Propane/N2O is an interesting combination. It needs to be pressurized quite a lot (the N2O in particular) to stay liquid, ruling it out for large systems like launchers. But for small thrusters, it means the system is self pressurizing, meaning (potentially) less mass and complexity for the entire thing.
HAN and ADN are also interesting options.
That being said, they should be tested and flown a lot more on cargo launches before they're ready to be put on crafts carrying humans.
HAN and ADN are also interesting options.
That being said, they should be tested and flown a lot more on cargo launches before they're ready to be put on crafts carrying humans.