Cesium reacts violently with water. So much so that just the moisture in the air is enough to make it react . Needless to say it’s not found in its elemental state naturally .
If you’re turning it into plasma with SR-71 exhaust, would you need elemental Cs?
True, but it is unlikely in the extreme that elemental cesium was used. Cesium COMPOUNDS are not (or not nearly) as reactive. An inorganic possibility is cesium carbonate, but I think more likely that some organometallic complex was used to enhance solubility in the organic fuel matrix.