In the order that they appear in the paper, these are a few of the parts that seemed iffy to me. Some of them may be easily shown to be either definitely iffy, or definitely not-so-iffy, with a little more research:
As for nuclear fusion, the standard fusion reaction is 3H +2H→4He +n+ 17.59 MeV. In MeV, the masses of deuterium and tritium are 1876 and 2809, giving an η of 17.59/(1876 + 2809) = 0.00375. We will take this η to be the correct value,because though no fusion reactor is likely to be perfectly efficient, there is also the possibility of getting extra energy from the further fusion of helium and possibly heavier elements.
I’m not sure what existed at the time the paper was written, but there are now proposals for fusion rockets, and using the expected exhaust velocities from those might be better than using the theoretical value from DT fusion.
The overall efficiency of the solar captors is 1⁄3, by the time the solar energy is concentrated, transformed and beamed back to Mercury.
I feel like I’m the only one that thinks this Dyson sphere method is a little dubious. What system is going to be used to collect energy using the captors and send it to Mercury? How will it be received on Mercury? The total power collected toward the end is more than 1024 W. If whatever process is used to disassemble the planet is 90% efficient, the temperature required to radiate the waste heat over Mercury’s surface area is about 7000K. This is hotter than the surface of the sun, and more than twice the boiling point of both iron and silica. In order to keep this temperature below the boiling point of silica, we would either need the process to be better than 99.98% efficient, to attach Mercury to a heat sink may times the size of Jupiter, or to limit power to about 1020 W. If melting the planet isn’t our style, we need to limit power to about 1019 W.
I don’t think this kills their overall picture. It “only” means the whole process takes a few orders of magnitude longer.
Of the energy available, 1⁄10 will be used to propel material into space(using mass-drivers for instance [37]), the rest going to breaking chemical bonds, reprocessing material, or just lost to inefficiency. Lifting a kilo of matter to escape velocity on Mercury requires about nine mega-joules, while chemical bonds have energy less that one mega-joule per mol. These numbers are comparable, considering that reprocessing the material will be more efficient than simply breaking all the bonds and discarding the energy.
The probes will need stored energy and reaction mass to get into the appropriate orbit, unless all the desired orbits intersect Mercury’s orbit. Maybe this issue can be mitigated by gradually pushing Mercury into new orbits via reaction force from the probes. Or maybe it’s just not much of a limitation. I’m not sure.
Because practical efficiency never reaches the theoretical limit, we’ll content ourselves with assuming that the launch system has an efficiency of at least 50%
This seems pretty optimistic. In particular, making a system that launches large objects at .5c. Doing this over the distance from the sun to Earth requires an average force of about 105 N per kg. For .9c and .99c, it requires about 8× and about 35× this force/mass, respectively. I don’t know what the limiting factor will be on these things, but this seems pretty high, and suggests that the launcher would need to be a huge structure, and possibly a bigger project than the Dyson swarm.
I also have some complaints about the notation, which I will post later, and possibly other things, but this is what I have for now.
I feel like I’m the only one that thinks this Dyson sphere method is a little dubious. What system is going to be used to collect energy using the captors and send it to Mercury? How will it be received on Mercury? The total power collected toward the end is more than 1024 W. If whatever process is used to disassemble the planet is 90% efficient, the temperature required to radiate the waste heat over Mercury’s surface area is about 7000K. This is hotter than the surface of the sun, and more than twice the boiling point of both iron and silica. In order to keep this temperature below the boiling point of silica, we would either need the process to be better than 99.98% efficient, to attach Mercury to a heat sink may times the size of Jupiter, or to limit power to about 1020 W. If melting the planet isn’t our style, we need to limit power to about 1019 W.
Doesn’t it mean that the last stage of the process takes a few orders of magnitude longer, not the whole process? The process consists of a series of N doublings. For all but the last… 7 or so doublings, the waste heat can be dumped into Mercury itself. Only for those last doublings does waste heat start to overheat Mercury. Right? So it’s just that those last 7 doublings or so need to slow down (or rather, stop growing exponentially)
More interesting proposal: Maybe actually a better strategy would be to just deliberately overheat Mercury at that point, turn it into an expanding cloud of superhot gaseous material, and then scoop up said material as it cools down? Not sure if that’s possible, maybe the cloud wouldn’t expand enough.
Another interesting proposal: Smash Mercury to bits by colliding other objects like Ceres into it.
Another, more practical proposal: Once Mercury is saturated (can’t double any more without overheating it) switch to mining those cold asteroids out in the asteroid belt, with your giant fleet of spaceships you’ve built with the infrastructure that blankets Mercury and orbits the Sun. The asteroids are all spread out, so it’s easier for them to radiate heat… right?
If growth is fast until the last 7 doublings, then things are fast until you get to ~1% of your final output. That’s still a ton of power, at least by our current standards. My guess is you don’t bother doing a dyson swarm until you’ve built a lot of infrastructure in more convenient places (e.g. somewhere closer to Earth’s orbit around the sun). But I dunno, it really depends on what the technology is like and how bottlenecked on energy you are.
Breaking apart Mercury is an interesting idea. Maybe there’s a way to, for example, shoot jets of molten material out, so that they’ll drift in a convenient direction and cool on their way. You’d also need to be careful about all the reaction force, because if you’re doing this on the scale of the entire planet, you could change its orbit. It wouldn’t be too hard to do some basic calculations on the feasibility of this, but I probably won’t take the time for that (yet).
And yeah, starting with smaller pieces like asteroids has less of an issue with waste heat, since they have more surface area.
In the order that they appear in the paper, these are a few of the parts that seemed iffy to me. Some of them may be easily shown to be either definitely iffy, or definitely not-so-iffy, with a little more research:
I’m not sure what existed at the time the paper was written, but there are now proposals for fusion rockets, and using the expected exhaust velocities from those might be better than using the theoretical value from DT fusion.
I feel like I’m the only one that thinks this Dyson sphere method is a little dubious. What system is going to be used to collect energy using the captors and send it to Mercury? How will it be received on Mercury? The total power collected toward the end is more than 1024 W. If whatever process is used to disassemble the planet is 90% efficient, the temperature required to radiate the waste heat over Mercury’s surface area is about 7000K. This is hotter than the surface of the sun, and more than twice the boiling point of both iron and silica. In order to keep this temperature below the boiling point of silica, we would either need the process to be better than 99.98% efficient, to attach Mercury to a heat sink may times the size of Jupiter, or to limit power to about 1020 W. If melting the planet isn’t our style, we need to limit power to about 1019 W.
I don’t think this kills their overall picture. It “only” means the whole process takes a few orders of magnitude longer.
The probes will need stored energy and reaction mass to get into the appropriate orbit, unless all the desired orbits intersect Mercury’s orbit. Maybe this issue can be mitigated by gradually pushing Mercury into new orbits via reaction force from the probes. Or maybe it’s just not much of a limitation. I’m not sure.
This seems pretty optimistic. In particular, making a system that launches large objects at .5c. Doing this over the distance from the sun to Earth requires an average force of about 105 N per kg. For .9c and .99c, it requires about 8× and about 35× this force/mass, respectively. I don’t know what the limiting factor will be on these things, but this seems pretty high, and suggests that the launcher would need to be a huge structure, and possibly a bigger project than the Dyson swarm.
I also have some complaints about the notation, which I will post later, and possibly other things, but this is what I have for now.
Doesn’t it mean that the last stage of the process takes a few orders of magnitude longer, not the whole process? The process consists of a series of N doublings. For all but the last… 7 or so doublings, the waste heat can be dumped into Mercury itself. Only for those last doublings does waste heat start to overheat Mercury. Right? So it’s just that those last 7 doublings or so need to slow down (or rather, stop growing exponentially)
More interesting proposal: Maybe actually a better strategy would be to just deliberately overheat Mercury at that point, turn it into an expanding cloud of superhot gaseous material, and then scoop up said material as it cools down? Not sure if that’s possible, maybe the cloud wouldn’t expand enough.
Another interesting proposal: Smash Mercury to bits by colliding other objects like Ceres into it.
Another, more practical proposal: Once Mercury is saturated (can’t double any more without overheating it) switch to mining those cold asteroids out in the asteroid belt, with your giant fleet of spaceships you’ve built with the infrastructure that blankets Mercury and orbits the Sun. The asteroids are all spread out, so it’s easier for them to radiate heat… right?
Just spitballing here.
If growth is fast until the last 7 doublings, then things are fast until you get to ~1% of your final output. That’s still a ton of power, at least by our current standards. My guess is you don’t bother doing a dyson swarm until you’ve built a lot of infrastructure in more convenient places (e.g. somewhere closer to Earth’s orbit around the sun). But I dunno, it really depends on what the technology is like and how bottlenecked on energy you are.
Breaking apart Mercury is an interesting idea. Maybe there’s a way to, for example, shoot jets of molten material out, so that they’ll drift in a convenient direction and cool on their way. You’d also need to be careful about all the reaction force, because if you’re doing this on the scale of the entire planet, you could change its orbit. It wouldn’t be too hard to do some basic calculations on the feasibility of this, but I probably won’t take the time for that (yet).
And yeah, starting with smaller pieces like asteroids has less of an issue with waste heat, since they have more surface area.