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Can Monomethylhydrazine and Unsymmetrical dimethylhydrazine be used one in place of another for the class of the existing orbital maneuver engines (with reduced performance may be)? Such as Shuttle's OMS (uses MMH), Soyuz DPO (uses UDMH) and ATV's main engine (uses MMH).

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    $\begingroup$ Both will kill you very quickly and painfully. So in that sense, yes. As propellants, not sure. $\endgroup$ – geoffc Aug 12 '13 at 0:43
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    $\begingroup$ A great and really funny story of propellants research is John D. Clark. Ignition! (1972) See, in particular, the "Hunting of the Hypergol" chapter. In short, it's about density, freezing point and thermal stability. Don't forget the venerable Aerozine. So, unless you use the fuel for cooling the nozzle, you should be fine substituting one for another, provided you keep the fuel warm enough to be liquid. $\endgroup$ – Deer Hunter Aug 12 '13 at 7:30
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    $\begingroup$ ...However, such substitutions generally necessitate redesign and retesting of much more than an engine or tanks: production, launch complex fueling sensors... Not worth it, am afraid. $\endgroup$ – Deer Hunter Aug 12 '13 at 18:49
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There are several considerations that lead to selecting a particular hypergolic fuel:

  • $I_{sp}$ specific impulse
  • $\rho$ density
  • $T_f$ freezing point
  • thermal stability (rate of decomposition vs. temperature)
  • vapor pressure
  • chemical compatibility (rate of corrosion)
  • presence (or absence) of well-characterized economical production capacity

The usual bi-propellant pairs have the following $I_{sp}$ (in seconds):

                    At 1    In 
                    atm    vacuum

Dinitrogen tetroxide ($\require{mhchem}\ce{N2O4}$)

MMH                 288.5   341.5
Hydrazine           292.0   343.8
Aerozine (50/50)    288.9   341.7

Inhibited red fuming acid III-A (83.4% $\ce{HNO3}$, 14% $\ce{NO2}$, 2% $\ce{H2O}$, 0.6% $\ce{HF}$)

Hydyne
 (UDMH/DETA 60/40)  269.2   318.7
MMH                 274.5   324.3
UDMH                272.2   322.1

Inhibited red fuming acid IV, HDA (54.3% $\ce{HNO3}$, 44% $\ce{NO2}$, 1% $\ce{H2O}$, 0.7% $\ce{HF}$)

Hydyne
 (UDMH/DETA 60/40)  274.4   325.2
MMH                 279.8   330.8
UDMH                277.4   328.6

Source: https://en.wikipedia.org/wiki/Liquid_rocket_propellants

DETA = $\ce{NH{(C2H4NH2)}_2}$

UDMH = $\ce{{(CH3)}_2 NNH2}$

Hydrazine = $\ce{N2H4}$

Density:

Hydrazine            1010 @ 20 deg. C
Hydyne                855 @ 15 deg. C
MMH                   878 @ 20 deg. C
UDMH                  789 @ 15 deg. C

Source: Modern Engineering for Design of Liquid-Propellant Rocket Engines, Huzel, Huang. 1992. P.21.

I heartily recommend a great and really funny story of propellants research - John D. Clark. Ignition! (1972). See, in particular, the "Hunting of the Hypergol" chapter.

Substituting one fuel for another leads to complications:

  • ballistics part of the design changes;
  • you have to rethink thermal limits imposed on the tanks;
  • MMH can be used to cool the nozzles due to higher thermal stability, UDMH cannot;
  • launch infrastructure may have to be adapted (different densities and thermal brackets => changes in procedures and sensors)
  • industrial production capacity cannot be switched to the new fuel at once
  • re-certification is a pain

All in all, an engine/propellant combination is a design unit, and you won't like changing parts of the combination without much thought and effort.

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