14 layer casting tubes weigh 13.64 grams per inch.
| Grain # | Tube Length | Recess Length | Propellant Length | Gross Weight | Tube Weight | Propellant Weight | |
| 1 | 6.25 | .18 | 6.07 | 4.4 | .1879 | 4.2121 | 6.25 |
| 2 | 6.75 | .19 | 6.56 | 4.8 | .203 | 4.597 | 13 |
| 3 | 8.25 | .2 | 8.05 | 5.8 | .2481 | 5.5519 | 21.25 |
| 4 | 6.625 | .17 | 6.455 | 4.8 | .1992 | 4.6008 | 27.875 |
| 5 | 7.0 | .19 | 6.81 | 5.0 | .2105 | 4.7895 | 34.875 |
| 6 | 7.0 | .18 | 6.82 | 5.0 | .2105 | 4.7895 | 41.875 |
| 7 | 7.1875 | .2 | 6.9875 | 5.2 | .2161 | 4.9839 | 49.0625 |
| 8 | 6.875 | .2 | 6.675 | 4.8 | .2067 | 4.5933 | 55.9375 |
| 9 | 6.75 | .2 | 6.55 | 4.8 | .203 | 4.597 | 62.6875 |
| 10 | 6.5 | .2 | 6.3 | 4.8 | .1955 | 4.6045 | 69.1875 |
| 11 | 6.5 | .2 | 6.3 | 4.6 | .1955 | 4.4045 | 75.6875 |
| 12 | 6.625 | .2 | 6.425 | 4.6 | .1992 | 4.4008 | 82.3125 |
| Totals | 82.3125 | 80.0025 | 58.6 | 2.4752 | 56.1248 |
Propellant Weight: 56.1248 pounds
Diameter: 4.22"
Core: 1.77"
Grain Length: 80.0025" (6.667" average per grain)
Density: 0.0609 pounds per cubic inch
Kn: 437, 474, 428
Throat Diameter: 1.45"
21 July, 2006: Test Day
We had a nice crew on hand for the static test and later in the day a small rocket launch. Ryland came down with his mother and sister, Kathy and her daughter Amy were there, and John and I. John was kind enough to dig out the hole in the ground for the motor, while Ryland and I set up the launch control system and the data acquisition hardware. Once all the cameras were in place, we all moved to the launch controller behind the earth dike around the pond.
Ryland had the honors and was at the controller. 5,4,3,2,1 and ignition!
Click Here for a video of the test from our spectator perspective.
Click Here for the close up video from the big Panasonic Camera.
Moments after ignition. The motor chuffed once, as I had expected, then came to life.
Less than one second into the burn, the dark gray smoke was not expected, and is usually a sign of high chamber pressure.
At the tail end of the burn, the EPDM rubber belched out very black smoke. Some small particles of rubber were also ejected.
Here is the time/pressure curve from the test.
The motor had burned much faster than expected, more than twice as fast actually. If you look at the graphic above, the pressure was flattening out at about 780 psi, then took off and peaked at 1,357 psi. The small sharp hump at about 3.1 seconds was some EPDM thermal liner being ejected. The motor survived the test just fine, but the reason for the fast burn rate needs to be looked at.
I did run this motor at a slightly higher Kn than previous motors, as I was trying to improve performance by increasing chamber pressure slightly. So the higher Kn would account for some of the increase. But the real reason for the fast burn, I believe, is the low core to throat area ratio I used on this motor combined with the large length to diameter ratio of this motor. You can sometimes push that ratio tighter on smaller motors, but a motor this size and length couldn't handle it as well. What happens is you get a lot of erosive burning, which increases the amount of propellant burning, which increase the chamber pressure, higher chamber pressure leads to faster burning... It's a vicious circle, and you can get run away chamber pressure in that condition. It also leads to a condition of the lower propellant grains (nozzle end) burning out well before the upper grains. That leaves the lower motor with the thermal liner only, I'm sure that's why I had some EPDM ejected. That, and I also used two wraps of EPDM, and the inside wrap would have been unsecured and "flapping in the wind" so to speak.
As it was, the motor performed well within its limits, and would have made for one heck of a flight had it been in a rocket. Can you say 0 to mach 4 in 3.5 seconds? The peak thrust recorded was 3,248.8 pounds, I'm really glad I didn't try using my 1,000 pound load cell on this test!
For the next test of this motor I'll use a larger core on the propellant grains, as well as drop the Kn back to my usual level. With a motor this long I'm sure I'll still have some erosive burning, but if kept in check the effect should be minimal and the motor burn more nominal.
Here are the numbers from the test data:
Burn Time: 3.858/3.0 seconds
Peak Chamber Pressure: 1,357.5 psi
Peak Thrust: 3,248.8 pounds
Isp: 125.4 seconds
Total Impulse: 7,037.04 pound seconds
This motor would have a commercial classification of an O 10,426, being 76% of a full O.
22 July, 2006:
After a failed attempt to get the motor apart the night of the test. I managed to get the job done today. I tried knocking the forward bulkhead out using a 10' length of EMT conduit, but it was impossible to get the conduit centered on the bulkhead, and the uneven pressure simply wedged the bulkhead tightly in place. So today I made an impromptu sliding hammer out of some assorted pipe fittings. This worked very well, and in a minute or two of light tapping the bulkhead was out.
Here's the sliding hammer and bulkhead after extraction.
Here is the thermal liner after removal from the motor. It really is in good condition, the only holes in it were holes I poked in it with the conduit while trying to knock out the forward bulkhead. There was a very slight burn through at the nozzle end too. It does look like a fair amount of the inside layer of the EPDM was ablated away.
The nozzle was a bit of a challenge getting out too. I ended up pounding it out using an 8' 2x4 and a sledge hammer. I had decided the nozzle was the most expendable piece of the motor. Turns out it didn't matter, look at the crack running the length of one side. The crack wasn't noticeable in the motor, but I'm pretty sure it happened during the burn because the o-ring closest to the convergent nozzle is burned through at the exact location of the crack. Obviously, the chamber pressure ran a little higher than the nozzle could handle.
Also of interest to me was the condition of the casting tubes. Or in this case lack of... I only found portions of 2 of the 12 casting tubes remained in the motor. I guess that shouldn't surprise me too much, considering the erosive burning would have played hell with the lower casting tubes. But even the upper two I did recover were badly charred, with little remaining. I had expected to find at least some of the tubes more or less intact. I have been playing with the idea of soaking the cardboard tubes in sodium silicate to fire proof them. Perhaps I should run some tests of the practicality of that. A heavier casting tube looks to be in order as well, just what I wanted, more layers of paper on my casting tubes!
It looks like inhibitor failure may have played a partial role in the fast burn rate of this motor. For the next test I'll use heavier casting tubes, a larger core on the propellant grains and I'll lower the Kn slightly.