Without a doubt, 1966 proved to be a banner year for lunar exploration. At the end of the first month of the year, the Soviet Union launched the E-6M spacecraft called Luna 9 which made the first successful landing on the Moon on February 3 (see “Luna 9: The First Lunar Landing”). This was followed two months later with the launch of the E-6S designated Luna 10 which became the first spacecraft to orbit the Moon (see “Luna 10: The First Lunar Satellite”). These flights were then followed by NASA’s Surveyor 1 mission, which landed on the Moon on June 2 (see “Surveyor 1: America’s First Lunar Landing”), and Lunar Orbiter 1, which entered orbit around the Moon on August 14 (see “Lunar Orbiter 1: America’s First Lunar Satellite”).

But this was only the start of Soviet and American missions to the Moon in 1966. These successes were followed by a pair of Soviet E-6LF lunar orbiters, known as Luna 11 and 12, which attempted to map potential future lunar landing sites in August and October, respectively, with mixed results (see “Mapping the Moon: The Soviet Luna 11 & 12 Missions”). Unfortunately, NASA’s Surveyor 2 was lost after it tumbled out of control when one of its vernier engines failed to ignite during the midcourse correction the day after it was launched on September 20 (see “Surveyor 2: Things Don’t Always Go as Planned“. Five weeks later, Lunar Orbiter 2 was launched on its successful mission to continue mapping of future Apollo and Surveyor landing sites (see “Lunar Orbiter 2 and the Picture of the Century”). In the closing weeks of 1966, the Soviet Union launched one more mission towards our neighbor this time to repeat the feat of Luna 9.

The E-6M Lunar Lander

Development of the E-6 lunar lander originally started in 1960 at the OKB-1 design bureau run by the legendary Soviet aerospace engineer, Chief Designer Sergei Korolev. After a string of frustrating failures (see “50 Years Ago Today: The Launch of Luna 5”) and with the resources at Korolev’s organization tied up developing the 7K Soyuz family of manned spacecraft to compete with Apollo, responsibility for the E-6 and other automated lunar and planetary spacecraft programs was transferred in April 1965 to NPO Lavochkin under Chief Designer Georgi Babakin — a newly independent organization that was spun off of OKB-52 that was well known for its intensive testing and quality control of the flight hardware it built.

As the remaining E-6 hardware built by OKB-1 was launched during the balance of 1965, engineers at Lavochkin began upgrading the E-6 lunar lander to become the E-6M. On its very first attempt, the Lavochkin-built Luna 9 succeeded following 11 failed attempts of the E-6 during the previous three years. Following these successes, the E-6M hardware was modified to support the E-6S and E-6LF lunar orbiter variants.

The 2.7-meter tall E-6M lunar lander, which had a launch mass of just over 1,500 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-5A 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 about 150 kilometers of its intended landing site. The second firing was for the final 46-second braking 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 a region centered 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 control and navigation system. 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 KTDU-5A engine had 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 the E-6M bus was the slightly egg-shaped lander with a diameter of 58 centimeters and a mass of about 100 kilograms. It was protected from the impact of landing by an inflatable airbag system similar in principle to the one employed decades later by NASA’s Opportunity Mars rover and other American Mars landers. The airbags would be inflated just after 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 the package. Inside were the lander’s transmitter, batteries, and other equipment to support several days of operations on the lunar surface. For the E-6-based orbiter missions, Lavochkin engineers replaced the lander with an instrument package which was placed into orbit using the E-6 bus carrying a light load of propellant.

Following the success of Luna 9, Lavochkin engineers made modifications to the E-6M lander such as to its thermal control system and to enhance its science return. Like its predecessor, E-6M No. 205 carried a SBM-10 radiation sensor to monitor any potential radiation hazard to humans on the lunar surface. But unlike Luna 9, the new lander was now fitted with two panoramic cameras instead of just one allowing stereo images to be acquired of its landing site. The new lander also carried a pair of extendable 1.5-meter long arms which would be deployed after landing. One arm carried a ground penetrometer to gauge the density and strength of the surface. This was accomplished by an explosive charge driving a 3.5-centimeter in diameter pointed rod made of titanium with a force of 50 to 70 newtons for 0.6 to 1.0 seconds and noting how far and fast it extended out to its maximum length of five centimeters.

The second arm on the new E-6M lander held a radiation density meter to measure the soil density to a depth of 15 centimeters. Inside the 25.8 by 4.8 by 1 centimeter detector head was a small amount of cesium-137 which emitted gamma rays with an energy peak near 662 keV. Three radiation detectors, shielded from direct exposure from the cesium-137 source, then detect the number of gamma rays reflected out to various distances. Also carried was a three-axis piezoelectric accelerometer to measure the landing forces allowing a determination of the surface’s mechanical properties to a depth of 20 to 30 centimeters. A set of four infrared radiometers rounded out the sensors suite of the lander whose data would allow the surface temperature to be determined. The mass of the E-6M No. 205 lander grew to 112 kilograms as a result of the modifications and additional instruments.

The launch vehicle for the E-6M was the four-stage 8K78M rocket which had been developed to launch the Soviet Union’s first planetary probes and Molniya communication satellites (see “The First Mars Mission Attempts”). It was from this latter payload that the 8K78 derived its popular designation, “Molniya”. The first three stages of the Molniya rocket would eventually serve as the basis of the Soyuz launch vehicle still in use today. The first two stages of the 8K78M 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 in orbit, 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.

The Mission of Luna 13

The 1,620-kilogram E-6M No. 205 spacecraft lifted off on board 8K78M serial number N103-45 from launch complex 1/5 at the Baikonur Cosmodrome (better known today as Gagarin’s Start) at 13:17:00 Moscow time (10:17:00 UT) on December 21, 1966. The first three stages of the Molniya launch vehicle successfully placed the Blok L escape stage and the Moon-bound payload into a temporary 171 by 223 kilometer parking orbit with an inclination of 51.8°. The Blok L escape stage ignited on command and placed what was now called Luna 13 on a trajectory towards the Moon. A routine trajectory correction the day after launch at 18:10 UT set the spacecraft on course to land just under 80 hours after launch.

With everything proceeding as planned, Luna 13 began to align itself with its approach trajectory at 16:00 UT on December 24 in preparation for landing. At 17:59 UT, Luna 13 ignited its KTDU-5A engine after its radar altimeter indicated an altitude of 70 kilometers had been reached. Two minutes later, Luna 13 bounced to a landing at 18.57° N, 60.00° W in the western part of Oceanus Procellarum before local dawn about 400 kilometers away from the Luna 9 landing site. Four minutes after landing, Luna 13 discarded its airbags, opened its four petals, deployed its pair of instrumented arms and began transmitting back to Earth for its first communication session. The Moon, for the first time, now had four spacecraft operating on it or in its vicinity: in addition to the newly arrived Luna 13, Luna 12 and Lunar Orbiter 2 were conducting independent surveys of the Moon’s lumpy gravity field and environment from orbit while intermittent contact was being maintained with Surveyor 1 on the lunar surface in the southern part of Oceanus Procellarum about 800 kilometers from Luna 13.

Shortly after establishing contact with ground controllers, Luna 13 fired the charge in its penetrometer at 18:06 UT. The instrument’s indentor rod extended to 4.5 centimeters indicating that the soil at the landing site was granular and slightly cohesive with a density of about 0.8 grams per cubic centimeter. These findings were confirmed by the independent readings from the radiation density meter and accelerometers measuring the forces as Luna 13 bounced along the lunar surface during landing. The lunar surface was found to be strong enough to support the weight of a cosmonaut and his lunar lander.

With local sunrise taking place at 00:30 UT on Christmas Day 1966, the second communication session started at 12:15 UT. With the Sun 6° above the horizon, it was time for Luna 13 to send back its first images of the lunar surface. Unfortunately, only one of the pair of cameras carried by Luna 13 operated but the 220° panoramas the working instrument returned revealed a relatively smooth landscape with few rocks or craters visible. With this information as well as images from orbit returned by Luna 12 in hand, Soviet planners would pick Oceanus Procellarum as one of the potential landing sites for a future manned lunar landing.

Over the course of the next three days, Luna 13 would return a total of five panoramas and other data during what would turn out to be nine communication sessions. Luna 13 confirmed the earlier findings of Luna 9 that the radiation levels on the lunar surface were not a hazard for humans. Based on the IR radiometer readings with solar elevation angles as high as 38°, it was determined that the noontime surface temperature would reach 117°±3° C at the Luna 13 landing site. Finally at 6:13 UT on December 28, contact with Luna 13 was lost during the ninth communication session as the lander’s batteries were depleted.

With this final success to round out the year, 1966 would prove to be the busiest year for lunar exploration since the start of the Space Age. But the Luna 13 mission would also prove to be the last for the E-6M lander. And after three more flights of the new E-6LS orbiter variant for tracking and communications tests culminating with the mission of Luna 14 in the spring of 1968, the E-6 series had reached the end of its usefulness. As work at the successor of OKB-1, called TsKBEM (Central Design Bureau of Experimental Machine Building), pushed forward with the development of the 7K Soyuz for missions to Earth orbit and the Moon, work at NPO Lavochkin had already shifted to the development of the new E-8 spacecraft series – the next generation of automated lunar spacecraft which would include rovers, orbiters and eventually sample return spacecraft initially to support and later supplant the cancelled Soviet manned lunar missions (see “The Last Lunar Sample Return Mission“).

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Related Reading

“50 Years Ago Today: The Launch of Luna 5”, Drew Ex Machina, May 9, 2015 [Post]

“Luna 9: The First Lunar Landing”, Drew Ex Machina, February 3, 2016 [Post]

“Surveyor 1: America’s First Lunar Landing”, Drew Ex Machina, May 30, 2016 [Post]

General References

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

Asif Siddiqi, Bart Hendrickx and Timothy Varfolomeyev, “The Tough Road Travelled: A New Look at the Second Generation Luna Probes”, Journal of the British Interplanetary Society, Vol. 53, No. 9/10, pp 319-356, September/October 2000

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