Some questions popped up about difficulties of creating throttleable liquid fuel engines.

I wonder why the obvious alternative isn't more common: multiple engines, from which only some will be lit; possibly ones of varied thrust so that you can fine-tune your thrust by selecting which engines are active simultaneously.

Several Soviet launchers used four engines. The Shuttles used three engines. Falcon Heavy uses 27 Merlin engines. Soyuz uses 20 combustion chambers across five engines - so the multi-engine solution is not unheard of - but not quite as common as single-engine ones. What factors decide upon its relatively low popularity?

  • $\begingroup$ LF is liquid fuel? $\endgroup$ – Hobbes Nov 30 '15 at 12:32
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    $\begingroup$ Efficiencies of scale, mostly. A bunch of tiny engines delivering x total thrust will mass more than 1 x thrust engine. $\endgroup$ – Organic Marble Nov 30 '15 at 12:35
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    $\begingroup$ @Hobbes: Yes - hydrogen/oxygen or such; as opposed to SRBs. $\endgroup$ – SF. Nov 30 '15 at 13:27

Having a large range of thrust settings is important for landings, not so much for launches. During a launch, you want some throttlability to reduce loads around max-Q, but otherwise a launch is most efficient when it's done as quickly as possible. So in non-reusable launchers, there's no point in being able to shut down some of your engines.
When you use more engines, your parts count, cost and weight goes up and your reliability goes down.
If you have enough engines (about 5 engines seems to be enough), you gain the option to continue the mission if one engine fails, but for most designs that decision hasn't been made (more launchers with a small number of engines than with engine-out capability).

  • $\begingroup$ Go ahead and post that as a new question, it's an interesting question that needs a larger answer than can be done in comments. $\endgroup$ – Hobbes Mar 18 '18 at 17:56

As discussed in this other thread, the biggest problem with having lots of little engines is that lots of little engines means lots of small fuel and oxidiser lines (whereas, if you use a few big engines, you can get away with using a few big fuel and oxidiser lines). Which, in turn, creates two problems:

  1. More complicated plumbing makes it a lot harder to ensure that the fuel and oxidiser flow is stable and uniform - especially when you have a lot more engines that all need to be producing the same amount of thrust. As a result, designs with huge numbers of engines often have severe problems with fuel- and oxidiser-flow instability, which leads to combustion instability, which causes the chamber pressure of the engine suffering from it to fluctuate, which affects the amount of fuel and oxidiser that enters the engine, which worsens the instability, all of which induces vibrations (called "pogo oscillation") in the vehicle structure, leading us to the second big issue:
  2. Having lots of small propellant lines makes the rocket's plumbing much more fragile - smaller, thinner lines break more easily than bigger ones, and there're more lines that can break. When you have a vehicle that's already prone to severe pogo oscillation, fragile plumbing is not your best friend.

Look at the Soviet N1 rocket for an example of the problems that can arise when using lots of little engines. Of the four N1 launch failures (which comprised the N1's entire operational history), three occurred after parts of the rat's nest of plumbing feeding the 30(!) engines in the N1's first stage ruptured for one reason or another (pogo oscillation in the first launch, an exploding turbopump in the second, the fuel/oxidiser equivalent of a water hammer in the fourth), setting parts of the stage on fire, which rapidly burnt through various important components.

TL;DR: Having lots of small engines means having lots of complicated, fragile plumbing, which is generally ungood for a rocket.

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    $\begingroup$ The Soyuz has 20 combustion chambers connected to 5 pairs of tanks. The Falcon Heavy has 27 engines connected to three pairs of tanks. But the N1 had 30 engines connected to only one pair of tanks for fuel and oxidizer. Several sucessful rockets have 4 to 9 engines per pair of tanks. $\endgroup$ – Uwe Mar 24 '18 at 20:05
  • $\begingroup$ The only thing we really learn from the N1 is that unreliable engines make for an unreliable rocket. I suspect that time will show the Falcon Heavy's 27 engines are fine. $\endgroup$ – user8269 Mar 24 '18 at 23:35
  • $\begingroup$ @PhilipNgai: The engines are perfectly fine - it was the rocket trying to feed thirty of them at once from a single set of propellant tanks that kept blowing up. $\endgroup$ – Sean Mar 24 '18 at 23:56
  • $\begingroup$ @PhilipNgai: The NK-33 engine had a later sucessful usage. $\endgroup$ – Uwe Mar 25 '18 at 10:53
  • $\begingroup$ First launch: defective engines. link As it turned out, six seconds after liftoff, high-frequency vibrations had torn off a gas pressure-measuring pipe located downstream from the turbopump in engine No. 2. To make matters worse, at T+25 seconds, a pipe for measuring fuel pressure before the gas generator had also broken off, spewing kerosene into the guts of the rocket. $\endgroup$ – user8269 Mar 26 '18 at 7:02

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