| Grain # | Casting Tube |
Propellant Length |
Propellant Weight | Density |
| 1 | 5.34" @ 27.77 gr. | 5.18" | 1.8171 | 0.0605 |
| 2 | 4.91" @ 25.53 | 4.77" | 1.7268 | 0.0624 |
| 3 | 5.26" @ 27.35 | 5.05" | 1.7852 | 0.0610 |
| 4 | 5.05 @ 26.26 | 4.92" | 1.7523 | 0.0614 |
| 5 | 4.83" @ 25.12 | 4.7" | 1.703 | 0.0625 |
| 6 | 4.99" @ 25.95 | 4.85" | 1.7094 | 0.0608 |
| Totals | 30.38" @ 157.98 | 29.47" | 10.4938 | 0.0614 |
Kn: 424 initial, 463 max, 426 ending
Burn Time: 9.5 seconds
Total Impulse: 1,200.8 pound seconds
Isp: 114
Peak Thrust: 203.89 pounds
Peak Chamber Pressure: ~340 psi
I thought I'd try a higher oxidizer ratio with the erythritol. This test used 70% KNO3 and 30% erythritol in the SBS-1350 motor.
Here's the thrust curve from the test.
The motor lit quickly, but as you can see it was not only slow to build any significant thrust, but it burned a long, long time. With a maximum chamber pressure of less than 350 psi, it's not a great surprise the Isp was so low. It was a bit of a surprise at how slow this mixture burned, obviously the Kn would need to be increased to shorten the burn and improve the combustion efficiency.
The long burn pushed the motor to its limit too. The nozzle throat glowed a bright red for at least six seconds of the burn, in the process the nozzle o-rings melted and at the very end of the burn there was a very slight amount of blow by at the nozzle, although there was no apparent damage.
The nozzle also had a heavy layer of deposits on it, while the throat diameter wasn't affected, it was another sign of a poor burn. To try to "tweak" standard KN/ER propellant a little more, I may try some Mg shavings at about 3%.