Chargeable Atomic Battery for Much Faster Space Probes

USNC-Tech has a NASA NIAC phase i study for a compact twenty thousand-watt, 500 kg dry mass, radioisotope-electric-propulsion spacecraft design powered by a novel Chargeable Atomic Battery (CAB) that is capable of ∆Vs on the order of 100 km/s with a power system specific mass of 5-8 kg/kWe. This would be over five times faster than the Pluto Express mission.

Ultra Power Dense Radioisotope Batteries

A spacecraft powered by this technology will be able to catch up to an extrasolar object, collect a sample, and return to earth within a 10-year timeframe. The data collected from samples and data from interstellar objects has the potential to fundamentally change our view of the universe and our place in it. Two of these objects ‘Oumuamua and C/2019 Q4 (Borisov) have passed through our solar system in the past three years, and we must be ready for the next one. The core innovation of this spacecraft architecture that makes this amazing mission possible is the CAB, which has a power density of over 30 times that of Pu-238. The CAB is easier and cheaper to manufacture than Pu-238 and the safety case is greatly enhanced by the CAB’s encapsulation of radioactive materials within a robust carbide matrix. This technology is superior to fission systems for this application because fission systems need a critical mass whereas radioisotope systems can be much smaller and fit on smaller launch systems reducing cost and complexity.

Written By Brian Wang,

17 thoughts on “Chargeable Atomic Battery for Much Faster Space Probes”

  1. I seem to remember that the best Stirling engine fluid is highly compressed helium.
    Light molecular mass gave better results because low specific heat of the fluid and low viscosity, helium is not chemically active like Hydrogen, nor does is diffuse straight through any reasonable material (as much), and high pressure means more of the working fluid mass per volume.

  2. The attached papers at that site reveal that they are looking at different sized systems and also different radioactive elements. So the large 1.7 tonne system with a short 100 day half-life will give far more power than the 100g system with the 10 year half life.

  3. The attached papers at give some more details.

    • They explicitly say that they are currently using solid state thermoelectrics, but that dynamic (probably Stirling engine) methods are a future possibility.
    • The different power levels are achieved by using different radioisotopes. This also gives a different half live in inverse proportion to the power (as you'd expect).
    • They also discuss radioisotope thermal rockets as being better than RTG→electric→electric ion drives. It gives a better overall efficiency, the improvement in ion drive ISP is not enough to overcome the low thermoelectric conversion efficiency and the gravity loss from low ion drive thrust.-
  4. An important consideration of free space power supplies is the ability to operate efficiently with a high heat rejection temperature. Heat transfer in vacuum is exclusively by radiation. Since radiated power is proportional to the forth power of the temperature difference in an absolute temperature scale, mass increases quickly as rejection temperature decreases. If it were not for mechanical complexity, a closed cycle Brayton engine using supercritical CO2 would do nicely. That may become the engine of choice for applications where maintenance is possible.

    I wonder if a Stirling engine using SC-CO2 has been attempted?

    Consider two equal size radiators of the same material, one operating at 300K, and one at 500k. The 500k radiator would radiate 7.7 times as much energy as the 300k model, presumably massing less than 1/7 of the 300k. Presumably, some of the waste heat would be used to keep electronics warm.

  5. You'd want a charged state atom that is an alpha emitter, with low gamma emissions so you would not have to carry much shielding. You'd want an atom with an uncharged state , that has a very high cross section of neutron capture, so it would charge quickly, and a charged state with a very low cross section of capture so it would not be a neutron poison(waste reactor neutrons).
    If it is to be rechargeable, it would presumably decay into the precharge isotope. A half life of 20-40 years would be good. The decay should release as much energy as possible. I'd think one of the nice things about plutonium 238 is there are several quick decays after the first that increase it's power.

  6. The difference in electrical output is in part due to different energy conversion schemes. A 2016 NASA report describes a free piston engine that could increase conversion from 5-8% from thermoelectrics to 20% for the engine.
    The thermal efficiency of free piston Stirling engine suffers from the lack of a regenerator, a heat exchanger that removes heat from the working fluid when it is traveling from the "hot end" to the "cold end", and reheats the fluid on the return trip. This means less added heat is needed at the hot end to maintain temperature. Such devices work well in alpha type Stirling engines, which have separate hot, and cold cylinders, with a tube between them.
    Stirling engines are a natural match for radioisotope heat sources, since they can operate with very high temperature heat input, which increases their efficiency, and high temperatures are easily obtained from radioisotopes.

  7. Likely, all those RTGs use thermoelectrics for their energy conversion. The CAB probably uses a free piston Stirling engine which has over an order of magnitude higher thermal efficiency, and is highly reliable due to it's mechanical simplicity.
    125-200 Watts electric would mean maybe 400-800 Watts thermal.

  8. Doesn't "patent pending" just mean that they've filed for a patent, but it hasn't been issued yet? Of course it could be denied for some reason I supposed.

  9. Alpha and beta emitting isotopes could provide an effective source of power. But I believe nuclear reactors are where we really want to go. Just have to learn how to make then cheap and safe enough for space. As long as you don't turn them on until you reach space they aren't that radioactive.

  10. Well, that cut off rather abruptly, but I see THEY regard "CAB" as standing for Commercial Atomic Battery, not "Chargeable". Though I suppose they could be equivocating in their literature.

    Obviously, it avoids the red tape by having no isotopes useful for bomb making.

    The illustration they use of their atomic battery is exactly the same as the illustration they use for their FCM, fully ceramic micro-encapsulated fuel. Searching on the web turns up very few details, unsurprising as one of the details was "patent pending". Use of that illustration could be accurate, could be deliberately misleading, could be just a lazy graphic designer.

    Assuming it accurate, they manufacture fuel slugs using an encapsulated non-(or mildly)radioactive isotope, then place them in a reactor to transmute that isotope into a radioactive one. The fuel slug then becomes hot enough to power some sort of thermal cycle. Maybe Thorium 230 to U233?

    Though I suppose it could be some kind of nuclear isomer, Hafium 178 has one that would maybe be suitable, if they can produce it by gamma irradiation.

    Given how long it's supposed to operate, we're looking at something with a half-life in the region of 1-2 decades, probably no more than 50 years.

  11. Here's a better description:

    It's "chargeable" because it's a non-radioactive material that is "charged" by sitting in a fission reactor. I assume that means neutron activation. But I don't know what isotopes are created.

    It claims an energy density between 1 and 100 MWh/kg. That's a very wide range. So maybe the details aren't worked out yet.

    They say the charging takes "years to decades", and that it avoids the red tape of plutonium-based systems.

    So I still have no clue what it actually is. But it sounds interesting.

  12. But I'm still curious. What fuel does it run on? Why is it called "chargeable"? How long can it run before its electrical output is cut in half (and is that a shorter time than the half-life of the fuel itself)?

  13. The article here says the new CAB system has a specific mass of 5-8 Kg/KWe, which is a power density of 125-200 W/kg. Wikipedia lists 11 RTGs used in space, which range from 1.3 to 5.4 W/kg. It also lists several terrestrial RTGs, each of which is less than 1 W/kg (probably because proliferation concerns prevent them from using plutonium or other good fuels). 

    So that 125-200 W/kg is very impressive. It's 30 times better than the best RTG, and 125 times better than the worst space RTG that was listed, and vastly better than terrestrial RTGs.

  14. Wow, what would this do for transit throughout our solar system (i.e., travel to the moon, Mars, Europa, etc)? Because, catching up to an extrasolar object and returning within ten years sounds rather incredible.

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