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Post by sevenperforce on Apr 6, 2017 22:29:32 GMT
Being able to use an onboard system to refill the propellant tanks in a relatively short period of time -- preferably less than 24 hours, though if it takes as long as a week that would just be something the scenario would have to deal with. It's assumed that you would touch down in an area with access to water, either to use as the propellant or to use to synthesize something else. Pure-water NTR is thus easiest because you only need to factor in the mass of the pump to get the water up into the tanks, and perhaps a filtration system. If you allow a little more time, you can use bimodal NTRs to crack the water into hydrogen and oxygen and then liquify both, permitting you to use a LANTR design. Or you can dump the oxygen and just go with pure liquid hydrogen. Another option would be hydrogen peroxide if you really want to ramp up your thrust and impulse density. With sufficiently advanced near-future tech you could conceivably even synthesize ammonia or hydrazine. If you can come up with a way to get your hands on some CO2 then there are a whole slew of additional options. But trying to fit an entire ISRU unit into the dry mass of an SSTO is starting to push the limits of the rocket equation, even with nuclear thermal power on your side.
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Post by Enderminion on Apr 6, 2017 23:10:18 GMT
just has to be bigger then, maybe build drop tanks with local materials?
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Post by newageofpower on Apr 6, 2017 23:13:39 GMT
If we raise the core temperature to 4400k (TaHfC encapsulating molten liquid fissile metals) we can probably get a decent eV (and thus dV) out of water.
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Post by The Astronomer on Apr 7, 2017 2:09:05 GMT
What's NTR's Isp in Earth's atmosphere?
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Post by sevenperforce on Apr 7, 2017 2:17:13 GMT
What's NTR's I sp in Earth's atmosphere? Depends on propellant and nozzle expansion ratio.
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Post by The Astronomer on Apr 7, 2017 2:21:42 GMT
What's NTR's I sp in Earth's atmosphere? Depends on propellant and nozzle expansion ratio. I wonder why LV-N NERV in the game gives off so horrible I sp.
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Post by n2maniac on Apr 7, 2017 5:00:37 GMT
Try heavy water.
(No, seriously, for some reason it seems to beat water ingame (dissociation assumptions suspected))
Though, I will admit, if RP-1 is out then heavy water should definitely be out.
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Post by sevenperforce on Apr 7, 2017 12:22:40 GMT
Depends on propellant and nozzle expansion ratio. I wonder why LV-N NERV in the game gives off so horrible I sp. IIRC, a hydrogen-rich exhaust is going to have a somewhat greater SL loss than an exhaust with more carbon/oxygen, so that's one source of losses. But the more likely culprit is chamber pressure. The NERVA built in real life was being designed for a Mars transfer propulsion stage, so they were depending on expansion to a vacuum and had no intention of firing it at sea level. I can't find numbers on the chamber pressure for the real-life NERVA, but it was probably very low. A given propellant/engine combination has a certain potential specific impulse, but the actual specific impulse is going to depend on how much of that potential can be extracted. If you're building an engine optimized for vacuum, you can go with a lightweight, low-pressure chamber and stick a large expansion nozzle on the end. However, if you need your engine to operate at sea level, you need to have a higher chamber pressure so expansion to 1 atm extracts a greater percentage of the potential specific impulse. As an extreme example: if your chamber pressure is 2 atm, then operating the engine at sea level (expanding to 1 atm) extracts only half the specific impulse that you'd get from operating the engine in vacuum (expanding to 0 atm). If your chamber pressure is 100 atm, then expanding to 1 atm gets 99% of the specific impulse you'd get from expanding to 0 atm. Of course, that's an oversimplification; it's not a linear relationship. A SL nuclear thermal engine with a higher chamber pressure would have far better performance than the real-life NERVA. Not necessarily. Cracking raw materials into RP-1 is a lot more involved (and takes a lot more energy and systems) than centrifuging heavy water out of light water. That could also be an interesting in-story twist: the engines can run on plain water, but if you have time to spin yourself up some heavy water, they get much better performance.
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Post by newageofpower on Apr 7, 2017 16:20:24 GMT
A quick and dirty build with the Tantalum Hafnium Carbide pellet/liquid fissile metal design allows me to extract 5.44 km/s from plain water at 12 meganewtons while maintaining 260 TWR, while maintaining a core meltdown margin of 10k. I am using a aluminum injector and amco nozzles for safety reasons, and the CoADE simulation does not allow for composite nozzle design, so an IRL NTR could easily surpass this design.
I can probably improve this, but as a proof of concept it's proven it's point.
Smaller thrusters can comfortably pass 5.52 km/s, but for an SSTO design we need multi-meganewton setups.
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Post by gedzilla on Apr 8, 2017 18:34:02 GMT
A quick and dirty build with the Tantalum Hafnium Carbide pellet/liquid fissile metal design allows me to extract 5.44 km/s from plain water at 12 meganewtons while maintaining 260 TWR, while maintaining a core meltdown margin of 10k. I am using a aluminum injector and amco nozzles for safety reasons, and the CoADE simulation does not allow for composite nozzle design, so an IRL NTR could easily surpass this design. I can probably improve this, but as a proof of concept it's proven it's point. Smaller thrusters can comfortably pass 5.52 km/s, but for an SSTO design we need multi-meganewton setups. Smaller thrusters can easily pass 5.5km/s ? For water ? I never got higher than 4.2km/s ish on water NTR's. Mind showing the design ?
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Post by ash19256 on Apr 8, 2017 19:26:59 GMT
A quick and dirty build with the Tantalum Hafnium Carbide pellet/liquid fissile metal design allows me to extract 5.44 km/s from plain water at 12 meganewtons while maintaining 260 TWR, while maintaining a core meltdown margin of 10k. I am using a aluminum injector and amco nozzles for safety reasons, and the CoADE simulation does not allow for composite nozzle design, so an IRL NTR could easily surpass this design. I can probably improve this, but as a proof of concept it's proven it's point. Smaller thrusters can comfortably pass 5.52 km/s, but for an SSTO design we need multi-meganewton setups. Smaller thrusters can easily pass 5.5km/s ? For water ? I never got higher than 4.2km/s ish on water NTR's. Mind showing the design ? I suspect that newageofpower increased the the throat radius beyond what was necessary to make the engine function, to allow them to use more of the dissociation energy of the water, which allows for more thrust and exhaust velocity without the need to modify the reactor beyond what is necessary to keep the reactor functional.
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Post by newageofpower on Apr 8, 2017 22:18:48 GMT
Smaller thrusters can easily pass 5.5km/s ? For water ? I never got higher than 4.2km/s ish on water NTR's. Mind showing the design ? At work right now. Read the goddamn posts before opening your piehole - I am using Tantalum Hafnia Carbide encapsulating liquid Uranium (modded fuel) for a core temperature of 4478k.
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Post by gedzilla on Apr 8, 2017 23:14:04 GMT
first off chill, i was just asking.
second; Ah, modded fuel.
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Post by RiftandRend on Apr 8, 2017 23:14:59 GMT
first off chill, i was just asking. second; Ah, modded fuel. Modded, but fairly realistic.
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Post by Enderminion on Apr 8, 2017 23:21:26 GMT
if you get anything hot enough it will go fast enough
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