Cryogenic Argon/Neon Ullage/SEP propellent gelled with Metalic Iodine
For Hybrid chemical/SEP in space stages
Synopsis; (A)Cryogenic LH2 LO2 boil off cools tank of LAr and LNe with particles of metallic Iodine infused.(B)During operations LAr/Ne gasify and pressurize the LO2 oxidizer tank(ullage gas).(C) In space liquefaction/fractionation device separates gases O2 from gases Ar/Ne and sends to IVF and ion engine systems.(D)Heater vaporizes Iodine into SEP engine propellent.(E) Most likely to avoid icing problems we need a 70% Ne or higher or lower to the Ar.Zenon is the best bet for SEP but most likely cannot be used in this scenario.(F)If metallic Iodine can’t be filtered out of the Ar/Ne ullage gas then stored in separate tank.
Iodine needs to bypass O2 oxidizer tank and instead to SEP engine for Mixing with Ar/Ne.In the case of Methane/Oxygen launchers Ar/Ne could pressurize both tanks in trade studies as to how much SEP fuel is needed for mission requirements.Cryogenic LH2 would cause icing problems with Ar/Ne as well.
We see Hybrid Chemical/SEP in space stages as spacecraft in their own right as long-lived value added propositions to remove themselves from GTO orbits, deploys or carry secondary payloads(ride share) and share nobel gases propellent with cargo by pressurizing cargo
In the case of the proposed Skylab-II , Bigelow and Cislunar habitats(all of them) we could pressurize them with xenon and its better properties as SEP propellent and not have to worry about icing as they are not required as Ullage gases, so in effect this a two stage in space system
For fun I estimate that the ISS if de-crewed could be pressurized with 6 metric tons of Xenon and docked with a ARRM derived stage with 11 Metric tons of xenon gets us to cislunar space( The ISS would be salvage) As for previous posts a Hybrid Chemical/SEP EUS stage for SLS with the Xenon enhanced Skylab-II could increase SLS payload mass to destination.
Edit 2 April 2016 Cryogenic Argon/Neon mix would have considerably less volume in a dedicated tank per this master list of Cryogenic liquid to gas volume.I wonder if the force of evaporation of cryogenic Argon/Neon could be used as cogenerated power when spacecraft is on internal power