The casing was all I needed, so I ordered some 6061 T-6 aluminum tubing from Online Metals. I ordered two pieces of 3.5" outside diameter, .125" thick by 36" long tubing. I used my Solid Motor Design Calculator to estimate the strength of the casing material. 6061 T-6 aluminum has an ultimate tensile strength of 45,000 psi, using the dimensions and the tensile strength, the casing has a burst pressure near 3,200 psi. Well above the maximum expected operating pressure (meop) of my motor.
One thing I did want to change with this motor was the forward closure retention. I have used pins or bolts with great success in the past. But the bolt heads extended beyond the casing diameter, which required a larger motor mount tube in a rocket. So I did a quick modification on my big old South Bend lathe, I installed bearings on the steady rest. That allowed me to cut a groove on the inside of the casing to install a snap ring.
Here is a picture of my first snap ring groove. The tube has a .125" thick wall, the groove is .0575" deep and .12" wide for a .11" thick snap ring. For nozzle retention, I went with my old method of retaining pins, I used (12) .25" steel rivets.
The formula for snap ring stress. Snap Ring Stress=(Bulkhead ejection Force)/(3.14 x [Snap Ring Thickness] x [Chamber Inside Diameter])
So in my case, using 2,000 psi chamber pressure, I have a bulkhead ejection force of (3.14x(1.625x1.625)) x 2,000= 16,583.125 pounds.
Snap Ring Stress= 16,583.125/(3.14x.11x3.25)
Snap Ring Stress= 16,583.125/1.12255
Snap Ring Stress= 14,772.73 psi
The snap ring I'm using is hardened steel, with an ultimate tensile strength of over 90,000 psi. So no problems there.
Next, I need to look at the stress the snap ring creates in the chamber. Here's the formula for that.
Snap Ring Stress in Chamber= Bulkhead Ejection Force / {(3.14/4) x (Chamber Outside Diameter(squared) - Groove Diameter (squared))}
Again, in my case.
Snap Ring Stress= 16,583.125 / (.785 x (12.25 - 11.458))
Snap Ring Stress= 16,583.125 / (.785 x .792)
Snap Ring Stress= 16,583.125 / .62172
Snap Ring Stress= 26,672.98 psi
So the stress created by a chamber pressure of 2,000 psi on the casing material is 26,673 psi. I'm using 6061 aluminum, with an ultimate tensile strength of 45,000 psi. So I'm well within the limits for this metal and pressure.
Here are some of the dimensions of the motor:
Overall Casing Length: 36"
Outside Diameter: 3.5"
Inside Diameter: 3.25"
Wall Thickness: .125"
Snap Ring Groove Depth: .0575"
Snap Ring Groove Width: .12"
Distance From Casing Edge to Groove: .361"
Forward Closure Thickness: .98"
Retaining Pin Diameter: .25"
Distance from Edge of Casing to Pin Holes: .81"
Nozzle Thickness: 1.054"
A couple of things I should note. I'm pretty conservative on most things. The forward bulkhead is much thicker than it would need to be. The distance from the snap ring groove to the edge is more than it needs to be, the nozzle is thicker than it needs to be, and the distance from the retaining pin holes to the edge are more than needed. So I did end up using more of the casing length for these items than I would need to. In all, I ended up using 3.575" of casing length for the nozzle and forward closure, leaving an internal length of 32.425".
My design is for 6 grains, 5" long. If I space the grains .25" apart, that totals 31.5", still leaving me with just under 1" of free space in the casing. I'm not a big fan of sealing the grains to the forward closure and nozzle, so I'll leave that 1" as free space.
Here is the forward bulkhead and nozzle ready to be installed in the motor. One of the grains can be seen to the left.
Here is the forward closure installed.
Motor Weight: 17.6 pounds "ready to fly"
I wanted to set up a casting stand like I used on the SBS-800, a wood block drilled out to hold the coring rod from the bottom, then a plastic disk inserted into the bottom of the casting tube to seal it. The plastic disk would also be drilled out in the center for the coring rod. I had a devil of a time getting the plastic disks cut. For some reason my circle cutter just wasn't doing the job very well. After two hours, I only had one usable disk, so I decided to start casting with just one casting stand.
I was using a 1.25" wood dowel wrapped in aluminum foil for a coring rod. I think a steel rod may still be the best, I think it draws the heat out of the propellant, and the propellant around the core rapidly cools and hardens, making removal of the rod easier.
I had already started melting a propellant batch for two grains, 1,500 grams in all. The KNO3 and erythritol seem to have a long pot life, so it wasn't a concern casting only one grain, and waiting to cast the other.
For some reason, I think perhaps because of the large batch size. The first batch never did seem to get as thin as it should. It also had a lot of bubbles in it. I suppose the thicker mixture wasn't allowing the bubbles to escape. So I applied a vacuum to the batch for about 20 minutes, releasing the vacuum and drawing it down again three times during the 20 minutes.
After casting the first grain I allowed it to cool for 23 minutes before attempting to remove it from stand. All the outer surfaces were hard, but when I went to remove the coring rod, the inside was still very soft. I deformed the inside some while removing the coring rod. After it set up, the core surface was still rough, but appeared to be usable.
For the second grain, after casting I allowed it to cool for 35 minutes. At this point the coring rod was a little hard to get out, as I had to rotate it while pulling. Once the rod was out, it was apparent the propellant still wasn't cool enough inside, again I had roughed up the core surface extracting the rod.
For the third grain I mixed up a batch of 750 grams, just enough for the one grain. This batch melted much better, was thinner, and didn't have the bubbles in it the first batch did. It seems I've reached the upper limit in which the melting pot can keep a full batch in a good, melted form. I waited 45 minutes before removing this grain from the stand, it was just about right, maybe a little longer yet. The coring rod came out easier than anytime before.
The fourth grain sat for 60 minutes before removing from the stand. The grain was almost completely cured, a very small spot in the center remained soft.
Grains five and six were cured for about a hour and fifteen minutes, and still showed a very small spot of soft propellant in very middle.
I recast grain one, as it had a very rough core. So the last two grains cast were numbers 6 and 1.
OK, so I must have been tired. If you read the earlier post I had the last two grains measuring very high density. That wasn't the case. I had used a core diameter of 1.75" rather than the actual core of 1.25". Like I said, I must have been tired. In the end, the densities all fell between .0605 and .0629 pounds per cubic inch.
3" cardboard tube weighs 5.2 grams per linear inch.
| Grain # | Casting Tube | Propellant Length | Propellant Weight | Density | |
| 1 | 4.72"@24.54 gr. | 4.41" | 1.5584 | .0605 | |
| 2 | 4.71"@24.49 gr. | 4.22" | 1.5503 | .0629 | |
| 3 | 4.5"@23.4 gr. | 4.13" | 1.5459 | .0641 | |
| 4 | 4.69"@24.39 gr. | 4.53" | 1.6191 | .0612 | |
| 5 | 4.67"@24.28 gr. | 4.56" | 1.6226 | .0610 | |
| 6 | 4.75'@24.7 gr. | 4.43" | 1.5853 | .0613 | |
| Totals | 28.04 | 26.28 | 9.4816 | .06183 |
OK, test day is finally here. Well ok, so I just got the casing in yesterday, but I was anxious to get this brute tested. The data on the grains is on the table above, so I won't repeat it all here.
Propellant Weight: 9.481 pounds
Nozzle Throat Diameter: .75"
Kn: 392, 417, 359
I kept the Kn a bit low for this test, it seems like a lot of times I get faster burns with larger motors. So to be on the safe side this test is using a load of about 9.5 pounds of propellant out of a designed 10.8 pounds maximum load. If I get a good Isp, that should still make this a low "M" class motor. The largest to date that I have built and tested.
I don't generally get too excited about static tests, and I didn't with this one either. Until I started cutting the EPDM rubber liner for the motor. For some reason, just then I got some butterflies as I realized I was going to get the motor tested today. I coated the upper four grains on all burning surfaces with my pyrogen, the lower two grains were coated on just the ends. I had a devil of a time getting the EPDM lining in the case, I ended up laying the grains on the liner, then rolling it up and inserting it into the case.
Once the nozzle was on and covered, I started loading things up for the static test. With a motor this size, I knew I wanted the maximum protection during the test. So I went to our farm pond which is over 1/2 mile from anything. I set up the motor on one side of an earth berm, and I located my van, launch control box and test equipment on the other side.
Everything was prepped and ready, all clear, data was recording, cameras were recording. 5,4,3,2,1 and ignition!
I held my breath... Almost instantly the motor came to life, and it burned, and it burned, I watched the data coming
in on my computer screen. No large spikes, no inverted "V", a nice long burn. I breathed again, a big
sigh of relief, then one word. YES!
The first thing I did was to take a quick look at the data from the burn, again, it looked good. If anything, it had lower thrust, but a longer burn than expected. Which is really exactly what I wanted. Now I was anxious to get home and analyze the data, to see exactly how I did.
Not a very good picture, the video was all washed out. you can see more from the shadow on the ground.
I just looked at the video from my second camera, it was on the big bale of hay you see in the picture above. I didn't think the video would be any good because it was shooting into the sun, but it's not too bad so I'll include it below.
Click Here for the camera 2 video of the test. Divx AVI 990KB
The codec's for Divx can be found at: Windows 2000/XP http://www.divx.com/divx/play/download/
For Windows 98/ME: http://www.divx.com/divx/divx_v6_me98.php
Here is the thrust/time curve from the test.
And the Numbers:
Burn Time: 6.684 seconds, ~6.1
Burn Rate: .1434"/second
Peak Thrust: 273.34 pounds
Peak Chamber Pressure: ~470 psi
Total Impulse: 1,247.92 pound seconds
Isp: 131.61
As you may have gathered, I'm very pleased with the test results. These low burn rates and good Isp are exactly what I've been looking for in a sugar propellant. The Prelude waits, should be a really fun flight.
Here's a photo of the motor liner removed after the test. It's really in pretty good shape for a 6 second burn. The Scotch tape holding the EPDM rubber is even intact. The nozzle shows a lot of heat discoloration, but no damage.
Here I cut the liner open, and you can see the casting tubes.
You can see a burn through of the EPDM lining after the far left grain. That is the grain next to the forward closure, which normally you wouldn't think would be prone to high heating. I think what happens with erythritol based propellant, is that at then end of the burn any small pieces of propellant left after thrust ends, drop down to the forward closure. Basically the Kn of the motor is too low for the propellant to completely burn, and it melts and drops down, where is sits and smolders for a while. I think that's where the residue I see comes from too. Although this test didn't result in a great amount of residue. I doubt if you'd see this phenomenon in a motor after a flight.
The forward closure also took the brunt of this slag propellant burning after the motor burn. I protected the forward closure with a layer of EPDM rubber glued to its surface with high temp RTV. While the EPDM rubber did burn through in some spots, the RTV was intact and protected the closure.
All in all, a very good test with no problems to be seen with the motor. The motor is ready to test again or fly.