I’m trying to write a book on my experiences on working on the Shuttle BFS SW. I’ve looked all over the web and every diagram on an OMS/RCS pod shows the 2 fuel tanks for each, OMS and RCS. What seems odd is that there are 2 Helium pressurisation tanks for RCS, one of each of its two hypergolic fuel tanks, but there is only 1 Helium pressurisation tanks for its 2 hypergolic fuel tanks. What do I not understand about these two systems that would have the OMS system requiring only one pressurisation tank?
Sometimes, as an instructor, it seemed like that we had found the Orbiter on the runway left by an ancient civilization, with no answers as to why things were done the way they were.
This is one of those things I've wondered about as well - I don't have a referenced answer, so consider it informed speculation.
In the OMS with the single helium tank per pod, there was a danger that over long periods of time, propellant vapors could diffuse upstream through the quad check valves into the helium lines. If the fuel and oxidizer vapors get together in the helium lines, it's a KABOOM case. So OMS had to provide additional valves - the Vapor Isolation Valves - to prevent this. This schematic from the 1995 OMS training manual shows the vapor iso valves.
In the RCS, providing a separate helium tank for each of the fuel and oxidizer systems designs out this failure case, because the helium lines for the two systems do not interconnect. It seems like the better design from a safety standpoint.
The OMS training manual says that having one helium tank for the whole system provides better control of mixture ratio since both propellant tanks are pressurized from the same source. Perhaps that's a bigger deal for the OMS since it's a much more powerful engine than the RCS jets. I also wondered if packaging in the pods was an issue since the OMS helium tank was quite a bit bigger.
I think these were some of the main tradeoffs they considered when they came up with the design, and I'd love to see it actually written down somewhere. Some maybe more minor factors that might have been considered:
- Everything else being equal, two tanks that together hold X amount, weigh more than 1 tank that holds the same amount, plus plumbing, valves, etc.
- The RCS systems were active almost all the time in space for attitude control, so the vapor iso valve scheme wouldn't be practical for them. OMS got turned on relatively rarely compared to the RCS.
Sources (annotations mine):
- Shuttle OMS training manual
- SODB Volume 4 "Crash book"