Static Test 103

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This was a standard batch of 65% granular KNO3, 35% granular erythritol. The batch was mixed and melted at a setting of 250 degrees. I just love working with erythritol, it's non-hygroscopic, melts at a low temp, is fully cured when cooled, makes a very pourable melted propellant and isn't as sticky as xylitol or sugar. The only drawback I could see was that it was a little hard to get ignited and up to pressure.

With that in mind, I decided to paint on a pyrogen mixture to help get the grains burning. I used my standard igniter pyrogen, just thinned out a little. The top three grains were painted with this pyrogen on all burning surfaces, the bottom four grains were painted on just the ends.

Number of Grains: 7

Grain Length: 2.5" (17.5" total)

Grain Diameter: 1.51"

Core Diameter: .755"

Propellant Weight: 1.433 pounds

Kn: 435 initial, 461 max, 432 ending

Density: .061 pounds/cubic inch

Here is the time/thrust curve from the test.

Burn Time: 2.684 seconds (~1.7)

Peak Thrust: 173 pounds

Peak Chamber Pressure: ~950 psi

Total Impulse: 181.72 pound seconds

Isp: 126.81 seconds

Well, that's more like it! Finally a good burn from the KN/ER propellant. The painted on pyrogen seems to have done the trick. I wouldn't say the propellant took off instantly, but there was only a couple of seconds at most before thrust occurred. The longer casing helped no doubt too, this propellant is a bit like AN/Mg propellant in that it is slow burning, and benefits from a longer, larger motor.

If you look at the burn time, the time above 10% max thrust is about 1.7 seconds. With a web thickness of .3775", that's an average burn rate of .222" per second. And the propellant seems to have leveled off rather nicely, as opposed to a sharp peak. The Isp of about 127 is very nice as well.

I also found a web retailer that is selling erythritol at a much better price, http://www.store.honeyvillegrain.com Yes, we are still talking about $5 a pound for it. But as it gains in popularity I'm sure the price will continue to fall. I mentioned before, the only plant in the U.S. that makes erythritol is about 2 hours from me, and I have made initial contact in hopes of getting bulk quantities at reduced prices. But let's be realistic, even at $5 a pound for erythritol, the motor I just tested cost $2.63 for erythritol and $ .49 for KNO3 for a grand total of $3.12. That's still pretty cheap for a "J" class motor.

I'll say this, I really, really, really like the properties of erythritol based propellant. If the only down side is that I need to use a really hot igniter, or paint on an ignition aid, it's well worth it.