Model Rocket Motors
A Better Header Much easier than the method described previously
What was I thinking? Molding a header grain then gluing it in? Too much trouble! Someone, I think it was Jon Carter, asked if I couldn't just press the warm propellant in the end of the tube. Hmmm... That can't work! It's too easy! But after pondering awhile, I couldn't think of any reason why it would be any worse than the difficult method I had been using. After a few tests, I have decided it is at least as reliable, and far easier. Less glue used, less on the fingers too! Thus it passes the "Jimmy test" - cheap, easy, and it works.
So here is an empty casing with nozzle in place.
Another trick I have learned is to wrap these uninhibited grains with one wrap of fuse paper. It improves ignition dramatically.
Grain and its wrap are rammed down to press against the nozzle, and a square of plain typing paper is cut the size of the outside diameter of the body tube. Corners are cut off to make a crude stop sign. The paper swatch is pressed down against the propellant grain. This paper will serve to keep epoxy from draining down to the propellant grain, and will also keep the two grains separated, rather than becoming one.
The head-end of the casing is coated with 5-minute epoxy. Great care is taken to avoid coating the paper separator with epoxy. The propellant grain will have no difficulty burning through plain paper to ignite the header grain, but if it is covered with epoxy, it may well resist the flame and ejection will fail. It takes surprisingly little epoxy to form an effective flame barrier.
So I look at it real good before taking the next step. There are a few errant dabs of epoxy on the right, but there is also plenty of dry paper so I'm going to go with it. If there were much more epoxy on the paper, I would remove this swatch and put in a dry one.
A glob of warm, soft propellant is rolled into slug slightly smaller in diameter than the inside of the tube. This is so that it drops in easily and hits the paper without the end touching the sides of the tube. That's another way epoxy could get between the header grain and its flame source.
The soft propellant is rammed down gently but firmly. Another little glob is added to bring it up to the desired level, about 1/8th inch from the end of the tube.
The remaining epoxy is spread over the end of the tube, covering the header grain end, as well as sealing the raw end of the motor tube. I believe this makes the header grain much more strongly bonded to the tube, and also helps prevent forward leakage of gasses which could cause premature ejection.
Apologies for the mediocre photos. The new camera kept complaining that there was too little light.
Finally, I decided to try the flash and you know what? It works!
The header/delay grain is now nicely sealed against moisture, among other things. And the ejection charge is nicely sealed away from the delay grain, so I had better remember to make a hole in the epoxy before loading the ejection charge.
Click Here to see a video of this motor being test-fired. It worked fine! 0.45 second thrust duration, 4.8 second delay (1 meg .wmv file, 7 seconds of video)
