Wyatt,
Congrats on starting down the APCP path. Lots of fun stuff there. I've made
a lot of small motors (thru N, but most smaller), so here are my words of
wisdom. For what that's worth. The bulk of my chems were purchased from
Firefox.
1. Be safe. PPE. I always put my mixer someplace (like the middle of my
gravel driveway) at the end of a long extension cord. Another time of
danger is extracting the mandrel. No so much for small motors, but be
aware. Contain any inadvertent shrapnel. For small motors, a hole with an
inverted motor is pretty good. Or sandbags if you need to do things above
ground. Hose clamp it to a solid rod as anchor. A good addition is some
metal "net" above your hole. This will "catch" any errant nozzles, flaming
fuel grains, etc. Don't ask how I know.
2. It's a lot easier to make some decent propellant than it is to make your
motor hold together during firing. In particular, even small APCP motors
need some good insulation. More on that in a bit. And pay attention to how
you anchor your forward bulkhead and nozzle.
3. AP: A simple mix of 200 micron and a smaller size one is good enough.
4. Al: Small motors need very little. 2% to 5% is reasonable. The smaller
the Al, the more of a pain it is to get mixed into your mix.
5. Curative. We're assuming HTPB here. I've had great luck with Isonate
143L. With it, I get good grain strength and can pull the mandrels in 48
hrs. I only used IPDI in a mix 1:3 with Isonate when I needed longer pot
life. That didn't happen until I went from the Cuisanart to the 20qt beast.
For smaller motors I used 13% HTPB and 1.8% 143L.
6. Burn rate catalyst. Ferric Oxide works fine. I usually put 1% to 2%.
Cupric Oxide is also a cheap, simple one.
7. Liquid Additives: Tepanol, Castor Oil, DOA (or cousin), Lecithin. I used
around 3% DOA and 0.5% for the others. I added Lecithin when I tried some
high solids propellant (88%). It certainly made a difference.
8. Mixing. As others have said: liquids (except curative), metals, other
solids (Iron Oxide for example), then the AP. Finally the curative. While
mixing the liquids I used a heating plate (at a low setting) to make the
mixing easier. It was removed before adding any solids.
9. Mandrels. Luckily standard aluminum rod sizes fit small motors very
well. 3/8in for 29mm, 1/2 for 38mm, 3/4 in for 54mm, 1in for 98mm. I drill
a hole near one end. I put a screwdriver in it to twist the mandrel out. I
also use the teflon tape that Firefox sells to wrap the mandrels. I have a
teflon casting board that I drill holes in to hold the mandrels upright.
10. Insulation: Find good insulation for wahtever motor case you use.
Years ago I scored a bunch of G10 tubing that perfectly fit a 38mm motor
case. I just cast into that tube, cap it off, and I was good to go. Firefox
sells a great insulating material. It's a carbon impregnated cloth,
available in all sorts of sizes. If I can't find a liner tube, I make one
out of this stuff. One layer works fine. Seal with Hi Temp RTV.
11. Forward Bulkhead Insulation: I cut open a few failed single use motors
and saw that the insulation failed and the flame burnt through the motor.
So I added a G10 washer on top of the motor. I coat the motor chamber side
with the graphite cloth. I also found a fast simple way to make these
custom washers. I mount a roughly cut peice of G10 on my Dremel. I turned
the Dremel on and held it against my disc sander. I have the tube I'm
capping at hand. Just keep sanding it down till it just fits. Voila. Epoxy
that in.
12. Motor Cases: Nothing easy/cheap here. I always did a few simple PVC
(1in) nozzleless motors to make sure the propellant behaves as expected.
Not a sure thing. A few places make kits. Some pop up on Ebay from time to
time. Typically with Al cases, snap rings, and graphite nozzles. If you can
find reasonably priced fiberglass tubing that will work for single use
motors. I lucked out about 20 years ago. A small, now long defunct, motor
maker ordered a bunch of G10 HPR sized tubes. Then he didn't buy them. A
friend of mine found out and we bought a bunch. After your propellant is
well characterized, you can use Aerotech type cases. Not the "well
characterized" part, expensive otherwise. Don't ask how I know... I use
Aerotech nozzles for my single use motors. I just couldn't match the
price/quality of these versus anything I could make.
13. It's hard to improve on Terry's book!
OK, that's enough for now. I added all the granularity as you obviously
know your way around rockets. Anthony's job offer certified that! :)
Please feel free to ask any questions. There's an incredible amount of APCP
knowledge here. I'm a mere rookie compared to many of them.
Burn'em if you've got them.
Brian
