How dangerous are the “Green Propellants”?

Some time ago we attempted to address the question “How Dangerous are the Hydrazine Fuels” and so it is interesting to consider the same question for the “Green Propellants”. I don’t know if there is any generally accepted definition of the term Green Propellant. I think that for many people, it simply means a propellant that is neither MON, nor Hydrazine, nor MMH.

The two Green Propellants which seem to be making the headlines at the moment are the ‘AF-M315E’ formulation developed by Aerojet (for the US Air Force) in the USA and the ‘LMP-103S’ ™ formulation developed by ECAPS in Sweden.

These propellants have a lot in common; they consist of highly explosive substances (in pure form, crystalline solids) used in an aqueous solution. AF-M315E is based on hydroxyl ammonium nitrate (HAN) and LMP-103S is based on ammonium dinitramide (ADN). Initially, it was said that one of the key attractions of both propellants is that the vapour above the liquid consists only of water and thus eliminates the  problem of toxicity through inhalation problem which accompanies the traditional propellants. Actually, both formulations are believed to have constituents in them besides the main one and these secondary materials are present in the vapour phase, so giving rise to some toxicity risks, though not has great has with hydrazine. The early US formulations with HAN incorporated alcohols and glycols, as reported in various conference papers.

Toxicity is not the only issue which needs to be considered however. There are several other issues that need to be assessed and among these we could include the stability of the propellant when subjected heat, shock, electrostatic charge, and friction. We need to know exactly which materials can be used in the propulsion system and are fully compatible with the propellants and which are highly incompatible with the propellants and need to be scrupulously avoided. Additionally, we need to know what the decomposition products of the propellant are (formed by any foreseeable process) and how the propellant can safely be disposed of or treated after a spill, and what the effects of release in to the environment are. Some of the established propellants are adversely affected by exposure to air (e.g. hydrazine by CO2 and MON by H2O) and MON changes composition if repeated vented (through preferential loss of nitric oxide). Do similar issues arise with the green propellants?

When you start to try and find such information for these two propellant is that the information currently in public domain is “quite limited”. You might hope to be able to get a Materials Safety Data Sheet (MSDS), but the MSDS of both AF-M315E and LMP-103S still appear to be confidential. The ECAPS website advises that LMP-103S has been designated as a Class 1.4S explosive. AF-M315E is believed to have been designated as Class 1.3C explosive. In the UK, possession of such materials will require certification under the terms of the Explosives Act and may not be easy to acquire. Such certification is not needed for MON or hydrazine.

In the case of HAN, there is some information in the public domain dating from the time when people were trying to use that material as a propellant for guns. In the case of ADN, there appears to no significant information about the chemical properties of its aqueous solutions in the open literature.

So going back to my original question, figuring our just how dangerous these two propellants are is task that is still in progress and much of whatever data has been acquired is still under lock and key! While one can see these propellants with explosive classifications getting in to military applications (missiles) quite readily, it seems likely that getting them fully adopted commercial space platforms might take a little longer.

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