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Les Kramer's L-3 Certification Project
 
MOTOR MOUNT ASSEMBLY
The following parts make up the 75mm motor mount:
• 1 - 75mm x 32" phenolic motor tube PIC
• 3 - 7.51" x 76mm x 1/2" 9 ply birch centering rings PIC
• 1 – Aero Pack 75mm Quick-Change Motor Retainer Assembly
• 2 - 3" Sections Hardwood Railing Stock PIC
• 2 - 3/8" U-Bolts
All parts are assembled with West Systems epoxy (105 resin / 205 hardener / 404 filler) where required. PIC
Marking Fin and Centering Ring Locations on the 75mm Motor Tube
Fellow Sky Buster Jim Mullane made me some cool marking guides with a program called Fins Fun. The application can be found on the Apogee website. Basically, this free application imports a Version 7 RockSim file and prints either a fin guide or body wrap. Since my rocket uses a 7.5" diameter tube and I wanted to mark the fin locations on my 75mm motor tube, Jim simply created a quick 75mm rocket in RockSim and added 4 - 3/8" fins. Next, he imported the file and printed out the fin alignment guides for me.
Once I marked the fin locations, I used a piece of aluminum angle to draw the fin lines along the entire length of the motor mount tube. I used the fin alignment guide to mark the location of the fins on the forward face of centering ring #1 and the aft face of centering ring #2. I also scored a small slot in each line the width of the fin. I located the positions of the centering rings and marked the tube. PIC
Next I removed the centering rings from the motor mount tube and sanded at the centering ring and fin locations. The sanding roughs up the surface so that the epoxy would "grab" better.
Drill Forward Centering Ring Holes
Two 3/8" U-Bolts will be attached to the most forward centering ring for attachment of the “V” recovery harness assembly. The holes were drilled for these and the u-bolts installed. The excess threaded stock was cut off and epoxy applied to the nuts to lock them in place. The stainless steel u-bolts came from McMaster-Carr.
PIC 1, PIC 2, PIC 3
Motor Retention
The Aero Pack 75mm Quick-Change Motor Retainer Assembly consists of the body and motor retention cap. The larger 75 and 98 mm retainers are not bonded to the motor tube with JB Weld as are the smaller retainers. Retention bolts and threaded inserts are included with the motor retainer. The 75mm body is attached to the aft side of centering ring #1 using 6 - #6 bolts. The holes were drilled in centering ring #1 and 6 threaded inserts were installed following the instructions.
PIC 1, PIC 2, PIC 3, PIC 4
Rail Button Support
Later, during the booster assembly, I will install 2 large rail buttons. The rail buttons will screw through the rather thin wall of the booster tubing into a rail button support.
The rail button supports are made from hardwood hand railing stock. The hand rail was attached to the face of centering ring #1 and centering ring #3 with epoxy. I recorded the centerline measurements from the bottom of the booster tube so that I could find them later during booster assembly. PIC 1, PIC 2
Epoxy Centering Rings
Centering rings #1 and #3 were permanently attached to the motor tube with epoxy. At this point, I did not epoxy the area between centering ring #1 and #2. This area will be completed once the fins have been attached to the motor tube.
Completed Motor Mount Assembly PIC
Test Fit in Booster
The completed motor mount assembly was inserted into the 48" booster tube to test the fit.
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