My mind went there too. I didn't mention that one because it doesn't seem to be exponential. I actually found a past entry on the xkcd what-if blog: http://w

My mind went there too. I didn't mention that one because it doesn't seem to be exponential. I actually found a past entry on the xkcd what-if blog: http://what-if.xkcd.com/7/ http://what-if.xkcd.com/7/ For a given amount of ship mass, you need 20x that mass in fuel to get it into space. That seems to be multiplicative growth, unless I'm interpreting something wrong.