Designing the Nuclear-Fusion Reactor for my Generation Ship

[Strato Incendus]

Alright, time to turn to something I've neglected for far too long, but which I cannot afford leaving unaddressed:
My generation ship Exodus (accelerating and braking at 0.048 m/s2 over 25 years, maximum coasting speed 0.125 c) needs to decide on a specific type of nuclear-fusion reactor. This doesn't just refer to what type of fusion reactor would work the best in zero g vs. on Earth, but also what type of fusion reactor you personally consider the most likely to become a reality first, just in general.

For starters, I've included a few of the most commonly proposed fusion mechanisms in a poll. Notably, I've added the Boron-Hydrogen reaction, as @John Strickland proposed it to me (taken from a story of his) in the very first thread on the Exodus ship in this post.

Suffice it to say, I don't think I can simply put a plain old Tokamak or Stellarator into one of the 21 spherical tanks inside the 1-km-large aft sphere of the Exodus (sphere packing in a sphere) and call it a day. Anyone familiar with the problems the Tokamak and Stellarator are currently facing will be unconvinced, and thereby pulled out of the story quickly, if that's all I have to offer in terms of how both my ship engines and the general power supply for all the electricity on board work.

Recently, I watched a video from Real Engineering about the Helion design (using the Deuterium + Helium-3 reaction, which was also the one I had chosen for the Exodus thus far). For me, as a layman, this design seemed to be using the heat energy more efficiently, rather than heating water to hit a turbine as an interim step. However, I've also heard some of the criticisms of the Helion approach, such as the Deuterium + Helium-3 reaction being less efficient in general than Deuterium + Tritium.

So now, I'm of course somewhat confused and uncertain about how to proceed. Especially because it's still not fully clear whether the exact workings of the nuclear-fusion reactor will end up becoming plot-relevant (to book 1 or any of the planned sequels and prequels) or not.
Obviously, there is some creative liberty as long as no particular approach to nuclear fusion has been successful yet (in terms of becoming a reliable source of energy that can be upscaled to industrial levels). At the same time, that also makes it easy to "bet on the wrong horse" here, and postulate a type of fusion reactor for my ship that's of a different kind than the one that actually ends up becoming viable one day.

Keep in mind my ship is shaped like a dumbbell (credit to @DaveC426913), and that the spherical tanks inside the fore and aft sphere of my ship double-act as radiation protection from the front and back. Hence, so far, I would presume these tanks include "heavy water", i.e. liquid deuterium oxide.


Personally, I'm confident enough about the themes of my story to consider that part "timeless", so that the story doesn't become worthless immediately if any of its assumptions about the universe are disproved. That's because the thematic conflict and the character psychology is still at the heart of the story - it's not just speculative fiction about "what would life on a generation ship be like". You can still enjoy "The War of the Worlds" while fully knowing that Mars is neither inhabited by aliens, nor covered by a Red Weed.

However, I acknowledge that there is a subgroup of readers that approaches fiction this way - especially in sci-fi, and especially the "harder" sci-fi gets. And given how much time I (with your help) have already spent on designing the ship itself, so far it's pretty "hard" sci-fi; much harder at least than the works of fiction in the spirit of which it is written (such as Star Trek).

Thus, I can't just handwave the nuclear-fusion reactor away, after having already dedicated a large amount of attention on getting the details of artificial gravity via centrifugal force, acceleration to relativistic speeds, protection against radiation and space debris etc. right. Or rather, if I handwave something as integral as the fusion reactor now, the hard-sci-fi crowd will probably be quick to dismiss the story as a whole. Because some of them may end up being drawn to it for what they consider the speculative-fiction aspect.

If they end up being disappointed by the focus being more on theme an character psychology, that's one thing; however, I don't just want to lose them simply because I refused to do my homework on the technical aspects of the ship design. It's not something I care for in a story in and of itself; rather, I care enough about immersion in a story that I want to do this "chore" properly.

I even find myself applying this standard to other sci-fi works as a reader / viewer; for example, whenever I encounter a setting in which Mars has been terraformed (such as Elite: Dangerous), I must assume the author "kind of forgot" about Phobos and Deimos, the fact that Mars's gravity won't change, etc.
A terraformed Mars is kind of to sci-fi what the "one-handed longsword" is to fantasy: An often regurgitated trope that has little to do with actual research from physics or history / HEMA (historical European martial arts), respectively. Sure, every piece of fiction needs to handwave some things; but I prefer to handwave in areas about which fewer people have expert knowledge - so that the overall number of readers pulled out of the story by a disruption of their suspension of disbelief will hopefully be kept to a minimum.


So, without getting into controversial real-world speculation too much, but instead focusing on the fictional context of a hitherto overly ambitious project like a 3-km-long interstellar generation ship:
What type of nuclear-fusion reaction would you choose, and which type of reactor to go along with it?

 

[Drakkith]

Recently, I watched a video from Real Engineering about the Helion design (using the Deuterium + Helium-3 reaction, which was also the one I had chosen for the Exodus thus far). For me, as a layman, this design seemed to be using the heat energy more efficiently, rather than heating water to hit a turbine as an interim step. However, I've also heard some of the criticisms of the Helion approach, such as the Deuterium + Helium-3 reaction being less efficient in general than Deuterium + Tritium.

Yes, D-H3 fusion generates less energy than, say, D-T fusion, but that's really a moot point. You have the ability to make the size of the reactor and the power output nearly anything you want and you don't need to worry about all the pesky real-world details of actually designing and building it. If you want to use D-H3, then just use it.

As for readers being pulled out of the story because of some technical detail, I really wouldn't worry about it. During one of the seasons of The Expanse, the protagonists ship is shown to use Inertial Confinement Fusion, which is probably not going to be an actual method of producing fusion power in ships due to various issues. Despite me knowing this, I wasn't pulled out of the story. Besides, the number of people who know enough to be pulled out of the story because your fusion reactor might not be quite right is very small.

 

[Rive]

what type of fusion reactor would work the best in zero g vs. on Earth

I think that part about gravity is just unnecessary. Everything involved in fusion is just sooo stronger than gravity that the difference what that measly 1g would make is just ...

what type of fusion reactor you personally consider the most likely

One with a lot of handwavium. I think if you just honestly include a waving hand there (to make it clear where the science ends) would make your design better and more foolproof than trying to cook up something supposedly believable now.

Regrading fuel: diesels get their momentum by being able to blast on anything. I would keep the design fairly universal.

 

[Nik_2213]

Although your ship design significantly distances the fusion system from the hab-zone, I'd prefer aneutronic fuel mixtures for safety.

D2+ He₃ is the usual candidate, I'm not sure about that H2 + Boron brew. IIRC, it is significantly 'harder' to ignite. Upside, the hydrogen / boron ingredients are easy to get, though the latter will so require a planet with hydrological cycle and salt lakes...

That said, no sustained fusion route is easy. And, IMHO, you may have to begin with one of the messier mixes to 'run up' the main process...

 

[Vanadium 50]

How does this affect your story? What actions will your characters take that differs based on what is in the tanks? If the answer is "nothing", why describe it?

You say you don't want a Tokamak because anyone familiar with one will realize these are difficult to make work. Yet your solution is to pick something even less practical today.

Let me argue from analogy. James Bond is about to give chase to Blofeld's Henchman, who has kidnapped the Bon-Girl-Of-The-Moment. He gets behind the wheel of his iconic Aston-Martin, fires up the engine, and thinks "I am way overdue for an oil change. 10W-40? Winter is coming - maybe 0W-30 would be better. Pure synthetic? I mean, the owners manual doesn't say it's required, but I do drive pretty hard. And do I want to use Pennzoil or Shell - I do sort of owe it to the Dutch after that unpleasantness in Amsterdam, but Pennzoil is cheaper, and RN commanders aren't made of money."

 

[Strato Incendus]

Thanks for your replies!

How does this affect your story? What actions will your characters take that differs based on what is in the tanks? If the answer is "nothing", why describe it?

You say you don't want a Tokamak because anyone familiar with one will realize these are difficult to make work. Yet your solution is to pick something even less practical today.

It's still hanging in the balance - I may end up sending a repair team to the reactor at the midpoint of the first book; or it may become relevant once they start developing a singularity drive in the second book.

The point is, much like I took the constraints of the rotating ring habitats as a given, for the purpose of realism, the fusion reactor seems like one of those things that should be "hard worldbuilding" - and then, I have to work within those confines.

The ship layout in general is among the few things I didn't design directly in service of the story - rather, it's a setting attempting to be as realistic as possible, and then, the story takes places within it. A lot of the scenes of the story could have worked just as well on a ship with a classic Star-Trek-like stacked-decks architecture; however, the only way to explain such artificial gravity within the story would have been "something something field generator". So instead, I went with rotating habitats, since that's the only way of creating artificial gravity we can currently think of that should actually work without handwaving.

Yes, D-H3 fusion generates less energy than, say, D-T fusion, but that's really a moot point. You have the ability to make the size of the reactor and the power output nearly anything you want and you don't need to worry about all the pesky real-world details of actually designing and building it. If you want to use D-H3, then just use it.

I think that part about gravity is just unnecessary. Everything involved in fusion is just sooo stronger than gravity that the difference what that measly 1g would make is just ...

Great, thanks! These two points certainly sound reassuring.

Although your ship design significantly distances the fusion system from the hab-zone, I'd prefer aneutronic fuel mixtures for safety.

This is something I hadn't thought of at all yet, so thanks a lot for pointing it out!
What would be the danger otherwise? Radiation?

That's indeed an element of the worldbuilding that already affects the story - somebody pointed out in an earlier thread that radiation is more dangerous to female reproductive organs than male ones. This could be one reason why a generation ship specifically would only allow men to work near the fusion reactor - and would only send men there for a repair mission, in case something about the reactor breaks.

Thus, it might be more useful for the story to NOT use aneutronic fuel mixtures, as long as I can give a practical reason why - and the "safety measure" would indeed be the distance from the habitat ring, or even the closest ring, where the vertical farms are.

D2+ He3 is the usual candidate, I'm not sure about that H2 + Boron brew. IIRC, it is significantly 'harder' to ignite. Upside, the hydrogen / boron ingredients are easy to get, though the latter will so require a planet with hydrological cycle and salt lakes...

The reasoning @John Strickland gave was that the hydrogen-boron mixture would require less tank mass.
I haven't specified the tank mass for my ship thus far, so it's something I could handwave away.

As for where to get the Helium-3: So far, I noted down that it could be mined on the moon, bred on Earth, possibly taken from the clouds of Saturn. Would that make sense in principle?

I took the liberty to adapt the planned HAVOC stations from Venus to Saturn, too, under the name HASSLE = High-Altitude Saturn Solution of Levitating Elements. How do the HASSLE cloudposts deal with the wind speeds on Saturn? Yeah... I'll have to handwave that part, too.

Btw, people on Earth in my setting have later also adapted the HASSLE concept to Uranus. At least the wind speeds are much lower here than on Saturn (200 m/s rather than 1100 m/s). So if HASSLE ends up being too unbelievable, I could simply cut it (my setting has no cloudposts on Jupiter either), and only have cloudposts on Venus and Uranus, possibly Neptune.
On Uranus, the Balloon Units for Technology Transfer were installed as part of the Refinery Establishment and Colonisation Trial Uranus Mission.

 

[Drakkith]

The point is, much like I took the constraints of the rotating ring habitats as a given, for the purpose of realism, the fusion reactor seems like one of those things that should be "hard worldbuilding" - and then, I have to work within those confines.

But you can't design a realistic fusion reactor for a spacecraft, as we literally don't have them yet. So you are, by necessity, making stuff up.

As for where to get the Helium-3: So far, I noted down that it could be mined on the moon, bred on Earth, possibly taken from the clouds of Saturn. Would that make sense in principle?

Doesn't matter for your story. No one in the story will care, as they are on a generation ship light-years from Earth, and if anyone reading the story asks, just tell them the company/organization that built the spacecraft bought the helium-3 off of the market like everyone else in-universe does.

 

[Strato Incendus]

Thanks for your take! ;) I guess, overall, this sounds like a blank cheque for me to handwave anything I want about the reactor.

The choice between a Tokamak, Stellarator, or Helion design would be primarily relevant because of the reactor's shape. For example, in case I have somebody climb around or into that thing in order to fix something.

Which begs the question what the emergency power supply of the ship would be based on if the nuclear-fusion reactor actually fails - or if the ship would have multiple fusion reactors to begin with, given how much redundancy the architects included for most other things.

So to get to the core question: All things considered, would you currently consider the Tokamak, Stellarator, or Helion design more likely to ultimately be the design that ends up succeeding?

 

[Drakkith]

Which begs the question what the emergency power supply of the ship would be based on if the nuclear-fusion reactor actually fails - or if the ship would have multiple fusion reactors to begin with, given how much redundancy the architects included for most other things.

Redundancy would be preferable for such a long voyage, but it might depend on how costly, complex, and massive each reactor is. A single reactor that can be easily maintained might be better than two or three reactors of a different design that is harder to maintain.

So to get to the core question: All things considered, would you currently consider the Tokamak, Stellarator, or Helion design more likely to ultimately be the design that ends up succeeding?

Why not just make up a new design? Call it whatever you like. Name it after a person, a company, or whatever.

 

[Rive]

So to get to the core question: All things considered, would you currently consider the Tokamak, Stellarator, or Helion design more likely to ultimately be the design that ends up succeeding?

:sigh:

I would go with a beefed up Helion with multiple beams/chambers (CBF), kind of like the tri-alpha looks like.
No more credibility than anything else going on right now, but kind of looks cool and has plenty of loose text sample as basis to generate the necessary technobabble.

 

[Strato Incendus]

I would go with a beefed up Helion with multiple beams/chambers (CBF), kind of like the tri-alpha looks like.

Thanks, I hadn't heard of the tri-alpha yet! :) That one seems to be a design for aneutronic fusion.
So if I understand you correctly, the mechanism would still be that of the Helion design (Deuterium + Helium-3), but the shape would be that of the tri-alpha, right?

Would those multiple beams/chambers increase the reliability, or the efficiency, etc.? Meaning, are there any functional reasons why you would propose this design? Or was it just an idea because you liked the look of the tri-alpha, i.e., "the law of cool"? ;)

 

[Rive]

Would those multiple beams/chambers increase the reliability, or the efficiency, etc.?

I'm totally not qualified to assess any of that regarding a fusion device.
As pure technobabble, if it does not work with just two opposing beams then it still may work with more => beef it up and blast away.

Regarding the exact fuel/reaction, I really would keep that kind of universal. ISRU is a thing, and you can't expect to find always the right fuel everywhere.

Ps.: a beefed up tokamak or stellarator still would be just a doughnut-engine

 

[Drakkith]

In terms of 'favorite fusion reactor design' (irrespective of if it works), my personal favorite is the polywell.