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Post by lennson on Aug 20, 2017 16:21:04 GMT
I assume the reason hydrocarbon (e.g. methane and decane) have fairly good exhaust velocities in a NTR is that they effectively are behaving as hydrogen storage (the reactor causes the molecules to disintegrate and one ends up with an exit velocity that depends on the composite atoms).
Having said that has anyone considered lithium hydride in a NTR? It seems it may be better than methane (possibly by quite a bit since the lithium atom used by itself is much lighter than the carbon atom used in methane, which I assume could give a better average exit velocity).
edit: If one were to put lithium-6 deuteride through a NTR could any significant fraction of the lithium be converted to tritium and helium from neutron bombardment?
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Post by matterbeam on Aug 20, 2017 21:38:15 GMT
I assume the reason hydrocarbon (e.g. methane and decane) have fairly good exhaust velocities in a NTR is that they effectively are behaving as hydrogen storage (the reactor causes the molecules to disintegrate and one ends up with an exit velocity that depends on the composite atoms). Having said that has anyone considered lithium hydride in a NTR? It seems it may be better than methane (possibly by quite a bit since the lithium atom used by itself is much lighter than the carbon atom used in methane, which I assume could give a better average exit velocity). edit: If one were to put lithium-6 deuteride through a NTR could any significant fraction of the lithium be converted to tritium and helium from neutron bombardment? It 3000K+ temperatures, hydrogen bonds dissociate, releasing atomic hydrogen from hydrogen-containing molecules. Taking methane as an example, exhaust velocity at 3000K, as measured by the root mean square method, should be 2.16km/s. However, thermal dissociation means that each mole of methane releases 4 moles of hydrogen and 1 mole of carbon, lowering the average molar mass of the exhaust gasses down to (4*1 + 12*1)/5: 3.2g/mol. This raises effective exhaust velocity to 4.8km/s. If lithium hydride undergoes thermal decomposition, the average molar mass of the exhaust gasses is (7*1 + 1*1)/2: 4g/mol. This makes it a worse propellant than methane, in addition to being a solid metal.
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Post by lennson on Aug 21, 2017 0:26:30 GMT
It would seem that beating methane in terms of exhaust velocity is rather hard because of how it stores hydrogen.
If one were to use lithium-6 hydride and were able to split the lithium-6 into tritium and helium in the NTR, using excess neutron radiation, then in theory this would give lower average molar mass of the exhaust gasses, (1 + 3 + 4)/3: 2.7 g/mol, but it doesn't look like this would be at all practical.
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Post by n2maniac on Aug 21, 2017 6:45:00 GMT
It would seem that beating methane in terms of exhaust velocity is rather hard because of how it stores hydrogen. If one were to use lithium-6 hydride and were able to split the lithium-6 into tritium and helium in the NTR, using excess neutron radiation, then in theory this would give lower average molar mass of the exhaust gasses, (1 + 3 + 4)/3: 2.7 g/mol, but it doesn't look like this would be at all practical. Eh, given the lithium fission would substantially increase the exhaust gas temperature. Would work better so long as the fission fuel doesn't get too much of the heating (spoiler: it probably will). Do note that the reactor's reactivity would be stupid hard to control with such a potent neutron absorber flowing through it.
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Post by Pttg on Aug 21, 2017 16:45:50 GMT
Use the hydrogen in your cruise thruster, and use oxygen in your combat thruster.
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Post by bigbombr on Aug 21, 2017 17:04:25 GMT
Use the hydrogen in your cruise thruster, and use oxygen in your combat thruster. You have considerably more other atoms than hydrogen atoms. Example: if your storage molecule is water, you have 16 kg of oxygen for every 2 kg of hydrogen. So I don't really see the point.
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Post by The Astronomer on Aug 21, 2017 22:41:13 GMT
That's why methane looks preferable as heck. If you combine it with water, you get CO2, which can be used to produce ethanol (artificial photosynthesis) or glucose and starch (in your algae vats, if you have any).
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Post by Enderminion on Aug 23, 2017 2:09:09 GMT
That's why methane looks preferable as heck. If you combine it with water, you get CO2, which can be used to produce ethanol (artificial photosynthesis) or glucose and starch (in your algae vats, if you have any). space booze, would just shunting pure methane through the NTR be better though?
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Post by The Astronomer on Aug 23, 2017 5:03:37 GMT
That's why methane looks preferable as heck. If you combine it with water, you get CO2, which can be used to produce ethanol (artificial photosynthesis) or glucose and starch (in your algae vats, if you have any). space booze, would just shunting pure methane through the NTR be better though? Coking.
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Post by bigbombr on Aug 23, 2017 5:21:51 GMT
space booze, would just shunting pure methane through the NTR be better though? Coking. NTR's make fairly short burns though, only a few minutes. Maintenance can be done whenever you refuel.
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Post by n2maniac on Aug 27, 2017 3:19:06 GMT
NTR's make fairly short burns though, only a few minutes. Maintenance can be done whenever you refuel. Not by radiation-sensitive operators (eg. humans).
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Post by matterbeam on Aug 27, 2017 18:10:38 GMT
NTR's make fairly short burns though, only a few minutes. Maintenance can be done whenever you refuel. Not by radiation-sensitive operators (eg. humans). Maintenance would include either flushing hot oxygen down the tubes or going to a workshop, removing the engine core and scrubbing off the carbon deposits.
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Post by Enderminion on Aug 27, 2017 18:34:12 GMT
Not by radiation-sensitive operators (eg. humans). Maintenance would include either flushing hot oxygen down the tubes or going to a workshop, removing the engine core and scrubbing off the carbon deposits. our engine bells are made of carbon, and neutron embitterment would mean it would still be radioactive
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Post by 𝕭𝖔𝖔𝖒𝖈𝖍𝖆𝖈𝖑𝖊 on Oct 13, 2017 5:14:50 GMT
Hydrogen is great. It is light and simple, so most rockets prefer it. However, storing hydrogen is a pain in the ass, because due to its small size and extremely low boiling point, it tends to boil off and leak out of your tanks. Water (H 2O), Ammonia (NH 3) and Methane (CH 4) are some of the lightest hydrogen-containing molecules. They are far easier to keep in tanks than hydrogen, so if you can find ways to separate hydrogen from those molecules, they looks promising. Water can be electrolyzed, producing hydrogen gas and oxygen gas. I suspect the same can be done for ammonia...? 2H 2O + energy ---> 2H 2 + O 22NH 3 + energy ---> 3H 2 + N 2For methane, the process is a bit more interesting. The most commonly used method is steam reforming, which reacts hydrocarbons with steam water. In methane steam reforming, this produces carbon monoxide and hydrogen gas. Carbon monoxide can react with another water molecule to form carbon dioxide and more hydrogen gas. CH 4 + H 2O ---> CO + 3H 2 (endothermic) CO + H 2O ---> CO 2 + H 2 (exothermic) Other possible storage molecules includes lithium hydride (LiH), which reacts with water to form lithium, hydrogen gas and hydroxide ion. 2LiH + H 2O ---> 2Li + 2H 2 + OH -Acids like plain hydrogen chloride should also work. Reacting them with some metal. Simple middle school chemistry. If you manage to find some, though. Hydrogen moles per molecular mole, hydrogen grams per kilogram of substance (approx.)Lithium hydride: 8 g/mol. 0.5 moles of hydrogen molecules for each molecular mole, ~125 g for each kilogram. Beryllium hydride: 11 g/mol. 1 moles of hydrogen molecules for each molecular mole. ~182 g for each kilogram. Methane: 16 g/mol. 2 moles of hydrogen molecules for each molecular mole. ~250 g for each kilogram. Ammonia: 17 g/mol. 1.5 moles of hydrogen molecules for each molecular mole. ~176 g for each kilogram. Water: 18 g/mol. 1 moles of hydrogen molecules for each molecular mole. ~111 g for each kilogram. Ammonia Borane: 29 g/mol. 6 moles of hydrogen molecules for each molecular mole. ~ 207 g for each kilogram. good idea. water can give you a ridiclous tank mass to propellant mass ratio because it is a liquid, and you can harvest asteroids for ice to get the hydrogen and oxygen in the form of water. because all of our ships have nuclear reactors, we could use those to seperate them and give a ridiclously efficient fuel tank which only holds water but can be separated into oxygen and hydrogen.
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