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Masten Space Systems has a proprietary hypergolic bipropellant called MXP-351 intended for use in the XL-1 lunar lander. We seem to know the following information:

Since it's proprietary, it's probably impossible to pin down the exact composition, but can we narrow down the possibilities? I assume NTO, nitric acid, and perchlorates are too toxic, so the possibilities for oxidizer seems to be really limited-- maybe hydrogen peroxide?

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    $\begingroup$ Answer(s) to Does the Soyuz spacecraft chemically stabilize its hydrogen peroxide (H₂O₂) to remain ISS-dockable for 200+ days? suggest that when used as a monopropellant hydrogen peroxide should not contain stabilizers that could potentially allow it to be storable for "a few years" the reason being "because any impurities would deactivate the catalyst." However that doesn't seem to necessarily exclude the use of stabilizers when peroxide will be used in a bipropellant hypergolic combination. $\endgroup$ – uhoh Apr 11 at 11:02
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    $\begingroup$ Also: Hydrazines and NTO are semi-explicitly ruled out, and the reactants have low vapor pressure. Part of the problem with pinning this down is that "non-toxic" and "non-cryogenic" are marketing terms, while 322s ISP is suspiciously high for even normal hypergols.¶ Sigh. I hate it when companies do this. Like, your idea is not valuable. Your engineering and design is valuable. It's probably not even your idea anyway—chances are the likes of John D. Clark were playing around with it in the 1950s. $\endgroup$ – imallett May 2 at 18:04
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Until Masten Space Systems reveals the exact composition of the propellants, we can only speculate on the combinations they used. Based on what you have presented in question we can rule out options one by one, but we cannot be 100% sure especially for fuels because if we miss oxidizer than we will miss fuel to.

The smoking gun here is a sentence “prepared propellants sitting in open containers don't require a respirator”. It's a phrase often used by authors to promote propellants which may be toxic but with low vapor pressure (don’t fume), therefore they would not poison workers by inhalation even if they work without protective equipment. It’s often used as advantage for HTP, although fumes are toxic/irritant, compound has low vapor pressure therefore will cause no harm to workers.

When we speak about store-able non-cryogenic hypergolic oxidizers there are 4 options:

  • White fuming nitric acid (WFNA),
  • Red fuming nitric acid (RFNA),
  • Dinitrogen tetroxide (N2O4,)
  • Highly concentrated hydrogen peroxide (H2O2), or high test peroxide (HTP)

First two acids have fuming in their names suggesting that they have very high vapor pressure. At the same time fumes are toxic and irritant. Dinitrogen tetroxide is liquefied gas which boils at 21 °C and very toxic too. First three oxidizers are actually in practical use and many studies are searching for their replacement. Therefore, I find only hydrogen peroxide as viable option to satisfy all requirements from question. Also Masten Space Systems confirmed that they use same precautions for handling their propellants as for handling HTP.

HTP is a good choice due to its moderate toxicity, high density, low vapor pressure, and exothermic decomposition to water and oxygen. Pure hydrogen peroxide is most stable, but has freezing point −0.43 °C and there is low confidence that it could be stored safely without additives for few years. Because catalytic decomposition is not in consideration, I think that small amounts of freezing depressants and stabilizers would improve work in space, provide stability for years without significant impact on vacuum ISP. To achieve short ignition times many studies show that concentration of HTP should be in 94-98% range. It is possible that sodium salts are used as stabilizers (sodium stannate together with sodium nitrate) combination known to significantly increase storage life of HTP. Persulfates, ammonium nitrate (AN) and ammonium dinitramide (ADN) could be used for deep space missions as possible freezing depressants for HTP, although they increase ignition times and lower ISP because of poorer oxygen balance. Regenerative cooling of rocket engine is out of question if AN and possibly ADN are used because they can decompose explosively. Considering that Masten Space lunar lander is not designed to survive lunar nights, several % of water with proper selection of storage materials of 1 class compatibility, proper passivation of these materials, and good thermal isolation may be just fine.

As far of fuels which are hypergolic with HTP, there are two types: catalytically driven fuels and reaction driven fuels. In first, catalyst dissolved in main fuel decomposes hydrogen peroxide into oxygen and water, and then hot oxygen reacts with the main fuel. The list of possible catalysts: enter image description here

In reaction driven fuels the hydrogen peroxide reacts directly with the fuel or reducing reagents which are dissolved in fuel. Reducing agents are high energy dense and increase energy output of primary fuel but are generally poorly soluble. Also solutions are usually transparent liquids. The list of reducing reagents:

enter image description here

Theoretical ISP of MXP -351 is 322s which is very close to MMH/NTO ISP (96%). This is pretty high ISP for hypergolics and could not be achieved easily with HTP. The fuel must have at least one of following requirements: high hydrogen content, high positive enthalpy of formation (achieved through nitrogen energy carriers N-N, N3 or high strain energy between carbon atoms), or hydrocarbons of B, Al, Li, Si which are energy dense when combust in oxygen. Also fuel must not degrade when reagents are added during storage and in case of reaction driven fuels they shouldn’t be moisture sensitive. There are several proprietary fuel blends which I found in research papers that are close to this level of ISP:

  • Dimethylacetamide (or DMA) / Monoethanolamine (or MEA) mixed with NaBH4 gives the specific impulse equal to 321,56s, in terms of theoretical performance according to this paper. Both fuels have low vapor pressure and fuel blend has ignition delay time of 14ms with 98% HTP. Same source claims that another blend MEA/Piperidine(50/50) mixed with NaBH4 has ISP close to 320s but Piperidine is more toxic than previous fuels and has higher vapor pressure so this fuel blend can-not strictly fulfill all requirements. Both fuel blends are stable during storage and didn’t show signs of degradation. enter image description here

    • Stock 3 (proprietary fuel blend) – Diethylenetriamine (or DETA) / Tetrahydrofurane (or THF) mixed with NaBH4. This fuel blend showed very good compatibility with reducing agent without degradation after four months of storage. Ignition delay time is 9ms with 98% HTP. According to this source Stock 3 / HTP can theoretically reach 96.7% of MMH/NTO ISP and surpasses it in dISP. enter image description here
    • Ionic liquid 1-buty-3-methylimidazolium acetate (or BMIM Ac) with 10% Manganese Acetate Tetrahydrate (MAT). This proprietary blend is named BMA-10. Ignition delay time with HTP is 10ms, but ionic liquid doesn’t mix with HTP very well so ignition is not reliable. To improve ignition 10-20% of ethanol is added to BMIM Ac and new fuels are named BMA-9 E10 and BMA-8 E20. According to this source BMIM Ac has ISP of 322.1s with HTP. Adding ethanol up to 20% improves ISP but increases ignition delay times to 30ms. enter image description here

    • Dimethylaminoethylazide (or DMAZ) catalyzed with 8 % anhydrous Cobalt Butyrate. This proprietary fuel blend is called HF-57J and according to this source it comes very close to ISP of MMH/NTO if used with 96-98% HTP. Ignition delay time is in order of 1-2ms which is considered excellent. Also according to same source fuel blend is not prone to degradation during storage.

And at the end, there has to be space left for Masten Space Systems and their inventiveness and creativity, and that they did not follow any of these recipes, but that they came up with their combinations and therefore rightly do not want to reveal their compositions.

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  • $\begingroup$ Wow what a thorough, instructive, in-depth and well-supported answer! $\endgroup$ – uhoh May 25 at 23:39

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