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Grains: (3) 6.2" long, 3.0" diameter, 1.0" core
Propellant Weight: 7.091 pounds; 3,216.3 grams
Kn: 252 initial, 327 maximum, 320 ending
Nozzle Throat: .69" diameter, .3737 sq inches
Density: .0607 pounds/ cubic inch
This is the thrust/time curve from the test.
Here is the motor at full thrust.
Burn Time: 4.216 seconds
Peak Thrust: 925.35 pounds
Total Impulse: 929.69 pound seconds, 4,132 Newton seconds (Strong "L" class motor)
Isp: 131.1 seconds
Peak Chamber Pressure: ~1,700 psi estimated
Well, that was interesting! A much faster burn than predicted. The 4.2 second burn time is misleading as well, as the vast majority of the propellant burned in about 1 second. This is looking a lot like the inverted "V" thrust profile of sorbitol based propellant. I get the feeling there is a threshold point, pressure based I'm reasonably certain, where the propellant "takes off" at an accelerated burn rate. While I didn't find that in my strand burn tests, my motor tests are indicating this. I'm getting the feeling the Kn should be in the 260 to 270 range, and a much more neutral Kn is in order.
While the 1,700 psi chamber pressure is a lot higher than expected, it's no where near the casing pressure limit. The casing should have a burst pressure in the 3,200 psi range, so I'm still close to a 2x safety factor. The motor survived the test with no visible problems, the o-rings all sealed, nozzle throat didn't erode, casing is in mint condition and the forward bulkhead is fine. The paper casting tubes/inhibitors were mostly intact, what was lost I think was burning after the propellant had burned out. The EPDM rubber lining was mostly intact, though it too had a couple of burn through points. But again, I think they were caused more after the propellant burn. The motor smoked a black, rubbery smoke for about 30 seconds after the burn. I think that was the paper liners and the EPDM rubber lining smoldering.
For the next test of the motor I'm looking at the following grain configuration:
Number of Grains: 4
Length: 4.75"
Core: 1.5"
Estimated Weight: 6.1 pounds
Kn: 262 initial, 275 maximum, 253 ending
Throat Diameter: .8"
That leaves me about a pound less propellant, but a very neutral and lower Kn. I should have enough metal left on the nozzle to bore out the throat to .8", as I left it a little thicker in the throat region for just that purpose.