This batch was prepared by mixing and melting one grain at a time, using 850 grams per batch of 65% granular KNO3 and 35% granular erythritol. The chemicals were dry mixed by shaking in a sealed container for about 1 minute.
I had ordered some 1.25" Delrin plastic rod from McMaster-Carr, the guys at Trailer Trash Aerospace indicated they used it as a coring rod with great success on sorbitol, so I thought I'd give it a try.
I let the first grain cool for about 70 minutes, then I removed the coring rod. I did have to lightly tap it out, but it came out fairly easy. For the second grain, I wet sanded the Delrin rod with 600 grit paper, then applied some wax to the rod. After casting I waited about 70 minutes again, same as last time I had to tap the rod out. In the process, I noticed the propellant hadn't solidified completely yet, and I accidentally dug out a little gouge in the core.
The last four grains I waited until they were just about room temperature before attempting to remove the coring rods. As it turns out, the rods almost fall out of the propellant grain when it's completely cooled. My guess is the Delrin rods expand more than the propellant when hot, then when everything cools down the rods are now slightly smaller than the core, making removal of the rod very easy.
Casting Tube: 5.2 grams/inch
| Grain # | Casting Tube |
Propellant Length |
Propellant Weight | Density |
| 1 | 5.21"@27.09 gr. | 4.89" | 1.7333 | .0607 |
| 2 | 5.2"@27.04 gr. | 4.92" | 1.7557 | .06112 |
| 3 | 5.17"@26.88 gr. | 5.06" | 1.7795 | .06024 |
| 4 | 5.26"@ 27.35gr. | 5.11" | 1.8132 | .06078 |
| 5 | 5.15"@ 26.78gr. | 4.82" | 1.6797 | .05969 |
| 6 | 5.24'@ 27.25gr. | 4.98" | 1.7603 | .06054 |
| Totals | 31.23"@162.39gr. | 29.78" | 10.522 lb. | .06052 |
One thing to keep in mind with my measurements on these grains. I'm casting into a 3" diameter cardboard tube, and trying not to fill it completely. I'm leaving a little "head room" in the casting liners for grain spacing. The propellant "head room" surface is not completely flat, so I have to estimate or try to average the gap in the liner tube.
I'm still a little short on propellant in this load. I was shooting for 30" of propellant and 10.8 pounds. I ended up with 29.78" and 10.522 pounds. Close enough though, to give me a good sense of how this motor will perform.
All the grains look very good except for grain #2, which I gouged out a small area of the core while removing the coring rod. I debated using the grain as is, recasting it, or trying to fix the gouge. I opted to try to fix the gouge. I don't think the damaged area was enough to cause a problem anyway, but I was curious to see how a repair would work. So after a subsequent grain casting, I filled in the gouge with some melted propellant, pushing it into the gouge with a flat screwdriver blade. Once it cooled, I used a 1.25" spade bit drill to clean up the core. It looks like a good repair, and the core drilled out nicely.
All the grains were cleaned up, any drips of propellant on the outside of the casting tube were removed with a fine file. The grains were all given a single wrap of aluminum foil tape as final inhibitor layer.
Here's the grain numbers:
Total Propellant Length: 29.78"
Average Grain Length: 4.9633"
Total Propellant Weight: 10.522 pounds
Core Diameter: 1.25"
Nozzle Throat Diameter: .75"
Kn: 423 initial, 466 max, 430 ending
Web Thickness: .875"
Density: .06052 pounds per cubic inch
Final preparation of the motor included making the o-rings, RTV gluing of an EPDM rubber disk on the forward bulkhead and cutting an EPDM thermal liner. Followed by a last inspection of the casing, forward bulkhead and nozzle. Lastly, I painted on my thinned down pyrogen to the grain ends, and the cores of the top 4 grains. I thinned the pyrogen a little more for this test, to see if I could get by using less of the pyrogen.
This test was performed in pretty much the exact same location and manner as the first test, well protected by the earth berm surrounding our farm pond. I'll have to admit, I made a silly mistake during this test. I had everything set up for the test, all clear, then I went to do the continuity test and pop! What was that I wondered? The continuity light didn't come on, then I heard the motor come to life. I had accidentally pushed the wrong button, rather than the continuity button, I had pushed the ignition button. Not that it really mattered, but it does concern me. I think I'll add a flip cover to the launch button, to make sure that doesn't happen again.
Back to the test. When I heard the pop, there was about a three or four second delay before the motor came to pressure. So the thinner pyrogen I used didn't work as well. Once I realized what had happened, I quickly looked to the computer screen to watch the incoming data. It was apparent the motor was producing a higher level of thrust, as the thrust was well over 300 pounds for quite some time. If anyone wants to see the video of the test, drop me an e-mail and I'll upload it. But it wasn't much different than the first test.
Here is the thrust/time curve from the test.
Test Numbers:
Burn Time: 6.117/5.1
Peak Thrust: 370.22 pounds
Peak Chamber Pressure: ~625 psi
Total Impulse: 1,409.67 pound seconds
Isp: 133.97 seconds
The numbers are close to what I expected. A slightly shorter burn with higher thrust. Increasing the Kn/chamber pressure more would no doubt increase the Isp some, and shorten the burn. But I think I'm pretty happy with the performance the way it is.
On both tests of the SBS-1350 motor I've noticed a small hump at the start of the peak thrust, then it drops down and levels off. It's more pronounced on this test. I'm not sure of the cause. But I suspect it's the pyrogen I use to get the grains ignited. It may be the acetone in the pyrogen is soaking into the propellant outer layer, and causing a faster burn in that area. Which is fine, a nice little boost off the tower only helps get a rocket stable quicker. So long as that little peak doesn't become a mountain.
All said, another good test of the KN/ER propellant. The propellant continues to perform in a stable manner, very close to my software predictions. I've got my supply of erythritol now, so I could certainly do more tests of this motor. But it is a big job casting 10.5 pounds of propellant. I may do a few more tests using a smaller motor, and see if I can tweak the propellant just a bit, to squeeze a little higher Isp out of it.
Here are the casting tubes and liner after the test. They looked almost identical to the first test. The motor showed no signs stress, blowby or heat damage.