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Post by apophys on Sept 6, 2017 15:01:36 GMT
If we want to prioritize radiator performance over endurance, we could operate at much higher temperatures and just accept the vapor pressure losses. If I'm reading it right, not really. Extrapolating from the graph, at ~1250 K, tin would have 1000 times the vapor pressure, giving it a rated lifetime of 11 days. More liquids need to be tested; the ones in the paper are all unsuitable for ~2000-2500 K radiators. Titanium, cobalt, boron, and silicon would be top on my list of things to test.
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Post by jtyotjotjipaefvj on Sept 6, 2017 15:27:20 GMT
If we want to prioritize radiator performance over endurance, we could operate at much higher temperatures and just accept the vapor pressure losses. If I'm reading it right, not really. Extrapolating from the graph, at ~1250 K, tin would have 1000 times the vapor pressure, giving it a rated lifetime of 11 days. More liquids need to be tested; the ones in the paper are all unsuitable for ~2000-2500 K radiators. Titanium, cobalt, boron, and silicon would be top on my list of things to test. 11 days of combat endurance doesn't sound too bad. That's of course assuming your engines don't need the reactor to run at 100%. With a ship powered by NTRs, it should still be pretty usable. Of course, 1250 K is not really ideal for a reactor exit temperature, but it could be usable to have on a ship with less power-hungry weapons, such as conventional guns.
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Post by matterbeam on Sept 6, 2017 15:55:57 GMT
If we want to prioritize radiator performance over endurance, we could operate at much higher temperatures and just accept the vapor pressure losses. If I'm reading it right, not really. Extrapolating from the graph, at ~1250 K, tin would have 1000 times the vapor pressure, giving it a rated lifetime of 11 days. More liquids need to be tested; the ones in the paper are all unsuitable for ~2000-2500 K radiators. Titanium, cobalt, boron, and silicon would be top on my list of things to test. How would increasing the temperature by a factor 1250/1000: 1.25 reduce the lifetime from 30 years to 11 days, a factor 30*365/11:995? That doesn't sound right.
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Post by apophys on Sept 6, 2017 16:59:06 GMT
If I'm reading it right, not really. Extrapolating from the graph, at ~1250 K, tin would have 1000 times the vapor pressure, giving it a rated lifetime of 11 days. More liquids need to be tested; the ones in the paper are all unsuitable for ~2000-2500 K radiators. Titanium, cobalt, boron, and silicon would be top on my list of things to test. How would increasing the temperature by a factor 1250/1000: 1.25 reduce the lifetime from 30 years to 11 days, a factor 30*365/11:995? That doesn't sound right. Just from the easily visible data, a temp. rise from 900 K to 1000 K (1.11x) causes tin's vapor pressure to rise from 10 -9 mm Hg to 10 -7 mm Hg (100x). I understand that this means the mass loss is increased in speed by 100 times (I may be wrong; if so, please correct me). The 30-year duty rating for an enclosed LDR is apparently around 3e-7 mm Hg. If you continue the tin curve beyond the graph, it appears that somewhere around 1250 K it'll be 3e-4 mm Hg; 1000x the pressure (and presumably 1/1000 of the lifespan). Note that tin melts at 505 K and boils at 2875 K, according to CoaDE data. That's of course assuming your engines don't need the reactor to run at 100%. With a ship powered by NTRs, it should still be pretty usable. With current/near technology, electric drives are the best bet for getting very high velocity from your propellant, which is a huge gamechanger for decent travel times (no military wants to be a sitting duck; colonies could be serviced/populated faster) and actually reasonable mass ratios. We don't have CoaDE-tier lightweight reactors yet in reality, but we should get close enough at some point. Probably quite a while before fusion becomes a serious option. I'm pretty sure LDRs will be fine at >2000 K temperatures with the appropriate liquids.
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Post by matterbeam on Sept 6, 2017 18:31:26 GMT
apophys: I see now. While pressure increases with both temperature and the number of moles of vapor, meaning the mass loss rate isn't really linear with the vapor pressure, I do realize now that liquid droplet radiators have a tough time with high temperatures. Solid particle or 'sand' radiators might be the only option.
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Post by apophys on Sept 6, 2017 19:24:30 GMT
matterbeam I found a table with the vapor pressures of some materials ( www.physics.nyu.edu/kentlab/How_to/ChemicalInfo/VaporPressure/morepressure.pdf). Here is a comparison of the ones that get 1 Pa, ~7.5e-3 mm Hg, pressure (lowest in table) while liquid, at interesting temperatures. I arranged them by their melting points that I found and converted everything to Kelvin. Sadly, the four elements that I considered a few posts ago aren't quite good enough. [Edit: Found more at wikipedia: link. The data agrees, so they probably use the same sources.]
| 1 Pa temp. (K)
| Melting point (K)
|
| Niobium | 2942
| 2742
|
| Hafnium | 2689
| 2504
|
| Technetium
| 2727
| 2477
| All isotopes radioactive.
| Rhodium
| 2288
| 2236
|
| Zirconium
| 2639
| 2128
|
| Platinum
| 2330
| 2041
|
| Thorium
| 2633
| 2028
|
| SiO2
| 2239
| 1998
| Liquid glass, nice.
| Uranium
| 2325
| 1405
| DU LDRs? Sure, why not.
| Neptunium
| 2194
| 912
|
| Tin
| 1497
| 505
| (for comparison)
|
1 Pa is still many times too high pressure for consideration in an LDR, but zirconium looks promising so far. Uranium, neptunium, and thorium appear to be disturbingly good candidates for a high-temperature LDR liquid.
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Post by ironclad6 on Sept 6, 2017 19:31:10 GMT
apophys : I see now. While pressure increases with both temperature and the number of moles of vapor, meaning the mass loss rate isn't really linear with the vapor pressure, I do realize now that liquid droplet radiators have a tough time with high temperatures. Solid particle or 'sand' radiators might be the only option. Basically for warships it looks like there is no stopping point between solid radiators and dusty plasma.
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Post by bigbombr on Sept 6, 2017 19:59:39 GMT
matterbeam I found a table with the vapor pressures of some materials ( www.physics.nyu.edu/kentlab/How_to/ChemicalInfo/VaporPressure/morepressure.pdf). Here is a comparison of the ones that get 1 Pa, ~7.5e-3 mm Hg, pressure (lowest in table) while liquid, at interesting temperatures. I arranged them by their melting points that I found and converted everything to Kelvin. Sadly, the four elements that I considered a few posts ago aren't quite good enough.
| 1 Pa temp. (K)
| Melting point (K)
|
| Niobium | 2942
| 2742
|
| Hafnium | 2689
| 2504
|
| Technetium
| 2727
| 2477
| All isotopes radioactive.
| Rhodium
| 2288
| 2236
|
| Zirconium
| 2639
| 2128
|
| Platinum
| 2330
| 2041
|
| Thorium
| 2633
| 2028
|
| SiO2
| 2239
| 1998
| Liquid glass, nice.
| Uranium
| 2325
| 1405
| DU LDRs? Sure, why not.
| Tin
| 1497
| 505
| (for comparison)
|
1 Pa is still many times too high pressure for consideration in an LDR, but zirconium looks promising so far. Uranium and thorium appear to be disturbingly good candidates for a high-temperature LDR liquid. Remember that crazy idea about having completely molten reactor fuel that doubles as coolant (subcritical in coolant pipes and turbine, critical in the 'reactor' itself)? That might work decently with a LDR. But your LDR's might spill molten U-233 all over the outside of your spaceship when maneuvering. Oh well, we'll call it in-situ appliqué armour.
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Post by matterbeam on Sept 7, 2017 1:22:35 GMT
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Post by n2maniac on Sept 8, 2017 4:17:44 GMT
~1050 K at best? Those temperatures are too low for powerplant radiators. Should be great for laser radiators though. I'm curious about cobalt, for a Curie point LDR. Vapor pressure limits may be less strict when all solids are pulled back and recovered. That's a NASA study for droplet radiators supposed to last decades, with only low temperature components to cool down. If we want to prioritize radiator performance over endurance, we could operate at much higher temperatures and just accept the vapor pressure losses. We would probably pick a material that melts higher than tin, has good thermal conductivity, lower vapor pressure at the working temperature relative to its emissivity, and as low an atomic mass as practical (due to heat capacity per mass considerations). Which... starts looking like maybe the low tier refractory metals (ie. zirconium, molybdenum) or maybe silicon? Gah, the prospect of those freezing or reacting is bleh.
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Post by matterbeam on Sept 8, 2017 14:00:16 GMT
That's a NASA study for droplet radiators supposed to last decades, with only low temperature components to cool down. If we want to prioritize radiator performance over endurance, we could operate at much higher temperatures and just accept the vapor pressure losses. We would probably pick a material that melts higher than tin, has good thermal conductivity, lower vapor pressure at the working temperature relative to its emissivity, and as low an atomic mass as practical (due to heat capacity per mass considerations). Which... starts looking like maybe the low tier refractory metals (ie. zirconium, molybdenum) or maybe silicon? Gah, the prospect of those freezing or reacting is bleh. The simple solution is just to use solid-particle sand.
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Post by n2maniac on Sept 9, 2017 8:29:09 GMT
We would probably pick a material that melts higher than tin, has good thermal conductivity, lower vapor pressure at the working temperature relative to its emissivity, and as low an atomic mass as practical (due to heat capacity per mass considerations). Which... starts looking like maybe the low tier refractory metals (ie. zirconium, molybdenum) or maybe silicon? Gah, the prospect of those freezing or reacting is bleh. The simple solution is just to use solid-particle sand. If by "sand" you mean something like graphite or tungsten and by "solid particle" you mean that stuff that doesn't flow without vibration, clogs easily, does not wet surfaces (important for heat transfer!), and refuses to work with nice valves... wait, what was I saying? (In all seriousness, I would have some serious implementation follow-up questions if this was a proposal in a design review. Got any precedents for this in a non-slurry form?)
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Post by matterbeam on Sept 9, 2017 9:51:52 GMT
The simple solution is just to use solid-particle sand. If by "sand" you mean something like graphite or tungsten and by "solid particle" you mean that stuff that doesn't flow without vibration, clogs easily, does not wet surfaces (important for heat transfer!), and refuses to work with nice valves... wait, what was I saying? (In all seriousness, I would have some serious implementation follow-up questions if this was a proposal in a design review. Got any precedents for this in a non-slurry form?) Larger particles can be shaped like ball bearings and simply roll their way around. Smaller particles would need something like a conveyor belt or a bucket rain to move from heat exchanger to radiator and back. Tiny particles can be manipulated by electrostatic forces: www.sciencedirect.com/science/article/pii/S0921883110001640 or electromagnetic fields: www.google.sr/patents/WO2005069311A2?cl=en
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Post by treptoplax on Sept 10, 2017 13:02:54 GMT
If by "sand" you mean something like graphite or tungsten and by "solid particle" you mean that stuff that doesn't flow without vibration, clogs easily, does not wet surfaces (important for heat transfer!), and refuses to work with nice valves... wait, what was I saying? (In all seriousness, I would have some serious implementation follow-up questions if this was a proposal in a design review. Got any precedents for this in a non-slurry form?) Larger particles can be shaped like ball bearings and simply roll their way around. Smaller particles would need something like a conveyor belt or a bucket rain to move from heat exchanger to radiator and back. Tiny particles can be manipulated by electrostatic forces: www.sciencedirect.com/science/article/pii/S0921883110001640 or electromagnetic fields: www.google.sr/patents/WO2005069311A2?cl=enOnce you're up in ball-bearing size range, composite elements might be worth a look... Consider a ceramic shell filled with something that has a high melting point. It'll emit an large amount of heat at high temperature (melting point) before cooling any further. I want that boron slush radiator !
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Post by matterbeam on Sept 10, 2017 14:50:38 GMT
Once you're up in ball-bearing size range, composite elements might be worth a look... Consider a ceramic shell filled with something that has a high melting point. It'll emit an large amount of heat at high temperature (melting point) before cooling any further. I want that boron slush radiator ! Absolutely this!! The shells would better be something like a carbon-carbon material due to its high emissivity and resistance to the expansion and vapor pressure of hot fluids inside. Combined with high heat of fusion materials released just above boiling point and collected just as they solidify, we could probably reach the efficiency limits of a 'droplet' radiator. Maybe we should call it the encapsulated fusion radiator. It would go along nicely with other advanced radiators such as the carbon silk radiator or the dusty plasma radiator as designs ToughSF helped create
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