# Why would it be “less bad” for hydroxylammonium nitrate monopropellant to freeze than for hydrazine?

Answers to Is it bad if hydrazine freezes on a spacecraft? Is it always kept as liquid, or can it be safely allowed to freeze and then thawed when needed? explain that while the freezing of hydrazine on a spacecraft isn't necessarily catastrophic in all cases, it's "bad" because the contraction pulls in more liquid, then the expansion upon thawing can cause damage.

This answer to Is the EU really banning “toxic propellants” in 2020? How is that going to work? mentions:

NASA is testing hydroxylammonium nitrate in space which is expected to perform 50% better than standard propellants and can be allowed to freeze (hydrazine must be kept liquid). LMP-103S has been tested gets about 30%.

Question: what is it about hydroxylammonium nitrate that would make it "less bad" to freeze in space than hydrazine?

I've been unable to find any other reference to the quote in your question, that HAN can be "Allowed to freeze". I suspect what the asker meant was that the satellite could be allowed to operate in colder conditions.

Looking at the MSDS on HAN, at -80 degrees celsius, the fuel doesn't freeze in the traditional sense but forms an amorphous glass. Hydrazine freezes at +2 degrees.

Typically, satellites in earth orbit do not get much colder than $$0^o C$$, and the Green Propellant Infusion Mission will remain in earth orbit.

If HAN is, in-fact allowed to freeze, it would be because the difference in density between the solid and liquid forms is relatively small. Liquid hydrazine has a density of 1.02 $$g/cm^3$$ whereas solid hydrazine is at 1.19 $$g/cm^3$$, making the transition from solid to liquid result in a 17% volume expansion. In order for HAN monopropellant to be more suitable for freeze/thaw cycles, this expansion must be smaller.

Unfortunately, there's no a-priori way of determining the freezing point expansion/contraction, so the only way to determine if there is a difference is to measure it. I have not found any special precautions when it comes to freezing the material in storage manuals, which suggests to me that cold storage probably isn't an issue.

This probably doesn't fully answer your question, but unless someone has the density of glassed HAN propellant on-hand, I don't think you're doing to get a full answer.

A Master's thesis on HAN monopropellant

Article on hydrazine freezing in the Ulysses spacecraft

• wow! thanks for the carefully reasoned and well-sourced answer! – uhoh Aug 6 '19 at 4:10