19 thoughts on “A Space Nuclear Reactor For Mars”

  1. I’m not sure the size is in a sweet spot. One of the reactors, without shielding, weights 3,300 lbs (1,500 kg) and outputs 10 kW, which comes to 6.67 Watts/kg.

    You should get the same performance with about 30+ kW Mars-noon worth of solar cells, which should come in at about 150 kg using Vanguard Space’s 400 W/kg cells derated for Mars, along with about 850 kg of lithium-ion batteries for an 18-hour discharge time at 10 kW. The solar/battery package would weight two-thirds as much as the nuclear power source and wouldn’t have any moving parts, and that’s using numbers for batteries that are in common use.

    At such low power levels, sufficient to run four or five hair dryers, it might not be worth the hassle of going through all the hoops for launch approval, plus the enormous costs of anything associated with nuclear power sources.

    I’d expect nuclear’s big power-density advantages to kick in at much higher power levels.

    1. May I ask the source of your battery estimate?

      Because the numbers I find for the current ISS lithium batteries are about three times less efficient. About 15 Kwh per 200 kg battery ORU.

      1. 15 kWh per 200 kg is 45 Wh/kg, but some lithium batteries are 205 Wh/kg and higher. I went with 200 Wh/kg. Tesla Model 3’s batteries are 247 Wh/kg.

    1. Looking at some NASA data on optical thickness and diffuse insolation during the dust storms, upping the solar collector by a factor of four or so might be sufficient to get through them. If so, the particular solar panels I picked for the thought experiment (which are less than half the state of the art in W/kg) would bring the total mass even with the nuclear solution. Of course that would still require keeping the solar cells cleaned off.

      Also, during a dust storm nobody should be doing much EVA, so they won’t be getting helmet hair, and thus won’t be shampooing and using hair dryers nearly as much. This should dramatically cut their required power consumption during those periods.

      It would take digging into details, but solar would probably be a competitive solution for an early Mars outpost, which is a good thing because nuclear might hit lots of political roadblocks that could cause major program delays.

      1. Well let’s see. Solar electric vs Kilopower nuclear. How does it compare?

        590 watts per square meter peak Mars surface solar irradiance, noontime summertime. So with 45% conversion efficiency, plus doubling power production so excess daytime power can be stored in batteries for nighttime needs, equals a solar panel sized 75 square meters. And that solar panel will have to be steered to gain that much power, and even then power will be lower due to seasonal variation, panel dust and atmospheric dust, and panel degradation over 15 years time. Or you could multiply the number or size of the panels to compensate.

        And of course the baseline Mars mission requires 5 times that much power. Which means siting and deploying 5 solar power panels of that size at a location appropriate for the needs of the Mars surface base.

        Conventional solar electric power isn’t looking so great now.

        Another thing about nuclear electric power on Mars, is all the waste heat produced isn’t necessarily wasteful, and might be put to valuable use.

        1. “Mars is no place to raise your kids.
          In fact, it’s cold as hell.”

          The waste heat can be used to help heat the habitats, minimizing the amount of electricity you need to produce to sustain life. It can also heat water for bathing, washing, etc. and may be useful for purifying water. There are a lot of things you can do with that much waste heat. Most of it wouldn’t be wasted at all. To do those things with solar panels, you’d need to produce a lot more electricity.

        2. I went with no sun tracking because that adds complexity. Your 75 sq meter solar installation is only 30 feet on a side, and several of those isn’t much of a hurdle for a manned outpost. It gets tougher when you’re sizing an installation for a global dust storm, where it has to run off diffuse light like a badly overcast day, but I think it’s still competitive with tiny nuclear reactors.

          Solar definitely would become a major hindrance for a larger facility that’s actually trying to melt ice, refine ores, and make rocket fuel. There I’d suggest tens or hundreds of MW worth of nuclear power, preferably a molten salt reactor because the shell and high pressure piping of a light water reactor is insanely heavy.

          My point is that if you can use batteries and solar cells for a small outpost with little or no weight penalty, that’s probably the design route to follow because the schedule isn’t dependent on getting regulatory and launch approvals from agencies that might decide to drag things out with ten years of safety studies. Or they might keep adding new shielding requirements that blow your mass budget. Or other agencies might not give you reactor fuel when you need it because NASA’s program needed time to catch up, and they got a call from a particular Senator who discussed what would make him happy in that regard.

          So if you have an alternative, non-nuclear power source that can sidestep maddening bureaucratic hurdles, and do so using COTS equipment that can be purchased and delivered in a week, then that alternative is much clearer design path because any obstacles are technical, not bureaucratic or political.

          1. No tracking? Okay, so instead of a minimalist total solar array of 375 square meters, you need something more like 750 square meters. Getting awfully large now.

            The problem with even such “small scale” solar polar is your mission design begins to revolve around the solar power system instead of all the other factors a Mars mission requires. You’ve already mentioned putting the batteries inside the habitat to mitigate temperature problems. What kind of compromises and problems will that entail with the design of the habitat?

            I suspect that if solar power is used with a Mars habitat, that is exactly what will happen, the mission design will revolve around the solar power needs. That isn’t necessarily a bad thing. But it does mean a very conventional and simple solar power system for an orthodox spam-can-hab mid-latitude Mars base is unlikely.

            Solar power is likely to require a more exotic solution, like the Landis concept of a summertime occupied polar base. Or the concept of exploiting an orbiting Solar Electric Propulsion Mars spacecraft for use as a Solar Power Satellite to power surface needs. Or the concept of flexible solar power panels fully integrated with a huge inflatable habitat.

            As for the fear of nuclear politics delaying progress. Maybe. It hasn’t stopped the numerous nuclear power sources NASA has already landed on Mars.

            Considering the other factors delaying progress, such as NASA mired in porkbarrel spending, fear of nuclear reactors is far down on my list of concerns.

          2. Oh, I’m not worried about roadblocks for a NASA Mars mission. I’m worried about roadblocks potentially erected by somebody in a NASA safety or environmental office, or by like minded government folks a phone call away, if there was a private Mars mission that threatened to steal NASA’s thunder or upset some prime pork.

            If all the proper officials are fully on board, then no problem, but if they’re not, then designing in several nuclear power sources may be handing a veto to far too many bureaucrats who might link protecting the environment is far more important that some billionaire’s vanity project.

  2. In fact its cold as hell

    This also plays havoc with lithium ion batteries. The cursory web search I did on this with current commercial grade Li-ion batteries says they won’t take charge below -10C. There is some new experimental work going on that can take them down to -40C. I’ll let others run the daytime/nighttime temperatures on Mars.

    So Li-ion batteries would need a source of heat to keep warm enough to charge and possibly discharge. Now at the Mars equator during the noon time hours there’d be a window where they could charge if they weren’t being heated. Not likely elsewhere at higher latitudes. I don’t know how well they would take to discharge during the night-time when they are needed most. I suppose the current draw would heat them internally somewhat. I also don’t know Li-ion batteries respond to extreme temperature cycling. I suspect the answer is “not well”. So if you factor in the energy needed to heat them I suspect nuclear becomes far more appealing. There are of course other battery techs that deal with extreme cold better, but not at as good as the mass/energy ratios of Li-ion as George has already pointed out. I’d be curious to know if burying the batteries would help insulate them from this extreme temperature cycling. Anyone know what the mean subsurface temperatures are on Mars at say a depth of 10m? Might help with the heating requirements.

    1. I should elaborate. I’m thinking of remote uninhabited power stations of a ~24.5/7 variety. Like maybe an ISRU fuel processing plant. For a habitat obviously, the batteries would be kept inside the thermal protection/heating of the habitat itself. Possibly one dug into a cave for surface material insulation or purposely buried for the same reason or not. Everyone likes the geodesic dome designs for some reason. Maybe because structural CAD/CAM software used for creating futuristic pictorial illustrations offers easy to access libraries of these shapes? As others have pointed out there is still the problem of dust storms and solar.

    2. It’s a manned outpost, so why would you leave your batteries outside unless you were using early Boeing 787 model batteries that catch fire? My assumption is that the batteries are inside the habitat and running at room temperature. That would also provide a bit of extra warmth during the day from charging inefficiency.

      During development, the solar/battery system could be tested over and over, even putting plastic sheeting over the cells to recreate lighting conditions very similar to a Martian dust storm, and having interns in bikinis (who you won’t see testing most nuclear technologies) toss dust on the cells to compare various cleaning methods. You could also of course test all sorts of mounting and deployment methods, comparing various automatic and manual set ups, and design and test without any outside monitoring or supervision.

      With nuclear, I think design and testing are more problematic. Just arranging for the permanent storage of a couple of used test reactors is likely to be a bureaucratic nightmare for anyone outside of existing government agencies. Getting approval for even a minor design change might be a headache. There are many small nuclear start ups whose work mostly revolves around the possibility of one day getting permission to build or test something.

      A Mars reactor is certainly an attractive possibility, but as a requirement I wouldn’t want to insert it into an aggressive development schedule if there were other feasible options because the whole program might end up stuck in a warehouse for years waiting on final approval for that one critical element.

      The trade off on a reactor for a lunar base works out quite differently, though. Instead of designing for a 12 to 15 hour discharge cycle and adding more lightweight solar cells, you’d have to size the batteries for two weeks on the night side, which is about 710 hours, or 50 times longer and thus at least 50 times heavier than the Mars battery.

  3. As with the Moon, the optimum in start-small-and-scale-up, minimum-hassle-ground-installation source of power is going to be orbital powersats. Elon’s first expedition should have at least one Starship full of folded up powersats that can be quickly deployed to an optimum orbit. That Starship wouldn’t land, just return to Earth in due course. One of the freighters that lands should carry the rectenna.

    1. It’s been a while since I looked at SPS systems, but refreshing my memory, the problem is that they have to be very large to get efficient power transfer at the ranges involved. Wiki says a 1 km diameter transmitter and a 10 km diameter receiver for geosynchronous orbit. The altitude for a Mar’s orbit is half as much, which should cut those numbers in half, but an efficient system would still be far too large to be cost effective for a small outpost.

      So we’re probably back to nuclear vs solar. Of course the goal for Mars development down the road is wind and hydroelectric. ^_^

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