Why did the Soviet Union decide to use 30 small engines instead of a few large ones on the N1?

Question

The N1 was a big disappointment after exploding on every single launch attempt. From what I have read, it seems like most of the attempts went wrong because one of the 30 engines exploded causing a chain reaction on the rest.

30 engines of the N1

5 F-1 engines of the Saturn V

NASA on the other hand developed the F-1 engine. It was more powerful than the ones the Soviet Union used and the Saturn V only required 5 instead of 30.

Why didn’t the Soviet Union develop larger engines for the N1?

Answer 1

Developing new engines takes time, and time was a precious resource in the Moon race.

The major problem with making larger engines is the problem of combustion instability of large combustion chambers. This is a problem that gets dramatically worse as you scale up engines. It's not an unsolvable problem, it just takes careful injector design, focused R&D, and at the time completely new technology leaping into the unknown.

Progress on rocket engine technology was made extremely quickly back then. The actual engine used, the NK-15 is also an example of that. Just as the F-1 solved the problem of combustion instability in large chambers, the NK-15 solved the problem of oxygen rich staged combustion, making it an extremely performant kerosene/LOX engine still an attractive option up the the current day.

So what to do when under extreme time pressure?

• Option 1: Hope the issue of combustion instability can be quickly solved, develop a new cutting edge engine in record time. (Probable result: rushed engine technology having a high chance of launch failure)
• Option 2: Use an existing high performance engine, and hope that the complexity and integration problems of running many in parallel can be solved in a short time (Probable result: rushed integration having a high chance of launch failure)

Using components that actually exist seems like a reasonable decision with some chance of success. But the time crunch was going to cause problems anyway.

Answer 2

While the accepted answer explains the technical reason why many NK-15s were chosen over a few large, new engines, it isn't the whole story:

• The N1 program was only given the go-ahead by the Soviet Politburo halfway through 1964. That meant the Soviets were starting around three years behind Apollo, which when you consider Apollo took about nine years to succeed, is a massive and arguably unassailable deficit.
• Sergei Korolev, the architect of the N1, had a massive disagreement and falling-out with Valentin Glushko, the #1 designer of rocket engines in the USSR, over the type of propellants in engines for manned rockets; Korolev wanted liquid oxygen/kerosene which he considered safer, while Glushko firmly believed that higher-energy but more dangerous hypergolic propellants were the way forward. After a commission ruled in Korolev's favour, Glushko effectively threw his toys and refused to work with Korolev. Despite this being Soviet Russia, Glushko was considered too vital to military rocket engines for ballistic missiles to be forced to behave, thus Korolev was forced to look elsewhere for an oxygen/kerosene engine supplier.
• Nikolai Dmitriyevich Kuznetsov was ultimately the person chosen by Korolev. While Kuznetsov was an immensely talented engineer whose rocket engine designs ultimately did prove themselves, at this stage in his career he'd only ever designed airplane engines. Thus, expecting him to build a super-heavy oxygen/kerosene rocket engine that also overcame the then-unsolved problem of combustion instability was considered somewhat unreasonable even by the Soviets - hence he provided the relatively low-risk, but small and untested, NK-15.
• Korolev passed away in 1966, leaving his deputy Vasily Mishin to take over the N1 program. Mishin was by all accounts more of an administrator than anything else, and did not actively drive the N1 program as his predecessor had; rather he simply continued implementing Korolev's plans. In particular, this meant he did not consider e.g. re-engaging with Glushko in order to attempt to procure different engines to the then-untested NK-15, nor he did not obtain or retain the political backing that the program as a whole would would require to see it through what would undoubtedly be some initial failures.
• The first launch attempt of the N1, and also the first test of the rocket coupled to its engines, was in February 1969 - a mere five months before Apollo 11's historic moon landing in July of that year. This attempt failed, demonstrating that the lack of integration testing of vital components was a serious problem; but as there was no alternative design or components and it was far too late to go back to the drawing board if the Soviets still wanted to beat the Americans, the N1 team pressed on.
• The N1's second launch attempt occurred less than three weeks before Apollo 11's touchdown, and again failed - this time catastrophically enough to destroy the entire launch pad. At this stage it was likely obvious to all involved that massive design changes were needed for the N1 to ever make it out of the atmosphere, but...
• ... on 20 July 1969 the USA landed on the Moon, with the result that the Space Race was over, the Soviets had lost, and political support for the N1 quickly died - along with the possibility of securing the additional resources required for the redesign work it needed to succeed.
• While the final two launches of N1, in 1971 and 1972, could potentially have saved the program, these also failed despite significant redesign of numerous components. A fifth test flight was intended for the second half of 1974, but program cancellation occurred before this could occur.

Source of above information: various Wikipedia articles and YouTube documentaries that live rent-free in my head

Answer 3

The N1 was a big disappointment after exploding on every single launch attempt. From what I have read, it seems like most of the attempts went wrong because one of the 30 engines exploded causing a chain reaction on the rest.

This isn't quite correct.

1. Explosions were fully expected during the development program. NASA built a test stand where they could test the entire first stage. The Russians thought building such a test stand would take too long, so they decided to test the first stage by flying it. They were planning up to 14 of these test launches, initially with dummy stages. The actual launches all had payloads, I guess they were getting desperate (with the obvious progress of the Apollo project by 1968, it was clear there wasn't time for 14 test launches).

2. The NK-15 had a similar problem. The initial version of this engine used pyrotechnics to open some valves, which meant the engine could only be started once before needing a rebuild. This meant they could not test-run engines before installing them on the stage. Instead of testing each engine, they would build a batch of 6, test 3, and if those ran well, install the remaining 3. They were working on an improved version that was restartable, to be used from the fifth launch.

3. Most of the failures were not engine issues.

• N-1 3L: Pogo oscillation in the propellant lines to one of the engines started a propellant leak, starting a fire in the engine bay.
• N-1 5L: Debris (welding slag) entered the turbopump of an engine which caused it to explode, starting a fire in the engine bay.
• N-1 6L: rocket went out of control due to complex aerodynamics at the base of the first stage.
• N-1 7L: Shutting down 6 engines caused a shockwave in the propellant lines, breaking them and starting a fire in the engine bay.

So the 3 failures that involved the engines all had to do with the structures in the first stage that were attached to the engines. Testing individual engines would not have found this, but all-up testing of the first stage would have helped.

Sources for this are mostly Boris Chertok's memoir Rockets and People and Nick Stevens' book N-1 For the Moon and Mars.

Answer 4

The Soviet program (general program, aside from lunar program) was far more militarized, and in addition to Ian Kemp’s points a small engine had more military (missile) applicability. The Soviet powers that be would be more approving of the 33-engine scheme, being extensible (descensible?) to an ICBM or similar.