Bill:
Anthony nailed it.
There is no one perfect blend of 400/200/90 in APCP. You must consider ALL
solid particles used in formula greater than 1-2% of total. Optimizing
particle size blend is a complex statistical probability equation, best
visualized with CAD or 3D solids modeling tool. The Katz book is good starting
point.
IMHO - The ISP differences between bi-modal and tri-modal AP for amateur HPR
are not worth the hype wrapped around them. Most of the tri-modal blends I
used where only Tri-modal to help adjust burn rate, or increase mix viscosity
to eliminate particle size settling/stratification. BTW – One size AP can be
also used with bi-modal metal powders in highly metalized APCP. Think about
it…J
Oh look a rabbit hole appeared? J
To make APCP particle sizing even more complex, a perfect packed size blend is
horrible to mix as there is no extra space in matrix for movement. You almost
do not want a perfect 100% packing density matrix, as you get reduced shear
thinning. Processing a perfect blend successfully requires complex vacuum
blending/casting equipment, and usually entails processing aids to reduce
friction during mixing. It is quite common for formulator to design max
packing density, and then look for best way to keep energy density high, and
improve processing with adjusted size mix.
As Chuck Rogers noted: AP particle sizing is not as simple as it seems. :o)
Something called 200um AP is actually a range of particle sizes with statistic
average near 200um. There is even an allowable percentage of particles that
are well outside of a ‘200um’’ range, depending on purchase specification!
Have seen bottom of a surplus 200um AP drum that had 20%+ broken AP fines in it
due to rough transportation. These size variations will wreak havoc on your
packing density and propellant characteristics. You ALWAYS want to
re-characterize your formula with each and every new batch of raw materials.
It is only way to ensure that raw materials are making same thing every time.
If you look up the government sourcing documents on AP (remember one was MIL
822667 as it plastered on every drum?), they are probably best definition for
what is expected inside a drum of 200um AP as government buys more than anyone
else.
If you want to climb further down AP size rabbit hole:
Not sure if still true, as I am out of HPR game now, but; Majority of AP
bought and sold by amateur community used to be government surplus (or surplus
from a contractor working for government/aerospace contracts)?
In 20+ years I made HPR motors, personally inspected and processed hundreds
drums/totes of surplus AP. Everyone of small lot chemical distributors that
HPR folks use are all buying surplus AP from same (2) chemical brokers that
buy/sell AP in USA, or they have cash to buy large lots directly from exact
same government surplus auctions.
Back when my partner was alive, AMW used to split lots with one of brokers
occasionally. We had so much in inventory at times, one particular Florida
hurricane flooded 2 shipping containers holding 35,000+ pounds of AP,
destroying most all of it. L When a crazy dude (who has repeatedly threaten
to kill me) started funding AP buys with my partner and broker buying large
lots to supply amateur crowd, I stopped participating in surplus purchase
process and sort of lost track of it. I hear he actually resorted to buying
directly from the mfg at one point, as surplus market has disappeared? Good
for community.
One problem with surplus AP market that you need to watch out for: What is
really in the drum?
AP was sold ”surplus” for a reason.
Much of surplus AP is sold as remnants left over after a contract is completed,
years later. When Thiokol shuttle booster program used to sell surplus lots,
it was typically 50+ drum lots. When smaller missile programs sold surplus it
was 2-3 drums, or less. Reason being, they always buy enough to finish
contract from same mfg lot to keep same performance.
Another (scary) reason AP is surplus; because it failed to met particle size
specifications. This size failure could be as shipped from mfg, or could be
due excess/poor storage conditions that allowed excess clumping or particle
size changes. Chemicals are not “fun” to return to mfg nor cheap to dispose
of properly; so many times these out of specification materials get sold in
surplus chemical market “AS IS” for alternate uses.
Believe it not, when surplus AP was really cheap in 90’s due bi-annual Thiokol
AP surplus sales; one ammunition company would buy every AP lot listed below
their trigger price. They would separate AP into perchlorate acid and ammonia.
The separated perchlorate was cheaper than buying it new from mfg, and ammonia
was sold for fertilizer. We could always tell then perchlorate acid pricing
was lower than re-processed surplus AP, as there was less competitive bidding
on surplus lots. J
As Anthony noted: AP in USA is mfg by one monopoly mfg. L I hear pricing has
gone crazy lately?
Maybe this will help?
AP is also produced in China in large quantities. “Surplus” pricing became
available from China ~2010. I am aware of at least one of the high volume
fireworks folks have imported full containers of oxidizers from China that
included AP due US pricing increases. The material I saw was high quality with
tight particle size distributions, and performed like any other AP I used
before?
The challenge with China sourcing is not transportation costs, but logistics.
If you are not routinely shipping containers from China, it is very hard to
find a reliable broker to consolidate, generate customs paperwork in
quintuplicate , and watch over a successful shipment. Everyone in process has
hand outstretched to help you move faster through the system for higher cost.
Most of the big fireworks operations actually have a sourcing person in China
for many weeks at a time, that manages broker(s) & suppliers; to deal with
crazy international logistics issues. For anyone who needs large volumes of
AP, it might be time to call some fellow danger lovers from PGI, and see if
they can help with lower cost China AP sourcing? Just an idea from an old
man. Ignore me please.
OK, rabbit hole is probably too deep. Thanks for reading some ancient history.
YMMV, Stay safe, and Cheers!
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Anthony Cesaroni
Sent: Saturday, August 04, 2018 9:51 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Trimodal AP
This is very dependent on the size, distribution and quantity of other solids
in the formulation such as aluminum powder as well as ballistic modifiers such
as SSFO, ALO2, Guignet green etc. If you are using a surfactant, that also
comes into play. Most APCP is more than bi-modal or trimodal in terms of
solids. Only oxidizers seem to be referred to in this regard.
Most workhorse commercial propellants are bi-modal as far as oxidizers are
concerned BTW. This is primarily for mechanical properties, ballistics and
economics. 200 mu is the baseline feedstock with everything else being site
milled from that. 90 and 400 have expensive surcharges. Those sizes are
sometimes used in specialty formulations such as 90/10/1 for example or in the
case of 400, gas generators, slower burn rate propellants, delays etc. Until a
few years ago, 600 mu and 50 mu varieties were also produced. AP was also FDA
approved for use in cattle feed. It causes thyrosis so the bovine fattens up
quicker for market but I digress.
A good but expensive reference is “ The Handbook of Fillers for Plastics” by
Harry S. Katz. One method I experimented with many years ago to formulate
multi-modal start points was CAD believe it or not. It worked surprisingly
well.
Best.
Anthony J. Cesaroni
President/CEO
Cesaroni Technology/Cesaroni Aerospace
<http://www.cesaronitech.com/> http://www.cesaronitech.com/
(941) 360-3100 x101 Sarasota
(905) 887-2370 x222 Toronto
From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> On Behalf Of
William Claybaugh
Sent: Friday, August 3, 2018 11:51 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Trimodal AP
Any one know the optimum mix of 400 micron, 200 micron, and 90 micron AP?
Bill