| Grain # | Tube Length | Recess Length | Propellant Length | Gross Weight | Tube Weight | Propellant Weight | Density | |
| 1 | 6.9375 | .2 | 6.7375 | 5.1467 | .2796 | 4.8671 | 6.9375 | 0.0611 |
| 2 | 6.875 | .21 | 6.665 | 5.1522 | .2771 | 4.8751 | 13.8125 | 0.0619 |
| 3 | 76.5 | .2 | 6.3 | 5.2382 | .2821 | 4.588 | 20.8125 | 0.0616 |
| 4 | 6.9375 | .22 | 6.7275 | 5.2100 | .2796 | 4.9304 | 27.75 | 0.0620 |
| 5 | 7.0 | .21 | 6.79 | 5.2461 | .2821 | 4.964 | 34.75 | 0.0618 |
| 6 | 7.0 | .21 | 6.79 | 5.2089 | .2821 | 4.9268 | 41.75 | 0.0614 |
| 7 | 7.0 | .21 | 6.79 | 5.2062 | .2821 | 4.9241 | 48.75 | 0.0613 |
| 8 | 6.875 | .21 | 6.665 | 5.1055 | .2771 | 4.8284 | 55.625 | 0.0611 |
| 9 | 7.0 | .2 | 6.8 | 5.2111 | .2821 | 4.929 | 62.625 | 0.0613 |
| 10 | 7.0 | .2 | 6.8 | 5.2104 | .2821 | 4.9283 | 69.625 | 0.0613 |
| 11 | 6.935 | .2 | 6.7375 | 5.1760 | .2796 | 4.8964 | 76.625 | 0.0615 |
| 12 | 6.2 | .24 | 5.96 | 4.5631 | .2499 | 4.3132 | 82.825 | 0.0612 |
| Totals | 82.325" | 6.8604 average |
79.7625 6.64875" avr. |
61.3063 | 3.3355 | 57.9708 | - | 0.0619 |
Kn: 431 initial, 455 max, 408 ending
Core to Throat Area Ratio: 1.743 to 1
Weight: 92.4 pounds
This will be the second test of the Defiance rocket "O" class motor. In the first test I had a faster
burn than expected, after considerable thought and carefully going over the data I believe the fast burn was due
mostly to partial inhibitor failure and to a lesser degree erosive burning. The erosive burning is going to be
a factor in a motor this long regardless of what I do, but to help minimize the effect I increased the core from
1.77" to 2.02". I also opened up the nozzle throat slightly for this test from 1.45" to 1.53",
that should drop the Kn to a max of about 454. But the real change is in the casting tubes I'm using. I had some
tubes custom made with an internal diameter of 4.375" with a wall thickness of .12". The grain diameter
is larger for this test, offsetting the larger core diameter so I should be able to get a full propellant load
of 58 pounds in the motor.
I'm paying a little more attention to the casting process to get better density from this set of grains as well. I'm using my hand sander to vibrate and dislodge bubbles after casting the propellant. I'm also heating the propellant longer to reduce viscosity and allow more bubbles to float to the surface and break. One last thing I'm doing to increase density is to over pour the grains by about 1". I then cut the top of the grain down to the desired length. What small bubbles there are in the propellant rise to the surface, with the upper portions of the grain having the largest number of bubbles and lowest density. While these surface bubbles haven't really posed a problem in the past, it was one more area I felt I could work on to improve grain density and maintain uniform grain burn rates.
One last change to this test is that I'm going to paint on sodium silicate to the outer surface of the casting tubes. Sodium silicate has very good fire proofing properties, and should help prevent inhibitor failure.
All the grains outer casting tubes were painted with sodium silicate and allowed to dry. When dry, the sodium silicate leaves a clear, thick, hard coating on the cardboard tube surface. I considered adding a wrap of foil tape to the outside surface as well, but the sodium silicate layer was fairly thick and I was reaching the maximum diameter allowable on the grains the way it was. The sodium silicate also changes to a dry, fluffy powder after a flame has been applied, and I think the aluminum foil tape may have not have done much good after the motor heated up.
Here are the grains after the sodium silicate was applied and before the pyro mix was painted on.
I coated the top (forward closure end) 5 grains on all surfaces with my pyro mixture, the lower 7 grains were painted on the end surfaces only.
Here the grains are laid out on the EPDM liner after the pyro mix was painted on and allowed to dry.
I assembled this motor similar to the last test of this motor, only this time I only made one full wrap of EPDM around the propellant grains, again sealing the seam with aluminum foil tape. I dusted both the EPDM liner outer surface and the interior of the casing with talcum powder to make insertion of the propellant "package" easier. After adjusting the propellant package in the motor, I discovered I had lost a little linear length in the process of inserting it. I suppose the friction of inserting it slightly compressed the EPDM rubber, making it a little shorter. I tried removing a grain from each end and pulling the liner out a little, but it wouldn't budge. So I ended up cutting off 1/2" of the last grain to free up a little more space inside the liner. I could have gone with it the way it was, and I think it would have been fine. But I like to seal the liner to the casing with some high temp RTV silicone, and I was worried the RTV would seal the grain in place as well. I don't like sealing a sugar propellant grain to the casing (or thermal liner) because of the brittle nature of the propellant makes it much more likely to fracture from pressure on the core of the grain. So I ended up losing over a third of a pound of propellant, but felt better about the configuration.
Test Day:
It wasn't looking good for a test today, as the forecasters had predicted thunderstorms for most of the day. At about 1:00 pm I called off the test, if by some miracle things cleared off, maybe I could still get the test in later in the day. As fate would have it, the weather cleared for a couple of hours late in the afternoon and dried things off somewhat, so I decided to go with test. Bill had gone home, but John had showed up so John and I loaded up the van to head out to the test site.
Set up went well, John was at the controller and I had a video camera at our "safe" location.
5,4,3,2,1 and ignition!
Click Here for a video of a wide shot and close up in one video. wmv format, 3.42 MB
The motor made one quick chuff then came to life.
Here's a capture from the close up camera.
I counted as the motor burned, at about 6 seconds you could tell the burn was subsiding. Right at about 7 seconds the burn ended. It was a perfect burn, you could even tell from the sound it was a nice, stable burn. There was no evidence of dark smoke or particles being ejected and the tail end of the burn was fairly short, all indicating a good burn.
We left the casing to cool and loaded up the equipment and headed home to analyze the data.
Here's the pressure/time graph from the test.
Looking at the graph above, what a wonderful curve. Exactly what I had hoped for!
Here are the numbers:
Burn Time: 7.167/6.617 seconds
Peak Chamber Pressure: 547.5 psi
Peak Thrust: 1458.35 pounds
Total Impulse: 7610.15 pound seconds, 33827.12 Newton seconds
Isp: 131.2 seconds
Commercial Designation: O 5116, 82% O
Click Here for the DataQ original file
Click Here for the original file exported in CSV format
Click Here for an edited version of the CSV file
The data from the test is somewhat questionable, my pressure transducer was out of calibration at the time of the test. The transducer was reading 52.5 pounds of pressure with no pressure on it. So for the data calculations I deducted the 52.5 psi over the duration of the burn to come up with what should be at least very ballpark numbers. The graph above did not have the extra pressure deducted so it reads 52.5 psi high across the curve.
Later that evening I retrieved the motor, and brought it back for disassembly. I removed the transducer and forward bulkhead, then pulled out the EPDM liner and casting tubes.
Above are the remains of the propellant package. You can see the forward closure to the lower left, it's in good condition. Although the EPDM layer pretty much burned off, the RTV is almost entirely intact. To the lower right are the two casting tubes nearest the forward bulkhead, they are nicely charred but in good condition as well. When firing a motor upside down in a static test, the forward bulkhead takes a lot more heat and abuse than it would in an actual flight. The EPDM liner was largely intact, the liner did have some light burn through between the first and second grains from the bulkhead. At the top you can see the rest of the EPDM liner looks virtually the same as when it went in the motor. The rest of the casting tubes are in great shape as well, with no casting tube being ejected or burned through.
Here is the liner and casting tubes at the nozzle end. This is by far the nicest looking/performing thermal and inhibitor layer I've ever had. The casting tubes with the sodium silicate performed flawlessly.
Here's the liner and casting tubes in the light of day.
The nozzle came out easily this time, with modest pressure I simply pushed it out from the forward end using a long piece of wood. In the picture above, you can see the convergent end of the nozzle. The surface is perhaps slightly more coarse around the entrance to the throat, but there was no significant erosion of the nozzle.
This is the nozzle from the divergent end, it looks very good, I don't think I'll even need to smooth it up on the lathe before it's used again.
Final Thoughts:
I ran these grains a lot tighter in the chamber diameter than I normally do. But running grains at close to maximum diameter in a sugar motor can be a double edged sword. The fact that there is little room between the casting tube and the thermal layer allows for more propellant, and it also reduces the combustion gas flow behind the grains, helping to reduce inhibitor failure. But some combustion gas must be allowed on the outside of the grain or you'll end up pressurizing the core and cracking the propellant. As I have done in the past, I made a small notch in each casting tube in the area that separates the grains. This notch is intended to allow some combustion gas behind every grain. Obviously, this worked very well. The risk is allowing too much combustion gas, combined with too much free space which could lead to both inhibitor failure and thermal liner failure. It looks like I hit the optimum right on the head, as I didn't suffer any of the problems of too much or too little space between the casting tubes and the thermal liner.
Erosive burning was really minimized in this test as well. The initial hump I often see due to erosive burning was barely perceptible in the graph of the pressure/time trace. While there is still some erosive burning taking place, it's well with acceptable limits.
I'm comfortable with the performance of the motor in this configuration, and ready to fly it. The next burn of this motor will be in the Defiance rocket!