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 Post subject: Re: Space Solar Power
PostPosted: May 03, 2016 11:05 pm 
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I am always loathe to dampen enthusiasm about nuclear energy, but I think for distances of less than about 2 AU from the Sun, it's pretty hard to beat solar power for most applications. The reasons that solar power is lousy on the ground really go away, and nuclear power loses some of its greatest advantages in comparison.

Solar-powered spacecraft have no cloud cover to contend with. Eclipse periods are known perfectly even when the spacecraft is in the design phase. Proper depth-of-discharge of energy storage systems (batteries) can be sized with little problem. Geosynchronous satellites exhibit almost no eclipse periods and spacecraft in transit between planets are in full sun. Highly-efficient solar cells that are utterly cost-prohibitive on Earth are no big deal in space, since launch costs dwarf everything else.

Nuclear power sources, particularly in an orbital environment, suffer from several large drawbacks. The largest one of all is the loss of a convenient convective heat sink such as air or water. Waste heat must be rejected by radiation, whose effectiveness is proportional to the fourth power of radiative temperature. One can scarcely appreciate what ANYTHING to the fourth power means. But the upshot is this, the spacecraft must run hot, and the reactor REALLY hot.

The kinetic energy embodied in the orbit of the nuclear powered spacecraft is an ever-present risk, since it always must be considered an "energy-release" term that can breach the containment of the reactor system and lead to wide dispersal of radioactive materials. Thus the nuclear-powered spacecraft's orbital altitudes are always constrained to those of an exceptionally long lifetime, and this limits its usefulness in low orbits where eclipse times might otherwise give nuclear power an advantage. At higher "nuclear-safe" orbits the advantages of solar power only compound.

Of course, as one ventures deeper into space, the advantages of nuclear power accumulate and eventually become overwhelming. But most spacecraft operate at less than 2 AU from the Sun, and in this arena solar power works pretty darn well for spacecraft.

Now of course the surface of the Moon or Mars and deep lunar craters are something else entirely...


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PostPosted: May 03, 2016 11:08 pm 
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Kurt Sellner wrote:
Is it inconceivable to have something much like an Ohio class submarine in orbit? Much like the submarine it'd be a military space station with armor and weapons, a crew of over 150 people, powered by a nuclear reactor, used as a means to project power over the globe.


Yes, utterly, completely, totally inconceivable and flawed on so many different levels.


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PostPosted: May 04, 2016 12:40 am 
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Kirk Sorensen wrote:
Kurt Sellner wrote:
Is it inconceivable to have something much like an Ohio class submarine in orbit? Much like the submarine it'd be a military space station with armor and weapons, a crew of over 150 people, powered by a nuclear reactor, used as a means to project power over the globe.


Yes, utterly, completely, totally inconceivable and flawed on so many different levels.

Yes, I got carried away there. I do believe that at some point we will have more and bigger space stations besides the ISS. Space tourism is growing and people do expect to have a sort of orbital hotel at some point to visit. This station will need power to keep the occupants comfortable and entertained. Unlike an unmanned craft where power loss could be considered a temporary inconvenience the loss of power on a manned craft could be life threatening.

Solar panels and batteries are great for orbital operations but for a manned spacecraft it would be wise to have the means to power critical systems in case of a loss of solar panel output. RTGs have been shown to work and so that seems a logical choice. If this station is large enough then perhaps we might experiment with nuclear reactors in orbit again. If we limit ourselves to manned craft in orbit then shipping fuel from Earth is an obvious means to provide backup power. If we don't limit ourselves to earth orbit then nuclear power becomes much more attractive. There are a number of people that think they can make a business out of taking people to space, and beyond earth orbit. Some of these plans I've seen include nuclear power.

Should we see space tourism become more common then we can expect to see a sort of "coast guard in space" develop. I envision a kind of government agency that performs the duties much like the US Coast Guard provides at sea but within orbit, such as law enforcement, search and rescue, and also serve military duties when required. That's when my mind took me to the "submarine in space" idea.

If we do see military operations in space, and I believe that it is inevitable if we keep sending people there, then I expect these military craft will want a power source that is more compact and less fragile than solar panels. Fuel cells are great but RTGs can run for much longer at a given mass.

We've seen nuclear material fall back to earth from failed launches and aged satellites and I have to wonder how much of a threat this truly poses to people. Certainly the total mass of this material should be taken into account in determining the threat. I have to wonder if this threat is for real or just political, because no nation wants to see another drop trash within their borders. This is doubly so when that trash is radioactive.

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Disclaimer: I am an engineer but not a nuclear engineer, mechanical engineer, chemical engineer, or industrial engineer. My education included electrical, computer, and software engineering.


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