I found some old 1" diameter aluminum tube in my metal rack, I'm not sure, but it's likely 6061. The wall thickness is .045", which seems incredibly thin, but in a casing this size it is more than adequate with an estimated burst pressure of over 4,000 psi if it's 6061. Even if it's the worst aluminum alloy made, the burst pressure is about 1,100 psi, and I only intend to run the motor at Pc of less than 500 psi.
I thought I'd try something a little different with this motor and make an epoxy forward closure. Using my Solid Motor Design Calculator and an UTS of 7,500 psi for the epoxy, a .35" thick plug should handle 3,000 psi.
Here is the casing getting ready for the epoxy forward closure. I drilled 8 small holes in the casing, then taped over the holes. The holes are intended to give the epoxy plug a better hold on the casing. I also roughed up the inside of the casing, again to give the epoxy a better grip. A layer of aluminum foil tape was then applied over the opening to keep the epoxy in the casing. For good measure, I poured the epoxy plug about .75" deep. I used Mr. Fiberglass brand epoxy.
For casting tubes I made 2 wraps of 100# card stock on a 3/4" mandrel, I glued the starting edge with Elmers wood glue, then finished wrapping the card stock and attached the end of the card stock with regular transparent tape. The paper tubes were then given two wraps of aluminum foil tape and removed from the mandrel. The foil tape did a good job of keeping the tubes more or less cylindrical.
You can see the epoxy plug in the casing, along with the four tiny grains, the nozzle and a snap ring.
This is the same shot only this is the nozzle end with the snap ring groove cut.
There is one thing I forgot to do, I intended to add some fiberglass strands to the epoxy to give it more strength. I guess there's always next time. If this one holds up to firing, a plug with glass strands added should be that much stronger.
I used my standard APCP formula, 18% R45M, 12% Al (325 mesh), 68% AP (200 um), 2% Isonate 143L. I made a 260 gram batch, which should have been enough for 3 reloads of this motor, but I ended up with better density and was left one grain short of 3 reloads.
Each of the grains were spaced about 1/8" apart and taped together with aluminum foil tape, then I used another 1.5 wraps of 100# card stock to wrap the "grain stick" for thermal insulation.
I sharpened the end of an extra piece of the motor casing to make a "cookie cutter" out of it. I used this cutter to make a disk of EPDM rubber to thermally protect the epoxy bulkhead from heat. After the EPDM rubber disk was in place, I inserted the propellant/insulator package into the casing. I applied some high temp RTV to help the nozzle seal to the cardboard thermal liner, then the nozzle with it's single o-ring was inserted. I got a little carried away inserting the nozzle and drove it in past it's desired position. The risk is that I damaged the cardboard thermal layer in that area, which is also the most likely area in a motor to suffer heat damage. I guess if I have a casing melt through near the nozzle I'll know why...
Here are the grain numbers:
Number of Grains: 4
Grain Length: 1.9375" each; 7.75" total
Propellant Weight: 60.8 grams
Casting Tube Weight: 2.2 grams each; 8.8 grams total
Total Motor Weight w/Propellant: 159.6 grams
Nozzle Throat Diameter: 3.1"
Kn: 168 progressive to 200 max
The weather finally changed back to winter here in Iowa, after 6 weeks of almost Spring like conditions the cold and snow are back. Despite a rather blustery day, I decided to test the new motor. The motor is a little small for my test stand, so I clamped it to my antenna tower in my yard with steel hose clamps, using a bolt head on the tower to keep the motor from sliding out of the clamps. This would give me a chance to use my new launch controller and my new (to me) video camera.
I used the internal battery in the launch controller, and as expected I had instant ignition using a 100' cord to the igniter. The motor burned for a second, then came to pressure and had a very nice, perhaps one second burn. I did shoot a video of the test, but I forgot to change the filter on the camera from inside to outside, so the actual video of the burn sort of really stinks. Shortly after the test I realized my mistake, so I shot a few seconds of video using the proper filter for the title. This camera is really big and heavy, which also makes working the launch controller and camera at the same time a bit tricky. I really need to find a good video tripod for this camera...
Here's video, poor quality but good audio. Click Here for the video, 1.2 MB in wmv format.
Here's the motor after the test. Seems to be in fine condition.
The motor disassembled, again it appears to suffered no damage from the test.
All said, it was a good test. I'd say this little motor is ready to fly in a small rocket. In an under 1 pound rocket with a body diameter of 1.6", this motor should propel the rocket to nearly 3,000'.