If I recall correctly... my very first post on Reddit was doing calculations for a (practically immortal) person eating beans and storing the flatus for a trip to the moon (searching shows that this is a not-infrequent request). It was only concerned with quantity - not storage or the engine.

... and the source document for the numbers was based on a paper that is fairly easy to find given the proper keywords in google search... https://pubmed.ncbi.nlm.nih.gov/1648028/ (and I learned that methane more rare in flatus than not).

Did you account for the weight of storage? Because I would think the tyranny of the rocket equation for such a low ISP would cause various levels of impossibility, not to mention the problem of getting enough thrust.

It was based on “how long would it take to fill the tanks of a Saturn V?” … with the additional “let’s pretend we can 1:1 swap methane and kerosene.”

Well you just activated a neural pathway that's been dormant for several decades... you wouldn't happen to remember the result would you? ;)

I think the scale was on the hundreds of thousands of years. We're dealing with 700 ml of hydrogen and 70 ml of methane at standard pressures and scaling this up to 90,000 kg of hydrogen and 635,000 kg of kerosene (with the 1:1 methane).

Hilarious!