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.