Failed swan song of the Soviet Moon rocket

The fourth launch of the N1 rocket (Vehicle No. 7L) carrying an unpiloted LOK crew vehicle and a mockup of the LK lunar lander took place on Nov. 23, 1972. Despite a failure after just 107 seconds in flight, it was the closest the giant rocket ever came to completing the operation of its troubled 30-engine propulsion system on the first stage.

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Fourth launch of the N1 rocket (No. 7L) at a glance (537):

Launch vehicle designation

11A52 No. Kh15007L (N1-L3 No. 7L)

Payload designation

L3 complex with vehicle 11F93

Launch date and time

1972 Nov. 24, 09:11:54 Moscow Time

Maximum altitude reached
approximately 70 kilometers
Flight duration
approximately 107 seconds
Launch site



In the 17 months between the failed third launch of the N1 rocket in June 1971 and the fourth attempt in November 1972, engineers at the TsKBEM design bureau, which led the project, introduced many upgrades into the vehicle, first of all into its first stage.

To improve the stability of the rocket in flight, four hastily developed movable thrusters replaced the steering nozzles which previously controlled the roll motion of the first stage by taking gas from gas generators of its main engines. Installed in movable gimbals, the new 11D121 engines were designed to be powerful enough to counteract the kind of force that doomed the third N1 rocket immediately after liftoff in 1971. The engines burned kerosene fuel mixed with hot oxidizer gas tapped from the gas generators of the main engines, thus negating the need for a separate ignition system.

The aerodynamics of the rocket were also improved with new fairings covering the propellant lines on the exterior of all three booster stages and with a newly designed cylindrical skirt at the base of the rocket, replacing the older conical shape.

Inside the rocket, a new freon-based fire-suppression system was installed along with improved protective shielding on instruments and their cables.

On the part of the flight control system, a new gyroscopic platform was providing signals for the dual main BTsVM computer, which itself was a long-delayed Biser machine. One flight control computer was installed on the third stage and was responsible for the powered ascent to orbit and another aboard the LOK crew vehicle, controlling the entire flight of the L3 lunar expeditionary complex.

A smaller, lighter Orbita-4D radio-telemetry system, developed at OKB MEI, was installed on the third stage to gather and transmit data from around 13,000 sensors spread around the vehicle. (231)

All the improvements boosted the payload of Vehicle 7L, assigned to fourth launch, to 90 tons giving some badly needed extra payload to the mass-strapped L3 complex.

Flight program of the fourth launch

In accordance with the protocol of the State Commission meeting on April 4, 1972, that included key officials responsible for the launch, Vehicle 7L was being assembled with fully operational three booster stages. The L3 payload assembly also included operational Block G and Block D upper stages, the partially equipped LOK crew vehicle, a mockup of the LK lunar lander and the payload fairing for the 6A operational variant.

The flight program of the 7L launch called for a flight to the Moon, the insertion of the L3 complex into a near-equatorial lunar orbit with the subsequent return of the LOK vehicle to Earth and a splashdown of its Descent Module in the Indian Ocean.

Final preparations for launch


Modified first stage of Vehicle No. 7L on the launch pad in Tyuratam.

On Aug. 15, 1972, Head of TsKBEM design bureau Vasily Mishin chaired a meeting of the Chief Designer council, where all the leading engineers and subcontractors confirmed the readiness of their systems for 7L launch. On August 21, the State Commission which gave the go ahead to the launch of Vehicle 7L and approved the final processing schedule, with the launch planned for the end of October 1972.

However, the program still faced numerous technical issues. For example, the integrated testing of the fully assembled N1/L3 complex in horizontal position in the vehicle assembly building in August 1972 revealed interference between various instruments and led to unstable operation of the flight control computers. The situation put enormous stress on all participants in the program from rank-and-file workers to top managers.

Around a week after the August 15 Chief Designer Council meeting, Mishin was hospitalized at the elite Kremlin clinic in Kuntsevo, perhaps due to overwork, and his deputy Sergei Okhapkin took over the responsibilities of the Chief Designer. However, soon, Okhapkin himself joined Mishin in Kuntsevo with a stroke. At the beginning of September 1972, the overwhelming responsibility for the risky 7L launch ended up in the lap of Boris Chertok, another Mishin's deputy and old associate of Sergei Korolev, who had led the early development of the project.

When Chertok arrived at Tyuratam, the fully assembled Vehicle No. 7L with the L3 complex had already been rolled out from the vehicle processing building at Site 112 to the launch complex at Site 110 on Aug. 24, 1972. Its on-pad testing started on Aug. 30, 1972. As usual, it revealed a multitude of problems. During one of the technical meetings reviewing initial results of the tests, Lt. General Aleksandr Mrykin, the veteran of the range, reported 17 issues in the flight control avionics and more than 100 problems with the telemetry system. With this kind of statistics, Mrykin questioned the wisdom of continuing the preparations on the pad. Replacements of many instruments had followed but the team proceeded toward launch. (685)

Launch and flight


The N1 No. 7L rocket lifted off from "Left" pad at Site 110 in Tyuratam on Nov. 23, 1972, at 09:11:54 Moscow Time.

In the cloud of fire and dust erupting from the three flame trenches of the complex, the giant rocket safely cleared the towers of the launch complex and, after a few seconds of vertical ascent, began nominal tilting toward its prescribed flight azimuth across clear sky. In the crowded firing control bunker of Facility 103 on the edge of the launch complex, Chief Engineer Boris Dorofeev and head launch officer Mikhail Belizin were glued to the periscopes, while top officials were listening to the reports from the loudspeakers.

As the rocket passed the point were the 3L vehicle had failed 69 seconds after liftoff in the winter of 1969, it was flying higher and farther than all three of its ill-fated predecessors. When the announcer reported the cutoff of the central engine cluster around a minute and a half into the flight, a sigh of relief was heard in the bunker and some had a glimpse of hope that the rocket could finally beat its odds. But moments later, tracking monitors registered a bright flash of light and the disintegration of the rocket. (233) Simultaneously, all communications channels stopped receiving the data.

The vehicle failed about 107 seconds after liftoff, just seven seconds before the scheduled shutdown of the 24 still firing engines on the first stage and the separation between the first and second stages.

As required, the Emergency Escape System, SAS, pulled the top sections of the LOK spacecraft away from the failing rocket just in time, resulting in the subsequent soft landing of the ship's Descent Module. The upper stages, also apparently separated from the booster stages before impact.

Investigation begins

The first attempt to find the culprit in the failure of the fourth N1 launch uncovered a few signs of problems in the moments preceding the accident and the specialists had only 0.2 seconds of comprehensive telemetry coverage as the catastrophe unfolded and before most data stopped. That initial uncertainty provided a fertile ground for competing theories, pushed forward first of all by developers of the propulsion system, which practically immediately became the prime suspect in the accident. The probe was further clouded by the growing realization of the precarious political position of the program.

Nevertheless, a picture of the accident emerged in the subsequent careful analysis of the available data and the pain-staking inspection of recovered debris, even though there would never be a full agreement of all parties involved about the root cause of the accident.

By 15:00 on Nov. 24, 1972, members of the State Commission and other specialists, who were involved in the failed launch, gathered in the main hall of the calculation center in the residential area of Tyuratam. Exhausted by a sleepless night spent deciphering available data, the officers from the NII IT enterprise who developed the Lotos telemetry system began the grim task of presenting the early results of the investigation. (685)

The available data showed that the flight of Vehicle 7L was fully nominal until L+106.94 seconds and all its parameters looked normal up to that point.

At L+94.5 seconds in flight, six engines at the center of the first-stage booster received the planned command to shut down, in order to alleviate structural loads on the vehicle ahead of the scheduled separation of the first stage. In the meantime, the 24 other engines, forming the outer circle on the periphery of the booster, continued firing. The four steering engines also performed normally.

The KORD system, which was responsible for real-time diagnostics of all engine parameters, had not issued any emergency cutoff commands until L+106.7 seconds.

Up to that time, the recorded loads on the rocket were lower-than-calculated and the vehicle was stable with all its motions within specifications. However, many signs of trouble appeared at the 107-second mark in the flight.

After L+106.9 seconds, the telemetry showed a sharp pressure drop in the fuel and oxidizer tanks and there was no sign that the second stage got the command to activate its propulsion system (preceding its separation from the first stage).

The telemetry system on Block B (Stage II) stopped talking to the ground at L+107.28 seconds, or 0.33 seconds after the drop of telemetry from Stage I.

The data from the gyroscopic stabilization platform at the top of the third stage, showed sharp deviations of the rocket along all three axis reaching 18 degrees as late as L+110 seconds. At that moment, the rocket’s main BTsVM computer registered an emergency situation.

Most crucially, acceleration sensors recorded a burst of unexpected loads on the structural ring of the first stage at L+106.95 seconds. The highest loads were recorded along Plane II in the rocket’s coordinate system. Just 0.05 seconds after the loads increase, almost all telemetry channels stopped transmitting data. Fortunately, Aleksei Bogomolov, the head of OKB MEI, broke the news at the meeting that the transmitter developed at his organization and installed on the third stage, continued sending data until L+282 seconds. As it already had become clear, thanks to the operation in the centimeter bandwidth, Bogomolov's Orbita system had the capability to penetrate the plasma that surrounded the burning vehicle during its fatal plunge to the ground. However, its data was yet to be deciphered at the time.

In any case, the preliminary information indicated that at L+106.97 seconds, there was an impact on the structural ring of the first stage between Planes I and II, leading first to the interruption and then to a complete loss of most communications.

After some inconclusive discussions on the exact origin of the anomaly, Chertok proposed to reconvene at the same time on November 25, after some rest and the in-depth analysis of the latest data. The gathering also assigned individual specialists to be responsible for various aspects of the investigation. Still, as the event was adjourned, Ivan Raikov approached Chertok and said that after looking at all the relevant information he was convinced that the oxygen pump on Engine No. 4 had exploded causing the launch failure. (685)

As it transpired, Engine No. 4 failed and 200 milliseconds later, Engine No. 5 failed as well. During the period from L+106,932 to 106,942 seconds, the telemetry showed a sharp increase of pressure at the entrance into the oxidizer pump of Engine No. 4. Later, a fragment of a nearly completely unwrapped and flattened oxidizer line from Engine No. 4 was found among the debris. (233)

Nikolai Kuznetsov, the head of engine development, argued that the abrupt cutoff of central engines with a total thrust of 900 tons at L+94.45 seconds could have produced a hydraulic shock inside the propellant lines that caused their rupture, followed by a fire quickly spreading inside the first stage. Although the liquid oxygen and kerosene from the ruptured lines would be spewing onto the already deactivated engines, they were hot enough to trigger the blaze that eventually led to the explosion, the proponents of the theory apparently said. However that hypothesis had to explain the nearly proven fact that the explosion of Engine No. 4 was subsequent rather than the original failure event. The proponents of the theory argued that because the combustion chamber pressure of Engine No. 4 remained normal until L+106.956 seconds, its explosion was the result of some prior failure. (The theory obviously neglected the fact that the engine's turbopump, featuring a fast-spinning turbine, was a much more likely source of the explosion than the combustion chamber.)

At the request from Chertok, Mikhail Ryazansky, the Chief Designer of avionics systems, assigned his engineer Anatoly Churkin to compile a very detailed timeline of the events leading to the failure with an accuracy of 0.1 microseconds.

During the second meeting of the investigative team, specialists tried to pinpoint the source of the massive explosion, which almost instantaneously enveloped the rocket into a thick cloud of plasma and caused the loss of communications. Notably, the breakdown of communications happened faster than during the first launch of the N1 rocket in 1969. The explosion theory was supported by the fact that large-diameter power cables between first and second stages were severed almost instantly from L+107.45 to 107.5 seconds. In total, engineers reviewed around 5,000 different parameters, many of which pointed at the explosion. Still, Kuznetsov insisted on investigating alternative scenarios, such as a rapid structural failure of the rocket.

However, at the second gathering, flight control engineer Georgy Degtyarenko presented indirect, but convincing evidence that the very initial “dynamic event”aboard the rocket originated between engines No. 3 and 5, or, more precisely, at Engine No. 4, where three powerful jolts were registered by the axial acceleration sensors within the 0.15-second time period between L+106.95 and +107.1 seconds.

Other specialists reported that all engines, but No. 4, operated normally until L+107.1 seconds. On Engine No. 4, the spin rate counter and other channels detected data breakdowns when other engines were still sending data.

It was shown that the spinning of the turbine in Engine No. 4 was interrupted 0,022 seconds earlier than on the adjacent engines No. 5 and 6. The first telemetry interruption was detected on local connector No. 13 at L+106,848 seconds, while most other channels stopped sending data at L+107.21 seconds. As a result, investigators focused their further efforts on a 0.362-second period of the last available data. By the end of the day on November 25, the investigative team formed four sub-commissions to detail various aspects of the accident.

Still, during another overall review of the investigation on November 26, Nikolai Kuznetsov, yet again demanded the evaluation of a theory involving structural failure resulting from the abrupt cutoff of six central engines. Proponents of the theory cited the fact that the explosion took place soon after the rocket’s fire-suppression system ran out of freon.

Kuznetsov got immediate push back from Dmitry Kozlov, head of the N1 production at the Progress factory in Kyubyshev. Kozlov said that the first stage had plenty of structural redundancy to withstand this type of shock and stressed that the failure took place far beyond the point of maximum dynamic pressure on the vehicle. Still, Kozlov ordered the re-check of all theoretical and testing data, as well as promised to conduct all necessary demonstration tests simulating the conditions of the flight.

With overwhelming evidence against Engine No. 4, most specialists at the November 26 meeting had no doubts about the real culprit, but, as the final argument, one of the officers of the calculation center wrote down on the chalkboard the chronology of data interruption, which clearly illustrated how the failure had cascaded in just three hundreds of a second from Engine No. 4 to Engine No. 22, located on the opposite side of the rocket's base, some 14 meters away:

Engine No.
Data loss time after liftoff, L+
Elapsed time from Engine No. 4 data loss
No. 4
L+106.932 seconds
+0.000 seconds
No. 3
L+106.936 seconds
+0.004 seconds
No. 20
L+106.948 seconds
+0.016 seconds
No. 22
L+106.962 seconds
+0.030 seconds

Still, to the chagrin of their colleagues, propulsion specialists continued advocating alternative scenarios even in the face of overwhelming evidence, prompting Chertok to schedule another meeting on November 27. This time, key presentation was assigned to Valentin Likhushin, from Thermal Processes Research Institute, NII TP, who had the reputation of being an expert in the issue.

Likhushin re-confirmed that the catastrophic explosion originated at Engine No. 4 and then spread to other engines within 0.04 seconds as shown by the dynamics data and confirmed by turbo-pump rate sensors. However, the question remained, whether it was an explosion inside the engine's combustion chamber or somewhere else. The combustion chamber was considered an unlikely source, because there were no signs of leaking oxygen or kerosene. According to Likhushin, investigators made an attempt to chronicle the process of disintegration of the engine with an accuracy of one hundred or even one thousand of a second and correlate it to the design of the engine in order to pinpoint the source of the explosion to one of the components, such as propellant lines or the turbine. Although there was no clear answer, Likhushin saw the turbopump as the most probable culprit. The same component was known to have caused failures during ground tests and it was also found to be responsible for the failure of Vehicle No. 5L in 1969.

Other specialists at the meeting added more supporting information. For example, flight control specialists showed that at L+110.847 seconds, or three seconds after the explosion, the main computer on Block V (Stage III) issued an emergency engine cutoff command. It proved that the flight control system continued functioning and detected the loss of control over the vehicle.

After another day of hot debates, the Minister of General Machine-building Sergei Afanasiev, who oversaw the industry, told Chertok that going forward he could "take it slow" while preparing a very detailed and thorough account of the accident for delivery to Moscow. A few more details came to light in the following weeks, though they did not change in any major way the understanding of what had happened. For example, the further analysis of telemetry showed that the power-producing fuel cells, that used the reaction of liquid hydrogen and oxygen to supply electricity to the L3 spacecraft, kept operating until the crash on the ground. (685)

Final conclusions

Between December 1972 and the end of January 1973, the majority of the State Commission members were confident that the failure of the turbopump in Engine No. 4 had been at the root of the accident. The commission moved to wrap up the probe despite continuous protests by Kuznetsov's engineers, arguing that the conclusions had been made before thorough examination of the debris, which they believed could prove their theory about the hydraulic shock of the central engines' cutoff causing the violent oscillations of the vehicle and its structural failure. Kuznetsov summarized this scenario in a report No. 1388ss dated March 21, 1973.

According to the records of the State Commission, the search and cleanup operations at crash sites associated with the loss of Vehicle No. 7L were completed on Jan. 26, 1973. These activities included the recovery of some components pertinent to the investigation, first off all, the propulsion system, but also the destruction of other debris in order to maintain secrecy around the project. However, between May 15 and July 2, 1973, more debris from propellant lines, structural elements of the tail section and measurement instruments had been discovered and delivered to investigators. It is not exactly clear how much of engine No. 4 had been recovered beyond a pump flange and a sensor mentioned in the earlier official accounts of the failure.

On July 2, 1973, seven months after the failed launch of Vehicle No. 7L, the State Commission gathered to approve the final results of the investigation. Protocol No. 38 of the meeting signed on July 3, 1973, stated that "the N1-L3 No. 7L accident took place as a result of damage in the aft compartment of Block A (Stage I) caused by the disintegration of the 11D51 engine No. 4. The cause of the disintegration of that engine was the burning through of its oxidizer pump. The presumptive cause of the oxidizer pump burn through could be a disintegration of the radial support bearing due to the sheering off rivet heads in its separator or an axial force imbalance in the rotor of the turbo-pump, TNA, due to wearing off sealing bundles, or the disintegration of the BDO shaft," the document said. (BDO probably referred to the booster pump).

The commission rejected the hypothesis that blamed the accident on the excessive axial vibrations caused by the abrupt simultaneous cutoff of the six-engine central cluster. Interestingly, among multiple corrective measures listed by the July 3 protocol for implementation on Vehicle No. 8L being constructed for the next launch attempt of the N1 rocket were two items seemingly addressing the issues raised by Kuznetsov:

  • The evaluation and implementation of the most rational scenario for the engine cutoff on Block A (Stage I) and Block B (Stage II) in order to reduce dynamic oscillations during stage separation;
  • The exclusion of sectional assemblies in rocket's pipelines with their replacement by welded connections.

Before the July 2 meeting went into the discussion of the root cause, Nikolai Kuznetsov and his colleague A.A. Tanaev left the gathering in protest against drawing conclusions before completing the analysis of the recovered hardware, according to the protocol.

The document also said that along with Kuznetsov and Tanaev, a representative of the TsIAM research institute, Levin also disagreed with the outcome of the probe. (TsIAM was the leading propulsion research center of the Soviet aviation industry, which also encompassed Kuznetsov's engine conglomerate, unlike the rest of the N1 development team, which belonged to the rocket-building Ministry of General Machine building, MOM.)

The document also cited premature departure of Kuznetsov, along with serious problems with the development and deliveries of the engines, the July 2 meeting deferred the discussion of the work plan with next N1 rocket – Vehicle No. 8L. Kuznetsov apparently never signed the official conclusion of the investigation into the failure of Vehicle No. 7. (1000)

End of the program

Perhaps not coincidently, the fourth launch of the N1 rocket, which happened to take place just weeks before the final lunar expedition of the Apollo program, turned out to be also the last attempt to fly the Soviet moon rocket. Although proponents of the N1 project believed that they were closer than ever to resolving the propulsion problems on the first stage and work was progressing briskly toward completing the fifth flight-worthy vehicle, No. 8L, the underlining political climate had changed irreversibly. The Kremlin's patience had finally run out and the Soviet leadership had decided to move on...


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Page author: Anatoly Zak; last update: November 25, 2022

Page editor: Alain Chabot; last edit: November 23, 2022

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The N1 rocket is erected on the launch pad in Tyuratam.



A gondola with one of four steering engines added to the N1 rocket ahead of its fourth launch.




The N1 No. 7L rocket on the launch pad in Tyuratam.



The N1 rocket prepares for liftoff.







The ignition of the N1 rocket.







The N1 rocket lifts off from Tyuratam.






The N1 rocket climbs to orbit under power of the first stage before failing 107 seconds after launch.



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