Complex sentence: Korolev was suffering from chest pains and close to a nervous breakdown, as this was the 24th Soviet space launch and the 16th involving a Luna/Vostok booster, and to that point 12 launches had failed, for a success rate of exactly 50%.
Split into simpler sentences: Korolev was suffering from chest pains and close to a nervous breakdown. So far, 12 launches had failed, for a success rate of exactly 50%.