I should also remind you that a year ago NASA didn't actually know "how" the j-2 worked. Their numbers said the bell should melt during flight. Eventually they realized that helium is injected on a outer ring of injectors to insulate the bell.

I think this is remarkable, given that film cooling of the nozzle is a known aspect [0] of the first stage engines of Saturn V.

[0] Thrust Chamber Nozzle Extension section on page 3 http://history.msfc.nasa.gov/saturn_apollo/documents/F-1_Eng...