Oh, so you meant....
...a Bussard ramjet! Why not simply say so.
Bussard ramjets make many appearances in Niven's "Tales of Known Space" series of novels and short stories as well as Poul Anderson's "Tau Zero".
If you remember Niven’s Down and Out*, then here’s a concept that will sound familiar: a machine powering itself by harvesting hydrogen from its environment. The jellyfish-like robot jellyfish demonstrated by researchers from Virginia Tech and the University of Texas, Dallas isn’t quite up to the task of gathering its own …
You obviously read my post before submitting yours, but look at the timestamps!
I wish I knew what is going on: I'd expect the timestamps to be added by the server, but the only way I can explain them is if they are local posting times set by the browser and you're in a different timezone to me unless, of course, you're using Winders and its suffering from the usual clock drift: my system is synced to NTP.
However, I digress. I've always assumed Niven deliberately wrote technological progress into the Known Space scenario and history line before he wrote more than one or two of the stories. The earlier, in Known Space time line, stories (Gil Hamilton, etc) only use fusion torches, then the earlier interstellar stories (Protector, The Warriors) are using Bussard ramjets or laser drives, with FTL drives following contact with the Outsiders. Putting the novels and short story collections in copyright date order, i.e. from 'World of Ptavvs' to 'Ringworld Throne', more or less confirms the hypothesis: "Ringworld" was copyrighted before "Protector" and it was copyrighted before most of the Gil Hamilton stories.
BTW, Poul Anderson's 'Tau Zero' was copyrighted a year or two after "A Gift From Earth" and so, if there was any interaction between them, Anderson got the Bussard drive idea from Niven, not the other way round.
In the context of the robot itself, there's no prospect that any feasible number of these could exhaust the ocean's resource of hydrogen - IF the problem of harvesting was solved.
So I won't lose sleep over letting the qualified "self-fuelling" statements stand.
By that thinking, it is also easy to convert hydrogen to helium - clearly possible, and done on unimaginable scale every day - but that is not necessarily what the robot guys had in mind now, is it?
Converting electricity to hydrogen and that in turn into heat seems like a dumb idea even to me, since there are much simpler ways of creating heat from electricity.
Thus, the way I would understand what they are trying to do is to simply collect ambient molecular hydrogen from the ocean and use that to drive their system, though they might be better off trying to convert ambient methane to hydrogen and use that.
(for more info see: "Methane and hydrogen in seawater (Atlantic Ocean)", Deep Sea Research Part A. Oceanographic Research Papers; Volume 35, Issue 12, December 1988, Pages 1903–1917)
gotta love internet search engines....
No, but those that do eat them include critically endangered species like sea turtles - that already die in significant numbers from mistakenly ingesting plastic bags. So, if it's all the same to you, no thanks. Unless you want to make your robots nutritionally viable and biodegradeable - that'd help.
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