James:
Glad I can help. If you get near Purdue University in Indiana, hit the
library. They have had active 'rocket' programs since 60's and have most of
old AIAA/JANNIF in stacks.
IME - Arrhenius equation extrapolation of service life is reasonably accurate
for PU binders, even when thermal cycled. Temp cycling was handled by
simulating both extreme temperatures. Usually find actual measured values
between the two curves?
What changes the service life results is when cycling of temperature creates
stresses that exceed the bond strength of polymer to filler or composite
mixture to the liner; creating extra surface area that changes a rocket motor
into a bomb upon ignition. The challenge I found in small hobby size motors is
always the polymer to filler bond.
Been out of the game awhile, but DOT used to require certified HTPB propellants
to have 10 year service life. This data point is not officially monitored by
DOT. And never seen DOT ask for test data on a new product? :)
Out of curiosity, cut apart some 8-10 year old motor grains from my earliest
AMW development motors for destructive analysis. Did Instron pull testing on
dog bone samples. Results align with your inspection and were as expected? I
used combination of perchlorate and nitrate oxidizers for color generation. In
early days tested several different wetting agents to improve filler bonding,
and had some old data on what worked best. Got best results with combination
of amine and ethyl-silane. Age samples showed a much larger gap between good
and once marginal - now bad; bonding agents. Testing showed any propellant
exposed to moisture showed lower strength due weak bond on 'wet' nitrates. The
large diameter perchlorate showed slight adhesion degradation due moisture,
with filler dropping out of matrix during testing. All of the samples properly
stored in a moisture barrier bag with desiccant; only showed slight PU
degradation resulting in crack formation during pull, despite a higher strength
due loss of plasticizer. One strange result? It seemed like the moisture
reduced the plasticizer leech out, as' wet' sample PU was not as strong as dry
samples? None of grains show evidence of liner debonding, even the 5.5" OD
grain.
Also put some of these same aged motor grains on test stand. The nice part was
despite a stiffer PU; the burn time and thrust curves were within 10-15% of the
original test data averages (typically with a slightly faster burn rate). I
have since flown several 15+yr old hobby motors and have yet to have a failure
with anything stored in 0-110°F storage.
Cheers!
On November 30, 2021 at 7:33 AM J P <james.padfield@xxxxxxxxx> wrote:
Jim, many thanks for that advice!
Unfortunately, where I work now doesn't have good access to a library.
The university I used to work at mayhave access to some of the old
AIAA/JANNAF, and I get back there occasionally since I still work on a few
projects with them, and get asked to give guest lectures sometimes. So I may
be able to find something there. I'll also look into the other missile
systems you mention.
Yes, the tactical missile motor compositions that use RDX or HMX are
closer to my PBX compositions, however my understanding from the literature
that I have read is that the predominant mechanisms of ageing are (i)
plasticiser loss, and/or (ii) oxidation of the PU binder (most likely, IMO,
probably a combination of both). Ageing of the nitramines isn't usually a
factor in ageing of the composition (as I understand - I may be completely
wrong!). In fact we just pulled some bombs loaded in PBXN-109 (RDX, Al,
HTPB-IPDI, DOA), after 15 years in the magazines, and the RDX doesn't appear
to have aged at all. The mech props of the PU though have changed - it has
become significantly stiffer, less flexible. More or less what we expected,
but it is nice to actually get our hands on the samples and confirm it!
Annex C to AOP-46 has an interesting discussion that uses the Arrhenius
equation to relate accelerated ageing studies to long-term storage life,
which (if this kind of thing interests you) you may find useful, if you
weren't already aware of it. It's only really applicable to constant
temperature ageing/storage though, applying it when the temperatures are
cycled is a little trickier...
Thanks again,
James
On Mon, 29 Nov 2021 at 17:57, Jim Rosson <jmrosson@xxxxxxx
mailto:jmrosson@xxxxxxx ;> wrote:
> >
James:
You want solid propellant service life data? I might be able to
help:
See if you get your hands on a copy of text book: Volume 170 of
Progress in Astronautics and Aeronautics, title Tactical Missile
Propulsion. Chapter 5 has treatise on service life predictions .vs actual.
Can also find articles in old AIAA/JANNAF journals from many
rocket/missile programs service life studies. Most of these antique
publications are not online, even if you are member of organization; so
might need to visit a decent science tech college library to find them.
Look for dedicated studies on Polaris, Minuteman, Titan, and Shuttle SRB
solid booster motors. These studies usually detail the polymer blend,
oxidizers, and additives.
The amateur community focus is on "safer" HTPB polymer system, and
leave the DOT 1.1 materials to experts. Regardless, Tactical missiles often
use HMX/RDX modifiers that might be more interesting to you? Couple
current missile programs like Aim-9 Sidewinder and AIM-120 AMRAAM; have
published service life studies. Couple of vehicles in the Nike missile
project https://en.wikipedia.org/wiki/Project_Nike had several service life ;
studies published. There are also some NATO member and other international
service life reports on non-US missile systems. These are all hiding in
same old AIAA/JANNAF periodicals. Some of newer declassified articles
published since 2000 can be found online.
Another source of solid propellant service life data is gas
generators used in automotive airbags, and aircraft exit slide inflation.
They used different oxidizers to lower flame temperatures, but they use
polymer binders that can be useful comparison. Have to hunt around the
automotive/aircraft tech journals for the papers, but they have published
extensive measured .vs. predicted service life studies at command of US
federal gubermint and NHTSA/FMVSS regulations.
Cheers!
Jim
>