This was the second static test of the ER1.75I motor. A couple of things I wanted to try in this test. First, I
added 1% Fe2O3 to the propellant. The iron oxide acts as a catalyst to accelerate the burn in most sugar propellants,
and may slightly increase the ignitability of the propellant. With that in mind, the second test was to try igniting
the propellant without a painted on ignition aid.
The propellant grain geometry was pretty much exactly the same as in static test 121. I added about .7 grams of some black powder green meal to the motor before inserting the igniter.
Here's a capture of the motor at full thrust.
Here is the time/thrust graph from the test.
Burn Time: 2.35/1.4 seconds
Burn Rate: .3086"/second
Peak Thrust: 107.86 pounds
Peak Chamber Pressure: ~950 psi
Total Impulse: 105.28 pound seconds
Isp: 109.67
You can see a small spike at about .3 seconds, that would be the black powder I added no doubt. Looking at the
curve, I'd guess the grain ends were slow to ignite, causing a more progressive burn profile. Without a painted
on ignition aid, the motor was slow to build up full thrust, then there was a slow tail off in thrust, again, likely
from the propellant not igniting fully at ignition. Disregarding the early and late low thrust, the burn rate was
dramatically increased, as was the peak thrust. The Isp increase was likely a result of the increased chamber pressure,
I do believe the Isp would have been much better had a painted on ignition aid been used. Still, the rather small
expansion ratio of the graphite nozzle causes a significant drop in Isp from a low nozzle thrust coefficient.
Conclusions: I learned two very important things from this test. First, I still need a painted on ignition aid even with 1% Fe2O3 added. Second, the added iron oxide speeds the burn rate considerably. For smaller motors of this size, the slow burn rate of KNER propellant can be problematic. The simple addition of 1% red iron oxide brings the burn rate to a level where a motor this size could still be used in a fairly good sized rocket.