In July 1970 Kuznetsov was given authorization to design substantially improved versions of the N1 rocket engines. These would include filters in the propellant lines to prevent ingestion of debris in the turbo-machinery; improved vibration isolation; autonomous operation; and many detailed reliability and durability improvements. So substantial were the changes that they received a completely new series of designations (NK-33, NK-43, NK-39, NK-31 in place of NK-15, NK-15V, NK-19, NK-9V). However it would take over three years of design, development, and test before these new engines would be available.
The N1 that would utilize these engines was designated the N1F (this had no relation to the much more powerful N1F design of 1965 - it was more like the N1U 'perfected' N1 design of the same year). With a payload to a 225 km orbit of 105,000 kg, the N1F would use the new engines, higher density superchilled propellants in all stages, lighter stage structures and numerous detailed changes. Following extensive wind tunnel studies, the boat tail was again redesigned in detail to cope with gas dynamics problems. A cylindrical base reduced the vehicle maximum diameter from 16.9 m to 15.8 m. Four high-thrust roll 'steering' engines were later added to prevent the loss of control that would destroy the 6L launch vehicle. The KORD was completely redesigned, a fire extinguishing system was installed, improved isolation of cabling and electronics was introduced. The telemetry system was reduced in weight while increasing the number of points measured from 700 to 13,000.
Full go-ahead to develop a liquid hydrogen/liquid oxygen high energy upper stage for the N1F finally came in June 1970. The decision was made to develop a multi-engine Block Sr with a propellant mass of 66.4 metric tons. This single stage would be used in place of the previously-planned Blocks S and R to insert spacecraft of the Lunar Expeditionary Complex (LEK) into low lunar orbit. It was also to be used to insert heavy spacecraft into geosynchronous orbit and on interplanetary trajectories. The increased payload of the N1F, combined with the Block Sr stage, would allow a single N1F launch to place 27 metric ton spacecraft on the lunar surface. The L3M lander could be increased in capability to allow three month stays on surface.
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Following the fourth N1 failure, Glushko and Kosberg were brought in to provide independent opinions on how the problems could be solved. Neither came up with inexpensive solutions. Kosberg wanted to equip the rocket with new-development 250,000 kgf engines using N2O4/UDMH. Glushko suggested using the RD-253 engines that he had developed for the UR-500 Proton launch vehicle. Interestingly, this very successful design was the final form of the RD-250 engine that Korolev had rejected for the N1 ten years earlier. But even Glushko had to admit that using the RD-253 would mean converting the N1 to N2O4/UDMH propellants, thereby lowering payload performance to unacceptable levels (just as Korolev had argued). Glushko was also rather dubious of successfully synchronizing the operation of 30 engines.
The cash-starved project continued to limp along. Funding to proceed with design and construction of the L3M was not forthcoming. But Kuznetsov's new series of engines had been subjected to huge amounts of ground testing. First/second stage engine testing was completed in September 1972 and third stage testing in November 1973. The next vehicles, 8L and 9L, would be the first to use these new modernized series engines and fully reflect the N1F configuration. The plan was that both 8L and 9L would be launched in the fourth quarter of 1974 in a demonstration of the N1F-L3M dual launch scenario. Confidence was high, based on the massive telemetry received on the 7L flight, that all problems had now been rectified.
On May 18, 1974 the Minister of Medium Machine Building Afanasyev attended a routine meeting of the management of OKB-1. In a few clipped sentences he informed the group that the Politburo had decided to remove Mishin, replace him with Korolev's old nemesis Glushko and combine the two OKB's into a new entity known as NPO Energia. Afanasyev wished the stunned managers every success, and left the room. The N1 program was cancelled