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Arrgh! It's not the cost of the fuel

14 Dec 2013, 21:49 UTC
Arrgh! It's not the cost of the fuel
(200 words excerpt, click title or image to see full post)

"What's the cost of propellant from earth vs getting it from the moon?" always comes up in discussions of lunar water. Or the cost of near earth asteroid propellant vs earth propellant.Propellant is cheap, typically a small percentage of spacecraft expense. Spaceflight is expensive because vehicles are disposable. How much would a plane ticket cost if a 747 were thrown away each trip?Well, how come we don't re-use our spaceships? It's due to constraints imposed by the rocket equation.As delta-V budget climbs, dry mass fraction shrinks. We can't eliminate engine or payload mass. We cut dry mass by making walls thinner and structure more tenuous.Thinner walls mean fragility. Designing upper stages is like designing egg shells.Upper stages are like cascarónes, confetti eggs. While cascarónes are fragile by design, upper stages are fragile due to the constraints imposed by the rocket equation and high delta-V budgets. An upper stage plunging into the atmosphere is like a cascarón plunging onto a friend or relative's head. But the conditions of re-entering earth's atmosphere at 8 kilometers/second are much more extreme than the back of her mom's head.Given propellant depots at LEO, GEO, and EML1 or 2, ferries between orbits would have delta V ...

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