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It should have similar characteristics to sorbitol, and perhaps even have a slightly lower burn rate to boot. I expect xylitol to be similar to sorbitol in that it shows an odd burn rate behavior, a very sharp inverted "V" thrust curve. Fortunately, Chuck Knight did extensive research on sorbitol, and seems to have solved the sharp peak problem. Mr. Knight has his information hosted on Nakka's site.
Sorbitol is fairly expensive, and a little hard to find in quantity. Xylitol is about as expensive, but I did find a company on the web that sold large quantities at reasonable prices, Xylitol For You.com. Very pragmatic name isn't it? Update Note: I ordered the xylitol from Xylitol For You.com, they shipped the same day I ordered it, wonderful service. The 10 pound bad was $27.50 plus $8.90 shipping.
One of my goals is to get a reduced burn rate with a sugar propellant. A small amount of AN is reported to slow burn rates significantly with sorbitol, I'm hoping the same holds true with xylitol. AN has a melting point of 338 (F), and a decomposition temperature of 410 (F). I had tried AN with sucrose based propellant in the past, but was unable to keep the AN from melting, then decomposing (big pooof!). The xylitol has such a low melting point, it should be possible to safely incorporate some AN in the propellant. In fact, the xylitol should about be able to melt in boiling water. Perhaps one of those cooking/boiling bags could be used to melt the propellant. Cut a corner off the bag and squeeze it out to cast.
XL-1:The first batch of propellant using xylitol was a small 100 gram batch. The xylitol in the as obtained form was very sugar like, so I ground the xylitol to a fine powder in a coffee grinder. The KNO3 was used in the as obtained state, again, rather sugar like in texture. I preheated my presto cooking pot to a setting of 200 degrees on the thermostat. The KNO3 and xylitol were weighed into a container at a ratio of 65/35 KNXY, then mixed by shaking for several minutes.
The mixture was poured into the cooker and immediately began to melt, within several minutes the powder was completely melted to a rather thin, syrup like consistency. I cast the propellant into two small cups, and a couple of thin strands on waxed paper.
As expected, and similar to sorbitol, the propellant once cooled was still pliable.
XL-2 65/35 KNXY Granular KN, Powdered XY- 500 grams
With the first test batch seemingly going as planned, I decided to do another batch. The second batch was similar to the first, only this time I used a total of 500 grams, and reduced the heat on the cooker to just under 200 degrees. Casting tubes were set up for grains for the T=3 motor, 1.5" x 5.5" with a .625" core. Once the cooker was preheated I added the dry propellant mix, again, the dry mix started melting at once, within 5 minutes the mix was completely melted. Using a candy thermometer I measured the molten propellant temperature at 207 (F). I poured the molten propellant into the casting molds, then inserted the coring rods. A few taps of the mold to settle the propellant and I set them aside to cool.
As the propellant cooled, it started to contract, that's the same thing I had seen with KNSU using non powdered KNO3. In this case, the propellant had formed a skin on the surface, and as it contracted it left that hard skin at the top of the mold, and the propellant contracted below the surface skin. If the propellant behaves like sorbitol, I'll have to wait 24 hours or so for the grains to cure to ascertain the final product.
After 48 hours I trimmed the grains to load them in a motor. The grains were cut to about 2.5" each. The crust layer on the top of the grains didn't appear to affect the propellant below it. As the grains showed no signs of voids or bubbles, and the density was measured at a good .06533 lb/cu. in.
Update: These grains have been used in static test 62.
XL-3: 65/35 KNXY Powdered KN, Powdered XY- 500 grams
The next test batch was again for the T-3 motor, using a .5" core this time. I wanted to see the difference powdered KNO3 would make. All other parameters remained the same. The propellant seemed to melt slower than before, with a slightly smoother texture and clearer appearance. But the main difference was apparent once the mixture melted. Instead of a smooth flowing liquid, the molten propellant was a thick paste. I quickly decided there was no way this would pour into the molds, so I grabbed a 1" diameter steel rod to pack the propellant into the mold. After the coring tool was inserted, I used a small dowel to prod and pack the propellant even more.
As anticipated, the propellant did not contract to a great degree as it cooled. After cooling, the propellant remained pliable, but seemed somewhat more plastic than the propellant using granular KN.
I left about a tablespoons worth of propellant in the cooker, just to see what would happen, I added a pinch of dried, powdered AN to the molten propellant and mixed it in. It mixed in easily, with no noticeable reaction. That's a good sign for future tests.
After 24 hours I decided to remove the grains from the casting stands. I was a little premature I think, as the grains from both castings were very soft in the middle, and easily deformed with little pressure. One of the grains using the powdered KNO3 adhered to the casting tube, and I destroyed the grain trying to remove it. It looked like the propellant had seeped through the paper casting liner, sticking to the PVC casting tube. I may need to rethink how I cast these grains.
XL-4 For test batch 4, I used powdered KNO3 and used XY in the granular, as obtained form. I also increased the heat to 225 (F) on my cooker thermostat. If there is residual moisture in either the KNO3 or the xylitol, I wanted a little extra heat to remove it. I made 4 casting liners out of heavy Kraft paper, six turns on each tube and glued together with Elmers yellow wood glue. Once the liners were dry, I inserted them into 2.5" lengths of 1.5" EMT tubing to keep them round.
The mix was melted, then stirred for several minutes before being scooped into the casting liners. I used a craft stick to gently prod the propellant into the tubes after each scoop of propellant was added. The casting tubes are about 2.6" long each, with no provision for a coring rod, so these grains will need to be drilled out after they are cured. The small amount of extra propellant left over from casting was formed into several strips and allowed to cool on wax paper. After the strips of propellant cooled, a grabbed one to check its appearance, it looked and felt more elastic than the previous three batches. It could be using the XY in the granular form may be a better choice.
Update: After 4 days of curing I checked on these grains. The outside surface was pure white and hard, so I trimmed about 1/4" off the end of the grain. Inside, the propellant was still soft and more translucent white than the solid white of the outside surface. Only the outer 1/16" to 1/8" had cured to a hard state. The inside could be easily pulled apart with minimal force.
The above pic was taken over a week after the grain was cast, it's been curing in a dry box. If you look closely, you can see a white ring around the outside edge. That white ring is very hard and brittle, in fact, if you look at the 11 o'clock position on the grain you will see some chips on the outside surface caused by cutting the grain.
New Update: It's been almost four weeks since these grains were cast, and I decided to check on them. Interesting, they were rock solid. So I cut one open to see what the inside was like, it too, was rock solid. I had cut about 1/4" thick wafer from the 1.5" diameter grain. I tried to break the wafer by pressing an edge to my work bench by hand, to my surprise I couldn't break it. Only after I used two pliers would the disk break. So, it seems the xylitol will set up, at least after an extended time, and is very strong once set.
XL-5 65% powdered KNO3, 35% granular xylitol
With the last test indicating the propellant was not curing to a hard state, I decided to try heating the propellant to a higher temperature, to see if that would result in a better cure. The dry ingredients were hand mixed dry for several minutes, then added to the preheated cooking pot at 275(F). The propellant melted quickly into a thick, white paste. I gradually increased the heat over a period of about 25 minutes until the propellant reached 320(F). Little change was observed, perhaps the propellant became slightly more waxy in appearance.
The propellant was cast into 2 paper lined 1.5" casting tubes with a .625" coring tool.
I played with a little of the extra propellant once it cooled. It was very pliable and clay like, easily being formed and reformed into different shapes. After being reformed 5 or 6 times, the propellant almost instantly went from translucent and pliable to a white powder. Did it get hot when it changed to the powder? I played with another small sample, same thing happened, and yes, it did get hot. Interesting, xylitol is endothermic when added to water, it really makes a small amount of water very cold. And now it's exothermic as the xylitol recrystallizes, (heat of crystallization) I found that information on the web too.
That's all very interesting, but what about the propellant. It doesn't look to me like the extra heat during casting helped the propellant firm up at all. I'll let the grains cure for a couple of days and reassess the grains.
After about 20 hours I removed the grains from the casting stands and trimmed them to length. They may be a little firmer than the previous batches, but not much. They are still very soft and you can easily pull some off the grain.
Here are the grain numbers:
All are 1.51" diameter with a .625" core
1) 2.586" @ 113.8 grams
2) 2.587" @ 116.5 grams
3) 2.458" @ 106.0 grams
4) 2.428" @ 114.4 grams
XL-6: 65% powdered KNO3, 25% granular Xylitol, 10% powdered sugar
Not overly optimistic the KNXY would prove to set up hard enough for use in large motors, I thought I'd give a mixture of Xylitol and sucrose a try. I mixed up a very small batch of 50 grams, heated at a setting of 300 degrees on my cook pot thermostat. The dry mix melted very quickly, shortly after melting the sucrose started to caramelize so I turned off the heat at that point. Six strands were poured onto a sheet of wax paper, three strands were put into a desiccant box, the other three left in the open air.
First impression after the propellant cooled. It seemed firmer, and much more elastic than the pure xylitol. I burned a one inch strand at one atmosphere, the burn rate was about 10.4 seconds (.0961"/sec). The strand burned in a very stable manner.
After allowing the propellant strands to sit overnight, I did some pushing, pulling and tearing "seat of the pants" tests on the strands. It was quickly evident the propellant was much firmer, yet very elastic. This may well be just what I've been looking for in a sugar propellant if the burn rate stays fairly low. Next, I need to cast a larger batch for use in a motor.
XL-7: 65% granular KNO3, 25% granular xylitol, 10% powdered sugar
A 600 gram batch was prepared and dry mixed by shaking for several minutes. I used granular KNO3 for this batch mainly to keep the propellant viscosity down, it should help lower the burn rate some too. I preheated the cooker to 275(F), then added the dry mixture. Once again it quickly melted. I used a candy thermometer to keep an eye on the temperature. After the mixture was fully melted, I increased the cooker temp to 300(F). After an additional 5 or 6 minutes of stirring and heating the temperature reached 260(F), at that time I noticed a slight color change to tan, indicating caramelization of the sugar had started. I turned the heat off and stirred for an additional one or two minutes, then poured the mixture into my prepared casting tubes, and several strips of propellant on wax paper.
This batch uses a little more sugar than the last batch (XL-7), I was actually pretty happy with the last batch in terms of grain properties, it was firm yet flexible, and didn't exhibit the surface brittleness of the pure xylitol grains. We'll see how the extra 5% sugar affects the properties. This batch was weighed out and mixed by shaking for several minutes, then added to my preheated cooker set at 275(F). Melting was slower, as expected with the larger sugar amount. I used a candy thermometer to monitor the temp after the mixture had melted. It was interesting to note, the sugar seemed to melt at the same time as the xylitol, which shouldn't be the case as sugar melts at over 300(F). My guess would be the sugar is dissolving in the melted xylitol. So I didn't push the heat with this batch, only heating the mixture to a maximum of 260 degrees.
The propellant was very thick, as I have noticed when using powdered KNO3 in past batches. I spooned it into casting tubes set up for the T-4 motor. 1.5" diameter by 6" long. I will cut the grains in half after they cool.
I'm becoming a little more optimistic I'm going to like this propellant. I'm really getting close to that elusive sugar propellant that's not brittle and burns slower.
After 24 hours the propellant was well cured, it looks like I went with too much sugar as the propellant is starting to get brittle. See the results of the static test here.
XL-9: 65% granular KNO3, 24% granular Xylitol, 11% powdered Sucrose
For this batch I cut back the sugar to 11%, it was mixed the same as previous batches. The dry powder melted quickly, I stirred and monitored the temperature. At 225(F) I turned off the heat. If the sugar indeed is dissolving in the melted xylitol, there is no reason for further heating. It will be interesting to see how the cured grain is affected by the reduced heat. I also used granular KNO3 and xylitol in this batch. I don't think the xylitol matters what form it is in, but the granular KNO3 results in a much more pourable melted propellant, the slight loss of Isp may well be offset by better density of the less viscous molten propellant.
See the results in Static
It's way too early to draw any definitive conclusions. In these early tests I've been working more towards tweaking the physical properties of the propellant, then to a lesser degree looking at burn rate and performance. By changing more than one parameter in each successive test, it's hard to draw solid conclusions. But I'm going to make a few guesses anyway.
First, it seems the xylitol based propellant is more pressure sensitive than I had expected. Reasonably small changes in Kn have resulted in fairly large changes in burn rate and chamber pressure.
The physical properties are interesting. The sugar added to the propellant helps, it makes the propellant more elastic and prevents a hard, brittle surface layer from forming. Even with 10% sugar the grains are very elastic, easily shaped by hand pressure. A cool grain is firmer than a warm grain, I guess that's to be expected but I'm not sure what would happen on a hot summer day. Off the top of my head I would say the grains heated to 260 degrees are firmer than the ones only heated to 225 degrees.
The longer casing length used in static tests 65 and 66 didn't seem to increase the Isp much.
I think I'll do another test batch at 10% sugar, then run a lower Kn in the motor. I'll make some strands with the extra propellant and do a series of strand burn rate tests.
Batch XL-10: 65% Granular KNO3, 28% Granular XY, 7% Powdered Sugar
For this batch I thought I'd see how low I could go on the sugar and still maintain the physical properties I wanted in the propellant. 7% seemed reasonable, not too much, but hopefully enough to do the job. I mixed up another 600 gram batch, hand mixed dry for several minutes then melted in my cooker preheated to a setting of 275 degrees. I stirred the entire time, heating until the propellant reached 260 degrees on my thermometer. The propellant was then cast into two prepared casting stands for the T-4 motor. I wanted to go a little higher on the temperature with this batch, it seems like the propellant is somewhat firmer when heated to this temp. That may make sense, as when cooking real candy (the kind you eat), 260 degrees is what's called the hard ball stage. Not yet brittle like a hard candy, but firm and still a little flexible.
Update: I wasn't happy with this batch, the 7% was not enough sugar to prevent
the grain from developing a brittle outer surface. So I destroyed this batch and moved on to batch XL-11.
With 7% sugar leaving the inside of the grain soft, and the outside brittle, I made this batch using 9% sugar. Hopefully I'm zeroing in on the correct amount of sugar. This batch was again 600 grams, heated to 260 degrees (F) and cast into 1.5" PVC tubes for the T-4 motor. Instead of regular copy paper lining the PVC pipe, I used a heavy coated paper to cast the propellant into. The copy paper liners tend to absorb the xylitol and stick to the PVC pipe. The coated paper (magazine cover) worked much better.
With the new revelation that batch XL-4 finally set up after four weeks, it looks like I better start casting some grains and get them curing. It may be this propellant will work as is, if you are patient enough. I cast this batch in a heavy coated paper again, to keep the xylitol from sticking to the casting tube walls. I ran into more problems with this batch, the xylitol is so sticky, and it seems to soak through about anything, so much so that I destroyed one grain trying to remove the coring tool. So I broke up the first two grains and remelted them. I re-cast the propellant into 1.5" PVC pipe, then made another batch and cast it into 2" PVC vacuum tubing. These grains will need to have the cores drilled out, which shouldn't be a problem as I drilled out the cores on the grains from XL-4 and used them in static test 70.
Notice To Pet Owners: In my research of xylitol, I found xylitol is poisonous in moderate quantities to dogs. Keep your xylitol out of the reach of your pets and small children. In large amounts it acts as a laxative in humans, you don't need the kids eating xylitol by the cup!