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Post by nihilrex on Jan 24, 2017 12:57:21 GMT
law of conservation of energy In this place, it seems more like a loose guideline...
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Post by caiaphas on Jan 24, 2017 13:20:01 GMT
I think the main stumbling block is figuring out how to keep the chamber temperatures cool enough that the heating coil doesn't melt, and so far the only way I've seen to do that mass-effectively is to dump a whole lotta propellant into it all at once, which seriously hurts its ability as an endurance thruster. If anyone could help further optimize, I'd be obliged. (Coil is Tantalum Hafnium Carbide.) The power input and the melting point of the coil material determine the amount of propellant that goes in. If you want to put 10 GW through a single engine, that's how much propellant it needs. If you want to reduce the amount of propellant it takes, you need to put less power in. I tried tweaking your design. Gimbal radius is 4.955. Diamond chamber has greater thermal conductivity and allows thicker wall, smaller throat radius, and less mass. Increased chamber length also helps cooling, for some reason, although 8.4 cm seems to be near a local maximum. I couldn't increase wall thickness/reduce throat radius any more without running into thermal problems. Increasing the chamber contraction ratio to 6 avoids the missing thrust problem. Increased expansion ratio to non-bottle-rocket levels, for better Isp. Of course, if you use decane (or worse, RP-1) resistojets, don't forget to cross to the other side of the street if you see the law of conservation of energy coming your way. This one's 8090% efficient. Where in blazes is the extra mass coming from in your design? Yours is ten times more massive than my second design.
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Post by vegemeister on Jan 24, 2017 20:04:28 GMT
The power input and the melting point of the coil material determine the amount of propellant that goes in. If you want to put 10 GW through a single engine, that's how much propellant it needs. If you want to reduce the amount of propellant it takes, you need to put less power in. I tried tweaking your design. Gimbal radius is 4.955. Diamond chamber has greater thermal conductivity and allows thicker wall, smaller throat radius, and less mass. Increased chamber length also helps cooling, for some reason, although 8.4 cm seems to be near a local maximum. I couldn't increase wall thickness/reduce throat radius any more without running into thermal problems. Increasing the chamber contraction ratio to 6 avoids the missing thrust problem. Increased expansion ratio to non-bottle-rocket levels, for better Isp. Of course, if you use decane (or worse, RP-1) resistojets, don't forget to cross to the other side of the street if you see the law of conservation of energy coming your way. This one's 8090% efficient. Where in blazes is the extra mass coming from in your design? Yours is ten times more massive than my second design. From the nozzle. TWR is still well over 1k, and my design has 10% more exhaust velocity. Your second design has an expansion ratio of only 3.2. The extra mass of the engine will be easily made up by the reduction in propellant (and armor, if used) mass.
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Post by newageofpower on Jan 25, 2017 0:25:31 GMT
Of course, if you use decane (or worse, RP-1) resistojets, don't forget to cross to the other side of the street if you see the law of conservation of energy coming your way. This one's 8090% efficient. I suspected that a large fraction of the 'additional' thrust was coming from the disassociation energy of the heavy hydrocarbons disassociating into lighter ones, but I'm dubious of 80x efficiency. *shrug*
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Post by caiaphas on Jan 28, 2017 4:41:34 GMT
Bugger me. Has anyone managed to get a 10 MW decane-fueled MPD above 73% efficiency? I keep hitting a wall with mine.
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Post by deskjetser on Feb 2, 2017 22:45:34 GMT
caiaphas Sorry, can't help you with MPDs. May as well be powered by magic for all I know.
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Post by newageofpower on Feb 3, 2017 1:22:41 GMT
Bugger me. Has anyone managed to get a 10 MW decane-fueled MPD above 73% efficiency? I keep hitting a wall with mine. Don't think it's possible via Decane. amimai apophys and thorneel probably know more, but you might have to switch to a lighter hydrocarbon to get a decent efficiency at such a low power output.
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Post by caiaphas on Feb 3, 2017 6:37:41 GMT
Bugger me. Has anyone managed to get a 10 MW decane-fueled MPD above 73% efficiency? I keep hitting a wall with mine. Don't think it's possible via Decane. amimai apophys and thorneel probably know more, but you might have to switch to a lighter hydrocarbon to get a decent efficiency at such a low power output. Sod. Well, there goes my entire drone fleet. Oh well. Back to the drawing board.
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Post by apophys on Feb 3, 2017 8:48:32 GMT
Oh well. Back to the drawing board. Here's 79.8% efficiency. 10 MW is a bit weak to be doing MPDs with, though. Thanks for tagging me, newageofpower .
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Post by caiaphas on Feb 3, 2017 19:21:59 GMT
Oh well. Back to the drawing board. Here's 79.8% efficiency. 10 MW is a bit weak to be doing MPDs with, though. Thanks for tagging me, newageofpower . You are a saint, thank you.
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Post by kelarkir on Mar 20, 2017 18:22:30 GMT
The 80% efficiency is not the limit
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Post by RiftandRend on Mar 21, 2017 8:39:52 GMT
Oh well. Back to the drawing board. Here's 79.8% efficiency. 10 MW is a bit weak to be doing MPDs with, though. Thanks for tagging me, newageofpower . Why have you armored the gimbal? Any sort of thermal attack would surely destroy the foil thin potassium anode and cathode before the armor, making it useless.
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Post by Enderminion on Mar 21, 2017 11:04:40 GMT
Here's 79.8% efficiency. 10 MW is a bit weak to be doing MPDs with, though. Thanks for tagging me, newageofpower . Why have you armored the gimbal? Any sort of thermal attack would surely destroy the foil thin potassium anode and cathode before the armor, making it useless. its scilicalgel, so it masses praticly nothing
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Post by RiftandRend on Mar 21, 2017 21:16:08 GMT
Why have you armored the gimbal? Any sort of thermal attack would surely destroy the foil thin potassium anode and cathode before the armor, making it useless. its scilicalgel, so it masses praticly nothing It masses at about 200 g and costs ~20 cr. That's a significant waste if deployed on thousands of drones.
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Post by Inglonias on Mar 22, 2017 12:47:57 GMT
I've started experimenting with sodium propellant NTRs because of sodium's impressive results in cooling reactors and results appear very promising. The exhaust velocity is nothing to write home about (best I can do is 2.5 to 2.7 km/sec) but the TWR is INSANE (I was easily able to get engines with 500 to 1 TWR). When you need crazy acceleration for things like drones and knifefight range missiles, these engines appear to be superior to chemical rockets in every respect except for the kilowatts of radiation they spew everywhere, but they're for remote controlled vehicles so who cares?! I've only made 1 and a half engines with this design so far (1 engine, and I tweaked it so my flak missile wouldn't have 38g acceleration, which I don't think the AI can handle) but I hope to continue my experimentation later.
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