This test came on the heels of a first test of this propellant in a small "F" class motor. Frankly, this
propellant doesn't hold much interest for me, as I have access to HTPB and I do feel it's a much better binder.
But for others this may be a viable option that don't have access to HTPB binders.
This propellant uses 75% 200 um AP, 5% 325 mesh spherical aluminum and 20% Mr. Fiberglass epoxy. The epoxy was
mixed 80/20 resin to hardener. The resin was added to a cardboard bowl, then the aluminum was added and thoroughly
mixed in. Then the hardener was added and mixed in, finally the AP was added and incorporated by knifing it in
using a heavy fork. The ammonia gas released didn't seem excessive, although I did have good ventilation.
After the initial mixing, I let the propellant rest about 10 minutes, mixed again, let rest 10 minutes then did
a final mixing. The mix/wait cycle is intended to allow the ammonia gas to release before casting. The consistency
of the mixed propellant is very dry and crumbly, but once packed the propellant cures to a very solid, dense grain
with no visible voids.
Propellant Diameter: 1.34"
Core: .51"
Web: .415"
Number of Grains: 4
Grain Length: 2.4"
Throat Diameter: .42"
Propellant Weight: .61 pounds
Kn: 181 initial, 209 max, 198 ending
Density: .0527 pounds/ci
Click Here for a video of the test.
Here is the motor at full thrust.
The thrust/time curve from the test.
Looking at the thrust curve it's very obvious the upper thrust data was clipped. I had expected a slightly longer burn, and only used a 100 pound load cell with the gain set to record a maximum of 143 pounds. The longer tail off of pressure was likely due to one grain having an unintentional offset core, oh well... Using just the data acquired, the total impulse was 99.08 pounds with an Isp of 162.4 and a burn time of about .8 seconds. With a web thickness of .415" the burn rate was .51875"/second. Of course there looks to be a lot of data lost, so I'd guess Isp was in the 190 range and total impulse close to 116 pound seconds.
I really contemplated using a .5" throat rather than the .42" throat I used for this test for two reasons.
You could also see and hear some instability in the burn. It's almost like a crackling sound, and I've heard that in other propellants using epoxy. One guess as to the cause is propellant chunks breaking off and impacting the nozzle, then burning in the exhaust stream as they exited the nozzle. There is a healthy dip in the thrust curve early in the burn that could have actually been a slight blockage in the nozzle. I used the ER1.75I motor for this test, and it has excessive internal casing length. So it's also possible the grain(s) impacted the nozzle as they moved in the gas stream towards the nozzle.
If I were to test this propellant again, I think I'd drop the Kn to a max of about 175 and not cast the grains with the cores to keep the density up. I'd also use a shorter motor or spacers to keep the grains in place.
All in all I'd say this could be a viable propellant with only a little fine tuning. Although as a mentioned earlier, this isn't a propellant I really intend to pursue.