Predicting non-standard grains with SRM.xls

Background information:

Hit Reload if you don't see motor burn animation image

These pages were written to learn and retain (leaky memory!) some principles in central/cylindrical cored grain performance.

Note on these pages I might use the term "Bates" grains a little loosely.
(The stricter description would be a center cored grain of appropriate length that burns from the core and both ends to give a neutral burn.)


When I started the 24mm sorbitol motor project I chose to pour the grains into the plastic autoclavable test tube caps because it made it easy to construct. The grain size is consistent yet can be purposely varied. The plastic propellant filled liners then slide into the paper casings and act as an ablative liner. Since the paper/clay motor casings can be made and cut to any length I like to think of these motors as flexibly modular.
The main goal was to make reliable functioning single-use low-power motors that are reproducible.
In order to attain this goal quicker I wanted to be able to sim the theoretical motors prior to construction so as to shorten the learning curve and hopefully reach successfull motors sooner than using trial and error. The seat of the pants approach never worked well for me.
I was encouraged by the SRM.xls file from Richard Nakka's site. It is a great Excel based sugar motor prediction program.
It allowed me to quickly attain a working motor with a minimum of problems. But when I tried to fine tune the accuracy of the sim thrust vs time curve to the static results a problem arose. These grains are not neutral burning Bates grains. For one thing they are usually inhibited on one end and not inhibited on the other. Another nonstandard approach was that the grains could be of different lengths in one motor.
The standard use of SRM.xls has the multiple grains (N) the same size/length as each other. It also has all the grain ends inhibited (Ends=0) or all ends not inhibited (Ends=1).
To try to sim these non-Bates grain motors I tried to use BurnSim from Greg Deputy's site.
This program allowed multiple grains in one motor each with different lengths and different inhibition strategies. Nice.
However I had troubles correlating the static thrust curve to the Burnsim predictions.
I tried fiddling with the Burnsim "a" and "n" values but with little success.
One problem was/is my newbieness :-) and perhaps another problem was that Burnsim did not allow more than one "a" or "n" value for a propellant at different pressure ranges. In other words it assumed a straight logarithmic relationship of burn rate to pressure.
Sorbitol is described in Richard Nakkas site as having an "inverted mesa" curve of burn rate versus pressure.

2007 Update: Greg has a beta version of Burnsim now that does allow for varying ballistic parameters.

I decided to find a simple way to make SRM.xls work for my nonstandard grains.


At the top of the page, and below, is an animation that shows the presumed burn sequence in one of my 24mm "two" grain motors.
Notice that the nozzle end grain is inhibited on the end near the nozzle and not inhibited on the other (mid motor) end
The second grain has a shorter "dead end" core and the one end near the other grain (mid motor) is not inhibited. The "propellant" at the uncored section of the second grain becomes the delay/smoke tracking grain. It burns at ~1 atm giving a delay of 6 to 7 seconds.

The animation is close to real time in that the pressurized thrust burn time is around 0.8 seconds and the delay smoke burn time at 1 atm is about 6 seconds.

Now let's take a look at a static thrust curve of the motor described in the above animation/description:

The thrust curve in lbs thrust vs time in seconds:



Notice the burn is somewhat progressive.
This would be expected because the first grain is inhibited on one end and a bit long for a Bates.
The oscillations are most likely due to the spring in the weight scale used.

Now... on to the experiments for using SRM with these motors:
Click here to see my approach to adjust SRM.xls sims to these non-standard "Bates" grains







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