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Both the the fuel and oxidiser preburners ran fuel rich. In order to prevent the preburner gas from entering the LOX pump and blowing it up, they had this complex double seal with a gas purged void in between.

  1. Why would one expect preburner gas to enter the pump at all? Shouldn't the pressure gradient prevent it? In any case, aren't there valve designs that can prevent backflow?

  2. Since LOX is found to the preburner anyway, why not have it go directly down the shaft? Why this detour? It could flow through a gap that us much smaller than the gap leading to the main combustion chamber. The laws of physics would then ensure that the right amount of LOX flows. Moreover, the combustion chamber will run at a lower pressure than the preburner, so it will naturally ducj more LOX.

This would eliminate the complexity of the seal, which also wears down quickly and causes friction with the pump

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Really, there's a whole lot of aspects that make your proposed setup less reliable than desired:

  • The laws of physics have no knowledge of how to run a preburner at exactly the right mixture ratio at all possible power levels and transient conditions. Your proposal would necessarily eliminate the oxidizer preburner oxidizer valve, which is the sole means of controlling the power input to the HPOTP turbine. Too much power and the pump overspeeds, too little power and the HPOTP outlet pressure drops below combustion chamber pressure (bad and bad).
  • Injectors were invented for a reason. At the power levels of a rocket engine, you can't just pour fuel and oxidizer together and light it and expect reasonable results. Rapid and thorough mixing is necessary to ensure stable combustion, a stream of LOX flowing down a shaft is unlikely to ensure this (again, bad for the engine and anyone standing too close to it)
  • The pressure gradient along the shaft is unlikely to be appropriate under all sorts of situations. With any centrifugal pump, the pressure is the lowest near the center (and the highest at the circumference). It is almost certain that there is a regime (probably during startup or shutdown) under which the pressure gradient reverses and hot preburner gas enters the LOX side (bad)
  • Before startup (or after shutdown), LOX would freely flow along the shaft into the preburner, causing a hard start when it eventually comes into contact with fuel (bad)
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    $\begingroup$ You nailed it with that third point. This simply cannot work with a centrifugal pump. That's the real deal breaker. Thanks. $\endgroup$
    – Abdullah
    May 15 at 11:01
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    $\begingroup$ @Abdullah I kinda disagree with your assesment, the third point is not nearly the most important one. They're all similarly important or perhaps even roughly in decreasing order of importance. You could devise complicated solutions to prevent backflow, but there would still be the complete lack of control, and that one is much harder to address. $\endgroup$
    – TooTea
    May 15 at 12:22
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    $\begingroup$ This is the sort of answer that reminds us that it is rocket science. $\endgroup$
    – Wayne Conrad
    May 15 at 22:05

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