This is even more vividly shown in this NASA plot of booster development: http://upload.wikimedia.org/wikipedia/commons/6/67/USAF_ICBM_and_NASA_Launch_Vehicle_

This is even more vividly shown in this NASA plot of booster development: http://upload.wikimedia.org/wikipedia/commons/6/67/USAF_ICBM_and_NASA_Launch_Vehicle_Flight_Test_Successes_and_Failures_%28highlighted%29.png http://upload.wikimedia.org/wikipedia/commons/6/67/USAF_ICBM...