I was reading a similar question as mine here. But no one talked about nitrogen dioxide (NO2). Its boiling point is 21 °C, so it is easy to store. But how easy it is to handle compared to hydrogen peroxide or other oxidizers?
Are there any better liquid oxidizers that are relatively easy to use? At least, a combination of oxidizers or chemical stabilizers?
There certainly seems to be no near-safe liquid oxidizer; all require extreme care and handling. Is this an exception?
Not a good idea for amateurs; it can kill you without you realizing you're dying:
10–20 ppm can cause mild irritation of the nose and throat, 25–50 ppm can cause edema leading to bronchitis or pneumonia, and levels above 100 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema.
Nitrogen dioxide is a chemical that forms an equilibrium with dinitrogen tetroxide, which in the world of rocketry is often just called "nitrogen tetroxide" so as to increase the amount of confusion. If you have a tank of one, it will change between being one or the other based on temperature and pressure.
This is one of the commonly used hypergolic storable oxidizers, and, like all commonly used hypergolic storable chemical propellants, it's quite dangerous and probably not suitable for the amateur. Specifically, it's quite deadly if inhaled.
Nitrous oxide, N2O, is much safer and liquefies under pressure similar to propane, and is commercially available due to its use in the whipped cream and motorsports industries. However, it has lousy performance and is not hypergolic with typical fuels. Additionally, it definitely can explode under the right circumstances. See this presentation about safety of N2O usage.
NO2/N2O4 is quite poisonous by the standards of amateur propellants, not an expert but not aware of any amateurs using it. N2O is common but at room temperature it's only a liquid at high pressures so if you want something that's liquid near STP like NO2 it might not be a good option. Liquid at STP and probably somewhere in between in terms of safety would be concentrated (fuming) nitric acid, but it's corrosive, still far from friendly (in no small part due to the substantial amount of NO2 usually dissolved in it) and, IIRC, generally not as performant as N2O or NO2. Still, unlike NO2, it's seemingly quite frequently made and used in reasonable quantities by chemistry hobbyists, and forced to choose one I'd certainly rather deal with it than with straight NO2.
In general, low-pressure room-temperature storable liquid oxidizers are all pretty crummy in terms of safety.
