It depends on flight rate. If your vehicle is going to fly just once a year (or less), it's too big.

Most of the benefit of heavy launchers can be obtained by in-orbit mating of parts launched on smaller launchers, and most importantly in-orbit propellant transfer. "Tank farms" in orbit containing oxygen and hydrogen (or methane) would be a real game changer for getting to the Moon and beyond, particularly if they can be refilled with propellants processed from ET sources (lunar polar ice, for example). The one thing SLS could sell itself for would be large unitary payloads, for example heat shields for landing on Mars. But I strongly suspect the problem of assembling a large heat shield in space from smaller segments could be solved.

The satellites will let you determine your position in relation to her enormous (gestures with hands) tracts of land.

I think that once the process is identified to achieve the orders of magnitude required to scale up fusion to commercial quantities that there will be a period of very expensive reactors that will perfect the required industrial processes to bring the cost down.

Your comment there brings up a very important general point about how technology develops.

ALL successful technologies develop by iteration, and this iteration can only happen if the cost of an iteration is sufficiently small. This means benchtop, or garage scale, technologies advance. If a technology starts at a point where an iteration costs $20B, it will go nowhere.

If fusion is to have any chance at all, it will be with technologies that can be investigated for $20M, not $20B. And once an iteration gets sufficiently expensive relative to the size of the market (see, for example, passenger airliners), advancement slows way down or stops.

See the Charpin report: “Economic forecast for nuclear power” by Jean-Michel Charpin, Benjamin Dessus, René Pellat, Report for France's Prime Minister. September 2000, Paris. Translated quote:

The extra cost associated with reprocessing and MOX (mixed-oxide) fuel fabrication, compared to direct fabrication of UOx (uranium oxide) fuel fabrication (from enriched uranium) is not offset by savings of natural uranium through plutonium use (or savings) resulting from reduction in the direct cost of disposing of final wastes," the experts wrote. "In other words, this strategy, from the viewpoint of the utility, represents an increase in the cost of a kilowatt-hour, which appears as an obstacle to its competitiveness, an element that is increasingly intolerable in (an electricity) market opening to competition.

So far, unless you're on an island and have to ship in your diesel fuel, solar doesn't make economic sense without massive subsidies.

This is false. Solar has made sense for people living even modest distances from grid for quite some time now. As solar has gotten cheaper, the distance has dropped. In some sunny locations (for example, much of Australia), the breakeven distance is now zero.

If you have tritium and can do boosting, there is no need for 'weapons grade' Pu to make weapons. This is the key point!

So-called weapons grade Pu is called that because that's the isotope mix you get when you maximize Pu production in a thermal reactor (leave it in longer and too much Pu gets burned up). It's not because more Pu-240 makes the material unsuitable for weapons.

The Pu used in very early weapons, before they had boosting, had very low Pu-240 content (so-called "super weapons grade"). Once boosting was invented it was no longer necessary to make the Pu so pure.

Pu-240 is more than just an impurity, btw. It fissions too, with a higher cross section (and lower critical mass) than U-235.

France, and the other countries that went ahead with fast breeders, discovered that at current uranium prices fast breeders make no sense. Fast breeders are kind of like fusion, in that they trade higher capital cost for lower fuel costs. This is penny wise, pound foolish. Fast breeders aren't quite as bad as fusion, but they're bad enough to be impractical. BTW, France also has admitted that even reprocessing fuel from thermal reactors doesn't make economic sense. It's more economical just to store the spent fuel as is, probably in dry casks, on the off chance it might be worth reprocessing a century or two from now.

Actually, all plutonium bombs have a mix of isotopes in them. You can't make Pu-239 without also making some of the higher isotopes. "Weapons grade" Pu is about 6% Pu-240. "Reactor grade" Pu can be as high as 26% Pu-240. The important thing about DT boosting is that it enables weapons to be designed that are immune to fizzles from premature initiation of the chain reaction. Even if the chain reaction starts at the moment of criticality, enough fusion neutrons are generated to produce high yield as the expanding core becomes subcritical. From a proliferation point of view, this means a country with a large reactor-grade Pu stockpile, like Japan, could "break out" and quickly make large numbers of nuclear devices, if they have a tritium supply.

They talk about the break-even point because it's the key to fusion power.

No, it's only the first step to fusion power. The real killer is going to be making a practical, reliable, economically competitive reactor. No one knows how to do that. At this point, tokamaks and ICF, even if they achieved breakeven, would be practical dead ends.

Well, CERN went ahead and did the heavy lifting to discover the Higgs. So the US got the value of that discovery without having to build the SSC. It seems we came out ahead.

Yes, an actual working commercial reactor will be even more expensive, since it will include things not present in ITER (like tritium breeding blankets, exotic materials that can withstand the neutron load, robotic systems for changing out damaged reactor segements when they reach their neutron exposure limits, and a turbine/generator set.) But even if $20B were too much, understand that a fusion reactor making 500 MW(th) would be uncompetitive even if it cost $2 B, an order of magnitude less. And ITER does not have $18 B of instrumentation in it.

In particular, it's by Lawrence Lidsky, who went on to be Todd Rider's thesis advisor. Rider showed in his thesis that aneutronic fusion (particularly based on non-Maxwellian plasmas) wasn't realistic. Lidsky switched over to fission reactor research, and pursued that for the rest of his life. Rider left nuclear energy entirely (what was he to do after his thesis basically chopped off an entire major branch of design space), changed fields, and has been doing interesting work in biology.

This confuses cause and effect. The lack of funding is caused by the extremely tenuous case for fusion, not the other way around. The real issue with fusion is not that too little has been invested in it, but that too much has been wasted on it. The fusion budgets should be greatly reduced, or even eliminated.

Right, currently small scale because there is no demand for large amounts of heavy water. They tore down the old Girdler Sulfide plant because if such demand ever does materialize, they will want to build a new CECE/CIRCE plant instead.