76mm Custom Motor
 
February 3, 2004
 


Project Parameters:

Where to Start:

Well, it was time to move up to a larger motor in order to fly my PML Ultimate Endeavor on sugar propellants. Since this rocket weighed about 13 lbs it was going to require some serious lifting power. I ended up "boring out" the 54 mm motor mount already built into the rocket and refitting it with a 76mm motor mount. But that is another story altogether! I decided to bypass all traditional motor retaining hardware and instead build a mounting flange directly into the aft closure. The motor then is screwed directly to the bottom of the rocket.

As with my smaller motors, I decided to utilize threaded closures for all of the hardware. One interesting piece of learning came when I did my first static test with two grains. After one second of nominal burn, the aft closure and graphite nozzle were liberated from the threads that bound them! The formula that I had been using for determining thread depth turned out to not provide a deep enough thread. It worked fine up to 38mm but the additional force built up when moving up to 96mm was too much. The deeper threads shown in the drawings below hold up just fine.

These are the finished pieces made from the drawings below. Note the use of a graphite nozzle that is secured by the threaded aft closure. I also get a little extra divergent cone with the aluminum and it seems to take the heat fine. Also note the forward closure is made of two parts. I find that with one part, the o-ring starts to bind as it is rotated in the threading. The two parts also allow me to make interchangable forward closures (delay elements, pressure gauge fittings etc.) without creating threads each time.

This is the two-part forward closure assembled to the casing.

Click the image above for the design of the propellant with Sorbitol and KNO3- Thanks to Richard Nakka's Spreadsheet!


This is me ready to load the motor on launch day (METRA September 2003 EX launch- Rich Kroboth took the pictures). Three grains like the one in my hand here were used.

Ready to go to the Launch Pad! (Sure hope this works!)

Click on the images above to see the altimeter/accelerometer data for the first flight. Note the 20G accelerations at both the drogue and main chute deployments


Click on the images above to see a closeup of the boost data. Took a little longer to come up to pressure than predicted which caused a slight instability coming off the rod but it quickly straightened out for a nice flight to 2700 feet. Next flight will utilize 5 grains for more boost and altitude.


Click on the images above to see the complete drawing of the threaded aluminum Casing


Click on the images above to see the complete drawing of the aluminum Aft Closure


Click on the images above to see the complete drawing of the graphite Nozzle


Click on the images above to see the complete drawing of the threaded Forward Closure


Click on the images above to see the complete drawing of the Forward Smoke Chamber


 

Conclusion:

Things do get a bit trickier as the motor gets bigger. Parts are a bit more dificult to handle on the lathe and tolerances become more important due to the increased forces on closures. The sugar propellants seem to scale up nicely though, with performance being very predictable and consistent. Richard Nakka's spreadsheet predictions held up very well and the flight was beautiful. A motor this size creates a lot of smoke on launch and is pretty spectacular at liftoff.

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