Search for green hypergolic propellants: Gas-phase ethanol/nitrogen tetroxide reactivity

Monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) is a well-known bipropellant combination system, the main advantage of which is low temperature ignition without ignition device, also called hypergolicity. The use of other chemical systems, less toxic, less hazardous, less corrosive, and more environmentally friendly than MMH/NTO are nowadays desirable. The replacement of the fuel MMH by ethanol (EtOH) is examined here. It is shown that ethanol/ NTO mixtures are not hyperbolic, but that reactions take place at room temperature and that these reactions form ethyl nitrite. Chemical reasons able to explain this reactivity, which does not lead to ignition, are discussed. In particular, it is shown that the EtOH/NTO initiation reactions going through H abstraction by NO₂, whatever this initiation reaction is, and especially the one leading to ethyl nitrite, are considerably more endothermic than the corresponding MMH/NTO initiation reactions.