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From: dietz@cs.rochester.edu (Paul Dietz)
Newsgroups: sci.space.shuttle
Subject: Re: hydrazine
Message-ID: <1990Nov9.193634.25572@cs.rochester.edu>
Date: 9 Nov 90 19:36:34 GMT
References: <7565@eos.arc.nasa.gov> <3250005@hpnmdla.HP.COM> <6015@mace.cc.purdue.edu>
Reply-To: dietz@cs.rochester.edu (Paul Dietz)
Organization: University of Rochester Computer Science Dept
Lines: 36

>Under the right conditions (1) heating in the presence of a catalyst, or
>(2) in the presence of a strong oxizizer and a catalyst, it decomposes into
>it's components, releasing a lot of hot gas.  
>
>Typically, monomethyl 
>hydrazine (MMH),  which is formed by replacing one of the H above with a CH3,
>is used in the first mode.  This is very useful for control thrusters 
>because it is simple, reliable, and reasonably clean.  

I believe hydrazine is mostly used as a monopropellant, I believe.
MMH is used with in bipropellants.  MMH and UDMH can poison the
catalyst in hydrazine monopropellant engines.

>A combination of MMH and unsymetrical dimethyl
>hydrazine (replace two of the hydrogens on the same side with CH3's),
>along with nitrogen tetroxide is used in the second mode.
>This is very useful for higher thrust applications, like the Titan core
>or the Shuttle OMS.

Shuttle OMS and RCS rockets use MMH, not a MMH/UDMH mixture.

>These types of systems are popular because
...
>2)  They aren't explosive per se like hydrogen is

Not really.  Not only are they flammable at a wide range of
concentrations (MMH's flammability range is 2.5% to 98% by volume in
sea level air), but they can spontaneously ignite when spilled on
surfaces or on cloth.  They also can explode all by themselves, as
they have positive heats of formation (as you would expect of
monopropellants).  Hydrazine decomposes explosively when subjected to
shocks at temperatures as low as 367 K. MMH is less sensitive, but
more toxic.

	Paul F. Dietz
	dietz@cs.rochester.edu