Why is there so much variation in the number of engines on launchers?

Question

There is much variation in the number of engines used in the liftoff stage of various launchers.

• Saturn V had 5
• Space shuttles had 5 (3 main + 2 SRB)
• Falcon 9 has 9
• Soyuz has 20
• N1 had 30
• Delta Heavy has 3
• Long March 3B appears to have 4 main + 2 or 4 boosters
• Ariane 5 had 1 main + 2 boosters

While the size and performance of these launchers varies, I can't see any straightforward correlation between those factors and the number of engines. The only correlation I can see is that Soviet/Russian launchers have a whole lot more and smaller engines.

What are the design tradeoffs when deciding to use many small engines vs few large engines?

Why does the Soviet/Russian calculus seem to be so different to everyone elses in this regard?

Answer 1

The number of engines and strapons are a series of tradeoffs made based on available technology.

The Americans, planning a series of monstrous rockets, started on the huge F-1 engine. (1.5 to 1.8 million lbs of thrust). The only things the Russians ever had that came close was the RD-170 engine, with similar thrust, but needs 4 thrust chambers instead of the F-1's one. The RD-170 came several decades after the F-1 as well.

Combustion instability in large scale rockets like this is very hard to solve, and the Americans were possibly lucky that they succeeded. The Russians were not.

So when the time came to build a large booster, the Americans were able to get by with just 5 F-1s (Which is an astonishing amount of thrust, 7.5 million lbs of thrust is truly awe inspiring) whereas the Russians with the NK-15/NK-33 in the 380,000 lbs of thrust needed to use 30 of them to get the needed thrust.

There is a pragmatic explanation for each of those examples.

Strapon solids are usually a sign that the design grew during development. Usually the main engine size is set early in the design, and the vehicle grows over time. The way to mitigate this growth, since usually the main engine design is hard to scale up, is to add strap ons. Solid strapons are often available from the makers of ICBM's.

Strapons are great for being high in thrust, usually with pretty low ISP, so that they are mostly used to get the vehicle moving off the pad, and drop off early.

In terms of Soyuz, that is a bit of a misnomer, since the engines have multiple thrust chambers/bells, but count as a single engine. So the side boosters look like they have 4 bells, but is really a single engine each.

Answer 2

Everything's a tradeoff.

Designing and building small, low-thrust engines is generally easier than designing and building large, high-thrust engines, but the larger engines may be more efficient.

The more engines you have, the more likely one or more are to fail in any given launch.

Once you have more than 5 or so, you may be able to continue on if one fails (depending on which engine, and at what point in the ascent it happens).

The more engines you have, the harder it is to predict how they may interact; this is likely part of what contributed to the failure of the N1.

Given that N1 never flew successfully, and Soyuz is really a 4 booster + 1 sustainer first stage, the "Russians use more engines" assertion is misleading. Energia was 4+4, but flew only twice; Zenit uses a single 4-chamber engine; Proton uses 6 single-chamber engines.

Almost every engine and launcher in production has a complex development history; sometimes your requirements change and you wind up building a much larger launcher than you had expected to.

Engine development generally takes longer than the rest of launcher development (tankage, avionics, airframe, etc.), so you often don't have time to develop the optimal engine for a new launcher. During the height of the Cold War/Space Race, if your government made a request for proposals for a new launcher, you'd look around at the engines you had already developed or had experience with, divide the needed thrust at launch by the thrust of one of those engines, and that's how many engines you'd design for.

SpaceX wanted to use a common engine design for Falcon 1, Falcon 5 and Falcon 9, and use the same engine in the second stages of the 5 and 9. The upper stage naturally needs less thrust, so they designed the Merlin to drive a single-engine second stage, and payload goals set the number of engines in the first stage accordingly.

The Saturn program was originally going to be a larger family of launchers. The Saturn C-3 was to use 2 of the enormous F-1 engines, the C-4 would use 4, and the C-5 became the Saturn V with 5 of them.