Post by John ReillyAndy, I suspect the reason you haven't had more responses to your
question on end burnin rockets, especially "4 lb." by which I assume
you mean 1-1/4" bore, is that using a black powder based fuel and a
clay choke, they are too inefficient to loft a large "heading" or
garniture of stars and burst. Were that not true, the traditional
skyrocket configuration with the tapered hollow grain would not be
necessary. Certainly, the first "rockets" invented by the Chinese or
the Arabs were end burning as that would have been the way the bamboo
tubes were first charged. The little Estes and Century motors that Orv
Carlisle invented and patented were end burning except for some some
boosters which had a small core. Looking at the surface area of fuel
burning per time increment of a given engine diameter, a hollow core
burns much more fuel/ time increment than an end burner. A cored rocket
can be as much as half empty and therefore weighs less than a solidly
charged motor of the same bore. The entire grain of a cored rocket
burns out before the rocket reaches 1/3 rd. of its' fnal apogee, the
tail streaming behind being the solid part of the grain, or the the
"delay" or "tracking" charge in a model rocket motor. As anyone who
launces small black powder rockets knows, they can't lift much. A
girondola driver (really an end burning rocket) can only lift perhaps
25% to 50% over its own weight. In fact, if you've watched "coronas"
taking off, many of the heavier ones stay spinning on their pivots
burning fuel and lightening the load enough to lift off! Look at how
little fuel and how lightweight an Estes BP motor is! A cored black
powder rocket can lift over twice its own weight to altitude.
All this being said, a good starting point for a 1-1/4" bore end
burning rocket would be choked down to about .30", or 25% of bore.
This is considerably less than a "standard" driver or fountain tooling
choke as made by Wolter or Greg of this newsgroup. You could use the
larger choke diameter but you won't get efficient lift. If I were to
orde one, I would want the nipple to be 3/8" tall before chamfer,
(1-1/4" O.D. of course), and a taper length of about 1/2" on the nipple
to the .30" diameter choke spindle. Spindle length would be about an
inch to an inch and a quarter above chamfered nipple, tapering from
.32" to about .28" at tip. A drilled drift with a .34" hole diameter
and depth of about 1-1/4" would be fine. The convex drift end should
have about a 30 degree bevel as well for efficiency. A shorter 1-1/4"
diameter solid drift would complete the tooling unless you want a
passfire drift as well with a stud to form a perforated clay bulkhead.
Were I having one made, I'd order the base and spindle assembly in
stainless steel and the drifts in hard aluminm alloy.
These numbers are somewhat arbitrary and more to MY liking than
anything else. Keep in mind that your fuel mixture and milling is
equally important to success. Make motors than are just short of
bursting and then back off the of the KNO3 percentage a tad for assured
success.
Hope this is of use. Be safe!
John
Great post, John.
end burners) using whistle. His videos and specifications are available
in their download areas. They are high-performance.
I watched his video and his explanation of the tooling was pretty good.
post. I've certainly duplicated that result when I made 1/2" and 5/8"
end burners. Still, they sound interesting.
post - I'm not that sophisticated with the tooling. For larger
cloth and greased with a light coat of vaseline. 'Tis the po' boy