The NK-15 engines had a number of valves that were activated by pyrotechnics rather than hydraulic or mechanical means, this being a weight-saving measure. Once shut, the valves could not be re-opened. This meant that the engines for Block A were only test-fired individually and the entire cluster of 30 engines was never static test fired as a unit. Sergei Khrushchev stated that only two out of every batch of six engines were tested, and not the units actually intended for use in the booster. As a result, the complex and destructive vibrational modes (which ripped apart propellant lines and turbines), as well as exhaust plume and fluid dynamic problems (causing vehicle roll, vacuum cavitation, and other problems), in Block A were not discovered and worked out before flight. Blocks B and V were static test fired as complete units.