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Post by zuthal on May 14, 2017 8:51:37 GMT
I think there is a relative consensus on this forum that methane (and the higher hydrocarbons) are basically the best NTR propellants, with a good mix of density and exhaust velocity. However, IRL, there is a significant problem with using hydrocarbons in a sufficiently-hot NTR: The thermal decompositions of hydrocarbons produces solid carbon, which can clog up the internal plumbing of the NTR.
Thus my question is, is there any way of preventing that coking, besides running the NTR cold enough that the hydrocarbons do not decompose?
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Post by nerd1000 on May 14, 2017 11:41:47 GMT
I think there is a relative consensus on this forum that methane (and the higher hydrocarbons) are basically the best NTR propellants, with a good mix of density and exhaust velocity. However, IRL, there is a significant problem with using hydrocarbons in a sufficiently-hot NTR: The thermal decompositions of hydrocarbons produces solid carbon, which can clog up the internal plumbing of the NTR. Thus my question is, is there any way of preventing that coking, besides running the NTR cold enough that the hydrocarbons do not decompose? The solution is simple: Don't use hydrocarbons as propellant in a NTR . If you really must use hydrocarbons: Run the NTR for a short enough time that coking isn't a major issue, add something to the propellant stream that reacts with carbon to form a gas or run the NTR so hot that the carbon deposits become vapour (It'll have to be a liquid or gas core NTR).
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Post by matterbeam on May 14, 2017 11:56:00 GMT
I think there is a relative consensus on this forum that methane (and the higher hydrocarbons) are basically the best NTR propellants, with a good mix of density and exhaust velocity. However, IRL, there is a significant problem with using hydrocarbons in a sufficiently-hot NTR: The thermal decompositions of hydrocarbons produces solid carbon, which can clog up the internal plumbing of the NTR. Thus my question is, is there any way of preventing that coking, besides running the NTR cold enough that the hydrocarbons do not decompose? If you really must use hydrocarbons: Run the NTR for a short enough time that coking isn't a major issue, add something to the propellant stream that reacts with carbon to form a gas or run the NTR so hot that the carbon deposits become vapour (It'll have to be a liquid or gas core NTR). Running oxygen through the reactor can scrub away the carbon, but hot oxygen voraciously attacks everything else too... Running the reactor hot enough to vaporize carbon soot requires temperatures of over 3000K. The solution is to physically remove it or just replace the exposed parts for cleaning. You can 'scrub' the soot with inert gas containing sand, or you can scrape it off. The problem with the latter is that the soot traps the radioactive particles emitted from fission fuels, so you have to handle it even more carefully than refined uranium.
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Post by zuthal on May 14, 2017 12:14:01 GMT
Hmmm... well, not using HCs means compromising either propellant density (LH2) or exhaust velocity (ammonia, water)... though I guess ammonia wouldn't be too bad a propellant, density of 682 kg/m^3 at the boiling point, and the breakdown product has an average molar mass of 8.5 g/mol (assuming full recombination to N2 and H2, compared to 5.33 g/mol for methane with hydrogen recombination) to 4.86 g/mol (assuming the hydrogen stays dissociated, but the nitrogen recombines, compared to 3.2 g/mol for methane). As exhaust velocity is proportional to sqrt(1/[molar mass]), an ammonia NTR would be expected to have about 80% of the exhaust velocity of a comparable methane NTR - which for our NTRs would mean about 5.1 km/s, comparable to decane NTRs, with a propellant density only slightly less than that of decane.
Also, regaring chemically clearing the carbon soot: Would running hydrogen through at a temperature below the thermal decomposition temperature of methane work, hydrating the carbon deposites into methane?
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Post by vegemeister on May 14, 2017 13:52:25 GMT
Maybe mix in some water? IANAC, but I think that should react with the coke to produce CO and hydrogen, both of which are volatile.
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Post by ash19256 on May 14, 2017 14:53:03 GMT
See, I just use Graphene coated fuel rods, then rely on the fact that the regenerative cooling loops generally don't get hot enough for decomposition and the nozzle is either too hot for decomposition to work, or the exhaust is moving so fast that any blockage is simply blasted out of the engine by the Mach 19+ exhaust. Although it probably doesn't help matters that I use icosane as a go-to propellant in my reactors.
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Post by Rocket Witch on May 14, 2017 15:56:03 GMT
Running oxygen through the reactor can scrub away the carbon, but hot oxygen voraciously attacks everything else too... I did mod in methanol and ethanol, which contain oxygen conveniently pre-bound to the carbon. Methanol in particular has a 1:1 ratio of C and O, which may yield carbon monoxide enough to carry away all the potential deposits in an engine tuned for exactly 80% dissociation (at least I assume the 4 hydrogens would separate earlier than the CO bond). Other alternatives are methane analogues. Silanes, boranes, et al. Since basically everything else is softer and melts more easily than carbon, they might deposit less in a solid core NTR and/or be easier to remove.
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Post by zuthal on May 14, 2017 17:26:10 GMT
Yeah, the boron that results from the decomposition of diborane melts at ~2300 K, so it will be definitely molten inside the core - and then come out as a fine spray, carried along by a jet of hot hydrogen.
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Post by apophys on May 15, 2017 0:36:26 GMT
Another possible alternative is methylamine (CH3NH2), which would have reduced carbon buildup due to cyanide (CN) in the exhaust.
Anything with any carbon in it will suffer some buildup, because a real thruster plume is not static, but contains a soup of viable compounds, of which the exact composition depends on temperature and pressure.
The easiest solutions to avoiding NTR carbon buildup are hydrogen deuteride, heavy water, and ammonia. With modding: diborane, silane, lithium hydride, lithium borohydride. Probably covers everything notable.
Ammonia borane would have the closely related issue of boron nitride deposition.
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Post by The Astronomer on May 15, 2017 7:53:45 GMT
Turns out, the best fuel that's not hydrogen-stuff or hydrocarbon-stuff is heavy water. It is significantly denser than RP-1, with 4.36 km/s exhaust velocity. Ammonia (~4 km/s for U-233 dioxide) sucks. Methane analogs (2-3 km/s for U-233 dioxide) sucks.
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Post by ash19256 on May 15, 2017 17:05:58 GMT
Turns out, the best fuel that's not hydrogen-stuff or hydrocarbon-stuff is heavy water. It is significantly denser than RP-1, with 4.36 km/s exhaust velocity. Ammonia (~4 km/s for U-233 dioxide) sucks. Methane analogs (2-3 km/s for U-233 dioxide) sucks. Unless, of course, you happen to like getting halfway decent performance out of your engines, at which point RP-1 is good enough. Of course, if you don't mind mods, then icosane is the way to go.
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Post by The Astronomer on May 15, 2017 17:07:48 GMT
Turns out, the best fuel that's not hydrogen-stuff or hydrocarbon-stuff is heavy water. It is significantly denser than RP-1, with 4.36 km/s exhaust velocity. Ammonia (~4 km/s for U-233 dioxide) sucks. Methane analogs (2-3 km/s for U-233 dioxide) sucks. Unless, of course, you happen to like getting halfway decent performance out of your engines, at which point RP-1 is good enough. Of course, if you don't mind mods, then icosane is the way to go. Isn't RP-1 hydrocarbon? The problem of them is coking, as stated in the thread's name.
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Post by ash19256 on May 15, 2017 17:09:02 GMT
Unless, of course, you happen to like getting halfway decent performance out of your engines, at which point RP-1 is good enough. Of course, if you don't mind mods, then icosane is the way to go. Isn't RP-1 hydrocarbon? The problem of them is coking, as stated in the thread's name. ... fair enough.
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Post by newageofpower on May 16, 2017 3:48:50 GMT
I've stated this before, but our rocketships have a NTR burn time measured in minutes, sometimes under 5 minutes. Coking builds up, send in cleaning robot, need deceleration burn, remove cleaning robot.
Plus, our rocket nozzles will likely be plated in Amorphous Carbon (thermal conductivity, melting resistance)... So carbon buildup wouldn't even be a big issue, imho.
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Post by matterbeam on May 16, 2017 6:30:28 GMT
I've stated this before, but our rocketships have a NTR burn time measured in minutes, sometimes under 5 minutes. Coking builds up, send in cleaning robot, need deceleration burn, remove cleaning robot. Plus, our rocket nozzles will likely be plated in Amorphous Carbon (thermal conductivity, melting resistance)... So carbon buildup wouldn't even be a big issue, imho. The way I understand it is that coking affects the reactor core's uranium rods by changing their thermal conductivity. This means that the hot uranium is not being cooled effectively by the propellant flow, and this causes a meltdown. Soot deposits on the nozzle is of minor consequence.
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