In another thread (under Dr. Kaku's EXPLORATIONS program page, the thread about the idea of a "Space Elevator"), the conversation turned to the prospect of using something like a "Space Elevator" as the basis for shifting from a petroleum based economic model to essentially a water based economy (or hydrogen based if you prefer), and we need some numbers for a few "back of an envelope" calculations. Thus the following questions: 1) If you take a gallon of water, and separate it out into h & o2 by way of electrolosis, how many cubic feet of hydrogen and oxygen do you get (at room temp and sea level psi)? 2) If you now take that h & o2 (that you got from one gallon of water), and you just burn it (turning it back into water), how many BTU do you get out of it? 3) How does that compare to gassoline? If you burn a gallon of gassoline, how many BTU do you get out of it? 4) If, instead of immediately burning the resulting h & o2 from a gallon of water, you instead cool it back down into liquid h and liquid o2 (for more compact transport as an energy supply), does the sum of the resulting amount of liquid h and liquid o2 still add up to one gallon? 5) How cold does h have to be before it liquifies at, say, 14.7psia? (I.e. would simply leaving h gas in a container in a shadowed area in orbit where temps can drop to several hundred degrees below zero be enough to liquify the gas?) 6) Ditto for the o2. How cold does o2 have to be before it liquifies at 14.7psia? 7) How much energy is required to separate a gallon of water into hydrogen and oxygen in the first place?