N2O has been viewed as a nontoxic, “safe” oxidizer that might be used in rocket propulsion systems (8-12). That is, N2O is a candidate “green propellant” oxidizer. Since N2O is an energetic oxidizer, it should be treated with at least the level of respect given to high pressure gaseous oxygen systems (16-17). Of special note in gaseous oxygen hazard experience was infrequent fires in stainless valves using viton® or silicone seats (17). Although electrostatic discharge was not cited as a cause for gaseous oxygen valve fires, it is logical that an electric spark through oxygen infused plastic seats could have started the fires. Environmental conditions of fast gas flow over plastic seats, high pressure, and low temperature through large expansion of oxygen gas were fairly optimal for producing electrostatic discharge (ESD) activity (18, 19). Since N2O can dissolve significantly in plastics, electrostatic sensitivity of such compositions seems probable (6). Probability of hazardous behavior with N2O in operational combustion uses has been recognized for decades. As a result, several explosive and fire studies have been supported by US Navy and AF organizations (20-25). Those N2O hazard studies did not show any strong violence by N2O runaway decompositions or continuation of monopropellant decompositions started by adiabatic compressions. Lack of violent events when N2O was abused by initiating means was, perhaps, partly due to use of small volumes for experiments where wall effects were adequate for limiting N2O speed or providing quenching for N2O decomposition processes. Approaches for safely utilizing N2O for rocket propulsion should involve measures countering N2O’s unique array of chemical and physical hazard characteristics. Concerns about pure liquid N2O reaction initiation by “water hammering” and “bubble collapse” (6) seem unlikely due to liquid N2O being a highly compressible liquid (8) and lack of response to 50 gram tetryl pellet detonation at 27 mm diameter (23). Main N2O traits to be concerned about are positive heat of formation (9), close proximity of its critical temperature to normal ambient temperatures (pressures above 40 atmospheres), its extraordinary solvent capability for dissolving or plasticizing even complex organic materials (25-31), and, perhaps, sensitivity to ESD when N2O flows rapidly over high electrical resistance materials, such as, viton® and silicone valve seats. Organic contaminated N2O fluid (gaseous or liquid) compositions might be particularly hazardous due to their ease of fire/explosion initiation by ESD and minor explosive stimulation.
Posted on: Fri, 31 Oct 2014 22:07:58 +0000
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