If fuel and oxidizer are pumped by turbines to the combustion chamber, then why there are so many pipes around? It should only have a two-cylinder shaped turbine assembly. I doubt there are other gases also pushed in other than fuel and oxidizer.

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    $\begingroup$ you only need one blood and how many vessels do yo have? $\endgroup$ Jun 20, 2020 at 4:16
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    $\begingroup$ Annotate your image to point out one (1) of the "pipes" on this SSME and I'll tell you what it's for. $\endgroup$ Jun 20, 2020 at 4:19
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    $\begingroup$ Jet engines don't even need oxidizer: have a look at the picture here... $\endgroup$
    – user21103
    Jun 20, 2020 at 10:04
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    $\begingroup$ The difference you see is the difference between science and engineering, or similarly, theory and practice. Science vs Engineering $\endgroup$ Jun 21, 2020 at 4:17
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    $\begingroup$ To get an idea of what some of those pipes are for, here is a fairly in depth explanation of SpaceX's new raptor engine: everydayastronaut.com/raptor-engine $\endgroup$
    – Philip
    Jun 21, 2020 at 5:20

2 Answers 2


While an ideal engine would just ingest fuel and oxidizer and produce exhaust gas real world engines will have some combination of regenerative cooling, film cooling, turbine exhaust, hydraulic power, ignition systems, pressure sensing, tank pressurization systems, drain/purge/test lines, and electrical connections that just look like pipes for heat protection, and are probably majority of the 'plumbing' visible in the question photo. Some of these may also have bypass or redundant connections.

The possible contents are liquid fuel, liquid oxidiser, vaporised fuel, vaporised oxidiser, fuel rich turbine exhaust, oxidiser rich turbine exhaust, fully burned exhuast, hydraulic pressure, hydraulic return (possibly multiples of these if valves outside engine, ignition system fluids, pneumatic system gases, electricity.

Examples of engines using various combinations Rs-25/SSME RL-10 F1/Saturn V Merlin Rutherford/Electron (possibly the simplest plumbing of a current engine)

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    $\begingroup$ Your link to film cooling isn't actually about film cooling! You'll note the exhausterator is at the end of the nozzle--there's nothing to cool beyond that. $\endgroup$ Jun 20, 2020 at 19:52
  • $\begingroup$ @Anton Hengest, you would be right, had thought it was at the end of the nozzle and then cooled the nozzle extension but that is not correct. There is a more relevant one there now though still have not found a really good ref for the concept. $\endgroup$ Jun 20, 2020 at 23:19
  • $\begingroup$ The F-1 is really the ur-example for film-cooled nozzles $\endgroup$ Jun 20, 2020 at 23:27
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    $\begingroup$ @Harper-ReinstateMonica Battery power to weight ratios have gotten drastically better in recent years, so the Rutherford approach isn't completely impractical, though I don't know if it's actually lighter or heavier than the equivalent power turbopump & turbopump fuel. $\endgroup$ Jun 22, 2020 at 2:53
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    $\begingroup$ @Harper-ReinstateMonica Well off topic now but my understanding was the batteries for electron weight more than the fuel+turbine would have, but it allowed them to side step a lot of design complications in a new engine so made sense. $\endgroup$ Jun 22, 2020 at 8:39

The tubes built into the nozzle perform to very necessary functions.

  1. the very very, super cold, super dense liquid in these pipes keeps the nozzle from melting or really deforming and becoming too inefficient. When wondering about rockets always consider cost to efficiency and performance to weight. Engines manufactured in higher volumes must meet these criteria. The liquid in the pipes that are part of the nozzle is the super super sub cold and dense fuel. To give up its energy the best way requires it to be a specific temp. The bigger nozzle pipes protects the integrity of the nozzle and preheats the fuel.
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    $\begingroup$ Can you confirm the fluid in the nozzle cooling pipes is "super dense"? For the SSME the H2 exiting the coolant circuit had a density of about 2.5 lbm/ft^3. $\endgroup$ Jun 22, 2020 at 2:00

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