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Post by SevenOfCarina on Apr 28, 2017 17:21:10 GMT
I'll go with a combination then of both, then. On a side note, how badly would a hydrocarbon like methane clog up an MPDT? Are there any other complications arising from the use of methane? I'd imagine that drives would be capable of using both hydrogen and methane, as, while methane is easily produced on Mars, I don't recall carbon being particularly abundant on Luna, and shipping methane or carbon up from Earth is not happening. Not when there's a ready supple of water-ice to disassociate to obtain hydrogen. Also, is it more efficient to shed excess velocity using aerobraking or to simply decelerate into orbit, considering heat shield masses and g-forces? If spacecraft could remain in a high enough orbit, NTRs become useless and are phased out quickly. Brawny NTR thrust would still be useful for operations within a gravity well, though.
And regarding fissiles, are there any reliable sources within reach, apart from Earth? Mars should have some, but unless reserves aren't exploited fast enough, interplanetary travel will regress to chemical-only.
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Post by princesskibble on Apr 28, 2017 17:24:03 GMT
Also, is it more efficient to shed excess velocity using aerobraking or to simply decelerate into orbit, considering heat shield masses and g-forces? Assuming you have the resources to design and build a heat shield for the mission, it will almost aways be more efficient to aerocapture
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Post by newageofpower on Apr 28, 2017 17:51:31 GMT
Also, is it more efficient to shed excess velocity using aerobraking or to simply decelerate into orbit, considering heat shield masses and g-forces? Assuming you have the resources to design and build a heat shield for the mission, it will almost aways be more efficient to aerocapture I wouldn't say always (low density bulk cargo requires gigantic inefficient heatshield) but for huuuge percent of cases it is a net gain. Methane distillation from CO2 and Hydrogen Sulfide is insanely energy intensive; unlikely to be used for local operations. Carbon buildup is a problem, but given the sheer amount of time in a transfer you can certainly send in a cleaning robot and turn the machinery off. With fissile fuel breeding, actual usable fissile reserves expand by more than two orders of magnitude. Fissile mining is one of the operations on Venus that would be very economically efficient due to the value of fissionable materials. Martian Uranium is likely to be less rich than Venusian or Terran deposits, but more accessible. Some Belter asteroid groups are likely to be rich in fissiles as well. EDIT: I believe I've said this before, but that's a hilariously lewd avatar.
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Post by princesskibble on Apr 28, 2017 17:53:49 GMT
Assuming you have the resources to design and build a heat shield for the mission, it will almost aways be more efficient to aerocapture EDIT: I believe I've said this before, but that's a hilariously lewd avatar. Oh thanks lol :3 You should see the whole picture!
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Post by SevenOfCarina on Apr 28, 2017 17:56:21 GMT
But heatshield mass alone isn't everything. With higher g loads, structural masses shoot up like crazy, since your spacecraft now has to handle tens of meters per second per second of deceleration, whereas with MPDTs, acceleration is in the milligees. Aerobraking might also necessitate a secondary high thrust engine to boost periapsis fast enough or to climb back out of the gravity well.
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Post by princesskibble on Apr 28, 2017 17:59:10 GMT
With higher g loads, structural masses shoot up like crazy, since your spacecraft now has to handle tens of meters per second per second of deceleration The G-loads never have to be that high, especially if you can do multiple passes
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Post by SevenOfCarina on Apr 28, 2017 18:05:05 GMT
But the initial pass, at least, needs to be in the hundereds of milligees. And a a secondary engine might still be needed. The problem might be exacerbated with a more rarified atmosphere, like that of Mars. And since we're using electrical drives that are using constant boost trajectories anyway, no reason not to complete the burn.
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Post by newageofpower on Apr 28, 2017 18:12:20 GMT
But the initial pass, at least, needs to be in the hundereds of milligees. And a a secondary engine might still be needed. The problem might be exacerbated with a more rarified atmosphere, like that of Mars. And since we're using electrical drives that are using constant boost trajectories anyway, no reason not to complete the burn. I was discussing low orbit-capable craft, and assumed you'd carry a set of nuclear torches because my god it takes forever to climb out of a deep gravwell with electric power. With nuke torches, you're going to need the structural integrity to endure high acceleration anyways.
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Post by SevenOfCarina on Apr 28, 2017 18:17:53 GMT
What I meant was that if you don't aerobrake, you don't need nuke thermal rockets, so you don't need high-mass load-bearing struts. No heatshield, no extra thrust truss, no heavy NTR/reactor hybrid, a little more propellant. I would expect that complexity should be significantly reduced, which would lower costs. The extra propellant mass can't be that bad.
Edit : "Facepalm"
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Post by Enderminion on Apr 28, 2017 19:23:45 GMT
a magnetic heat-shield works above mach 12, and at mach 12 heat shields no longer have to ablate
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Post by SevenOfCarina on Apr 29, 2017 5:42:43 GMT
But is it worth the mass cost and added complexity?
Also, how feasable is a spinning skyhook in orbit to assist launches and help reduce reentry velocities?
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Post by The Astronomer on Apr 29, 2017 5:55:34 GMT
But is it worth the mass cost and added complexity? Also, how feasable is a spinning skyhook in orbit to assist launches and help reduce reentry velocities? I don't see any problem with skyhooks. It just have to be very massive compared to the vehicle using it, and don't forget the momentum, you can't just launch without reboosting it back into original orbit.
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Post by SevenOfCarina on Apr 29, 2017 6:06:09 GMT
No need for reboosting if you're shipping cargo downwell as well. Asteroid mining and zero-gee manufacturing would mean significant exports from space to Earth.
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Post by The Astronomer on Apr 29, 2017 6:56:17 GMT
No need for reboosting if you're shipping cargo downwell as well. Asteroid mining and zero-gee manufacturing would mean significant exports from space to Earth. That's also counted as reboosting. Anyways, having some reboosting electrodynamic tethers (EDTs) on board should help too.
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Post by SevenOfCarina on Apr 29, 2017 7:07:29 GMT
Er, chemical/NTR reboosting. But having a reboosting tether would help too, at least as a backup. Also, assuming Earth population = ~2.86 billion, would a Lunar population of ~10 million, an Earth orbital/Lagrange point population of ~250,000-400,000, a Martian population of ~100,000, a Venusian population of ~4,000, a Belt population of ~5,000-10,000 and a Miscellaneous population of ~1,000 make sense? Is Earth population too low, assuming declining fertility rates and no 'hot' warfare in the interim?
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