You’re right on the details, there, but again, it’s the context.
Materials availability-wise: yes, carbon is more abundant. On the other hand, iron is post-post-scarcity anywhere asteroids are mined, as the process of extracting the interesting and actually scarce metals generates gigatons of the damn stuff as a side product, to the point that in the 'verse mining companies will basically give you the stuff for the cost of hauling it away.
But that’s not why they use steel, although it being dirt cheap doesn’t hurt: they use steel 'cause it’s easy to work with compared to other reasonable structural materials. Aluminum requires special welding techniques. Don’t even ask about what a pita titanium is to work with. Even plain old carbon fiber - leaving aside spinning it in the first place - requires specialized skills and equipment to work with, never mind fancy carbon composites or anything with nanostructure. But cutting and welding a steel hull can be done by pretty much any minimally trained tech with equipment that even a lot of people on Earth-now have in the shed.
And that’s very important, since the sort of people who fly this class of ship don’t have any kind of high-falutin’ engineering qualification. Hopefully, they have a base-level Master’s Certificate and Nuclear Engineering competence certification, and if you’re lucky, those weren’t actually issued to Captain Harbatkin by Space Liberia, then picked off his corpse after a bar brawl, cleaned, and resold.
Likewise, the deuterium. That’s because anyone who can pass chemistry class can separate deuterium from water - and because the stuff is found everywhere there’s hydrogen, which is everywhere - it’s easy to handle, it doesn’t decay, and when you do D-D fusion, it’s a monopropellant.
(The designers would probably have used protium if they could, but that’s too hard to build a cheap-ass drive to run on.)
It’s about as easy to deal with as starship fuel gets, and can be obtained anywhere in space, even on some backwater rim colony or technically undeveloped world where the locals look at you funny if you ask them for isotopically pure lithium.
Bear in mind, too, that this class of owner cares a lot more about running costs than final costs. Purified 6Li, say, costs a lot more than nigh-ubiquitous water, and that’s assuming that you can’t just drop your Cheap-Ass Separator ™ on the nearest ice asteroid.
This, incidentally, also explains a lot about disposal, because the captain-owners (look at the list of likely examples again) mostly don’t care about disposal. The average Hariven gets run until it catoes, gets sold at scrap prices to Shady Bob’s Wreckyard for what Shady Bob loosely calls “reconditioning”, or dumped on some environmentally-uncaring garbage heap like Brak Tífel . If one makes it to a reputable ship-breaker that disposes of radioactives conscientiously and appropriately, it’s time to throw a damn parade.
Ru <shearwater@gmail.com> on 2017-03-08 05:16:07 wrote:
If I were working metals in space and had a relaxed attitude to radiation I’d be using electron beams. They’re useful items to have for mining and for reasonably high thrust engines (that can run on almost whatever you can throw into the reaction chamber) so I’d expect them to be readily available in a range of colors and sizes.
Lithium purification might not be quite as hard as you suggest; there’s a poor man’s enrichment process which uses vacuum distillation, a mechanism readily available to anyone in space with a decent power reactor and some time to spare. On earth, at least, it is helped by the generous Li-6 to Li-7 ratio in natural sources, though I don’t know if that generalise to the solar system or elsewhere. The remaining depleted lithium probably makes for a good coolant. Poorly enriched lithium might not be much good as a fusion fuel, but it makes for a good combined coolant and neutron shield for your drive coils, or a good reaction mass for dirty neutron producing fusion reactions (eg. D-D or D-T)
when you do D-D fusion, it’s a monopropellant.
It’ll be adequate for high-Isp, low-thust regimes (and by low thrust I mean centigees at best) but deuterium is a poor choice for heating via neutrons (and that’s where most of your fusion energy is going after all) so you’ll have serious problems reaching a thrust of ~0.5g, let alone sustaining it. Incidentally, don’t discount low-thrust, long-burn flight regimes. A few centigees of sustained thrust could get you from Earth to Jupiter in a month or two
If you let me know the loaded mass of a typical loaded Hariven, I could run some numbers for you, if you like. (unless you’ve already done that yourself, of course!)
