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Post by darthroach on Mar 14, 2017 1:22:08 GMT
Space Command: We have just built a 200t, 9.3 km/s neon-based liner that can carry 40t with only 40t devoted to reaction mass. 40t Cargo Liner: Hello everyone! Consumer: We want you to delivery to our station in low Jupiter orbit. 40t Cargo Liner: Are you f**king kidding me later... 40t Cargo Liner: I'll just station in the asteroid belt... To be entirely honest, so long as you can continously reorient to thrust prograde, climbing out of gravity wells is not nearly as much of a problem - though it does take both way more time and energy. 9.3km/s is very underwhelming, however. I am rather surprised you managed to get an MPD to perform so low.
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Post by The Astronomer on Mar 14, 2017 1:24:39 GMT
Space Command: We have just built a 200t, 9.3 km/s neon-based liner that can carry 40t with only 40t devoted to reaction mass. 40t Cargo Liner: Hello everyone! Consumer: We want you to delivery to our station in low Jupiter orbit. 40t Cargo Liner: Are you f**king kidding me later... 40t Cargo Liner: I'll just station in the asteroid belt... To be entirely honest, so long as you can continously reorient to thrust prograde, climbing out of gravity wells is not nearly as much of a problem - though it does take both way more time and energy. 9.3km/s is very underwhelming, however. I am rather surprised you managed to get an MPD to perform so low. Well, as the title say, I devoted little mass to the remass.
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Post by newageofpower on Mar 14, 2017 1:43:07 GMT
At low energies MPDs are just very inefficient unless using Xenon or some other exotic propellant. Since I prefer hydrocarbons of some sort for compatibility with NTR propulsion, I have a hard time getting decent eVs out of a sub 20MW reactor.
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Post by The Astronomer on Mar 14, 2017 5:11:48 GMT
At low energies MPDs are just very inefficient unless using Xenon or some other exotic propellant. Since I prefer hydrocarbons of some sort for compatibility with NTR propulsion, I have a hard time getting decent eVs out of a sub 20MW reactor. Who'd even bother to design sub-100MW MPDs unless for probes and such?
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Post by newageofpower on Mar 14, 2017 5:24:22 GMT
At low energies MPDs are just very inefficient unless using Xenon or some other exotic propellant. Since I prefer hydrocarbons of some sort for compatibility with NTR propulsion, I have a hard time getting decent eVs out of a sub 20MW reactor. Who'd even bother to design sub-100MW MPDs unless for probes and such? Light Drones using weapons between .5-2 tons.
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Post by darthroach on Mar 15, 2017 6:57:04 GMT
Apophys did a calculation on this before; active cooling can circumvent the MPD overheating problem. How does one actively cool a 1mm diameter wire in the middle of a terawatt reaction chamber? As cool as these MPD torchships are, they might as well be FTL as far as realism is concerned. Heat rejection is probably the biggest hurdle in high performance propulsion.
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Post by newageofpower on Mar 15, 2017 8:40:19 GMT
How does one actively cool a 1mm diameter wire in the middle of a terawatt reaction chamber? As cool as these MPD torchships are, they might as well be FTL as far as realism is concerned. Heat rejection is probably the biggest hurdle in high performance propulsion. Well, my MPDs have cathodes 1 meter across... I'm not even sure you can have a 1mm wire, VanChrome or not, that won't get destroyed by EMF in a 1 TW setup. The example I have here is an average MPD with 2.6% wastage, meaning I need to dump 64 KW of heat, most of it from the cathode. MPDs require much less cooling than you might think, due to high efficiency. The system Apophys ran the calc with had some amazing 99.7% efficiency, and he noted it was possible to build 99.97% efficient (or better!) setups. Electrical drives have vastly reduced needs for heat rejection, in exchange for requiring ludicrous amounts of power and offering minuscule thrust.
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Post by vegemeister on Mar 15, 2017 9:36:31 GMT
newageofpower You can reduce mechanical stress by lengthening the thruster, and by reducing chamber thickness you can make the cathode smaller. That MPD can get a lot smaller: Thanks for showing me the magic of uranium anodes. It staves off the "thruster too short" warning, somehow. U-233 is slightly better. Maybe it has something to do with the radioactivity?
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Post by darthroach on Mar 15, 2017 9:56:54 GMT
How does one actively cool a 1mm diameter wire in the middle of a terawatt reaction chamber? As cool as these MPD torchships are, they might as well be FTL as far as realism is concerned. Heat rejection is probably the biggest hurdle in high performance propulsion. Well, my MPDs have cathodes 1 meter across... I'm not even sure you can have a 1mm wire, VanChrome or not, that won't get destroyed by EMF in a 1 TW setup. The example I have here is an average MPD with 2.6% wastage, meaning I need to dump 64 KW of heat, most of it from the cathode. MPDs require much less cooling than you might think, due to high efficiency. The system Apophys ran the calc with had some amazing 99.7% efficiency, and he noted it was possible to build 99.97% efficient (or better!) setups. Electrical drives have vastly reduced needs for heat rejection, in exchange for requiring ludicrous amounts of power and offering minuscule thrust. Your thruster is extremely tame compared to some of the stuff I see around these parts. This thead, for example: childrenofadeadearth.boards.net/thread/1039/thruster-exhaust-damage-forcesThat's a 456GW thruster. With a 1mm anode. 99.7% doesn't mean much when 0.3% is still 1.368GW of heat.
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Post by newageofpower on Mar 15, 2017 13:57:35 GMT
newageofpower You can reduce mechanical stress by lengthening the thruster, and by reducing chamber thickness you can make the cathode smaller. That MPD can get a lot smaller: Thanks for showing me the magic of uranium anodes. It staves off the "thruster too short" warning, somehow. U-233 is slightly better. Maybe it has something to do with the radioactivity? Your MPD is ludicrously heavy, my friend. @sithroach Duh. Some MPD designs are not plausible; like pre-patch kinetic weapons. But overall, massive multihundred GW designs with high efficiency and large components are reasonable; at 99.97% efficiency you'd be giving off 300kw waste heat per input GW, and I'll note my resistojets must radiatively reject far more heat per ratio of input power than MPDs. Granted, the resistojet bells aee fabricated from diamond and are operating above 3000 kelvins, far hotter than a temperature that would frag out current MPDs.
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Post by The Astronomer on Mar 15, 2017 13:59:05 GMT
I want MPD radiators. It'd look cool.
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ndeo
Junior Member
It's not a flashlight... It's a High-frequency relativistic boson cannon
Posts: 67
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Post by ndeo on Mar 16, 2017 1:22:45 GMT
When Terawatt MPDs can get 100%(Rounding error?) efficiency with no waste heat, something is definitely not right.
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Post by newageofpower on Mar 16, 2017 4:13:57 GMT
When Terawatt MPDs can get 100%(Rounding error?) efficiency with no waste heat, something is definitely not right. Probably a rounding error.
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Post by The Astronomer on Mar 16, 2017 4:28:02 GMT
When Terawatt MPDs can get 100%(Rounding error?) efficiency with no waste heat, something is definitely not right. Probably a rounding error. I'd like to suggest that qswitch should remove number rounding both displayed numbers (and change the unit to meter) and design box numbers.
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