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Post by thorneel on Sept 8, 2018 23:57:43 GMT
According to the game, hydrogen deuteride is a strictly better thermal propellant than dihydrogen, being denser, slightly less unwieldy and giving a higher Isp thanks to its lower energy of disassociation. According to this LOx-HD and LOx-D2 combustion mod, as a chemical propellant, it gives about 5% less exhaust velocity than H2 (and again 5% less for D2). However, it is still two thirds denser (and denser again for D2), density being the biggest problem with H2 (the other biggest problem, temperature, is alas unchanged). Ignoring for now the costs, would that make it a good choice for today's rockets, compared to existing options? Assuming it would, fuel costs are generally almost negligible with rockets. Would it be possible to mass-produce it enough to drive down costs to the same order of magnitude than currently used propellants? If not, would it be at least possible to drive the cost down enough to make them available for an hypothetical contemporary NTR? |
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Post by RiftandRend on Sept 9, 2018 8:53:06 GMT
I think so, but only if the cost of deuterium can be reduced by a factor of 10 or more. Hydrogen goes for 15 USD per kilo while 99.96% deuterium goes for about 130-700 USD per kilo.
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Post by treptoplax on Sept 9, 2018 16:07:22 GMT
Currently cost-per-kilo to LEO is well north of $1000/Kg anyway - maybe $1500 best-case with a Falcon Heavy? So that wouldn't seem an implausible fuel cost for an interplanetary stage.
Now, if SpaceX can actually deliver their BFR at anything remotely close to their targeted $75 (!) per kilo that's another matter.
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Post by AtomHeartDragon on Sept 9, 2018 16:25:39 GMT
I think an interesting flip question here is how much more expensive will our NTR/the rest of our stack get if we can't have HD? Is it going to be more or less than the extra propellant cost? What's the tipping point?
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Post by Anon1 on Sept 12, 2018 23:47:55 GMT
According to the game, hydrogen deuteride is a strictly better thermal propellant than dihydrogen, being denser, slightly less unwieldy and giving a higher Isp thanks to its lower energy of disassociation. According to this LOx-HD and LOx-D2 combustion mod, as a chemical propellant, it gives about 5% less exhaust velocity than H2 (and again 5% less for D2). However, it is still two thirds denser (and denser again for D2), density being the biggest problem with H2 (the other biggest problem, temperature, is alas unchanged). Ignoring for now the costs, would that make it a good choice for today's rockets, compared to existing options? Assuming it would, fuel costs are generally almost negligible with rockets. Would it be possible to mass-produce it enough to drive down costs to the same order of magnitude than currently used propellants? If not, would it be at least possible to drive the cost down enough to make them available for an hypothetical contemporary NTR? Actually, the biggest problem with liquid hydrogen is that it leaks so badly.
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Post by AtomHeartDragon on Sept 13, 2018 12:25:49 GMT
According to the game, hydrogen deuteride is a strictly better thermal propellant than dihydrogen, being denser, slightly less unwieldy and giving a higher Isp thanks to its lower energy of disassociation. According to this LOx-HD and LOx-D2 combustion mod, as a chemical propellant, it gives about 5% less exhaust velocity than H2 (and again 5% less for D2). However, it is still two thirds denser (and denser again for D2), density being the biggest problem with H2 (the other biggest problem, temperature, is alas unchanged). Ignoring for now the costs, would that make it a good choice for today's rockets, compared to existing options? Assuming it would, fuel costs are generally almost negligible with rockets. Would it be possible to mass-produce it enough to drive down costs to the same order of magnitude than currently used propellants? If not, would it be at least possible to drive the cost down enough to make them available for an hypothetical contemporary NTR? Actually, the biggest problem with liquid hydrogen is that it leaks so badly.
Is HD going to be any better in this regard? Container's walls are unlikely to see any difference between presence and lack of additional neutron. The only things that can make difference here seem to be dipole moment and rotation.
Edit: And molecule velocities - that might make *some* difference. |
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Post by Anon1 on Sept 13, 2018 19:24:21 GMT
Actually, the biggest problem with liquid hydrogen is that it leaks so badly.
Is HD going to be any better in this regard? Container's walls are unlikely to see any difference between presence and lack of additional neutron. The only things that can make difference here seem to be dipole moment and rotation.
Edit: And molecule velocities - that might make *some* difference. Even tritium leaks. If you need long term storage, then pure hydrogen propellant is not your best option.
As a note, if you want fusion, then don't store your deuterium as liquid deuterium. Instead, store it in a liquid compound with something else. The extra mass can be used to augment thrust because you don't need pure fusion exhaust velocities to get around the solar system anyway. As a side benefit, you will be able to store it denser that way, so that your ship doesn't look like a giant balloon.
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Post by AtomHeartDragon on Sept 13, 2018 19:58:11 GMT
Is HD going to be any better in this regard? Container's walls are unlikely to see any difference between presence and lack of additional neutron. The only things that can make difference here seem to be dipole moment and rotation. Edit: And molecule velocities - that might make *some* difference. Even tritium leaks. If you need long term storage, then pure hydrogen propellant is not your best option. Well, tritium is also an isotope of hydrogen. With fusion you are not going to be bitten too hard by the extra mass and extra thrust could definitely help. It might be nice to engineer your compound's stoichiometry to benefit from something like CNO cycle, but the requirements for fusion are already gnarly enough as it is. |
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Post by Apotheon on Sept 15, 2018 19:23:00 GMT
If hydrogen really leaks at 1% per day, that's 25% in a month... pretty frustrating... is this fixable by 2250? If not, I guess I should switch my ships back to methane again. If this number also applies equally in space?
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ghgh
Full Member
Still trying to make kinetics work.
Posts: 136
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Post by ghgh on Sept 15, 2018 20:33:12 GMT
I guess it depends on the rate at which you consume the propellant. I imagine having different tank ratios would change the loss rate.
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Post by apophys on Sept 15, 2018 22:23:33 GMT
The only way to fix it is to have a double-walled container, and use a vacuum pump to constantly remove and re-condense the hydrogen leaking into the gap. This of course worsens your already-poor mass ratio via the double wall, and adds a mechanical point of failure (the pump & condenser) along with constant electricity use. Methane is much more reasonable.
Fusion ships would want their fuel to be lithium-6 deuteride if possible, since that does not have the problem of hydrogen leaking out. (Farther future fusion could use hydrogen+boron, stored as a borane mixture. But that's hard to ignite.)
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