Sorbitol & Potassium Nitrate Rocket Propellant


   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).

Potassium Nitrate Powder, alongside Sorbitol Powder

     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.

KNSB Propellant mixed together

Step Two:  Then it is time to add them to the electric griddle (which is pre-heated to 250° F).

KNSB Propellant on the electric griddle

    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.

Molten KNSB Propellant

Then after about another 3 minutes of cooking, the propellant is ready for casting (step three)

Sorbitol Propellant ready to cast

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

KNSB Propellant fully cured with core

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.