strength of the sail is over 100 GPa, an difference of over 6 magnitudes. Now about forces. Your material is strong against shearing, but not at all rigid. Let's suppose that your sail's corner (0.1 of its total area) has 1 degree inclination to the rest of the sail. It will produce 34 newtons of thrust less than the opposite corner, which is enough to flex the sail even more increasing the torque further.
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Post by Kerr on Jun 23, 2017 14:21:19 GMT
99.9% Reflectivity is more than sufficient for 1000x bounces. Not even close to sufficient. There is no magic here, wherever you get your light, either directly from the laser or by bouncing it between mirrors, the irradiance of the sail is the same. Your configuration requires 19.6 kN of thrust, thus at 150MW per newton that means it receives 2.94 TW of laser energy. Bouncing only allows you to have less powerful laser source. 1 percent of that power is still 29.4GW per square meter. Even if you increase your area to 10 m 2, you still have 2.94 GW/m 2. So no, you need a few orders of magnitude better mirror. To drop it to 50MW/m 2, your mirror should be 58840 m 2. 1) Photon pressure = 20kPA, the yield strength of the sail is over 100 GPa, an difference of over 6 magnitudes. Now about forces. Your material is strong against shearing, but not at all rigid. Let's suppose that your sail's corner (0.1 of its total area) has 1 degree inclination to the rest of the sail. It will produce 34 newtons of thrust less than the opposite corner, which is enough to flex the sail even more increasing the torque further. 1. See my latest reply. 2. Those sails need extreme perfection, we already build a sphere with only 0.3nm roughness difference. It requires a lot of effort and time to create such an smooth surface, but it might be probably way easier several decades from now. Or for example the mirror of an space telescope, both need nearly perfect surfaces. Same goes for sails.
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Post by shiolle on Jun 23, 2017 14:43:40 GMT
Yes, you have posted it while I was writing mine. 2. Those sails need extreme perfection, we already build a sphere with only 0.3nm roughness difference. It requires a lot of effort and time to create such an smooth surface, but it might be probably way easier several decades from now. Or for example the mirror of an space telescope, both need nearly perfect surfaces. Same goes for sails. In other worlds, this technology with the specs you provided is firmly in the realm of purely theoretical, but there are no fundamental laws of nature that precludes it. There are many useful things you can do with near-absolute mirror though and I don't mean anti-laser armor. For example, with it you instantly solve the problem of huge radiators on fusion and antimatter-driven ships. And with it you get into the realm where there are even better weapons you can construct with this technology, like relativistic projectiles that use little fuel to get up to speed.
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Post by leerooooooy on Jun 23, 2017 14:44:42 GMT
I don't think you can create more thrust by bouncing the laser
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Post by Kerr on Jun 23, 2017 14:47:47 GMT
I don't think you can create more thrust by bouncing the laser It was already demonstrated to work. By NASA Innovative Advanced Concepts
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Post by Kerr on Jun 23, 2017 14:54:47 GMT
Yes, you have posted it while I was writing mine. 2. Those sails need extreme perfection, we already build a sphere with only 0.3nm roughness difference. It requires a lot of effort and time to create such an smooth surface, but it might be probably way easier several decades from now. Or for example the mirror of an space telescope, both need nearly perfect surfaces. Same goes for sails. In other worlds, this technology with the specs you provided is firmly in the realm of purely theoretical, but there are no fundamental laws of nature that precludes it. There are many useful things you can do with near-absolute mirror though and I don't mean anti-laser armor. For example, with it you instantly solve the problem of huge radiators on fusion and antimatter-driven ships. And with it you get into the realm where there are even better weapons you can construct with this technology, like relativistic projectiles that use little fuel to get up to speed. How is it "firmly in the realm of purely theoretical" ? With Amplification factors that were demonstrated in reality, and existing Dielectric mirrors with two magnitudes better than needed reflectivity, and an sail quality equal to the hubble mirror which was produced in the early nineties? I also have alternative specs which are scaled-up from Breakthrough Starshot and the 2015 NIAC PLT.
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Post by shiolle on Jun 23, 2017 15:44:05 GMT
How is it "firmly in the realm of purely theoretical" ? With Amplification factors that were demonstrated in reality, and existing Dielectric mirrors with two magnitudes better than needed reflectivity, and an sail quality equal to the hubble mirror which was produced in the early nineties? I also have alternative specs which are scaled-up from Breakthrough Starshot and the 2015 NIAC PLT. The same way being able to build a chemical rocket does not mean you can accelerate it to 99.99999% of the speed of light. Theoretically, you can. In practice it is unlikely. Or like if you can swim, that doesn't mean you can cross the Pacific without a ship. Breakthrough Starshot is not something that can be built tomorrow. It requires a lot of technological improvements, and yet its specifications are nothing like the system you were proposing.
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Post by The Astronomer on Jun 23, 2017 15:55:53 GMT
How is it "firmly in the realm of purely theoretical" ? With Amplification factors that were demonstrated in reality, and existing Dielectric mirrors with two magnitudes better than needed reflectivity, and an sail quality equal to the hubble mirror which was produced in the early nineties? I also have alternative specs which are scaled-up from Breakthrough Starshot and the 2015 NIAC PLT. The same way being able to build a chemical rocket does not mean you can accelerate it to 99.99999% of the speed of light. Theoretically, you can. In practice it is unlikely. Or like if you can swim, that doesn't mean you can cross the Pacific without a ship. Breakthrough Starshot is not something that can be built tomorrow. It requires a lot of technological improvements, and yet its specifications are nothing like the system you were proposing. How about 20 years' time, the proposed time range for such mission? Anyways, we're talking about hundred years' worth of technological advancement time. You've got a lot of points, but one thing we all might agree on is that the technological advancement is not something that can be easily predicted. (About the CDE setting: I assumed that the setting used present-day technologies. Factions were likely to suffered from the loss of Earth and all the information it contained.)
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Post by Kerr on Jun 23, 2017 16:02:44 GMT
How is it "firmly in the realm of purely theoretical" ? With Amplification factors that were demonstrated in reality, and existing Dielectric mirrors with two magnitudes better than needed reflectivity, and an sail quality equal to the hubble mirror which was produced in the early nineties? I also have alternative specs which are scaled-up from Breakthrough Starshot and the 2015 NIAC PLT. The same way being able to build a chemical rocket does not mean you can accelerate it to 99.99999% of the speed of light. Theoretically, you can. In practice it is unlikely. Or like if you can swim, that doesn't mean you can cross the Pacific without a ship. Breakthrough Starshot is not something that can be built tomorrow. It requires a lot of technological improvements, and yet its specifications are nothing like the system you were proposing. You can't scale up conventional rockets like a Photonic system. 1 newtons is one Newtons per x Watt. Breakthrough Starshot includes electronics. Which makes the whole thing harder to build. On the other hand my system is just a basic sail.
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Post by shiolle on Jun 23, 2017 16:08:27 GMT
How about 20 years' time, the proposed time range for such mission? Anyways, we're talking about hundred years' worth of technological advancement time. You've got a lot of points, but one thing we all might agree on is that the technological advancement is not something that can be easily predicted. (About the CDE setting: I assumed that the setting used present-day technologies. Factions were likely to suffered from the loss of Earth and all the information it contained.) That's exactly what I am getting at. I'm not arguing that project starshot is impossible. I am arguing that while the sum of our knowledge is increasing, and the rate at which we gain it is increasing, individual technologies usually do not progress indefinitely. They get invented, undergo explosive growth and then climb to some asymptote where they remain. You gen an S-curve. To follow my analogy, we knew how to swim for millions of years, yet we still can't swim across the Pacific. Instead we invented ships for that. Classic seagoing vessels are too a good example of that. By now, no one expect that some time in the future regular ships will move 100 times as fast as they do today. Instead, we try something new: wing-in-ground effect vehicles, hovercraft, hydrofoil, etc. You can't scale up conventional rockets like a Photonic system. 1 newtons is one Newtons per x Watt. Breakthrough Starshot includes electronics. Which makes the whole thing harder to build. On the other hand my system is just a basic sail. How do you measure scalability of that technology? Rockets are limited by the amount of chemical energy in their fuel. This system is limited by the material of its sail, among many other things. Electronics on-board is not what we've been arguing about, was it?
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Post by Kerr on Jun 23, 2017 16:17:56 GMT
How about 20 years' time, the proposed time range for such mission? Anyways, we're talking about hundred years' worth of technological advancement time. You've got a lot of points, but one thing we all might agree on is that the technological advancement is not something that can be easily predicted. (About the CDE setting: I assumed that the setting used present-day technologies. Factions were likely to suffered from the loss of Earth and all the information it contained.) That's exactly what I am getting at. I'm not arguing that project starshot is impossible. I am arguing that while the sum of our knowledge is increasing, and the rate at which we gain it is increasing, individual technologies usually do not progress indefinitely. They get invented, undergo explosive growth and then climb to some asymptote where they remain. You gen an S-curve. To follow my analogy, we knew how to swim for millions of years, yet we still can't swim across the Pacific. Instead we invented ships for that. Classic seagoing vessels are too a good example of that. By now, no one expect that some time in the future regular ships will move 100 times as fast as they do today. Instead, we try something new: wing-in-ground effect vehicles, hovercraft, hydrofoil, etc. As you said, invented, and undergo explosive growth. Laser sails didn't undergone the explosive growth as many other system did, it might or might not come in the future. How does this analogy compares to a Laser sail? Ships are limited by the drag of the water, planes by atmospheric drag, and rockets in vacuum are limited by the tyranny of the rocket equation. If a Laser sail can survive x amount of light per square meter we can scale it up, and the big difference between it and rockets, or your ocean analogy is that Laser sails scale linear with external laser power.
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Post by shiolle on Jun 23, 2017 16:24:25 GMT
If a Laser sail can survive x amount of light per square meter we can scale it up, and the big difference between it and rockets, or your ocean analogy is that Laser sails scale linear with external laser power. Then it won't be 1 gram, and thus won't have the acceleration you specified, and will take a lot longer to reach its target. Remember, we were arguing about a system with very specific parameters. Also, remember about unintended consequences. Technology that enables the kind of system you specified also basically unties many, many technologies from thermodynamics.
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Post by Kerr on Jun 23, 2017 16:35:25 GMT
How about 20 years' time, the proposed time range for such mission? Anyways, we're talking about hundred years' worth of technological advancement time. You've got a lot of points, but one thing we all might agree on is that the technological advancement is not something that can be easily predicted. (About the CDE setting: I assumed that the setting used present-day technologies. Factions were likely to suffered from the loss of Earth and all the information it contained.) That's exactly what I am getting at. I'm not arguing that project starshot is impossible. I am arguing that while the sum of our knowledge is increasing, and the rate at which we gain it is increasing, individual technologies usually do not progress indefinitely. They get invented, undergo explosive growth and then climb to some asymptote where they remain. You gen an S-curve. To follow my analogy, we knew how to swim for millions of years, yet we still can't swim across the Pacific. Instead we invented ships for that. Classic seagoing vessels are too a good example of that. By now, no one expect that some time in the future regular ships will move 100 times as fast as they do today. Instead, we try something new: wing-in-ground effect vehicles, hovercraft, hydrofoil, etc. You can't scale up conventional rockets like a Photonic system. 1 newtons is one Newtons per x Watt. Breakthrough Starshot includes electronics. Which makes the whole thing harder to build. On the other hand my system is just a basic sail. How do you measure scalability of that technology? Rockets are limited by the amount of chemical energy in their fuel. This system is limited by the material of its sail, among many other things. Electronics on-board is not what we've been arguing about, was it? Measure scalability of that technology? If 500W produces 500KW Beam power , then 1KW produces 1MW of beam power. To achieve 100GW beam power just use 200,000 PLT (2015) to get 100GW Beam power. 100GW Beam power always produces 667N when reflected. Breakthrough starshot is propelled by an 100GW beam. Same beam power. At 667N/1g I get 667km/s acceleration. Including relativistic effect this sail will reach 2582km/s after 8s. Again for Chemical rockets. If you want more delta-v for a rocket you need more fuel, more fuel makes the ship heavier, you need even more fuel to compensate and so on. The laser sail get it's thrust from external sources, that means the only thing that determines the Dv of the sail at that point is, how long it accelerates. As the Electronics on-board. You said it can't be done tomorrow. That counts for both the sail and the electronics (StarChip). The two things that prevent Starshot from being build are the following. 1) Money 2) Producing multiply square meter of the sail material, production challenge. 3) StarChip. What sounds harder to make? An Mirror made out of wonder-materials which can already be build in small scale and is only a matter on funding funding and patience or an 2MP camera with a laser thruster and a antenna with over 4 light years range and a 150mg RTG. All of that in a fraction of a gram.
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Post by Kerr on Jun 23, 2017 16:38:04 GMT
If a Laser sail can survive x amount of light per square meter we can scale it up, and the big difference between it and rockets, or your ocean analogy is that Laser sails scale linear with external laser power. Then it won't be 1 gram, and thus won't have the acceleration you specified, and will take a lot longer to reach its target. Remember, we were arguing about a system with very specific parameters. Also, remember about unintended consequences. Technology that enables the kind of system you specified also basically unties many, many technologies from thermodynamics. If 16m² can provide 667. Then 32m² can prodive 1.334KN. The acceleration stays the same, because it scales linearly with beam power. Twice beam power (twice thrust) needs twice as much sail mass. What do you mean with "unties many technologies from thermodynamics"?
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Post by matterbeam on Jun 23, 2017 17:30:34 GMT
Guys, let's work out an example, using equations and data from this discussion on laser sails: toughsf.blogspot.com/2017/04/interstellar-trade-is-possible-part-ii.htmlLet's use a 10MW laser firing far-infrared light (5 um). We will use Starwisp figures of 100kg/km^2 and 99.9% reflectivity, so a 2m wide sail will mass 0.31 grams. Let's make this military grade and all LCD reflector/absorbant strips that can steer the lightsail, so a 1 gram lightsail 2m wide can be made. Normally, a single bounce laser sail reflective 99.9% of 10MW produces 0.13N of thrust and would accelerate 1 gram at 133m/s^2. After 10 seconds, the laser sail would reach 1.33km/s, after a minute, about 8km/s. It accelerates over a distance of 239.4km and delivers 32kJ to the target. This laser heats up the laser sail at a rate of 10kW. This leads to an equilibrium temperature of 486K over 3.14m^2 (one side reflective, one side black). We can push this temperature as high as 800K for aluminium, 2000K for carbon-based meshes. At 2000K, we can dissipate 2.84MW of waste heat. So, we can reflect 284MW and accelerate a 2m wide laser sail to 266km/s within a minute, to deliver 25MJ upon impact. Bouncing a beam ten times allows us to generate ten times more thrust per watt. So, a 284MW/99.9%/2m/1gram laser sail will reach 2266km/s in one minute and deliver 2.56GJ impacts. The LCD shutters would have to be replaced by actuators that expose to hide extra segments of mesh for manoeuvring due to the high temperatures, but you only need to dedicate 0.0026% of the incoming light to match the movements of a target accelerating at 10m/s^2. I do not, however, understand how bouncing light violates the conservation of energy.
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