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In the recent Spaceflight 101 article Rocket Burn postponed for NASA’s Juno, to remain in elongated Jupiter Orbit until December it says:

Juno uses a bi-propellant main propulsion system with Hydrazine fuel and Mixed Oxides of Nitrogen as oxidizer while the craft’s Reaction Control System uses Hydrazine monopropellant.

I looked elsewhere there but couldn't find anything further. According to this article in Spaceflight Now the Leros 1C Liquid Apogee Engine uses Hydrazine and Nitrogen Tetroxide (N2O4), so I wonder, what are the "mixed oxides of nitrogen?

I have just read here that the Leros 1B uses Hydrazine and MON-3. Does that stand for "Mixed Oxides of Nitrogen-3"?

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    $\begingroup$ Do you mean MON-3 instead of MON-B? See section 2.3.2.1 of this document (USAF handbook on nitrogen-based oxidizers, 12 MB download) $\endgroup$
    – Andy
    Oct 18, 2016 at 12:28
  • $\begingroup$ @Andy I'm not at a reasonable internet connection and it's not loading. 3 Indeed. My question is about Juno, were there options for different oxidizer combinations, or is it pretty much fixed by the selection of the engine? $\endgroup$
    – uhoh
    Oct 18, 2016 at 13:21
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    $\begingroup$ I'm not sure, but I would guess most hypergolic engines would be tolerant of some variation in oxidizer composition. On an expendable launcher you might be more inclined to use NTO (N2O4) for maximum specific impulse without regard for corrosion, while on a spacecraft that needs to maintain tank integrity for years you'd prefer a MON formulation. I also suspect that a lot of engines that are casually described as using NTO are actually using MON. $\endgroup$
    – Russell Borogove
    Oct 18, 2016 at 14:14
  • $\begingroup$ Actually MON,10% NO, gives slightly higher ISP than straight NTO(N2O4) according to Aerojet's Propellant Performance and Properties of liquid propellants. KM $\endgroup$
    – rocketmentor
    Nov 29, 2016 at 3:54

2 Answers 2

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MON according to Wikipedia:

Mixed oxides of nitrogen (MON) are solutions of nitric oxide (NO) in dinitrogen tetroxide/nitrogen dioxide (N2O4 and NO2).

The addition of a small amount of nitric oxide (1%-10%) makes the oxidizer less corrosive, but slightly less powerful as well, and changes the freezing point of the liquid. MON3 means 3% nitric oxide by mass, which is a common formulation for American engines.

From Clark's "Ignition!":

NO is an extremely effective freezing point depressant for N2O4. ... G. R. Makepeace and his associates, at NOTS, were able to show, in 1948, that 25 percent of NO would bring the freezing point of nitrogen tetroxide down below the required -65 F, and that 30 percent would depress it well below the magic -100 F. However, the vapor pressure of the latter mixture at 160 F was unacceptably high, about 300 psi.

(The freezing point of plain N2O4 is 16 F; Clark says -9 F, but this likely varies with contaminants. Military services wanted reliable storable fuels for all climates.)

Several agencies tried the mixed oxides of nitrogen (MON-25 or MON-30 or whatever, with the number designating the percentage of NO in the mix) with various fuels, and discovered that it was more difficult to get a good performance with MON than with neat nitrogen tetroxide. ... For this reason, and because of its high vapor pressure, investigators turned away from MON for some years.

In full, the usual modern hypergolic oxidizer is "an equilibrium mixture of dinitrogen tetroxide and nitrogen dioxide with an admixture of nitric oxide"; it's no wonder this gets abbreviated to "mixed oxides of nitrogen" and then to MON.

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  • $\begingroup$ Thanks! Is the change in freezing point good or bad or not enought to worry about? $\endgroup$
    – uhoh
    Oct 18, 2016 at 13:29
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    $\begingroup$ OK I can read it now. It looks like the MON3 has a bit lower freezing point than pure N2O4, don't know about the NO2/N2O4 mixture without the NO. $\endgroup$
    – uhoh
    Oct 18, 2016 at 13:54
  • $\begingroup$ "Pure N2O4" doesn't exist; it's an equilibrium mixture with NO2. As temperature increases, the reaction favors more NO2; lower temps leave more N2O4. Presumably a lower freezing point is slightly beneficial in rocket tankage. en.wikipedia.org/wiki/Dinitrogen_tetroxide en.wikipedia.org/wiki/Chemical_equilibrium $\endgroup$
    – Russell Borogove
    Oct 18, 2016 at 13:59
  • $\begingroup$ The Wikipedia MON article says: "The freezing point of pure nitrogen tetroxide is −9 °C (16 °F)..." perhaps it approaches N2O4 as it gets close to freezing. There's a YouTube for everything! youtu.be/j1ALRRos-AA $\endgroup$
    – uhoh
    Oct 18, 2016 at 14:08
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    $\begingroup$ Yeah, but "pure" could mean either "nothing but NO2/N2O4" or "nothing but N2O4 because we've reached the extreme of the equilibrium mixture" -- IANAchemist so I'm not sure which. $\endgroup$
    – Russell Borogove
    Oct 18, 2016 at 14:17
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The use of MON, as distinct from N2O4, arose because the latter can cause stress corrosion cracking (SCC) of titanium alloys. This was first observed in the 1960's during the development of the Apollo systems. The mechanism for SCC is believed to relate to the presence of oxygen in N2O4. Addition of nitric oxide (NO) to the propellant eliminates O2 by reacting with it; 2NO+O2----->2NO2

The work of A.Z.Conner at the Hercules Corporation played a large role in sorting this out.

For Apollo, a mixture of 1% nitric oxide in N2O4 was used and this was just referred to as "Green NTO" (pure NTO is reddish brown) but when the Shuttle came along the decision was made to use 3% NO and the term MON3 came in to being (Mixed Oxides of Nitrogen, 3%NO). Adding NO to N2O4 does also depress the freezing point and MON10 and MON25 have been used where low temperature operation is required.

For a good overview of the problem:

  • "Titanium Alloy Pressure Vessels in the Manned Space Programme", R.E. Johnson, paper presented in "The Science, Technology and Application of Titanium", Proceedings of an International Conference organised by the Institute of Metals, London , May 1968.

And for the detailed chemistry:

  • A.Z.Conner et al, A Study of the Chemical Reactivity of Nitrogen Tetroxide with Titanium Alloys". Final Report, February 1969, NASA Contract No 8-21207.
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    $\begingroup$ There are a number of reports in the literature by Conner, all dated mid-1960's. I have them but I will need a bit of time to dig them out. The SCC problem caused a huge panic. The failed tanks looked like Easter Eggs smashed by a hammer. I did a huge lit search on NTO/MON back around 1988 when some of this stuff was not quite so obscure. $\endgroup$
    – Barrie Mellor
    Apr 20, 2019 at 23:56
  • $\begingroup$ Ok, the best overview of the SCC problem; $\endgroup$
    – Barrie Mellor
    Apr 21, 2019 at 0:04
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    $\begingroup$ Ok, the best overview of the SCC problem; "Titanium Alloy Pressure Vessels in the Manned Space Programme", R.E. Johnson, paper presented in "The Science, Technology and Application of Titanium", Proceedings of an International Conference organised by the Institute of Metals, London , May 1968. The detailed chemistry; A.Z.Conner et al, A Study of the Chemical Reactivity of Nitrogen Tetroxide with Titanium Alloys". Final Report, February 1969, NASA Contract No 8-21207. $\endgroup$
    – Barrie Mellor
    Apr 21, 2019 at 0:11
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    $\begingroup$ Thanks, that's fine. Somewhere, I think I have the Conner reports on microfiches. My recollection is that the London Confernce came out as a book. $\endgroup$
    – Barrie Mellor
    Apr 21, 2019 at 0:15
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    $\begingroup$ Also re Ti/N2O4, I have detailed literature review (about 20 cited papers) I produced on this in 1993, in hard copy. If any one is keen, may be I could scan and upload somewhere. $\endgroup$
    – Barrie Mellor
    Apr 21, 2019 at 0:21

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