What it was: Vladimir Chelomei’s plan for a direct-descent lunar lander. While never the forerunner for a Soviet Moon landing, it was always a strong alternative that Chelomei and his supporters kept pushing forward whenever they could get a step up on Sergei Korolev or Vasili Mishin.
Details: For a very short period of time Vladimir Chelomei was on the verge of becoming the top man in the Soviet space program, and used his influence to cut Sergei Korolev’s OKB-1 out of the USSR’s manned lunar flyby mission and replace it with his OKB-52. He never did manage to gain control of the manned lunar landing, which was always officially going to be the N1-L3 or a derivative, yet it’s clear that if Nikita Khrushchev had not been ousted from power October 1964 he would have pushed to take it over too—and very possibly would have got it. While strictly speaking the LK-700 didn’t come until after Khrushchev’s fall, it’s what we would have seen as the Soviet effort at a lunar lander if Chelomei had remained on top.
The LK-700 began as the LK-3, and was first formally proposed after Chelomei and Valentin Glushko had thoroughly studied their alternative to the N1, the UR-700. Unlike OKB-1’s rocket, which was repurposed from designs for a Mars mission, OKB-52’s proposed launcher had been built with the Moon mission in mind and though the LK-3 was not formally approved until October 1965—after Khrushchev’s fall— the two had apparently been worked on in lockstep since about 1962.
This meant that it had one intrinsic advantage when the N1-L3 program ran into weight issues. It had become clear in late 1964 that the first few N1 rockets were not going to be powerful enough to perform a single-launch Moon mission, and that OKB-1 was going to have to evolve their launcher into something that could do the job. As the UR-700 and what was now the LK-700 were designed for each other, they would have been able to go on an earlier flight and so—all else being equal—get to the Moon first. The October 1965 decision to stick with the N1 but also move ahead with Chelomei’s plan, albeit at a much lower level of funding, was specifically intended as a backup if the N1 turned out to be a failure. From then on the advancement or retardation of the LK-700 tracked the N1’s highs and lows.
The LK-700 also had the advantage of being quite conservative. It was a direct-descent lander, which meant no dockings in space, whether in Earth orbit or around the Moon; as that profile needs more mass the rocket itself had to lift a larger payload, about 150 tonnes, but would be based on the tried-and-true storable propellants nitrogen tetroxide and UDMH. So would the LK-700—the highly toxic nature of the fuel was glossed over.
A Moon mission on the LK-700 would see two cosmonauts (or three in later missions) be launched into a 200-kilometer parking orbit by Glushko’s proposed booster. There they would spend five orbits checking out the craft’s systems before committing to a trip to the Moon. The fully-fuelled craft would weigh some 154 tonnes, as mentioned, and be about 13 meters long (not counting its abort tower, which brought the length up to 21.2 meters during launch). This is immense compared to the L3 proposed by OKB-1, and would have even been larger in mass than the Apollo CSM and its S-IVB injection stage at trans-lunar injection if fuel is included.
The Apollo craft was considerably longer than the LK-700 would have been, though. Rather than use Apollo’s linear arrangement with one engine and tank on the injection stage and the actual spaceship perched on top, the LK-700 would have used a laterally clustered arrangement. Three of a proposed new engine, the 11D23, would be attached to tanks of propellant arranged in a trefoil around another 11D23 and tank attached to the aft end of the LK-700’s crew capsule (the VA) and lunar landing stage/ascent stage (the Block 1V). The three engines would fire to add another 3.1km/s to the LK-700’s speed and send it on its way to the Moon, at which point they would be jettisoned.
The fourth engine and its propellant (Block 11), still attached to the outbound craft, would be used for course corrections during the 80-hour journey to the Moon. Upon arrival the Block 11 would fire again to slow the craft down to about 30 meters per second somewhere between three and five kilometers above their destination—notionally the Mare Fecunditatis, though Chelomei’s bureau never got anywhere near actually picking a landing site.
At that height the Block 11 would run out of fuel and be ejected, exposing the Block 1V engine. The LK-700’s landing platform and gear (AKA Block 111) enclosed the Block 1V cylindrically, but let the rocket fire downwards to bring the craft to a soft landing on the Moon. The ship would have been designed to stay on the Moon for 12 to 24 hours, during which time the two cosmonauts it carried would make two surface excursions between two and two-and-a-half hours long.
When it was time to leave the Block 1V would fire again and launch the LK-700 back toward Earth while leaving the Block 111 behind. This would be a direct injection towards home, meaning that unlike the Apollo landings or the N1-L3 there would be no orbiting of the Moon either on landing or takeoff. This had the advantage of opening up a much larger fraction of the Moon’s surface for exploration, as there was no need to stay within the belt around the Moon’s equator where an orbiting mother ship would fly over the landing site with regularity.
The return journey would be somewhat slower than the outbound, taking four days, and after re-orienting the craft for re-entry at 150 kilometers above the Earth, the VA crew capsule would separate from the rest of the ship at 100 kilometers. The LK-700’s capsule was quite similar in shape to the Apollo CM, though considerably smaller: 3130kg as compared to 5809kg, and an interior volume of 4.0 cubic meters as compared to 6.17. Having the same outline and comparable small thrusters gave the VA the same rough steerability as an Apollo CM, and the crew aboard the last remaining component of the LK-700 could aim for a particular spot in the Soviet Union with about 11,000 kilometers of downrange and 300 kilometers of cross-range performance. Like other Soviet manned spacecraft, it was designed for a soft landing on land.
What happened to make it fail: Even though Chelomei was never able to get enough of the Soviet leadership to support his program over the N1-L3, the LK-700 trundled along at a low level for quite some time. The Central Committee of the Communist Party (at that time in the ascendance because of its support for Leonid Brezhnev’s takeover) re-authorized continuing work on it in September 1967. In the wake of the second N1 explosion in 1969, Chelomei even felt confident enough to push for the cancellation of the N1-L3 and its replacement with an LK-700/UR-700 based mission, making the good argument that re-designing and re-certifying the N1 so that it would stop blowing up on the pad would cost just as much as building the UR-700 anyway. Perhaps unfortunately for the USSR’s lunar landing ambitions that effort also failed to get enough backing and the N1 continued.
In the real world the LK-700 reached the mockup and early testing phase when it was killed definitively in 1975, along with all other Soviet Moon landing and flyby plans, by that shift in viewpoint towards space stations, Energia, and a Soviet space shuttle.
What was necessary for it to succeed: The LK-700/UR-700 was a very creditable attempt to make a Moon mission and certainly could have succeeded if technical skills were all that were necessary. Vladimir Chelomei had notable successes in his future, while the UR-700’s Valentin Glushko is arguably the greatest rocket engine designer of all.
Instead it never came to pass purely because of the poisonous politics of the Soviet space program from 1964-1975 (though of course if they hadn’t been like that it’s unlikely Chelomei would have been able to work on it at all once the decision was made to go with the N1-L3 in 1965). So at first the obvious answer to this question is “Vladimir Chelomei has to be able to maintain his remarkable drive to the top of the Soviet space program, rather than fall even more quickly than he rose”. To that end the continuing rule of Nikita Khrushchev would work very well, though it isn’t strictly necessary.
The main difficulty with this answer is Chelomei’s speed in developing his ideas. He had a strong tendency to go his own way and come up with unusual, if plausible, ways of solving problems. As a result his programs often required considerable fundamental work and testing as compared to more conservative approaches to the same problem. To his credit he took that time whenever it was politically possible to do so, but it meant long waits before missions were ready to go. While he would have been able to move considerably faster if OKB-52 had had the funds that OKB-1/TsKBEM had for the N1-L3 program, his deliberate pace on his other more successful projects strongly suggests that he would not have been able to beat the United States to the Moon by July 1969.
At that point the question becomes one of the Russian leadership’s attitude to a Moon landing after losing the race. It’s likely that the UR-700/LK-700 combination would have been less accident-prone than the N1-L3 (it hardly could have been worse), and so it seems that the Kremlin might have been greater tolerance for it if it ran late. Ultimately the success of the program would have come down to a race between Chelomei’s dream and a cancellation brought about by a desire to save money or (as in real-life) a re-orientation of the USSR’s space program toward military objectives. If the dream won the contest, a cosmonaut would have set foot on the Moon sometime around 1975-1980, with a likely Soviet Moon base to follow; if not, then we’d have seen an outcome rather similar to what happened in the real world, with only the doomed technology being different.