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February 21, 1969: serial number 3L – Zond L1S-1 (Soyuz 7K-L1S (Zond-M) modification of Soyuz 7K-L1 "Zond" spacecraft) for Moon flyby
A few seconds into launch, a transient voltage caused the KORD to shut down Engine #12. After this happened, the KORD shut off Engine #24 to maintain symmetrical thrust. At T+6 seconds, pogo oscillation in the #2 engine tore several components off their mounts and started a propellant leak. At T+25 seconds, further vibrations ruptured a fuel line and caused RP-1 to spill into the aft section of the booster. When it came into contact with the leaking gas, a fire started. The fire then burned through wiring in the power supply, causing electrical arcing which was picked up by sensors and interpreted by the KORD as a pressurization problem in the turbopumps. The KORD responded by issuing a general command to shut down the entire first stage at T+68 seconds into launch. This signal was also transmitted up to the second and third stages, "locking" them and preventing a manual ground command from being sent to start their engines. Telemetry also showed that the power generators in the N-1 continued functioning until impact with the ground at T+183 seconds. Investigators discovered the remains of the rocket 32 miles (52 kilometers) from the launch pad. Vasily Mishin had initially blamed the generators for the failure, as he could not think of any other reason why all 30 engines would shut down at once, but this was quickly disproven by telemetry data and the recovery of the generators from the crash site. They had survived in good condition and were shipped back to the Istra plant, where they were refurbished and worked without any problems under bench testing. The investigative team did not speculate as to whether the burning first stage could have continued flying if the KORD system had not shut it down. The KORD was found to have a number of serious design flaws and poorly programmed logic. One unforeseen flaw was that its operating frequency, 1000Hz, happened to perfectly coincide with vibration generated by the propulsion system, and the shutdown of Engine #12 at liftoff was believed to have been caused by pyrotechnic devices opening a valve, which produced a high frequency oscillation that went into adjacent wiring and was assumed by the KORD to be an overspeed condition in the engine's turbopump. The wiring in Engine #12 was believed to be particularly vulnerable to this effect due to its length, however other engines had similar wiring and were unaffected. Also, the system ended up drawing 25V instead of its designed 15V due to the ruptured power lines. The control wiring was relocated and coated with asbestos for fireproofing and the operating frequency changed. [33][34] The launch escape system was activated and did its job properly, saving the mockup of the spacecraft. All subsequent flights had freon fire extinguishers installed next to every engine.[35][36] According to Sergei Afanasiev, the logic of the command to shut down the entire cluster of 30 engines in Block A was incorrect in that instance, as the subsequent investigation revealed.[37][38]
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