Propulsion Polymers 38mm Hybrid
Experimental Motor
Background
I've got the Propulsion Polymers 38mm H/I/J combo set. They're great
sport Hybrid motors. I plan to continue flying them on sport days with
sport loads and thus continue lining Marcus's pockets with copious
amounts of cash. But I like flying on experimental days too.
Since this motor basically "fly's pipe" and stock pipe at that - it
should be relatively easy to EX it.
Injector & Nozzle
It's a monotube design, with floating injector.
Note the nozzle doesn't have a radial o-ring groove. The combustion
chamber seal is essentially entirely axially compressive.
I do notice that the OD of the nozzle is smaller than that of the
floating injector. I also note that sometimes pipe flows around that
area. I wonder if the smaller OD is to facilitate nozzle removal after
the plastic cools?
Floating Injector Specifications
I'm not planning at this point on building another floating injector,
but the various dimensions nearly specify the experimental nozzle I
want made. As such I don't detail the injector recess or any of that
detail.
The floating injector OAL is 0.890"
The o-ring shoulder OAL is 0.390
The Outside Diameter of the o-ring shoulder is 1.353"
The o-ring is a # xxx; the inside diameter of the o-ring groove is
1.120 and the groove is 0.150" wide for a total o-ring shoulder width
of 0.390"
The o-ring groove is roughly centered, but more precisely 0.130, 0.150,
0.110
The pipe shoulder OAL is 0.5"
The pipe shoulder diameter is 0.929" - sized for insertion into 1" SCH
80 PVC Pipe.
Nozzle Specifications
Stock Nozzle
The stock nozzle is a bit undersized with respect to the injector and
the tubing. Was it a fit for "more standard 38mm tubing"? Or does it
perhaps facillitate nozzle removal by shrinkage after the plastic that
sometimes flows around it?
Nozzle OAL 1.090
Nozzle Pipe shoulder - 0.930" diameter x 0.500" long
Nozzle tube bearing surface 0.270 long
Nozzle closure extension - 1.060 diameter
Nozzle throat diameter 0.390. I haven't mapped this back to any machine
drill size.
Experimental Nozzle
I am debating whether I need a nozzle closure extension. The closure
has a nozzle thrust area built in - but it's aluminum so might not take
the exhaust as well as a stainless steel thrust washer on a snap ring
case.
Nozzle pipe shoulder - 0.930 diameter x 0.500" long
Nozzle pipe bearing surface - 5/8" long with a 0.150" wide o-ring
groove in the middle, cut to 1.120" diameter.
Nozzle closure extension - 0.125"
Nozzle throat diameter 0.390. I haven't mapped this back to any machine
drill size.
Fuel Grains
Sport Grain Specifications
Older sport grains were white polyethylene and didn't come with a paper
insulator tube. Current sport grains are black polypropylene and come
with a paper insulator tube.
The Sport Grain is 1" SCH 80 Polypropylene pipe, with a chamfer milled
on each end. With the chamfer it's 7 9/6" long (or 19.2cm) it comes
with a paper insulating tube that's a snug fit against it and into the
motor. It's 7 3/4" long. The original white poly reloads didn't come
with the paper insulator tube.
A consumed "sport reload" from a J-140 flight. 1" SCH 80 black
polypropylene. You can see the regression starting just after where the
injector shoulder ends. Note the nozzle end. The nozzle sticks into the
grain/ pipe and there is that abrupt nozzle shoulder that causes
turbulence, and you can see the well and burn through that occurred
right at that spot. That's the limiting factor in this grain/nozzle
design. Click on it for a higher resolution image you can zoom.
Experimental Grain Specifications
Tact
Basically, I plan on flying pipe. Straight SCH 80 1" PVC. I'll probably
even try a flight without the end-chamfers he puts on the grains. I'm
sure they're there to help squeeze the o-ring, but I'm not sure they're
totally necessary, in particular if I put a radial o-ring groove on the
nozzle.