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A group of ships I've made are supposed to be modular in design with each modular hull segment being self sustainable both in terms of power needs and crew upkeep (oxygen, food, water, etc.)

I've been split between having the ships house massive reactors, with one or two (for redundancy) in each segment, or having that exact same space filled with hundreds or potentially thousands of smaller reactors.

Assuming the technology level is one that has been able to create fusion reactors that have far surpassed fission reactors, what are the benefits between having a smaller number of larger fusion reactors vs. larger numbers of smaller ones?

I'm looking for answers in terms of total power output per cubic meter of hull space used and manpower needed to maintain them.

Can fusion reactors be scaled up and down like this? Or are there other factors that make fusion reactors lean towards a specific size range?

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    $\begingroup$ Are the reactors being used directly for propulsion? Or just to generate power? Maybe you need both? $\endgroup$ – Spencer Aug 26 '17 at 12:34
  • $\begingroup$ They will be doing both. $\endgroup$ – Arvex Aug 27 '17 at 4:06
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what are the benefits between having a smaller number of larger fusion reactors vs. larger numbers of smaller ones?

If you have lots of small reactors then, for the same total volume of fuel used between them all, you have a much larger surface area than you do for one large reactor. This means you put more energy into heating up and maintaining your fuel at the ion density needed for fusion. (So larger>small)

I'm looking for answers in terms of total power output per cubic meter of hull space used and manpower needed to maintain them.

In terms of power output you will need to redirect more of that power back in to keep the reactors going if you use lots of small ones so the output will be lower. Manpower would be higher for lots of smaller ones too.

Can fusion reactors be scaled up and down like this? Or are there other factors that make fusion reactors lean towards a specific size range?

In principle there is no limit on size up or down (though you would probably need a few technological advancements if you want to start making something the size of the sun). The larger the plasma (more ions) the better since you've got a much higher chance they will hit another ion before attempting to leave the plasma (though magnets do direct them back in you will get some loss).

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As noted in the other answers, there are various trade offs to be made in terms of efficiency and manpower. I'd say the underlying calculus has to be in terms of overhead i.e. how much structure is needed to house the reactors, how many crew members are needed to attend to them and how much space is being used not just for the reactors, but associated plumbing, cabling, control wires etc. How much are you sacrificing for this compared to weapons, sensors, life support systems and so on?

This has to be balanced off by the various threats to the system. Is it very likely that reactors will be hit during a mission? If the supporting structures or infrastructures like radiators, fuel pipes etc. are hit, will this cause a cascade failure? If it is likely that rectors will be hit during a mission, then up to a certain point, having more is better. If the associated plumbing and structures are more likely to be hit, putting the reactors out of commission, then maybe having fewer, larger reactors will make more sense.

One other issue is what is the minimum amount of energy needed to run the ship or a module? If the individual reactors are too small, you may end up in a situation where the remaining ones are funning at full power but there isn't enough energy being produced to actually run the ship or its systems. Larger, redundant reactors which can shoulder the load may be a better choice (just run them at a fraction of their output under normal circumstances).

So a lot of this may depend on the circumstances you wish to introduce for your story, but in general terms, you want the lowest overhead and the ability to run the entire system even if one or more reactors need to be shut down or are destroyed. This would speak to the idea of using few, large reactors.

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Consider that a fusion plasma loses energy through its surface, but most of that energy has to be kept and used to fuse the atoms.

Therefore small reactors tend to be less efficient or even not operating at all, in the sense that they won't produce energy but rather absorb it to maintain the fusion.

Also, the more component you have, the more likely is a failure.

So, if you can build it, go single, go big.

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More reactors throughout the ship would require more in terms of maintenance and crew. Even small reactors would need attention, and if your ships are likely to engage enemies or take damage, you would need far more engineering crews to be able to support the reactors.

A single point of failure (one big reactor) can be dangerous too, since it will likely become the main target of enemies. On the other hand, too many small reactors could be less efficient to shield or protect, potentially meaning an enemy just has to target a few of them to really cause problems aboard(maybe your hypothetical reactors aren't dangerous, or maybe they are -I'm picturing small nuclear explosions all over a ship). Multiple redundant reactors would be a good in between. Not so many that every single person on board needs to be a reactor technician, but not so few that the failure of the reactor is complete power loss. Also, it could provide for a more optimum shielding solution. Yeah, you have to heavily shield a few more things, but at least you won't need that armor all over every little reactor spread through the ship.

With that, the redundant reactors could still be in your modular sections, as needed.

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  • $\begingroup$ @A.C.A.C. Yup, I agree that 2 reactors is still too few, but this is an optimization problem with plenty of constraint possibilities. The number and size of reactors will change somewhat or have an acceptable range based on what the OP has as his design considerations. $\endgroup$ – ryanrr Aug 25 '17 at 17:47
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Others have contributed good thoughts about the efficiency benefits of a small number of large reactors. Those points are valid and I think you should take heed of them.

I'd just like to contribute a point which heads the other way, as much as anything because I think it might make for some interesting plot devices for your story.

I have no direct knowledge of fusion reactors, but I did work for a while with the largest (by physical dimensions, not power output) nuclear Fission reactors ever made.

An interesting property of physically large reactors is that controlling the reaction becomes harder compared to smaller units. Neutron propagation time across the core becomes significant, so they don't respond to control inputs as quickly as small reactors do. Now I'm not even sure if this transfers to fusion reactors as they are quite different beasts, but if we suppose for a moment that it does, it seems to me that the following might be true

  1. your big reactors take time to change power level, so you have a delay before your ship can, say, fully charge it's weapons or employ full acceleration, or jump to light speed if your universe includes ftl travel

  2. Maybe you need a mix of large and small units to mitigate 1

  3. Large units might be more susceptible to control system damage or breakdown making them unstable and either causing a catastrophic explosion or requiring a shutdown for repairs.

Just some ideas to play with

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  • $\begingroup$ Does this apply to fast neutron reactors too? $\endgroup$ – Innovine Aug 30 '17 at 9:05

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