Masten Space Systems has a proprietary hypergolic bipropellant called MXP-351 intended for use in the XL-1 lunar lander. We seem to know the following information:
- Non-cryogenic, with storage life of "a few years"
- Isp of 322 s in vacuum, compared to 336 s for NTO/MMH
- Burns with an orange flame in atmosphere; one of the components is a clear liquid
- "Both propellants nontoxic", "prepared propellants sitting in open containers don't require a respirator"
- Developed by Masten, a small company who had previously only worked with LOX/CH4 and LOX/IPA engines
Since it's proprietary, it's probably impossible to pin down the exact composition, but can we narrow down the possibilities? I assume NTO, nitric acid, and perchlorates are too toxic, so the possibilities for oxidizer seems to be really limited-- maybe hydrogen peroxide?
