Sergei Korolev was the man in charge of the Soviet moonshot N1 rocket, and I'd argue that the root cause of that program's failure was his insistence on using oxygen/kerosene-powered (cryogenic propellant) engines for this endeavour - as opposed to the existing, proven hypergolic-propellant rocket engines designed by Valentin Glushko.

Glushko took this demand for cryogenic propellant engines... poorly, and consequently refused to work with Korolev in any way shape or form (including in supplying or assisting with the use of his existing hypergolic ones). Given that Glushko's engines were used to power the USSR's ICBM fleet, which were critical to deterring a USA nuclear strike, he was one of the most important men in the nation and this temper tantrum was thus ignored by the Soviet leadership.

So the Soviet moon program started without any engines (on top of being 3 years later than the USA's). In order to get the engines he needed, Korolev was forced to turn to Nikolai Kuznetsov, a brilliant aircraft engine designer. That's right, Kuznetsov had zero experience designing rocket engines - but he was able to relatively quickly create the oxygen/kerosene NK-15. While this was a fundamentally sound design later developed into the very successful NK-33 and -43 engines, it had never been fully tested - and especially, not in orbit.

Kuznetsov's engine was also a relatively small powerplant, with correspondingly low thrust; compared to the American Saturn V rocket that used five F1 engines, the Soviet N1 needed thirty NK-15s. Yes, thirty engines of a completely new design, that had never been verified... what could go wrong? Predictably, the first two N1 launches were complete failures resulting in the complete destruction of the launch vehicle (and the pad in the second instance), caused entirely by the engines and their interactions with each other and the rocket's other systems.

Given that the second launch attempt occurred only a few months before the successful American manned moon landing, by the time the NK-15 had been thoroughly debugged for the third launch in 1971 (which also failed, but this time not due to the engines), the Soviet moonshot was as dead as Korolev. Glushko eventually took over the entire Soviet space program, and one of his first acts was to (arguably mercifully) terminate the N1 program.

Sergei Korolev was the man in charge of the Soviet moonshot N1 rocket, and I'd argue that the root cause of that program's failure was his insistence on using oxygen/kerosene-powered (cryogenic propellant) rocket engines for this endeavour - as opposed to the existing, proven hypergolic-propellant designs of Valentin Glushko.

Glushko took this demand for cryogenic propellant engines... poorly, and consequently refused to work with Korolev in any way shape or form (including in supplying or assisting with the use of his existing hypergolic ones). Given that Glushko's engines were used to power the USSR's ICBM fleet, which were critical to deterring a USA nuclear strike, he was one of the most important men in the nation and this temper tantrum was thus ignored by the Soviet leadership.

So the Soviet moon program started without any engines (on top of being 3 years later than the USA's). In order to get the engines he needed, Korolev was forced to turn to Nikolai Kuznetsov, a brilliant aircraft engine designer. That's right, Kuznetsov had zero experience designing rocket engines - but he was able to relatively quickly create the oxygen/kerosene NK-15. While this was a fundamentally sound design later developed into the very successful NK-33 and -43 engines, it had never been fully tested - and especially, not in orbit.

Kuznetsov's engine was also a relatively small powerplant, with correspondingly low thrust; compared to the American Saturn V rocket that used five F1 engines, the Soviet N1 needed thirty NK-15s. Yes, thirty engines of a completely new design, that had never been verified... what could go wrong? Predictably, the first two N1 launches were complete failures resulting in the complete destruction of the launch vehicle (and the pad in the second instance), caused entirely by the engines and their interactions with each other and the rocket's other systems.

Given that the second launch attempt occurred only a few months before the successful American manned moon landing, by the time the NK-15 had been thoroughly debugged for the third launch in 1971 (which also failed, but this time not due to the engines), the Soviet moonshot was as dead as Korolev. Glushko eventually took over the entire Soviet space program, and one of his first acts was to (arguably mercifully) terminate the N1 program.