Q&A - Inter-propellant Seal

[fl1034]

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,

and

  2. 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)

[Bob Shaw]

Don't all modern US silo-based missiles use solids to get round all this?

[fl1034]

Don't all modern US silo-based missiles use solids to get round all this?

The US used plenty of hypergolic-engine-powered ICBM before it figured out how to do solids.

[Bob Shaw]

Don't all modern US silo-based missiles use solids to get round all this?

The US used plenty of hypergolic-engine-powered ICBM before it figured out how to do solids.

Er, yes - and they abandoned nightmare degradeable plumbing as soon as possible! How many Titan ICBMs are in service?

[fl1034]

Don't all modern US silo-based missiles use solids to get round all this?

The US used plenty of hypergolic-engine-powered ICBM before it figured out how to do solids.

Er, yes - and they abandoned nightmare degradeable plumbing as soon as possible! How many Titan ICBMs are in service?

Then how does the RD-180 intermediate seal work?

The Russians are (and have been a while) using even more insane design -  an engine submerged inside out in N2O4 or UDMH in SLBMs.

[Bob Shaw]

Don't all modern US silo-based missiles use solids to get round all this?

The US used plenty of hypergolic-engine-powered ICBM before it figured out how to do solids.

Er, yes - and they abandoned nightmare degradeable plumbing as soon as possible! How many Titan ICBMs are in service?

Then how does the RD-180 intermediate seal work?

The Russians are (and have been a while) using even more insane design -  an engine submerged inside out in N2O4 or UDMH in SLBMs.

You hit upon the right word when you used 'insane'. Hypergolics may have a place, but so did high-test hydrogen peroxide - and that place is mostly in the past. Even the Dr Strangelove brigade realise that these things are bad news. The ex-USSR military-industrial complex are another matter and are prone to outrageous environmental acts which make solids look like health foods!

[PhotoEngineer]

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,

and

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

The mechanism is very similar with non cryo fuels, instead of boiling though you cascade your seals so there is minimal leakage, which drops the pressure, and eventually the leak rate is countered by surface tension and the He2 purge gas. It's generally easier when its the hypergols because although its really bad if they come in contact you then don't have LOX compatibility and cryogenic thermal contraction to deal with in the design.

[fl1034]

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,

and

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

The mechanism is very similar with non cryo fuels, instead of boiling though you cascade your seals so there is minimal leakage, which drops the pressure, and eventually the leak rate is countered by surface tension and the He2 purge gas. It's generally easier when its the hypergols because although its really bad if they come in contact you then don't have LOX compatibility and cryogenic thermal contraction to deal with in the design.

Then what about UDMH/NTO engines for fast reaction launches? How do they contain leakage when inactive in silos? Can the mechanical valves achieve perfect seal indefinitely when turned off or there need to be wax seals?

[baldusi]

Then what about UDMH/NTO engines for fast reaction launches? How do they contain leakage when inactive in silos? Can the mechanical valves achieve perfect seal indefinitely when turned off or there need to be wax seals?

They have fuel and oxidizer pumps before the engine. The interseal only matters while running.

[baldusi]

You hit upon the right word when you used 'insane'. Hypergolics may have a place, but so did high-test hydrogen peroxide - and that place is mostly in the past. Even the Dr Strangelove brigade realise that these things are bad news. The ex-USSR military-industrial complex are another matter and are prone to outrageous environmental acts which make solids look like health foods!

Most of the "insane" hypergolic ICBM are submarine launched. Those usually are volume limited to existing tube sizes, thus the reason to minimize volume (e.g. submerging the engine in the tank, or concave tank bottoms).

[Jim]

Don't all modern US silo-based missiles use solids to get round all this?

The US used plenty of hypergolic-engine-powered ICBM before it figured out how to do solids.

Er, yes - and they abandoned nightmare degradeable plumbing as soon as possible! How many Titan ICBMs are in service?

Wrong.  Titan II was in service for more than 20 years