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Didn't see it listed as an option here, but are the travel-via-ICBM options that SpaceX has been touting going to actually be cleaner options than burning jet-fuel? If you remove the cruise speed requirements for longer-haul flights and put them in space, maybe that helps? Hard to say without any real numbers out there (and even if I had real numbers I'm probably not in a position to synthesize them vs. aircraft)

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Judging by the amount of volume of a rocket or ICBM that is consumed by its fuel, I would have to venture that it's nearly impossible that it's cleaner.

I would love to see the numbers for this.

A significant part of the volume in a ballistic rocket is oxidizer which a jet does not need to carry. Liquid methane is much less dense than jet fuel, so it makes sense it would take up much more volume. Also, methane has less carbon in it than jet fuel on a per energy basis.

Still suspect you are right.

As far as spacex visualizes them, no way in hell. Now, if you were to either use A) H2+LOX fuel generated via renewable means or B) non rocket propulsion (scram jet, railgun, etc) you could potentially do a lot better for long flights because once you're in space, there's no air resistance and you can coast as far as you'd like depending on your initial speed.

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