Hi Martin,
Back in 2007 I observed a similar graph (with dextrose rather than
sorbitol, and with bates grains rather than a core-burner). I then
undertook a long series of experiments and reported the results on this
list, but the archives from 2007 disappeared years ago.
Basically, the problem seems to be that ignition of the grain doesn't
happen the way I and most people seemed to think which was that as the
primer burns it ignites the propellant. Rather, it is a distinct
two-step process: the primer burns and *prepares* the propellant for
ignition by carmelizing it. Then, if conditions are right, whatever
propellant has managed to start burning (probably near the igniter) in
turn ignites the carmelized propellant.
This is well explained by Richard Nakka -- see
http://www.nakka-rocketry.net/ignexp.html and note the discussion
section at the bottom of the page.
Looking at your graphs, the clue is in the small pressure spike at the
beginning which is, in fact caused by the igniter and all of the primer
burning (as you've discovered). Then there is a sizable gap in time.
Then, if you're lucky, the rest of the propellant starts burning
vigorously producing the main portion of the thrust curve. So far, your
analysis is right on.
OK, so why doesn't the propellant ignite reliably?
As near as I could determine (by trying to "ignite" inert grains and
examining them post-ignition) it is ash from the primer that inhibits
the 2nd phase of the ignition. I found that
Nitrocellulose/charcoal/acetone, especially when applied in a thick
layer, created a lot of ash which acts as an insulator. Richard reminded
me back in 2007 that he used a *very thin* (0.005") application of
primer which was composed of 80/20 finely ground KN/Charcoal mixed with
70% isopropyl alcohol (sold as "Rubbing Alcohol").
I then performed some experiments with that and found that this approach
created far less ash, and, compared with the thicker application of
nc/c/acetone, ignition was far more rapid, uniform, and reliable.
So you basically need to ensure two things happen: (1) the primer burns
vigorously enough that it carmelizes the propellant surface and (2) that
it doesn't create so much ash that it inhibits the hot gasses from later
igniting the carmelized surface.
You may also find that ignition is aided by an igniter that burns longer
and more vigorously, the thinking being that it erodes whatever ash the
primer creates. You would have to be careful of overpressurization,
however, since the propellant in combination with the igniter may
produce more gas than the motor is designed to handle.
I hope this helps...
--Steve
On 10/22/2018 09:46 AM, M.C. Olde wrote:
Dear people,
For my master thesis I am characterizing both course and fine KNSB (KNO3-Sorbitol) propellants at various conditions (pressure/ grain initial temperature) as it is a readily available solid propellant that has not been characterized overtly much. In addition it would allow me to almost freely publish my test data/ propellant characteristics and other results which cannot be said of many commercial solid propellants or when collaborating with companies.
However during my experiments I have run into considerable issues with the fine (100 mu average KNO3) propellant, I cant get it to ignite while the course (350 mu average KNO3) propellant burns alright. I've tried most of the textbook solutions but currently I am completely out of ideas. I have the feeling that my current igniter charge burns too fast for the KNSB to properly start combusting. I'm looking for answers to for instance the following questions:
* What would be potential physical mechanisms that make the fine
KNSB more difficult to initiate than course KNSB?
* What igniter redesigns could I implement to give it a better
chance of ignition? (Other compositions/ different geometries/
slower burning suggestions?) - bonus points for reuse of current
hardware and non space-grade energetic materials such as BKNO3.
I have condensed my thoughts and a summary of the problem in a small Root Cause Analysis. I sincerely thank any suggestions in the right direction.
kind regards/ vriendelijke groeten,
Martin Christiaan Olde
/MSc student, DARE member
/
/Delft University of Technology/
