I came
across a page some years ago
detailing the use of sorbitol in rocket propellants in place of
sucrose. I then heard it wasnt as powerful and didn't even give it
another look... you could say I was a bit power hungry; and sorbitol
based rocket propellant didn't seem powerful enough to me at the time.
Then a few years later I was ordering some beer brewing supplies from a
vendor who also sold sorbitol at a very reasonable price (around $1.20
a pound) so I bought 3 pounds, and figured I would give sorbitol a try.
Upon receiving the Sorbitol, the first thing I did was to taste it.
Didn't taste quite like real sugar, but it was pretty damn good...
anyways,
back to rocket science. I found that the standard formula for KNSB
rocket propellant is 65 parts Potassium Nitrate to 35 parts Sorbitol. I
first tried making a half batch using 32.5 grams of KNO3 and 17.5 grams
of sorbitol, with an additional 2% (1 gram) Red Iron Oxide to act as a
burn rate catalyst. below are the two main ingredients in KNSB
propellant (Sorry; I forgot to include iron oxide in this picture).
The thing that most attracted
me to the process of making KNSB propellant was the fact that
I didn't have to use the recrystilization method that is typically used
for making sucrose based propellant; which takes a long time to do. To
make the KNSB propellant all that is needed is that you melt the
propellant at approx. 250° F ; I use a thermostatically
controlled electric griddle that I purchased at Wal-Mart for about $20,
and it works beautifully to melt the propellant, also it has a small
pour hole at the bottom, so you can just push the melted propellant
down the hole and into the casting tube you are using. So here are the
steps of the process:
Step
One: Measure out the KNO3 (65
parts), measure out the
Sorbitol (35 parts), and if you want to use iron oxide as a burn rate
catalyst add 2 parts Red Iron Oxide. Make sure you are using a scale
accurate to .1 grams; they can be found on eBay at very reasonable
prices just search "pocket scale". Then add the ingredients together in
a cup or bowl, and simply mix them together until homogeneous.
Step
Two: Then it is
time to add them to the electric
griddle (which is pre-heated to 250° F).
Above you can see the KNSB propellant melting on the griddle, this is
right after pouring it onto the griddle, so as you can see it melts
very quickly, and the great thing is that at this low working
temperature, there is very little chance of accidental ignition,
although you should still always wear a proper full-face shield, and
proper fire-retardant clothing. In the picture you can also make out
the small hole that I was talking about in the griddle, its absolutely
great for casting the propellant. now all that needs to be done is the
the mixture be stirred around until completely homogeneous on the
griddle, I use a cheap non-stick silicone spatula. After about 5
minutes you can see the propellant is now the consistency of a very
viscous fluid, much like molasses.
Then after about another 3 minutes of cooking, the propellant is ready
for casting (step three)
Step
Three: So now the propellant
is pushed over the small hole
in the griddle and allowed to fall into the casting tube(s) (in this
case, a cheap PVC tube, this batch will not actually be used in a
motor, so I didn't bother cutting a section of casting tube). Finally
the propellant is cored, by inserting a Teflon rod into the center of
the propellant mass while still hot (sorry, no picture). After 24 hours
the propellant is fully cured and is now workable, so the coring tool
may now be removed, and you are left with a perfect grain of KNSB
propellant. In a normal batch, I would use about two times this much
propellant to form multiple grains
Finally; here is a video of the grain burning uncontained:
Conclusion:
KNSB propellant is very easy to work with, much easier than any other
sugar propellant I have previously worked with in the past, so for now,
it is currently my favorite sugar based propellant. This propellant
also seems to
have a burn rate very similar to that of Sucrose based propellant, and
while not quite as powerful, it seems to be a very negligible about of
power loss.
