After the initial flurry of lunar mission launches in the opening years of the Space Age, the Soviet Union had amassed a significant list of Moon-related firsts: the first flyby of the Moon (Luna 1), the first impact on the lunar surface (Luna 2) and the first photographs of the previously unseen far side of the Moon (Luna 3). Of NASA’s many attempts to reach the Moon during this time, only the diminutive six-kilogram Pioneer 4 managed to make it anywhere near its target during this time (see “Vintage Micro: The Pioneer 4 Lunar Probe”). As the 1960s started, the next obvious lunar goal was to land a working payload on its surface.

To this end, NASA pushed ahead with its first in-house lunar project called Ranger which was officially approved in December 1959. The objective of this program was to impact the lunar surface while obtaining high resolution images during descent. The first batch of these spacecraft destined for the Moon would also attempt to hard land a simple seismometer package on the lunar surface (see “NASA’s First Moon Lander“). These Ranger missions would be followed by a series of more capable landers as part of NASA’s Surveyor program. At the same time, the Soviet Union pushed forward with the development of its own lunar lander designated E-6 which they hoped would beat the Americans once again.

The E-6 Spacecraft

Like the Soviet Union’s earlier Luna spacecraft, the E-6 was designed and built at OKB-1 (the Russian acronym for Experimental Design Bureau – 1) under the direction of the famed aerospace engineer, Chief Designer Sergei Korolev. While work on the E-6 lander began in 1960, it proceeded fairly slowly at first as the engineers at OKB-1 focused on the manned Vostok and related spacecraft (see “Vostok’s Legacy”), unmanned planetary spacecraft as well as the development of new launch vehicles. This delay gave NASA’s faster-moving Ranger program a shot at making the first lunar landing during 1962 and potentially take the lead in the race to the Moon.

The 2.7-meter tall E-6, initial versions of which had a launch mass of just over 1,400 kilograms, consisted of a two-part multi-mission bus and a lander package. The bottom half of the main bus held the propulsion system built around a KTDU-5 retrorocket developed by OKB-2 under Alexei Isayev. This propulsion system was topped with a toroidal aluminum alloy tank filled with an amine-based fuel and a 0.9 meter in diameter spherical oxidizer tank filled with nitric acid. The total propellant load for a landing mission was about 800 kilograms. Four outrigger vernier thrust chambers provided attitude control and thrust trimming during the firing of the main engine. The propulsion system generated up to 45.5 kilonewtons of thrust and was designed to fire twice: the first time was to provide a velocity change of up to 130 meters per second for a midcourse correction to ensure that the craft would come down within 150 kilometers of its intended landing site. The second firing was for the 43-second retrograde burn to decrease the spacecraft’s velocity by about 2,600 meters per second for the vertical descent towards the lunar surface for landing. Because of trajectory requirements and the need to approach the lunar surface nearly vertically, potential E-6 landing sites were restricted to just north of the equator in the western part of the lunar near side centered on Oceanus Procellarum.

On top of the propulsion module was a cylindrical equipment section pressurized to 1.2 Earth atmospheres to provide a laboratory-like environment for the equipment inside. Although this resulted in a heavier spacecraft, this standard Soviet practice simplified design and testing of spacecraft systems as well as aided in thermal control. This section contained communications equipment, power supplies, batteries, as well as the I-100 control and navigation system built by Scientific Research Institute NII-885 under Nikolai Pilyugin. This section also supported the Sun and Moon sensors needed for attitude reference during the coast to the Moon. Strapped to either side of the spacecraft bus were 300 kilograms of lightly constructed packages containing radar equipment to initiate retrorocket fire, additional batteries and the cruise attitude control system. This attitude control system consisted of sets of nitrogen gas jets mounted on three arms that fed off of three gas bottles. Once the engines ignited for the final descent to the lunar surface, these items were no longer needed and were discarded to save weight. Unlike the Soviet Union’s planetary spacecraft of this era, the E-6 had no solar panels and relied solely on its batteries for power during its relatively short mission.

Mounted on top of this bus was the slightly egg-shaped lander with a diameter of 58 centimeters and originally with a mass of about 82 kilograms. It was protected from the impact of landing by an inflatable airbag system similar in principle to that employed decades later by NASA’s Opportunity Mars rover and other spacecraft. The airbags would be inflated just before the retrorocket started firing and the lander would be thrown clear of the main bus upon contact with the lunar surface. After the bottom-heavy lander rolled to a stop and the airbag deflated, four petals would open to stabilize it. Inside were the lander’s transmitter, batteries, and other equipment including a simple facsimile-style camera which could return panoramic images of the lander’s surroundings. The E-6 lander was much more complex and certainly more capable than the 43-kilogram package NASA was trying to land on the Moon with Ranger.

The launch vehicle for the E-6 was a modified version of the four-stage 8K78 Molniya rocket which had been developed to launch the Soviet Union’s first planetary probes and Molniya communication satellites (see “The First Mars Mission Attempts“). The first three stages of this rocket would eventually serve as the basis of the Soyuz launch vehicle still in use today. The first two stages of the 8K78 consisted of the Blok A core surrounded by four tapered boosters designated Blok B, V, G, and D. The engines of the four boosters and core would ignite on the launch pad to generate 4,054 kilonewtons of thrust. After two minutes of flight, the four boosters would shut down and separate from the rising rocket. After another 175 seconds of flight, the Blok A core would exhaust its propellants leaving the Blok I third stage to take over. The Blok I would burn for four minutes to place the E-6 payload and its Blok L escape stage into a temporary Earth parking orbit. After a short coast, the Blok L escape stage would ignite to send the E-6 on its way to the Moon. The 8K78 was 42.1 meters tall and had a liftoff mass of about 306 metric tons. Before the first orbital test flight of the American Block II Saturn I rocket on January 29, 1964, the 8K78 was the most powerful rocket in the world (see “The Largest Launch Vehicles Through History“).

For the E-6 Luna flights, a special version of the 8K78 Molniya was employed designated the 8K78L. Unlike the original 8K78 and the 8K78M introduced in 1964 which had separate control systems for the Blok I and L stages, the 8K78L used the I-100 system of the E-6 to control the upper two stages. The elimination of the upper stage control systems decreased the total mass and improved the launch vehicle’s lunar payload performance. Unfortunately it also created another version of the failure-prone 8K78 with a unique control system that would require debugging.

The First E-6 Missions

With the failure of all three of NASA’s Block II Ranger lunar landing attempts during 1962 and the first launch of NASA’s Surveyor lander still years away, the Soviet Union had a chance to be the first to land a working payload on the lunar surface in 1963. The first of their E-6 spacecraft, serial number 2, was launched from the Baikonur Cosmodrome using 8K78L number T103-09 on January 4, 1963 at 12:49 Moscow Time. The first three stages of the launch vehicle worked as intended placing the Blok L escape stage and E-6 into a 178 by 194-kilometer parking orbit with an inclination of 64.7°. Unfortunately, a power supply failure in the I-100 control system prevented the escape stage from igniting stranding what would become unofficially known as “Sputnik 25” in a short-lived orbit that decayed a week later.

The next Luna launch attempt, carrying E-6 number 3, fared even worse. Launched at 13:29 Moscow Time on February 3, 1963 on 8K78L number G103-10, the ascending rocket received erroneous pitch commands from the I-100 control system during the ascent. After separation of the Blok A core stage, the Blok I third stage failed to ignite and the rocket broke up during reentry with its remains crashing in the Pacific Ocean near Midway Island.

The problems with the I-100 control system were subsequently diagnosed and fixed for a third launch attempt two months later. On April 2, 1963, 8K78L number G103-11 lifted off at 12:04 Moscow Time carrying the 1,422-kilogram E-6 number 4. The lunar-bound payload and its Blok L escape stage were successfully placed into a 167 by 182-kilometer parking orbit. On command, the Blok L stage ignited successfully sending what was now called Luna 4 on its way to the Moon. Unfortunately, by the next day it was apparent that the navigation system had malfunctioned and it would be impossible to perform the midcourse maneuver required to place Luna 4 on course for a lunar landing. On April 5, Luna 4 passed 8,336 kilometers from the Moon about 77 hours after launch. The off-course probe then entered an extended 89,250 by 694,000-kilometer orbit around the Earth. Luna 4 was last heard from on April 6 and its path was eventually perturbed into solar orbit.

After three E-6 failures in four months, a review of the E-6 and its launch vehicle (which had operated as intended only three times in 13 attempted lunar and planetary launch attempts) was started. Following an almost year-long hiatus to correct quality control issues and upgrade key E-6 systems, another launch attempt was finally made. This time the late-March 1964 E-6 launch window overlapped with that of a pair of 3MV-1 spacecraft carrying landers bound for Venus requiring juggling of the launch schedules to get all three spacecraft off the ground on their respective Molniya rockets (see “The Soviet Zond Missions of 1963-65: Planetary Probe Test Flights“). On March 21, 1964 E-6 number 6 lifted off from the Baikonur Cosmodrome at 12:15 Moscow Time atop of Molniya number T15000-20. Unfortunately, a failure in a LOX valve in the Blok I third stage prevented its engine from reaching full thrust and caused it to shutdown prematurely. The rocket and its E-6 payload failed to reach orbit and were destroyed upon reentry.

Another E-6 launch attempt was made the following month. This time, Molniya rocket number T15000-21 lifted off at 12:08 Moscow Time on April 20, 1964 carrying E-6 number 5. Yet another malfunction in the I-100 control system caused the Blok I third stage to shut down early with the rocket and its payload breaking up during reentry. These two successive failures forced yet another reevaluation of the E-6 and another stand down as ongoing issues with the spacecraft and its launch vehicle were addressed.

The Flight of Luna 5

After another eleven-month, self-imposed hiatus to correct issues with the I-100 and other E-6 systems, Korolev and his team at OKB-1 were finally ready to launch another E-6 lunar lander on March 12, 1965. The 8K78L carrier rocket number R103-25 successfully placed E-6 number 9 and its Blok L escape stage into a 201 by 287-kilometer parking orbit. However, a power supply failure associated with the control system prevented the escape stage from igniting stranding the lunar lander in a quickly decaying parking orbit. Soviet authorities designated this spacecraft Kosmos 60 – the first in a string of failed lunar missions disguised in the series.

After this latest failure, the power supply for the entire guidance and control system was modified. After so many problems with the I-100, separate guidance systems for the upper stages were installed at the Cosmodrome to save the extra time of shipping the stages back to the factory. The modified 8K78L number R103-26 lifted off on April 10, 1965 carrying E-6 number 8. This time a failure in the system that keeps the Blok I stage’s LOX tank pressurized resulted in the stage shutting down early with the rocket and payload destroyed during reentry.

Undeterred, another launch attempt was prepared for the following month. Just before 11:50 Moscow Time on May 9, 1965, 8K78M number U103-30 lifted off from the Baikonur Cosmodrome carrying E-6 number 10. The lunar lander and its escape rocket were successfully placed into 151 by 217-kilometer parking orbit. On command, the Blok L escape stage worked as intended placing the 1,476-kilogram Luna 5 on course to the Moon. The first communication sessions after launch indicated that everything was finally going well.

Early tracking indicated that Luna 5 was placed on a trajectory that would strike the Moon albeit not in the desired location. Luna 5 would be required to make a minor midcourse correction during the second day of flight to set it on it proper course. But as Luna 5 attempted to turn from its cruise attitude and orient itself for the mid-course correction burn, the spacecraft began to roll about its long axis. Engineers regained control of the spacecraft and set about trying to diagnose the problem. They found that the gyroscopes in the I-100 control system were not given sufficient time to warmup and stabilize resulting in a loss of attitude reference. The ground controllers attempted to perform a course correction a second time but incorrect commands were inadvertently issued to Luna 5 and the engine burn failed to occur. Unfortunately, with the extra time required to diagnose the cause of these problems, it was now too late to perform a mid-course correction. Luna 5 was destined to be yet another failure.

Hoping to salvage at least some much needed engineering data from Luna 5, it was decided to perform the terminal descent maneuver and retroburn as the spacecraft approached the lunar surface. Although Luna 5 was approaching the lunar surface at too oblique an angle for a successful landing, it was a chance to exercise all of the systems required for descent for the first time in flight. But when the time came to perform the required maneuvers for the descent starting at an altitude of about 8,300 kilometers, the I-100 control system failed to perform and the newly upgraded KTDU-5A engine never ignited. After a flight of 83 hours and 20 minutes, Luna 5 crashed into the Moon at 1.6° south, 25° west in Mare Cognitum about 700 kilometers from its intended landing site in Mare Nubium.

Without missing a beat, another E-6 and 8K78M rocket were prepared for launch the following month. With the first lunar landing attempt by the American Surveyor spacecraft still many months away as issues with its Atlas-Centaur rocket were being addressed (see “Surveyor 1: America’s First Lunar Landing“), the Soviet Union still had a chance to be the first to land a working payload on the lunar surface and beat the United States to another important space goal.

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Related Reading

“NASA’s First Moon Lander”, Drew Ex Machina, January 26. 2016 [Post]

“Luna 9: The First Lunar Landing”, Drew Ex Machina, February 2, 2016 [Post]

General References

Boris Chertok, Rockets and People Volume III: Hot Days of the Cold War, NASA SP-2009-4110, 2009

Brian Harvey, Soviet and Russian Lunar Exploration, Springer-Praxis, 2007

Wesley T. Huntress, Jr. and Mikail Ya. Marov, Soviet Robots in the Solar System: Mission Technologies and Discoveries, Springer-Praxis, 2011

Nicholas L. Johnson, Handbook of Soviet Lunar and Planetary Exploration, Univelt, 1979

Timothy Varfolomeyev, “Soviet Rocketry that Conquered Space Part 5: The First Planetary Probe Attempts, 1960-1964”, Spaceflight, Vol. 40, No. 3, pp. 85-88, March 1998