Table 1. Performance of MMH/RFNA vs. TEAB/WFNA. I sp values estimated at P c = 1000 psia with the listed exit conditions. IDT represents minimum values observed at Purdue 11,12 Fuel Oxidizer I sp s ρI sp [s-g/ml] IDT ms Fuel NFPA Health Rating 1 atm ϵ=50 1 atm ϵ=50 MMH RFNA 274 314 350 402 2 to 3 4, Carcinogenic TEAB WFNA 265 303 343 394 3 to 4 3 C. Humidity Effects on Propellant Performance Pourpoint and Anderson have shown that the ambient gas composition, including different inert gases, can measurably affect IDT. 13 Similarly, changing the water vapor in the testing environment can alter the IDT directly. Water vapor has a higher specific heat capacity than dry air, thus humid air can absorb more heat from the hypergolic reactions to reach the same temperature change. The water molecules in the environment may partake in the chemical reactions and the presence of water may change when hydrogen and hydroxyl radicals are liberated. This effect could change the chemical pathways of the hypergolic reaction. Allen et al. found that the overall oxidation rate for moist carbon monoxide and nitrous oxide gas mixtures increases as water vapor rises from 10 ppm to 1300 ppm. The water vapor reacts with nitrous oxide to form hydroxyl radicals which then react with carbon monoxide to form hydrogen. The authors goon to state that the direct reaction of nitrous oxide with carbon monoxide is negligible in the presence of water Alternatively, hypergolic propellants can absorb moisture from the environment. MMH and WFNA are known to be hygroscopic and TEAB maybe as well Although no information on whether TEAB is hygroscopic is available at the time of writing, ammonia borane (a solid hypergolic amine borane with low IDT) is known to be hygroscopic. 3,15
