> You could also say that space based solar makes total sense "if you can do it, it's just a really hard engineering problem"

The thing is that while they are both really hard engineering problems, space solar (which includes energy transmission via lasers) is also a massive ongoing logistics and coordination problem.

Solar power without wireless transmission makes a lot of sense the closer you get to the sun and in the inner solar system it almost certainly makes more sense than fusion but once you get to the asteroid belt or further out, inverse square law would suggest that it quickly becomes infeasible.

"solar roads for sci fi fans"

Solar roads are expensive and worked out economocially nowhere, where they were build. Unless you mean roads that have roofs of solar panels - that is a sound concept, because you can have standard cheap panels and replace them at will, but is not what is generally understood with solar roads.

Also about fusion:

"when it has all the radioactivity problems of fission"

Not at all the same. What exactly are you referring to? Fission with heavy isotopes has radioactivity by design - but Fusion not really. There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity.

"massive scale challenges"

And where exactly are they? Once a small reactor runs, what is the problem to build a big one?

"and no reason to think that it will ever be anything except massive construction project"

Because fusion bombs work since 70 years. And fusion in labs are not hard either. We can do fusion - we just cannot do it in a controlled way yet. But recent records with controlling hot plasma indicate that there is real progress. So (allmost) no one is currently betting that we will soon have fusion power, but at some point we will, if we keep on working.

>There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity. Per unit energy D-T fusion, the fusion reaction most likely to be viable near term, generates more neutrons than the fission of U235. The U235 fission reaction[0] releases 3 neutrons and 202.5 MeV of energy, 8.8 MeV escapes as antineutrinos, so let's call it 193.7 MeV, so 64.5 MeV per neutron.

D-T fusion generates 17.6 MeV and 1 neutron, so 17.6 MeV per neutron. So for the same amount of energy produced it makes about 3.5x more neutrons. Worse yet, in D-T fusion, 14.1 MeV of the energy released is in the kinetic energy of the neutron. Compare this to the 8.8 MeV energy of all three neutrons in a fission reaction. Neutron activation and damage are significant engineering concerns for fusion reactors.

[0]https://en.wikipedia.org/wiki/Uranium-235 [1]https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusi... https://en.wikipedia.org/wiki/Uranium-235

Thank you for some much needed solid information.

And then you need to use that neutron to breed new tritium from lithium-6, which is only about 2% of lithium. And since our only feasible source of tritium is a single tritium atom from that single neutron from a D-T fusion reaction that consumes a tritium, we had better be damn efficient at making sure the neutrons go only to breeding new tritium. Which is good, because then they cause less neutron radioactivity of all the rest of the reactor parts. But bad, because it's pretty unrealistic to capture all the neutrons via lithium blanket.

Which means we need ultimately need an outside tritium source, external from the fusion reactions, which means another neutron source, which means, fission reactors!

The whole idea of controlled terrestrial fusion being an inexpensive source of electricity is just absolutely baffling to me. As is the idea that it will somehow be a good extraterrestrial energy source, or power some sort of spaceship. Fission is far more realistic, likely to be compact, etc. Direct conversion of aneutronic fusion is complete sci-fi at the moment, we may as well be talking warp drives.

"Per unit energy D-T fusion, the fusion reaction most likely to be viable near term"

Ok well, I see the use of tritium only as a step towards normal hydrogen as fuel in fusion reactors. Tritium as fuel is just the "easy" target and yes, it is in limited supply and causes problems. But "in theory" we could have higher temperatures and not be dependant on tritium.

> Solar roads are expensive and worked out economocially nowhere, where they were build.

This is exactly my claim for fusion: an uneconomical idea that sounds really cool in theory.

> the basic idea of fusing hydrogen into helium works without radioactivity.

Where does the deuterium come from? There's going to be neutrons bombarding everything, by design, all sorts of radioactivity. It's literally at the core of the entire thing. This point is swept under the rug, but ensuring that the radioactivity is short lived will be a major design requirement of most components for the reactor.

> scale challenges

The scale challenge might be building a small reactor, instead of a large one. We don't know what sort of containment will work, and at what sort of physical scales. Which is why massive construction projects are a problem. We are terrible at massive scale construction, and much better at small scale manufacturing.

> fusion bombs

Not sure why fusion bombs would indicate we could ever do sustained and controlled fusion reactions. These are entirely different processes with almost no connection to each other. This is an error on the order of magnitude of thinking that fusion is not radioactive.

And this is why fusion is solar roads for sci fi fans. People love solar roads because they don't know the particulars about solar panels and roads and why they are a bad fit for each other. People love fusion power because they don't know the specifics about fusion or power and why they are a bad fit for each other.

> Where does the deuterium come from?

Sea water, it's surprisingly plentiful. What I'm not convinced by is that the neutron emissions that come from D+D fusion are in any sense "safe" or claims that that the neutrons are in a form that "can't" ever be used for development of fission weapons…

That was a basic goof, should have been tritium there.

Fair enough.

Tritium comes from D+D reactions, which… make a lot of neutrons in the process.

> Unless you mean roads that have roofs of solar panels

This would be really cool and may make sense with cheap/efficient enough panels.

But of course, you have to solve the 11'8" problem wherever these things get installed.

"Solar Roadways[tm]" is a specific startup, and is all but certainly what was referenced upstream.

The proposal and technology were obviously nonviable to anyone with the least technical knowledge from the beginning.

Wikipedia: <https://en.wikipedia.org/wiki/Solar_Roadways>

Phil "Thunderf00t" Mason on YouTube had a particularly brutal critique. Repetitively repetitious and occasionally repeating itself (as is his wont), but on point:

<https://yewtu.be/watch?v=H901KdXgHs4>

(Several follow-ups, including an on-site visit.)

The point I was trying to make is that fusion makes sense, the only issue is actually doing it.

In contrast solar roadways and space based solar don't make sense. Sure you could build them. But it would never make sense to do so because there's always a better way to spend your money. It's never* going to make sense to build a solar panel into a road than just on a field. And it's never going to make sense to build space based solar rather than just Earth based solar + batteries.

*standard HN pedantry disclaimer

And my point is that it's never going to make sense to do fusion to replace any current energy generation method. There's always going to be a better way to spend your money than fusion.

This is inherent to the way fusion is even being developed. It's always assumed that the fusion part is going to be more expensive than the equipment to convert thermal energy to electricity, which is why it makes sense to make it as energy dense as possible, and therefore more cost efficient.

But from that very starting point, it's clear that there will always be better ways to generate electricity than fusion: solar and wind combined with batteries.

We have much more reason to be optimistic about the cost case of extraterrestrial solar than the cost case for fusion.

"But from that very starting point, it's clear that there will always be better ways to generate electricity than fusion: solar and wind combined with batteries."

I am 100% for renewable energies and would not wait for fusion - but I am very hesistant to use the word always here. If there is a breakthrough in handling the plasma - and recent developements indicate that this might be possible - it could make fusion very cheap. So when you have a cheap, compact and reliable energy source - most people would favor it over something unstable that takes a lot of space.