[avmich]

Right, but at the time to save on mass they used tricks like working with negative safety margin, that is, engines were calculated to serve particular number of seconds and performed slightly outside of elastic deformations... They did move towards multi-start engines for first stages eventually, but not during 1960-s. The original idea of using rockets was military, and those guys had hard time to understand why such a thing should work multiple times, I guess.

Vacuum engines can actually be tested on Earth, some special devices which produce external pressure decrease when the engine is running (like, if you run engine in a tube, the hot gases will push all the air from the tube making a pressure drop).