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I just read about the "green" propellant Hydroxylammonium nitrate writing this answer. It will be tested in the Green Propellant Infusion Mission (also see GPIM AF-M315E Propulsion System).

Catalytic Decomposition of Hydroxylammonium Nitrate Monopropellant discusses preparation and testing of HAN-water blends and the use of Iridium as catalyst.

Is Hydroxylammonium nitrate propellant "green" enough to ever be used in a cubesat? In this context, "green" refers to its lower toxicity when compared with hydrazine.

The question is not so much compatibility with the cubesat itself as it is with the safety requirements of the most common space deployment options and cubesat guidelines related to stored chemical energy.

According to Wikipedia NH3OHNO3 is a fuel/oxidizer blend, also known as AF-M315E. It would likely be used in a spacecraft as an aqueous solution liquid propellant (the solid salt has stability issues), and may be more efficient (per volume) than Hydrazine.

According to GPIM (Green Propellant Infusion Mission) / STP-2

Delivering approximately 50% higher ρΙsp than hydrazine (5% higher Isp combined with a 46% higher density, AF-M315E offers comparable performance to traditional storable bipropellants for low V missions while employing roughly half the number of components, thereby retaining the well-established increased reliability and reduced cost of traditional monopropellants. Many design issues and failure modes associated with long-duration interplanetary missions (e.g. control of mixture ratio, of propellant vapor diffusion and reaction, oxidizer flow decay) do not apply to an equally capable AF-M315E system.

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It probably couldn't be used on a cubesat, but not primarily for environmental reasons.

The restrictions on cubesats generally relate to the risk of it damaging the launch vehicle, primary payload or people involved in the wider mission (including astronauts for cubesats deployed from the ISS). The system being developed for GPIM would break several of the restrictions:

  • While it's a lot less unpleasant than hydrazine, the safety data still says "Explosive, Carcinogenic, Toxic, Harmful, Irritant, Dangerous for the environment". I would guess that this would completely rule it out from being deployed from the ISS, and is likely to be challenging for other launcher systems. I haven't looked into it enough to say how big the explosion risk is - doubtless less than hydrazaine, which basically doesn't want to exist at all - but expect a lot of "you're not putting that amateur explosive device anywhere near my $100M satellite".

  • Feed pressure of 6.8 to 27.6 bar; cubesats need a waiver for anything above 1.2 bar (for good reason: a pressurised tank going bang can make a mess of anything nearby).

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    $\begingroup$ I see what you mean. I guess that rules out the nail-polish remover (acetone) and hairspray I was going to use for my "cosmotology-sat" project. The pressurization is a problem I hadn't even thought about yet either. It would have to be done with a pump - the flow rate is so tiny that it could be a small pump, but the tank wouldn't have to be pressurized. Overall I think your conclusion is pretty solid. $\endgroup$
    – uhoh
    Oct 30, 2016 at 18:04
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    $\begingroup$ Solid Iodine is a "newish" cube-sat Ion propulsion alternative (which also mitigates the pressurization problems). $\endgroup$
    – Magic Octopus Urn
    Oct 9, 2019 at 17:35
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CubeSat propulsion systems employing HAN-based propellants are currently under development and slated for launch in the next couple years. AF-M315E, the most developed HAN-based propellant, has very low shock and impact sensitivities. In light of this (along with the negligible vapor toxicity), range safety management has designated AF-M315E in a lower hazard classification than hydrazine, making it logistically well-suited for CubeSats missions. The real obstacle to full infusion is that ionic liquid thrusters require greater preheat power to operate, and must be constructed of expensive refractory metals due to the much increased reaction temperatures.

Incidentally, the Aerospace industry substance with the most lethal history is regular nitrogen gas. No one associated with handling hydrazine as a propellant has ever been significantly harmed, because it is easily handled in ways that isolate personnel from the relatively diffuse vapors.

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    $\begingroup$ You might want to check out ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100038321.pdf Table 1 shows 7 injuries or deaths from hypergolic fuel incidents. $\endgroup$
    – Organic Marble
    Oct 9, 2019 at 17:01
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    $\begingroup$ Welcome to Space! It's important to support fact-like statements in Stack Exchange answers with links and/or references so they can be confirmed by readers. Otherwise readers are not able to tell if information is reliable or not. You can see how many sources I've added in the question already. Safety of energetic materials stored in cubesats for months or years before launch is a complex issue, there would have to be an approval process, etc... Thanks! $\endgroup$
    – uhoh
    Oct 9, 2019 at 17:04
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The problem with HAN is that it wasn't anywhere near as stable and "detonation resistant" as it's proponents suggested it was. That's what doomed the Air Force's ALASA program.

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