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This comment about the Houston Space Center's Rocket Garden drew my attention to the cropped image below, from that link.

I'm under the impression that some liquid propellant engines don't have a quite as extensive layer of tiny tubes running everywhere, perhaps more modern ones, or of different designs?

Question: Why do some engines have so many little tubes, and others, not so much?

Cropped and processed from original https://i.stack.imgur.com/IuVM2.jpg from here.

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  • $\begingroup$ One-shot vs reusable? $\endgroup$
    – Organic Marble
    Sep 11 '18 at 12:52
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    $\begingroup$ I think lots of the required mechanical complexity of older designs has been reduced due to reliable electronics, cheap and ubiquitous sensors, and modern computer systems $\endgroup$
    – Dragongeek
    Sep 12 '18 at 9:50
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There are several factors at work here:

  1. Some engines have their plumbing hidden (e.g. the Shuttle SSME) while here it's all in plain view.

  2. The J-2 engine has a control system that uses pneumatics to operate the valves. I get the impression modern engines use electrical control instead.

The control system included a pneumatic system and a solid-state electrical sequence controller packaged with spark exciters for the gas generator and the thrust chamber spark plugs, plus interconnecting electrical cabling and pneumatic lines, in addition to the flight instrumentation system. The pneumatic system consisted of a high-pressure helium gas storage tank, a regulator to reduce the pressure to a usable level, and electrical solenoid control valves to direct the central gas to the various pneumatically controlled valves. The electrical sequence controller was a completely self-contained, solid-state system, requiring only DC power and start and stop command signals.

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    $\begingroup$ Even cars have this "illusion", I owned an intrepid with a 2.7L engine, it had vacuum tubes all over the place, wires hanging out, etc... the 3.5L model that came out 4 years after the 2.7L looked slick as a baby's bottom in comparison, but as soon as you removed the engine cover, and a few other plates it was the same thing, a tubey mess. $\endgroup$
    – Magic Octopus Urn
    Sep 11 '18 at 13:05
  • $\begingroup$ The situation reminds me a little of pipe organs, where there are so many keys, pedals, and stops, and so many pipes each with a valve. There were strictly mechanical systems of course (before electricity) and there are electrical ones as well. Probably not so many that were 100% pneumatic though. $\endgroup$
    – uhoh
    Sep 11 '18 at 13:08

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