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Post by newageofpower on Nov 30, 2017 7:28:07 GMT
This is for the Advanced Solar Energy series, so criticality is less of an issue. The heat source is a grid of microchannels heated from the top and bottom by concentrated sunlight. Gas passed through the heat exchanger. I am unsure whether simply heating the gas this way will cause it to increase in pressure, or whether turbo-generators have both a compression (pre-compress the gas, heat it further, then release the pressure) and an expansion stage. Assuming the gas is behaving anywhere similar to an ideal gas (which, Helium is fairly close to) heating it will always increase pressure. Additionally, you'd want as much coolant per volume in your cooling system as possible (well, while maintaining safe pressure in coolant lines and pumping machinery) as that dramatically increases heat flow rates, so an optimized design would be under significant pressure. I'm uncertain about proposed Helium working fluid; a quick Google search gives me a dozen articles from 1968 to 2011 on high temperature helium embrittlement. Perhaps selecting a working fluid with a reduced tendency to diffuse into solid materials? It's not like Helium has excellent thermal characteristics as a coolant...
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Post by newageofpower on Nov 30, 2017 0:56:31 GMT
I am not sure than liquid coolants are a good idea. It would impose an upper limit on the possible operating temperatures, it would create a lot of waste drag on the turbine blade and you cannot make the flow too fast because otherwise you'd get cavitation. I am also unsure of how a turbine would extract energy from a closed loop incompressible flow in the first place. It cannot expand the fluid, doesn't cool it down and cannot slow it down either...? Hmm. Less dense coolants suffer from supercriticality issues, which tends to reduce ideal thermal properties and increases pressure stress on the system. I just tried to use a gaseous coolant in a 50 MW CDE reactor; cooling efficiency drops, so I'm forced to bloat up volume required. Furthermore, pump size jumps up dramatically, and power efficiencies for a given outlet temperature decreases. Perhaps liquid metal within the reactor core itself, then the gas or water outer loop (if we're using expansion turbines) using a heat exchanger... Now we're back to high Brayton specific power levels.
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Post by newageofpower on Nov 29, 2017 0:37:57 GMT
matterbeam Personally, I think we need to look up literature in existing liquid-metal turbines; the properties of a compressible gas coolant are extremely different from a liquid coolant. Liquid metal turbines have been built before, but I'm having trouble finding scientific papers on the subject. IIRC, the Soviet Alfa-class submarines used a lead-bismuth design. Once you can extrapolate the differences between, say, a naval gas turbine and a naval liquid metal turbine, we can then use that to extrapolate a hypothetical min-maxed Very High Temperature reactor turbine that would be mounted on a spacecraft.
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Post by newageofpower on Nov 28, 2017 20:48:36 GMT
matterbeamNotice my last line includes the entire reactor system - the comparison of a reactor's ancillary equipment has to be figured in; for example, a liquid metal/ceramic turbine would need a 'warming engine' that would likely be more complex and heavier than a starter engine, although I suppose that could be an externalized system that would perform the same function would let us save weight at the expense of making powerplant restarts without support facilities implausible. Furthermore, aircraft/rocket propulsion systems do not include apparatuses for translating mechanical energy into electrical power; though this should be reasonably light. From literature I've read, the most critical limitations in modern turbomachinery is that temperature rises as compression ratio increases; maximal temperature (and thus compression) is limited by the heat tolerance of the turbine blade alloy. The other limitation is that most alloys suffer increased creep fatigue when operating near maximal safe temperature and under high stress. Ceramics are far more resistant to high heat load and many exhibit superior creep fatigue mechanics. For these reasons, significant research is already taking place in ceramic composite turbines.
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Post by newageofpower on Nov 28, 2017 17:47:21 GMT
Graphene wont change anything because you can't use it for anything, yes it has a very high specific strength but that doesn't change the fact that you can only have a single sheet of Graphene before it becomes graphite By Graphene, I should have clarified - I meant nanostructured Carbon. And it's unlikely you'd use such substance by itself. For example, you could thread a turbine blade with pre-tensioned CNTs, massively enhancing tensile strength along the desired axis. Another advantage would be adding Graphene flakes ( which have been shown to significantly enhance mechanical properties in many materials) to the base material.
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Post by newageofpower on Nov 28, 2017 16:06:45 GMT
Before we start, let's consider the design differences between powerplant turbines and aircraft turbines. Civil power plants attempt to minimize cost, downtime and maintenance demands while maximizing reliability and efficiency. A relatively new GE power station gas turbine weighs 330t while generating 293 MW; this is less than 1kw/kg. Meanwhile, a similarly modern GE civil aircraft turbine develops over 10kw/kg, and military aircraft engines have far higher power/weight ratios.
Military spacecraft will almost certainly seek to optimize around power to weight ratio over other concerns.
Like RocketWitch has said, high temperature turbine materials - or even the turbine itself - are unlikely to be functional at lower temperatures. As a general rule, ceramics have a reduced (read: superior) thermal expansion coefficient compared to metals and alloys; but the expected operating temperature will be so high that designers will be forced to sacrifice low-temperature usability. Using an well-known real life example, consider the SR-71 Blackbird. It's thermal expansion at operating temperature was so significant the designers were forced to build it with gaps that only sealed when the airframe had been heated sufficiently.
Working fluids in very high temperature reactors (which I assume are the heat sources for your turbomachinery) are likely to be some sort of liquefied metal, meaning that a safe reactor design must be sufficiently hot to keep the metal molten even in 'off' state. Luckily, liquid metals are usually less corrosive than high temperature steam - though care must be taken to select a coolant that will not absorb & dissolve the selected ceramic at expected temperature ranges - for example, I recall reading about liquid Tungsten having a tendency to absorb Carbon, leaching it out of multitude of carbide-based ceramics, greatly reducing their lifespan.
Though significant weight savings over civil power turbines are plausible with compromises (reduced operational lifespan, narrow bands of operational temperature or even narrow bands of 'safe' temperature) in design, I'd expect a decrease in power/weight compared to a similarly designed lower temperature turbine, simply because the known very high temperature ceramics have a lower strength-to-weight ratio than known high-strength alloys.
TLDR Conclusion: As such, actual spacecraft turbomachine based reactors are unlikely to approach CDE thermocouple based reactor power density until the advent of large-scale Graphene fabrication; CDE thermocouple-based powerplants can pass 100 kw/kg (over 50 kw/kg even with armored radiators).
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Post by newageofpower on Nov 23, 2017 19:16:43 GMT
What. A 10,000 sqm hull radiating at 500k disposes of less waste heat than a. 100 sqm radiator at 4000k. Orders of magnitude less.
I don't see how a low temperature hull radiator is *at all* beneficial if you're using very high energy powerplants. Unless it's a non-Carnot powerplant like Fission Fragment.
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Post by newageofpower on Nov 21, 2017 3:41:06 GMT
Note that even with empty hands, Mikasa is definitely military grade firepower. I'm not sure if we'll count her as Infantry, though...
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Post by newageofpower on Nov 15, 2017 1:38:18 GMT
matterbeam I believe Kerr 's point is that a tactician sitting in the middle of a constellation of spinal gun drones watching a sensor plot as his constellation dances with the enemy battle constellation is hardly 'Star Wars' exciting. At least, for the unwashed, unintellectual plebian masses ;p One vehicle I suggested is the 'pellet fighter'. It uses a smaller pellet gun capable of lower velocities. It approaches targets and uses high maneuverability to evade. If you can catch a spacecraft at 3000km accelerating at 0.1g, then you can dodge up to 300km at 1g and 30km at 10g. This hard-burning fighter wiggles and winds its way up to the target and pops a few rounds into it from practically point blank range. Of course, the best way to counter this fighter is with... another fighter. They can approach each other to very short ranges due to their low velocity guns. Add a bit of human-in-the-seat-to-beat-predication-algorithms flair and you're got the spiralling dogfights and gundam-inspired twitchy combat that can capture the masses. Uh. The mass of the human, life support system is just unacceptable. As for dodging, fuel endurance is an issue. If my megameter-per-second frag linear accelerator can accurately hit a dinner plate from say, 100 megameters away, you need to be able to continuously evade until within your own effective range of my ship. It just doesn't seem feasible.
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Post by newageofpower on Nov 15, 2017 0:31:33 GMT
matterbeam I believe Kerr 's point is that a tactician sitting in the middle of a constellation of spinal gun drones watching a sensor plot as his constellation dances with the enemy battle constellation is hardly 'Star Wars' exciting. At least, for the unwashed, unintellectual plebian masses ;p
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Post by newageofpower on Nov 13, 2017 2:18:11 GMT
IIRC Amimai and apo-chan did some experiments about a year ago; for warships with armored or redundant radiators, 2500k-2600k is preferable. For civillian ships with paper thin zero redundancy radiators, 2400k was optimal.
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Post by newageofpower on Nov 11, 2017 3:21:20 GMT
I hate this forum. I've spent an entire hour typing the following reply. It took me six attempts between user verification, losing links when copying, not being able to control-Z and control-a sometimes wiping the entire message. I write chunks of my stuff in NPP first, when dealing with non-xenoforo forums.
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Post by newageofpower on Nov 7, 2017 18:10:16 GMT
I laughed so hard. Thank you, thorneel.
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Post by newageofpower on Nov 5, 2017 16:32:21 GMT
So you're asking me to admit that the infantry man will become obsolete right around the same time that the human race does? No. In a total war, human infantry are already obsolete. Of course, total war starts with ICBM launches, so... Already, naval warships don't use naval infantry for things other than getting rid of pirates in the equivalent of upgunned rowboats; they're basically more like cops than main battle weapons.
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Post by newageofpower on Nov 3, 2017 18:39:11 GMT
Some industrial processes are done better in zero or microgravity, but overall it is far easier to develop an industrial base around Luna than an asteroid.
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