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I had done some tests in the past with magnesium, at about 5% the Isp did indeed go up, as much as 8 or 9 seconds at that rate. I didn't really pursue the Mg heavily, as Mg is becoming very hard to acquire. My first impression was that sugar propellants burn at too low a temperature for the aluminum to burn, that seems not to be the case, and the aluminum appears to be burning.
I used some -325 mesh atomized aluminum for this test. Still not entirely convinced the aluminum would burn with just KNO3 as an oxidizer, I added 5% KP to give it a little kick. The aluminum and KP give a theoretical increase in Isp of some 35 seconds (170 Isp), a nice boost if it actually does. The really nice thing is, the xylitol with the aluminum really makes for a thin, pourable propellant. I tried a small batch at 65% KNO3, 25% xylitol and 10% aluminum and it was still very much a liquid. It didn't burn very well in the open air, but that was expected.
For this batch I used all the ingredients in the "as obtained form". No grinding or ball milling was done. The chemicals were simply dry mixed, then added to a preheated cook pot at 225 degrees (F). They were stirred continuously until fully melted, left to sit for a few minutes, then stirred again. I thumped on the side of the cook pot to bring any bubbles to the surface, slowly stirred again for a minute then poured the propellant into casting tubes lined with thin cardboard on the circumference and aluminum foil covering the bottom. I've found it easier to drill out the cores with xylitol propellant, so once these grains cure I'll core them on my lathe.
Total Propellant Weight: .8852 pounds, 401.5 grams
Number of Grains: 4
Grain Length: 2.06'" each, 8.24" total
Grain Diameter: 1.51"
Core Diameter: .5"
Kn: 185 initial, 202 max, 160 ending
Density: .06741 pounds/cubic inch
Here is a single frame capture from the test. This was during a brief moment when there was a large visible white flame.
Here is the thrust/time trace from the test.
As you can see from the test, the motor was slow to develop full thrust. My guess is that for the first 2.5 seconds or so of the burn, the temp/pressure was not high enough for the aluminum to be burning, the aluminum more or less acting as an inert solid in the propellant. The Kn was low enough in the motor that the early burn was very similar to plain KNO3/xylitol at low pressure. Once the temp/pressure got high enough, it looks like the aluminum started burning. Exactly why the sharp spike in thrust is a bit of a mystery. One would think that once the motor came to pressure, the burn wouldn't have slowed down, but it did.
Here's the numbers:
Burn Time: 3.967 seconds
Peak Thrust: 77.85 pounds
Total Impulse: 67.64 pounds
Isp: 76.4 seconds
Peak Chamber Pressure: ~480 psi
Post test inspection of the motor was also interesting. The casing and nozzle were both steel, with the casing showing signs of greater than normal heat saturation, although with the long burn that's to be expected. The casing was in very good condition, with no signs of blowby or heat damage. The nozzle too was in good condition, although there was significant build up of a thick, crusty dark gray residue on the inside (convergent area) of the nozzle. The divergent cone and throat had a minimal layer of build up.
Here comes a new thought on that pressure spike. My guess now, is that the layer of gray residue was building up on all areas of the nozzle (convergent, throat and divergent), as the throat surface area of the nozzle was reduced, the pressure started building in the motor, when the pressure built up enough, it started allowing combustion of the aluminum, and the extra pressure and heat cleared the nozzle throat, indicated by the drop in pressure after the spike. Makes sense anyway...
After this test I have serious reservations about this propellant. It looks like I'll have to raise the Kn of the motor to get the KNO3/xylitol part of the propellant up to pressure, once it's at pressure, the Al starts burning and it's a fairly clean burn. But, the problem is, once the aluminum is burning the burn rate will probably increase dramatically and the Kn should be lower. A fix to this may be a hot, fast burning igniter that will bring the motor up to pressure quickly, and allow the use of a lower Kn from the start. The risk of such an igniter is fracturing the propellant from the burst of pressure.
Just last night I made a batch of just that kind of igniter, I had intended to use it on KNO3/Erythritol propellant. So I'll do a couple of tests to see how much pressure the igniters induce, then decide what direction to go with this propellant.