Crthuky

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?

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.

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.

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.