Put stuff you don't want to burn on top of lots of stuff that burns, hope it doesn't burn

Rockets at 85% propellant and 15% structure and payload are on the extreme edge of our engineering ability to even fabricate (and to pay for!). They require constant engineering to keep flying. The seemingly smallest modifications require monumental analysis and testing of prototypes in vacuum chambers, shaker tables, and sometimes test launches in desert regions.

For a Space Shuttle launch, 3 g’s are the designed limit of acceleration. The stack has been certified (meaning tested to the point that we know it will keep working) to 3.3 g’s. This operation has a 10% envelope for error. Imagine driving your car at 60 mph and then drifting to 66 mph, only to have your car self-destruct. This is life riding rockets.
— http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html

A NASA engineer gets lyrical on just why rockets are so insanely fussy to build, test and operate. Remind me of Neal Stephenson's lament on the path dependency of orbital lift, how the need for instant intercontinental annihilation settled us too firmly on this burn-fast-and-try-not-to-detonate style of getting things from here to way, way there.