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Post by goduranus on Dec 17, 2016 5:39:19 GMT
Gigawatt MPDs so strong and efficient, this creates an unrealistic possibility that ships could be designed with very little mass devoted to propellant. I think this possibility can be best prevented by checking for Joule heating on the MPD's cathode and anode components. For a 150km/s hydrogen MPD I made, there is usually 300kA flowing through a 1mm cathode, I think this means the MPD should be instantly vaporized? Random example, ship by newageofpower, which can get between any two stellar bodies within one year.
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Post by n2maniac on Dec 17, 2016 7:33:42 GMT
Hmm, what does wiki have? 100 A/cm2, yea, that seems strong. Ingame stuff is like 1mm radius, 1cm long or so, multiply that out we get... 0.6cm2. At 100 A/cm2 we have... 60A. Ingame we use... 300kA. A factor of 5... wait, 5000 off. Hmm, I might be missing something, but something seems odd. Oh yea, dumping current into a 1mm radius wire at the cathode. That is 12AWG, NEC rated for 30A at 90 deg C. But NEC is lame, so lets assume it has to last 32ms. That gets us up to... 5.6kA before it melts within that time. Hmm, more questions...
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Post by The Astronomer on Dec 17, 2016 14:54:43 GMT
"Right now we have these megawatt gigawatt MPDs so strong and efficient that nowhere in the solar system is more than a year away..." Oh, okay. I think MPD tech is very, very cool, unless you notice that abysmal thrust that bars your ship well away from 'danger zones' or deeper in the gravity wells. Anyways, this seems to be an interesting case.
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Post by goduranus on Mar 12, 2017 3:04:02 GMT
Bump. Now that railguns are fixed, gigawatt-level 150km/s MPDs are one of the few remaining items where glaringly unrealistic designs still could significant impact gameplay. Checking for cathode/anode melting temperature in MPD designs can prevent unrealistically small ships with hundreds of Km/s worth of deltaV that propelled purely by MPDs.
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Post by newageofpower on Mar 12, 2017 4:27:12 GMT
Apophys did a calculation on this before; active cooling can circumvent the MPD overheating problem.
Granted, pumping away 300 MW of heat is a non-trivial task, but the MPD used in the example required 10 GW total power; you could easily afford a GW or two for heat pumping.
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Post by goduranus on Mar 12, 2017 5:23:45 GMT
Ahh, so they can actually work, my bad then.
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Post by argonbalt on Mar 12, 2017 19:33:23 GMT
The Reactor driven MPD revolution was actually the coolest thing i think that happened in this game, it actually convinced me that you don't need Hydrogen Deutrium fusion drives to get anywhere at a fair pace
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Post by newageofpower on Mar 12, 2017 20:25:30 GMT
The Reactor driven MPD revolution was actually the coolest thing i think that happened in this game, it actually convinced me that you don't need Hydrogen Deutrium fusion drives to get anywhere at a fair pace Well, the thrust is goddamn awful, so you have to use Resistojet/NTR to break low orbit before you begin Brachistrone Manouvers. And as The Astronomer determined, it's very hard to use in High-G settings, like his TRAPPIST-1 sim.
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Post by bigbombr on Mar 12, 2017 20:30:21 GMT
I have a 168 t craft, 110 of which is crewmodule and 50 is methane. It can pull 19 mG, which is sufficient to escape most gravity wells.
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Post by vegemeister on Mar 12, 2017 20:46:15 GMT
The Reactor driven MPD revolution was actually the coolest thing i think that happened in this game, it actually convinced me that you don't need Hydrogen Deutrium fusion drives to get anywhere at a fair pace Well, the thrust is goddamn awful, so you have to use Resistojet/NTR to break low orbit before you begin Brachistrone Manouvers. And as The Astronomer determined, it's very hard to use in High-G settings, like his TRAPPIST-1 sim. We just don't have the best tools for plotting low-thrust trajectories. You can get away with very little acceleration, so long as you don't need to land and don't care about taking advantage of the Oberth effect. To climb out of low orbit with a low-thrust ship, you have keep burning prograde and slowly spiral out. You can approximate it in-game by making a series of back-to-back prograde burns that individually don't go more than 1/6 of the way around the body you're orbiting.
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Post by newageofpower on Mar 12, 2017 21:25:04 GMT
It's what I stated in another thread; you can climb out of most gravity wells with hundreds (or thousands, depending) forwards + outwards burns.
Takes awhile, though.
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Post by Rocket Witch on Mar 12, 2017 21:54:19 GMT
I think it's implied by the dev blog that MPDTs and resistojets use the same coolant loop as the reactor powering them, since it says they require radiators but none are required ingame.
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Post by underwhelmed on Mar 13, 2017 13:46:01 GMT
Apophys did a calculation on this before; active cooling can circumvent the MPD overheating problem. Granted, pumping away 300 MW of heat is a non-trivial task, but the MPD used in the example required 10 GW total power; you could easily afford a GW or two for heat pumping. What were the dimensions of the MPD involved?
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Post by The Astronomer on Mar 13, 2017 16:11:17 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...
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Post by bigbombr on Mar 13, 2017 17:46:29 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... 20+ km/s at 19+ mg of acceleration is perfectly possible with a 1 GW MPDT. E.g. my favorite minilaserstar weighs 168 t, 110 of which is crew compartments, 50 is methane and the remaining 8 t are a 1 GW reactor, a 1 GW laser, the MPDT, the radiators, a drone launcher, and a magazine with 37 drones. Removing all weapon systems would drastically reduce crew requirements, so the acceleration and delta-v would be considerably more.
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