I get that the inter-propellant seal on the SSME LOX HP pump is simply a series of cascading regular seals trying to eliminate bleed-through by venting and letting boil at each stage, so that when what little is left of the bleed-through H2 and O2 meet at the central seal chamber their stagnation pressure is low enough so that they both can be contained and pushed back by a comparatively LP He2 source. However, this seems to work only on engines running on two cryo fuels. Barring an FFSC design, how then is inter-propellant seal achieved when

  1. only one propellant is cryo, e.g. RD-180 which runs on kerosene/LOX and powers the Atlas V,


  1. both propellants are non-cryo, e.g. the UDMH/NTO-fueled RD-275Ms on the Proton?

If the propellants are storable and hypergolic and suppose you are a rocket engineer to build one of these engines for military people who want rockets to remain in silos for years and launch at a moment's notice, can you simply turn the fuel/oxidizer main valves to "off" position and declare it safe in the silo? Does there need to be any kind of "wax seal" that really absolutely completely seals a pipe with no clearance instead of mechanical valves, because whatever little UDMH/NTO bleed-through and mixing can cause gas accumulation in an enclosed space and leads to an explosion? (I'm not talking about the fuel/oxidizer flow regulator)


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