Static Test 87

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This will be the second test of a KNO3 erythritol based propellant. The first test fell pretty close to the software predicted burn, so I'm increasing the Kn significantly for this test to try to bring the chamber pressure up to at least reasonable levels. Software predicts a maximum chamber pressure of about 400 psi and a burn time of 3 seconds. It remains to be seen if this propellant follows the sorbitol and xylitol odd burn rate acceleration at higher chamber pressures. I'm going to guess it will, though I'm not sure if the Kn is high enough to exhibit this behavior or not. I guess that's why I do the tests!

For this test I'm using a longer 1.5" EMT steel casing, with one of the LW-2 forward closures. The nozzle is a graphite nozzle used on previous composite tests. The grains were cast the same as in static test 86, only this batch I used granular KNO3, the granular (as opposed to powdered) KNO3 makes a much more fluid molten propellant. I also used a catalog cover for the casting liner, it doesn't allow the propellant to seep through it. Although I don't think the propellant bonds as well to it. The grains were again wrapped in foil tape as an outside inhibitor layer. I also used the foil tape to space the grains, applying a layer of tape around each grain joint while the grains were spaced about 1/4" apart.

I made a mixture of green meal mixed in alcohol as an ignition aid. The mixture was painted to all the grain ends, as well as the first 5" of the core in the forward propellant grains. The green meal mixture should improve the ignition.

Grain Numbers:

Number of Grains: 4

Aver. Grain Length: 2.595"

Grain Diameter: 1.55"

Core Diameter: .53"

Total Propellant Weight: 1.0562 pounds

Propellant Density: .06112 lb./ cubic inch

Nozzle Throat Diameter: .3281"

Kn: 368 initial, 423 maximum, 388 ending

Here is the thrust/time trace from the test.

Burn Time: 6.183 seconds

Total Impulse: 97.17 pound seconds

Isp: 92

Peak Thrust: 23.25 pounds

Peak Chamber Pressure: ~200 psi

That was interesting, the Kn was increased on this motor significantly, and it burned twice as long. I would guess using granular KNO3 had more to do with that than anything else. I guess in reality, this motor burned for a lesser time, but it produced thrust the entire time. Of course, I didn't do many strand burn rate tests at these low pressures, so my software predictions were based on higher chamber pressures. With that in mind it seems I'm going to have to increase the Kn a good deal more again.

Considering the low chamber pressure, the Isp of 92 is good. I think once the chamber pressure is up to snuff, I'll get Isp's in the same range as other "sugar" propellants.