Comments by "Stephen Villano" (@spvillano) on "Real Engineering"
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Kerosene and methane are more energy dense fuels, with greater power than hydrogen, but were harder to ignite in the 1960's - even with the hypergolic ignition system that the Saturns used. But, soot accumulation shouldn't be an issue with pure oxygen as an oxidizer, it wasn't for the Saturns.
The trick is making it reliable in a vacuum or near vacuum, otherwise someone's staying behind with a flare to light the engines for the rest of the crew to get home... ;)
But, hydrogen won't freeze when kerosene or methane does, it's all about trade-offs and critically, reliability of ignition. Still, that shouldn't be a problem with today's technologies and methane is ubiquitous in the solar system.
For CO2, I'd go with compression of the atmosphere and steal off the triple point CO2. It'll take a long time to get much, but time is plentiful in space exploration.
As for Martian cities, not gonna happen. We evolved for 1G, not less. Live outside of earth's gravity for much longer than a year, return to earth is pretty much a death sentence and radiation is quite high on mars, due to the thin atmosphere and no magnetic field. One can work around some of the problem, but colonizing mars means exile from earth
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