In this video of a Korea Aerospace Research Institute (KARI) rocket engine test, you can clearly see a second exhaust where thick smoke comes out.

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Is this an essential part of the engine, or just an extra attribute required when testing? In both cases, why is a separate exhaust necessary?

Here is a second video from a SpaceX tests of the Merlin 1D rocket engine with the same feature.


That is the exhaust of the turbopump drive. They burn a small amount of propellant, those exhaust gases are used to drive the turbopump that pumps the propellant and oxygen to the engine.
There are engines where this exhaust is fed into the main combustion chamber (staged combustion), but this is expensive to get right so many engines use the cheaper system with a separate exhaust (gas-generator cycle) and slightly lower performance.

  • $\begingroup$ Why is it hard to build a staged combustion engine? $\endgroup$ – DP_ Jun 18 '16 at 5:19
  • 2
    $\begingroup$ from the linked article: Disadvantages of staged combustion include harsh turbine conditions, exotic plumbing to carry the hot gases, and complicated feedback and control. In particular, ORSC requires advanced metallurgy due to extremely corrosive oxidizer-rich gas, while FRSC requires use of a fuel that will not coke, such as liquid hydrogen. $\endgroup$ – Hobbes Jun 18 '16 at 7:41

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