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Raiz Aerospace raised an interesting question during one of his KSP Realism Overhaul play throughs.
If the RCS propellant Cavea-B is better than hydrazine and less toxic why does no one use Cavea-B, while hydrazine is a defacto industry standard for RCS and orbital maneuvering systems.
Monopropellant systems such as catalyzed hydrazine thrusters are attractive at very small sizes, where the simplicity of a single propellant tank outweighs their relatively low performance.
According to Wikipedia, Cavea-B requires a small amount of UDMH or a similar hypergolic to begin ignition -- every time you want to fire it, which can be a frequent occurrence for RCS thrusters.
This means you need a UDMH tank and a set of valves, so you lose the simplicity advantage of a monopropellant, and the requirement to store and handle UDMH negates the non-toxicity advantage.
At that point you might as well switch to a NTO/UDMH or NTO/MMH bipropellant thruster, which are very mature technologies and outperform Cavea-B.
John D Clark, Ignition!, p.165, on the testing of a Cavea B motor:
“Well, through a combination of this and that, the motor blew on startup. We never discovered whether or not the [detonation] traps worked —we couldn't find enough fragments to find out. The fragments from the injector just short-circuited the traps, smashed into the tank, and set off the 200 pounds of propellant in that. (Each pound of propellant had more available energy than two pounds of TNT.) I never saw such a mess. The walls of the test cell—two feet of concrete—went out, and the roof came in. The motor itself—a heavy, workhorse job of solid copper— went about 600 feet down range. And a six-foot square of armor plate sailed into the woods, cutting off a few trees at the root, smashing a granite boulder, bouncing into the air and slicing off a few treetops, and finally coming to rest some 1400 feet from where it started. The woods looked as though a stampeding herd of wild elephants had been through.
“As may be imagined, this incident tended to give monopropellants something of a bad name.”
I think it comes down to this: you need both attitude control and thrust for a spacecraft to be able to do its job, and Cavea-B is only good at one of those jobs. Hydrazine, though less efficient, can power both your attitude control system and your primary thrusters, and so you only need one big pressurized tank. I do think it could be used for orbital maneouvers in things like space tugs since it's storable, dense and almost as good as bipropellant systems, but since there's no space tugs the point is, as of now, moot.
