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)
