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L1 No. 9L (Zond-5)


L1 No. 12L (Zond-6)


L1 No. 13L





7K-L1: Soyuz for circumlunar mission

In 1964, the USSR started a new program aimed to fly two cosmonauts around the Moon. It had a purely political goal of denying the US the most ambitious achievement in space short of the actual landing on the lunar surface. In order to meet mass restrictions for a flight toward the Moon on the UR-500 (Proton) rocket, the Soyuz had lose all its unessential systems, resulting in the 7K-L1 variant.

Previous chapter: Chelomei's LK project


The unpiloted version of the L1 spacecraft, officially known as Zond. Credit: RKK Energia


Key components of the 7K-L1 spacecraft and its payload section designated 11S824: 1) 7K-L1 spacecraft; 2) Block D space tug; 3) Payload fairing; 4) Launch vehicle adapter; 5) Emergency escape system; 6) The 8K82K (Proton) launch vehicle; 7) Support cone.

Known specifications of the 7K-L1 spacecraft:

Flight duration
7 days
Spacecraft mass (after separation from the Block D upper stage), including...
5,500, 5,630 (774), 5,680 (52) kilograms
  • Spacecraft mass in low Earth orbit
19,040 kilograms
  • Descent module mass
3,100 kilograms
  • Instrument compartment mass
2,250 kilograms
  • Fairing attachment cone mass
150 kilograms
Block D liftoff mass
13,360 kilograms (774)
L1 spacecraft propellant supply
~400 kilograms (2)
Spacecraft length
*4.796 meters
Spacecraft diameter
2.183 - 2.72 meters (52)
Solar panels span
~9 meters
Solar panels area
11 square meters
Block D length
5.5 meters
Block D diameter
3.7 meters
Spacecraft internal volume
5.5 cubic meters
Internal free volume
2.6 cubic meters
7K propellant components
AK-27 (oxidizer), UDMH (fuel)
Block D propellant components
Liquid oxygen (oxidizer), Kerosene RG-1 (fuel)
L1 spacecraft propulsion system thrust
4,033 kilonewtons (2); 425 kilograms (52)
Block D propulsion system thrust
8,500 kilograms (52)
L1 spacecraft propulsion system specific impulse
276 seconds per kilogram (52)
Block D propulsion system specific impulse
346 seconds per kilogram (52)

*4.5 meters without a support cone

The L1 was essentially a two-seat Soyuz spacecraft stripped of its habitation module to bring its mass down to as low as five tons. The spacecraft also lacked a backup parachute container, which was apparently replaced with an entrance hatch!

Because the development of the 7K-L1 variant trailed the work on the original Soyuz 7K-OK spacecraft, Korolev's engineers had a chance to integrate a number of new systems into the L1, which were not present on the Earth-orbiting Soyuz. For example, the L1 would be equipped with the first Soviet flight control computer -- Argon-11s.

The Block D (11S824) upper stage had minimal changes compared to its original design developed for the N1 rocket. A spherical oxygen tank had a cylindrical insert, which increased its volume. It also had a two-section payload fairing. (84)

The official "basic specifications" of the L1 upper composite (11S824) were approved on Dec. 31, 1965. This document listed following key development activities and upgrades within the 7K-L1 project:

  • The use of the Block D stage from the N1/L3 complex;
  • The use of the 7K spacecraft sans a habitation module, BO, with an upgrade of its descent module, SA, to reenter with a second cosmic speed. In order to install attachments of the emergency escape system, which normally interface with now missing habitation module, it would be replaced with a small external module. Named "base cone," the compartment would contain extra power batteries;
  • The removal the orientation and berthing thrusters, DPO, from the 7K spacecraft. Their functions would be transferred to the SOZ (engine ignition) propulsion system on the Block D stage;
  • The development of the new cone-shaped payload fairing is developed specifically for the 7K-L1 spacecraft;
  • The introduction of an additional engine firing of the Block D stage to enter an initial parking orbit. A subsequent firing of Block D would be used to develop a second cosmic speed;
  • The development of a flight sequence of the Proton/payload composite system to ensure the normal functioning of the emergency escape system and the launch vehicle safety system (monitoring the health of the vehicle during the ascent with the crew);
  • The development of the flight trajectory taking the spacecraft around the Moon and returning the descent module back to Earth with the second cosmic speed;
  • The testing of the L1 complex in the Earth orbit during the launches of vehicles No. 2P and No. 3P (where "P" probably stood for "prosteishiy" -- simplest (or "probny" - testing or "prototip" - prototype.) Both ships were to have simplified flight control system but equipped with Block D stages.

The same basic specifications also planned activities on the 7K-L1 spacecraft:

  • The development of the 7K spacecraft variant for a one-launch circumlunar flight scenario.
  • Phased fulfillment of flight testing including following vehicles:
  • Technical mockup complex 1M1 with the vehicle No. 1P;
  • A fully operational version of the L1 spacecraft and its payload section but without a crew for taking photos of the Earth and the Moon during launches of vehicles No. 4, 5, 6, 7, 8 and later No. 8 and 9)
  • Preparation of vehicles No. 11, 12, 13 and 14 for manned flight during a one-launch scenario, but with a possibility of launching them unmanned (if flight testing reveals problems in earlier flights).

At the time, the spacecraft was expected to have following specifications:

7K-L1 spacecraft mass at liftoff
depending on variant 5,200-5,700 kilograms
7K-L1 spacecraft mass at Earth escape maneuver
5,000-5,550 kilograms
Maximum length of the spacecraft body (without antennas)
4,796 millimeters
Descent module diameter
2,183 millimeters
A total volume
5.5 cubic meters

At the time, a provisional flight of the L1 spacecraft was expected to last from eight to 10 days, or up to two times longer than the maximum flight duration achieved by the USSR during the Vostok-5 mission in 1963.

For the L1 missions, the descent modules would be equipped with new, more extensive heat shield than the one developed for the Soyuz

L1 propulsion system

The L1 spacecraft would be equipped with a KTDU-53 propulsion system (also known as the S5.53 engine) developed at Isaev's design bureau.

Flight profile for the L1 project


1) Initial orbit insertion; 2) Initial parking orbit; 3) Trans-lunar insertion maneuver; 4) Trans-lunar trajectory correction; 5) Lunar flyby; 6) Second trajectory correction; 7) Third trajectory correction; 8) Separation between the descent module and the aggregate compartment; 9) Reentry and landing

During a typical L1 mission, the three-stage 8K82K rocket (a.k.a. UR-500K Proton) lifts off from Tyuratam. The payload fairing would be jettisoned with the help of small solid propellant motors during the firing of the second stage.

After the separation of the third stage, Block D (11S824) would make its first of two firings with its 11D58 engine lasting 140 seconds to enter a 205-kilometer parking orbit, with an inclination 51.5 degrees toward the Equator. (52)

After a passive flight, Block D fires again to insert the 7K-L1 spacecraft into a trans-lunar trajectory. The 7K-L1 would then separate from Block D as it speeded toward the Moon. After the ship's actual trajectory was carefully measured against the planned parameters, the spacecraft would have an opportunity to make a correction with its own engine at a distance of around 250,000 kilometers from Earth. It would then swing behind the Moon at a minimal distance of around 2,000 kilometers conducting photography and remote-sensing experiments.

The spacecraft would head back to Earth and make another trajectory correction maneuver at a distance of around 150,000 kilometers from the home planet. The descent module with the crew would then separate and make an aerodynamic reentry into the Earth's atmosphere to land on the Soviet territory between seven or eight days after launch.


Missions of the 7K-L1 spacecraft:

Official name
Industry name
Launch date
Landing date
Mission details
7K-L1 No. 2P
March 10, 1967
Tested systems of the Block D upper stage
7K-L1 No. 3P
April 8, 1967
A second firing of the Block D failed in Earth's orbit
7K-L1 No. 4L
Sept. 28, 1967
Proton's 1st stage failed; escape system saved the reentry craft
7K-L1 No. 5L
Nov. 22, 1967
UR-500K's 2nd stage failed. The escape system saved a reentry craft
7K-L1 No. 6L
March 2, 1968
The descent module self-destructed during reentry
7K-L1 No. 7L
April 23, 1968
Escape system self-initiated during launch
7K-L1 No. 8L
An on-pad explosion of the upper stage killed one person; craft damaged
7K-L1 No. 9L
Sept. 15, 1968
Sept. 21, 1968
Flew around the Moon; splashed down in the Indian Ocean
7K-L1 No. 12L*
Nov. 10, 1968
Nov. 17, 1968
Flew around the Moon; reentry craft depressurized during landing and crashed
7K-L1 No. 13L
Jan. 20, 1969
UR-500K's 2nd stage and 3rd stages fail; the escape system save reentry craft
7K-L1 No. 11L
Aug. 8, 1969
Aug. 14, 1969
Flew around the Moon
7K-L1 No. 14L
Oct. 20, 1970
Oct. 27, 1970
Flew around the Moon; landed in the Indian Ocean

*Vehicle No. 10 was converted into the 7K-L1A No. 3 experimental payload used for the first launch of the N1 Moon rocket in 1969.


Page author: Anatoly Zak; Last update: June 18, 2024

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Scale model of the launch complex for the Proton rocket with the L1 spacecraft in Tyuratam. Copyright © 2002 Anatoly Zak


Testing of the emergency escape system, apparently for the L1 (Zond) version of the Soyuz spacecraft. Credit: 152


A 7K-L1 spacecraft used a descent vehicle identical to the Soyuz 7K-OK spacecraft.


A 7K-L1 spacecraft, is ready for integration with its Proton launch vehicle, in preparation for the Zond-5 mission. Credit: RKK Energia


The Proton rocket with the L1 spacecraft for the circumlunar mission is poised for launch in Baikonur on Nov. 22, 1967.


Block D fires its BOZ thrusters before igniting its main engine. Copyright © 2017 Anatoly Zak


Cosmonaut Aleksei Eliseev, a veteran of the Soyuz-4 and -5 mission, works inside the 7K-L1 simulator.


The Salyut-1 computer and its control panel developed specifically for the L1 project.


The RD-58 engine propelled the Block D stage during L1 missions.


L1 spacecraft during pre-launch processing.


A night launch of an UR-500K rocket with a 7K-L1 spacecraft. Credit: RKK Energia


A still from a Soviet era animation showing the trajectory of the Zond spacecraft. Click to enlarge. Credit: Roskosmos