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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?

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2 Answers2

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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).

Hobbes
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  • 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. – Hobbes Mar 18 '18 at 17:56
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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.

Vikki
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    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. – Uwe Mar 24 '18 at 20:05
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    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. –  Mar 24 '18 at 23:35
  • @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. – Vikki Mar 24 '18 at 23:56
  • @PhilipNgai: The NK-33 engine had a later sucessful usage. – Uwe Mar 25 '18 at 10:53
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    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. –  Mar 26 '18 at 07:02
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    Second launch: defective engines. link

    analysis of the available telemetry, photos and film recordings revealed that as the propulsion system had been firing with the rocket still on the launch pad, a turbopump supplying liquid oxygen to engine No. 8 exploded a quarter of a second before liftoff. The turbopump of engine No. 8 had signs of melting and damage from an internal explosion, unlike the other 29 engines. The force of this blast was fatal for the entire rocket. Various arteries leading to other engines were severed.

     –  Mar 26 '18 at 07:09
  • Still more info: link

    the actual flight engines could not be static tested. Many valves were opened and closed with pyros instead of hydraulic or pneumatic power. After a test run the valves were welded shut and could not be opened.

     –  Mar 26 '18 at 07:18
  • The test policy was to accept engines from the factory in lots of 6. 2 were sent to the Kuznetsov Bureau's test facility and tested, then scrapped. If these two completed one full burn, the other 4 were sent to Baikonur and installed in the stages without any testing. Thus it is not surprising that out of 120 flight engine runs, there were two turbopump explosions and one hot gas leak (the actual cause of the first flight failure). –  Mar 26 '18 at 07:18
  • The idea that N-1 could have been a success with the improved NK-33 engines is disproved by the terrible record of these engines in the Antares program. That failure board tore down many NK-33s in stock at Aerojet and found that about 1/3 had metal missing from the turbopump shaft. This was probably a machining error by the night shift at Kuznetsov's factory that was missed by quality control. –  Mar 26 '18 at 07:19
  • @PhilipNgai: First launch: parts of the plumbing were torn apart by vibrations. That's a problem with the fragility of the plumbing. Second launch: The exploding turbopump doomed the rocket because it tore through enough of the plumbing to start a fire that damaged several of the other engines badly enough for them to be automatically shut down; the N-1's buggy engine control system did this by shutting down the entire stage (except for one engine that failed to get the message - a broken cable, perhaps?). Sturdier plumbing would have withstood the shockwave, ergo no fire, ergo no kaboom. – Vikki Mar 26 '18 at 17:51
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    Part of THE ENGINE's plumbing were torn apart. –  Mar 27 '18 at 03:12
  • @PhilipNgai: Exactly. THE PLUMBING LEADING TO THE ENGINE. NOT the engine itself. – Vikki Mar 28 '18 at 19:09
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    That plumbing WAS part of the engine. –  Mar 29 '18 at 19:06