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The idea is to build a cybernetic body for a human. Only the brain and a few glands related to emotions, like adrenal glands, will be kept. The brain and the glands will be in a closed, plug-and-play, container, that can be connected to compatible bodies, to turn humans into cybernetic space elves capable of living long enough to cross the void between the stars without relying on relativistic ships (assume for now that it is possible to keep the brain healthy for thousands of years with the correct chemistry).

But these bodies need power sources. Compact, energetic power sources. I'm thinking fission reactors.

Is it possible to build a fission reactor so small that can be possible to fit it in a human chest cavity? Would such a miniaturized reactor be better, in terms of power output and autonomy, then a combustion engine or batteries?

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  • $\begingroup$ How much do you care about radiation shielding? $\endgroup$
    – Philipp
    Commented Apr 9, 2019 at 14:17
  • $\begingroup$ Only enough to not damage the brains and the circuitry of the other systems. In the case of the brains, the brain case can have it's own shielding. It is acceptable to have less shielding in areas that aren't facing the brain, for example. $\endgroup$
    – Geronimo
    Commented Apr 9, 2019 at 14:20
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    $\begingroup$ This idea of the fantastic cyborg power core is integral to the Iron Man movies. Iron Man has got one wedged in his non-iron parts. ironman.fandom.com/wiki/Arc_Reactor $\endgroup$
    – Willk
    Commented Apr 9, 2019 at 15:54
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    $\begingroup$ This idea was made popular by Isaac Asimov in his Foundation stories where they evolved into something the size of our button lithium batteries. $\endgroup$
    – JBH
    Commented Apr 9, 2019 at 17:41
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    $\begingroup$ I would worry about the closed cycle bio reactors that you will need to synthesise the solution sort of like oxygenated glucose syrup with all those other micro nutrients for the brain. Self repair would be a bonus but eventually the design team will throw their hands up and propose self replicating humanoids. $\endgroup$
    – KalleMP
    Commented Apr 10, 2019 at 0:00

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Fission power requires critical mass to work, making it difficult to miniaturize power plants, even ignoring problems with radiation and radioactive waste. Californium-252 has the smallest critical mass of 2.73 kg, but it is expensive to synthesize. In addition, fission doesn't produce electricity directly, but instead evaporate water that produces electricity in turbines, adding to the weight of the power plant. Did I mention shielding? Cutting gamma radiation in half requires 1 cm of lead, and reducing it to (say) less than two percent will thus require 6 cm lead shielding, which quickly becomes very heavy.

An alternative might be fusion power, which has no critical mass. In return, however, fusion requires very high pressure and temperature, which might be difficult to contain in a small reactor. The smallest experimental reactors being build are the size of small houses. You would probably need fusion processes that don't emit free neutrons, which make stuff radioactive and are difficult to shield. Examples are deuterium-lithium and proton-boron processes, but these typically require greater pressure and temperature than neutronic processes. They also tend to require steam turbines to produce electricity. A theoretical exception is 'focus fusion', which produces electricity directly. This might be your best bet.

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    $\begingroup$ If I understand correctly the critical mass is due to the amount of neutrons that, after leaving a fracturing atom, successfully ignites fission in another atom. Won't materials that reflect neutrons increase the amount of fission events, lowering the mass thresholds, while at the same time shielding the rest of the body from dangerous neutrons? $\endgroup$
    – Geronimo
    Commented Apr 9, 2019 at 14:51
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    $\begingroup$ About the gamma radiation: If use materials denser then lead the thickness of the shielding lowers. But the mass will still be a problem and i'm beginning to see that this a variant of the tyranny of the rocket equation - the "hotter" the reactor, the heavier the shielding, demanding even more power to move the cyborg and even more shielding. $\endgroup$
    – Geronimo
    Commented Apr 9, 2019 at 14:52
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    $\begingroup$ @Geronimo You are correct that neutron reflectors allow for a lower critical mass/size and add shielding to the core, but good reflector materials are also very dense, so the mass savings may not be huge. $\endgroup$
    – Nuclear Hoagie
    Commented Apr 9, 2019 at 15:03
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Fission reactions (as opposed to reactors) is already how we power deep space probes, it should work for cyborgs in space.

A type of radioisotopic power system known as radioisotope thermoelectric generators employ radioactive materials such as plutonium-238 to produce heat and make use of a principle of electrics known as the Seebeck effect to produce power. Put simply, the Seebeck effect is when two different semi-conductive metals are connected. With one end heated, electrons will move from the hotter metal to the colder one.

https://cosmosmagazine.com/technology/where-do-deep-space-probes-get-their-power-from

Advantages: It works, basically forever. If it's not enough power then just have more of them.

Disadvantages: You're producing heat and then harvesting it, so it's bigger than "fitting inside of a human chest".

However for cyborgs in space there's no need to keep your power source next to your brain. The ship has a power source, we can ship electricity to various places real easy, a cyborg can have a power cable for most situations and batteries for when it has to be disconnected.

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    $\begingroup$ So autonomy will have to be sacrificed because the batteries won't have the autonomy a nuclear reactor embedded on the body would have, even taking into account the linear growth of battery capacity and increasing eletrical and motor efficiency. But this compromise may be acceptable if the battery is good enough and the body efficient enough. Recharge at the ship's power sockets, bring extra batteries when doing long missions or install a temporary generator when settling somewhere for some time. That generator may exploit the best power sources the place has, like oil if installed in Titan. $\endgroup$
    – Geronimo
    Commented Apr 9, 2019 at 16:17
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    $\begingroup$ @Geronimo Yes, all of that. For a cyborg, power is food. But we don't need to carry around 80+ years of food in a backpack, it's acceptable if we've got a separate vehicle which holds the bulk of what we need. $\endgroup$
    – Dark Matter
    Commented Apr 9, 2019 at 16:55
  • $\begingroup$ You could use wireless power transmission for your androids. Laser beams (less penetration through ship walls, but has plot potential), microwave laser beams, big loops of wire... $\endgroup$
    – John Dvorak
    Commented Apr 9, 2019 at 16:57
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    $\begingroup$ The other downside to an RTG is that it's hot. To give you an idea of how hot, the Apollo astronauts reported feeling the heat given off by the ALSEP RTGs through the insulation of their space suits. $\endgroup$
    – Mark
    Commented Apr 9, 2019 at 21:55
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    $\begingroup$ They have a finite life, they make more heat than electricity and they require generous radiation shielding. $\endgroup$
    – KalleMP
    Commented Apr 9, 2019 at 23:58
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If you want to keep the fleshy bits of your space elves, steer well clear of pretty much any kind of nuclear power supply, fission, fusion or annihilation. Highly penetrating x-rays and gamma rays you may be able to shield against (though I wouldn't bet on it) but the spray of fast neutrons you can expect to find flying out of any fission reaction and many fusion reactions (yes, even proton-boron fusion, where about 0.1% of the reactions will shoot out nice, fast, highly-penetrating and highly destructive neutrons) will be basically unstoppable and everyone will die of cancer before arriving at the new world. Even if you have super space medicine to fix cancer, those brains are gonna be pretty fried a lot of cells are gonna die. Not good news.

But to flip it around, how much power do you think you really need? "nuclear reactor" sounds like the sort of thing you'd want to power a rocket engine, or an energy weapon, not a person. A human body has an average power use of about 100W. An efficient robotic chassis with similar performance could be driven with modern batteries and recharged from time to time (daily, probably). On a spacecraft designed for such things, you might have charging points pretty much anywhere and everywhere people might congregate or rest. Super future batteries or fuel cells could either be recharged much less frequently, or provide much more power, if you can think of something to do with it all.

Remember also that a lot of the power draw will be moving around, but on a non-relativistic spacecraft (or even a plausible relativistic one, to be honest) there will be little to no acceleration from thrust so the only gravity forces you have to work against are artificial ones that you can dial up or down to your heart's content. Just turn the spin decks down and relax in microgravity.

(also if you really, really want fission power, be aware that fancy isotopes with small critical masses tend to have half-lives much, much shorter than your projected thousand year flight time. bring a big breeder reactor and a good supply of parent isotopes to work from)

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Nuclear Batteries

A nuclear battery can last for thousands of years and doesn't have the issues of trying to miniaturize a fusion or fission reactor.

Currently their power output is pretty low but if you have the tech to build cyborgs, the power output of a nuclear battery would also improve.

See Diamond Battery

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    $\begingroup$ It isn't necessarily a given than the power output would improve that much; the amount of energy released by radioactive decay just isn't that much. That diamond battery provides 16MJ of energy over 5000 years. A tesla powerwall does it over a few hours. Even if your diamond battery becomes a thousand times better in the future, it is still a thousand times less useful than a boring old Li-ion in this context. $\endgroup$
    – Starfish Prime
    Commented Apr 10, 2019 at 6:46
  • $\begingroup$ Yes but the diamond battery isn't much bigger than a pin head and a Tesla powerwall is bigger than the chest of the cyborg. What you do is couple the continuous output of the nuclear battery with capacitors to store and release as needed. $\endgroup$
    – Thorne
    Commented Apr 10, 2019 at 10:43
  • $\begingroup$ That battery looks intersting. I have read about the hypothetical "hafnium granades" but I discarded them. They can store energy exciting the atom and release it as gamma rays but that release doesn't happen when you want but when the nuclear forces want. I have no idea if, even with 10000 years of technology advancement, Mankind will be able to rush nuclear decay. If I handwaved that the cyborgs have something like the nuclear accelerator from Asimov's Robots And Empire I would use that. But I'd rather not use this handwave. $\endgroup$
    – Geronimo
    Commented Apr 10, 2019 at 11:00
  • $\begingroup$ continuing... But it's power output is low, even if it's size is small. I can see some uses as low power, low space, energy storage for battery-powered cyborgs - basically snacks and candies, their popcorn. $\endgroup$
    – Geronimo
    Commented Apr 10, 2019 at 11:02
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    $\begingroup$ @Geronimo they aren't energy-dense enough for that, really. Plenty of technology for reasonable-density, fast-discharge power storage out there. $\endgroup$
    – Starfish Prime
    Commented Apr 10, 2019 at 18:53
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Stick with chemical engines.

In a cyborg you have a whole body cavity to work with. The fact that is is a cyborg means to me that it will be in human-type circumstances; the only reason I can think of for a robot to emulate the human form. Human circumstances means there will probably be opportunities to eat and to breathe.

I propose that you put an internal combustion engine in there at the site of the heart. "Breathing" will provide the oxygen and also a mechanism to vent heat. The cyborg can eat fuel. Cyborg fuel might be kerosene or even rocket fuel; a cyborg in circumstances where it is unable to breathe might eat rocket fuel with oxidizer included. Other combustibles might include butter, chicken, or rum (carried in a hip flask for emergencies). RPMs of the engine will increase as dictated by caloric value of food and energy needs.

The exhalations of the cyborg will depend on what fuel it is currently running on. There might be a lot of smoke, or sometimes even flames.

It is worth noting that the varying heat of exhalations will correspond to varying gas density and so variation in vocal pitch, assuming the cyborgs use vibrations and the exhaled airstream to make sounds. A cold cyborg at rest will have a low voice; possibly quite low depending how cool they get. A hot cyborg exhaling flames will have a very high squeaky voice.

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  • $\begingroup$ Yes, they will be in human-type circunstances. The second reason to preserve the human form is psychological. They are stil humans and will probably have a "larval" stage, from birth to adulthood, in full flesh bodies, being cyborgfied when they become old enough, probably between 30 and 40 years old. But one problem that chemical engines might have is that we may soon reach the limits of power generation they can have, while batteries keep getting better linearly with occasional exponential jumps and nuclear is inherently more powerful. $\endgroup$
    – Geronimo
    Commented Apr 9, 2019 at 18:02
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    $\begingroup$ A robot running on alcohol sounds exactly like Bender...... $\endgroup$
    – Thorne
    Commented Apr 10, 2019 at 4:41
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The brain and whatever other original organic matter is still aerobic and are going to require supporting mechanisms to regulate the flow of blood, oxygen, nutrients, filter out waste, produce white blood cells, etc... and presumably some way to directly release the essential nutrients for life into the bloodstream without a digestive system. Once those challenges are solved, they will need significant shielding of those components from heat and radiation - from their own power supply, and from radiation in deep space which is considerable.

Whatever power supply you settle on, it's ability to dissipate heat is going to be critical.

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