[AR] Re: Stopping/restarting solids
- From: Terry McCreary <tmccreary@xxxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 27 Mar 2020 19:39:01 -0500
On 3/27/2020 5:43 PM, Henry Spencer wrote:
On Fri, 27 Mar 2020, ken mason wrote:
Not the same mechanism, Keeping F&Ox apart is different than solid
propellant which already has F&Ox intimately mixed.
But combustion still takes place largely in the gas phase, so things
introduced into the gas could still have an effect. Halon does *not*
separate fuel from oxidizer, it chemically clogs up the combustion
reactions themselves.
Pardon my remarks below, most of you probably know this stuff already,
but I can't help it, taught college chem in one place or another from
1976-2018. (Elder daughter once commented rather ruefully, "Dad's a
teacher... *anywhere* is the classroom." :-))
Virtually every chemical reaction occurs as a series of what are called
elementary steps, each step involving collision or decomposition of one
or two (rarely three) species---molecules, atoms, ions, free radicals.
The products of one step are involved in a subsequent step unless
they're final products of the reaction. The sum of the elementary steps
is the overall reaction. (Rocket significance: possibly the most common
example of an un-catalyzed vs. catalyzed mechanism used in chem
textbooks is hydrogen peroxide decomposition, alone and in the presence
of iodide ion.)
Often one or a few of the steps is significantly slower than the others.
That step(s) is the rate-determining step. Speed it up or provide an
alternate, faster step with a catalyst and the reaction goes faster.
Throw in a 'poison' (like Halon) that slows the step and the reaction
slows or stops completely.
One of the more important (rate-determining) steps in AP combustion is
the dissociation of solid NH4ClO4 into gaseous ammonia and perchloric
acid (HClO4) vapor--you'll see references to "A/PA vapor" in the
literature. It's an *endothermic* process, part of the reason the stuff
can be so hard to ignite. Those two species migrate away from the
surface, then undergo all sorts of other collisions.
Even simple fuels burn by complicated multi-step reaction chains, not
in a single step, and partially-burned products of one fuel molecule's
combustion catalyze combustion of the next one. The details are quite
complicated, and not too hard to interrupt if something grabs hold of
an intermediate product and won't let go.
Quite complicated indeed; a proposed mechanism for one combustion
reaction--I don't recall which one, octane maybe?--involved over 200
elementary steps.
What probably *would* be a problem with Terry's idea: while
combustion takes place largely in the gas phase, the key part of it
takes place not far above the surface, and the wind is blowing
*outward* from the surface as the solid decomposes. So even though
you don't need much Halon to interrupt combustion, getting even a low
concentration of it into the right place might be difficult.
True. It depends in part on whether there are other rate-determining
steps that occur farther away from the surface, and might be subject to
poisoning.
Quiz on Monday. ;-)
Best -- Terry
--
Dr. Terry McCreary
Professor Emeritus
Murray State University
Murray KY 42071
Other related posts: