Static Test 88

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This is the third test of the new KN/ER propellant. The mixture was 65% KNO3 and 35% erythritol, both were in the granular form. I used a heavier paper as a casting liner this time, it was office folder type of thin cardboard with a thickness of .01". In order to increase the Kn of the motor and create a fairly neutral Kn I used a 3/4" steel rod for a coring tool. Once wrapped in wax paper the actual core diameter was .775". The combination of the wax wrapped steel rod and the heavier paper worked very well. The coring tool and wax paper was easy to remove, and the thicker paper prevented the molten propellant from seeping through. These are by far the best grains I have cast using the polyol sugars.

Once the grains were cast, I thought I'd try a little test. I had about 10 grams of melted propellant left, so I added some previously made KN/XY propellant to it. I melted them together, stirred for several minutes, then poured several thin strands on wax paper to cure. I was curious, xylitol based propellant cures so slowly, as to what would happen when the xylitol and erythritol propellants were mixed. I didn't weigh them, so it's not very scientific, and I think I had more XY than ER, maybe 60/40 or so. It turns out the XY won the battle, as the propellant once cooled stayed a gummy mess.

Grain Numbers:

Number of Grains: 5

Grain Diameter: 1.532"

Core Diameter: .775"

Total Grain Length: 12.5"

Total Propellant Weight: 1.0573 pounds

Density: .06171

Kn: 522 initial, 550 maximum, 513 ending

Thrust/Time curve.

Burn Time: 2.367 seconds

Total Impulse: 108.38 pound seconds

Isp: 102.5 seconds

Peak Thrust: 119.35 pounds

Peak Chamber Pressure: ~1,000 psi (estimated)

I didn't use a painted on ignition aid in this test, I probably should have. The first igniter failed to light the motor. After waiting several minutes I added a little of my gray shot ignition powder, and installed a new igniter. This time the motor lit, but burned for over 16 seconds before producing thrust. Once chamber pressure was established, the motor burned very quickly and steadily. The burn time was really closer to 1.5 seconds, with the last .8 seconds being low thrust. The Isp of 102 isn't bad considering the motor burned so long without producing any thrust.

As for the curve itself. You can see that at about 60 pounds of thrust the burn was ready to stabilize, more than likely indicating all surfaces were now burning at a steady state. This point relates to about 550 psi of chamber pressure. Then the burn accelerates and peaks very quickly. This is the same phenomenon we have seen with sorbitol and xylitol based propellants.

So what's up with this weird burn behavior? I really don't know what causes it. I don't think it's either erosive burning or flushing as proposed by others. This burn rate behavior is not seen in strand burn tests, so I look there to find the possible cause. What is different about a strand burn test than a motor test?

For starters there is a much larger flame front in a motor as opposed to a single strand burning its length. If I had to venture a guess at this point, I'd say a possible cause is heat, I think the propellant may exhibit a more rapid burn rate under higher temperatures, and pressure of course. Perhaps the opacity of the propellant is part of the problem, I think I'll try adding a small amount of charcoal to the propellant. Conventional wisdom would make one think it would increase the burn rate, and that may well be the case, but it may also stabilize the burn. We'll see...