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Not knowing how Static Test 100 will turn out, and hoping for the best. This was just too tempting not to try. The above formula has a theoretical specific impulse of 194 seconds at 1,000 psi. With an unbelievable density of 0.0717 pounds per cubic inch. At a cost of $2.34 per pound, and at the high density, this would make a propellant that volumetrically rivals AP based propellant at a greatly reduced cost and is much easier to cast.
I cast this propellant identical to the propellant cast in the last test (ST-100). The 25% xylitol is still not the lower limit, as the propellant while much thicker than the last cast, was still slightly thinner than standard (no PG added) 65/35 KNO3/sugar propellant. I could tell the density was better too, as the casting tubes filled to about an inch less in volume than the last cast.
I'll hope for the best, but plan for the worst. It'll take a week or so for the propellant to cure I'm sure, then I'll test this in a motor at a remote test site.
Ok, so I chickened out again. I decided to do some strand burn tests on this propellant before the motor test. A small batch of 50 grams was prepared. The propellant was spooned into (5) 1" long 1/2" CPVC pipe sections. The CPVC pipe segments were measured and weighed, the average weight of the CPVC is 3.0477 grams per inch. Inside diameter is .486".
I did a quick density check on one of the strands. Length 1.051", weight 9.3 grams, weight of CPVC 3.203 grams. Propellant weight 6.097 grams. Density 0.06876 pounds/cubic inch out of a theoretical density of 0.0717 pounds/cubic inch. That's about 95.9% of theoretical best density.
These small strands will cure in a day or two, then I'll do some burn rate tests under pressure.
| Test# | Length | Start Pressure | End Pressure | Aver. Pressure | Time | Burn Rate | Density |
| st101-x1 | 1" | 603 | 1,795.4 | 1,199.2 | 1.617 | .6184"/sec. | .06876 lbs./cu in |
| st101-x2 | 1" | 216.35 | 692.31 | 454.33 | 2.784 | .3592 | |
| st101-x3 | 1" | 399.04 | 1158.7 | 778.87 | 1.834 | .5453 | |
| st101-x4 | 1" | 519.23 | 1873.1 | 1146.165 | 2.033 | .4919 | |
| st101-x5 | 1" | 500 | ? | ? | . |
c= 0.0115, n= 0.568 (After three tests)
The first strand test, x1, the pressure was started at 603 psi (nitrogen). The pressure in the strand burner reached marginal high limits for the apparatus, and got me a little nervous. The combustion appeared very complete, with little residue on the strand tube or in the burner itself. The burn rate was comparable to regular KN/SU, so the burn rate wasn't out of line.
For test x2, I lowered the pretest nitrogen level significantly to 216.35 psi. As a result the burn rate showed a dramatic decline, as did the maximum chamber pressure. Upon opening the chamber, there was a lot of fluffy dark gray powder residue, both in the chamber and on the strand tube. I also discovered small bright colored balls of what appears to be aluminum.
For test x3, I started the chamber pressure at 399.04 psi, the results of the burn rate and peak chamber pressure fell nicely in the middle of the first two test. After opening the chamber, I found a lot of light colored gray powder residue this time. About the same amount as in test x2 but much lighter in color. No balls of aluminum were found after this test.
Test x4 didn't fall into the burn rate expected. It was slower than it should have been, and also resulted in higher pressure than expected. I exceed my transducer max limit for some 50 samples (.833 seconds) so while the burn data is odd, it is also flawed.
Test x5 went over the transducer limit even worse this time as it was over range for 2.15 seconds. So it's a little hard to draw an conclusions from this test, other than it was a nice long burn. With that in mind, perhaps the upper pressure ranges don't increase the burn rate as dramatically as I thought earlier. More fine light gray colored powder after this test with no signs of unburned aluminum.
My conclusions: It seems the aluminum in this formula will burn at pressures of 400 to 600 psi and higher, it also appears the higher the pressure, the more complete the combustion. The c (or a) and n values are about what would be expected considering the use of KP in the propellant. That is, a fairly high pressure exponent. The coefficient is reasonably low, making this propellant appear to be useable, although a fairly neutral Kn would be in order I think.
I can see I should have used a higher range pressure transducer for these tests, basically I wasted two tests because the transducer was out of range high.