Static Test 98

KN/ER Propellant w/.2% Fe2O3

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For this test I'm revisiting the erythritol based propellant. It has just about everything you could want in a sugar propellant: non hygroscopic, fully cured upon cooling, low melting point, easy casting, good physical strength and low burn rate. The only two drawbacks are: it's hard to ignite and it's expensive. The hard to ignite problem is what I'm working on now, I'm adding .2% red iron oxide to try to increase the burn rate some, and make it easier to ignite. As for the expense, at some point I still want to drive down to the plant where the erythritol is made, one of only two plants in production in the world is within a two hour drive of me. Perhaps I can beg for some floor sweepings if nothing else... We'll see, for the time I have enough erythritol to do a few more tests.

For this batch I mixed up 500 grams of 65% granular KNO3 and 35% granular ER, 1 gram (.2%) of red iron oxide was then added and mixed by shaking. I know ball milling would be more effective, but I really like the smooth pouring of the propellant when using granular chemicals.

The dry mix was then dumped into my Presto Multi-Cooker which had been preheated to 275 degrees (F). As usual, the dry chemicals soon became a nice, smooth flowing thick liquid. The propellant was cast into casting stands lined with thin cardboard, and cored with 5/8" wax paper covered dowels. Once cooled, the grains were removed from the casting stands, trimmed to length and covered with a wrap of aluminum foil tape as an added inhibitor layer.

Number of Grains: 4

Total Grain Length: 11.2", 2.8" each

Grain Diameter: 1.51

Core Diameter: .63"

Propellant Weight: 1.0234 pounds, 464.2 grams

Density: .06181 pounds per cubic inch

Nozzle Throat Diameter: .32"

Kn: 423, 484, 466

Test Results:

Burn Time: 2.434 seconds, >10% ~1.7 seconds

Total Impulse: 106.29 pound seconds

Isp: 103.86 seconds

Peak Thrust: 110.72 pounds

Peak Chamber Pressure: ~1,100 psi

As with the previous KN/ER tests, the motor failed to come to pressure quickly, this time it took about four seconds before thrust was recorded. Once the motor came to pressure, the burn seemed very stable. As you can see from the test results, this was a shorter burn than previous tests. The .2% Fe2O3 obviously increased the burn rate.

The Isp isn't too bad considering the time the motor spent burning before it produced thrust, also, the nozzle has a very small divergent cone, causing an additional loss of total impulse. The actual thrust profile isn't unexpected, it's about the same as I've seen with the other polyol sugars, and the high peak can be worked around if need be.

I'd still like to see the propellant come to pressure a little better. More iron oxide may do the trick, or perhaps a little sulfur may work better.