WizardOfBoz
Active Member
So I purchased a Madcow DX3, with a 38mm motor mount and a PML motor retainer. I don't have level 1 cert, so I got a 38-29mm motor adapter so I could actually test launch before putting an H into the thing. In dry fitting the adapter together, I realized that if I put it together as the instructions suggested, I would need a very complex retainer to hold the 29mm motor in place. I made a drawing to help me think, and to communicate the thoughts. If the discussion below is useful, great. If not, ignore. If I'm wrong, I'd love to hear why (a better or more proven idea would be super).
So in diagram a), I show a standard 29mm motor, with a shoulder on the aft end to serve as a thrust ring. RM indicates the registration line of the shoulder. This is where the aft of the motor tube meets the motor and RM sets the position of the motor and motor nozzle.
As I'm fairly new to high powered (or medium powered here) rockets, I'd be a bit reticent to create an adapter that moves the motor forward or aft. I figure that the rocket designer knew where they wanted the motor, and put it there. So in diagram b), I've put the adapter registration line (RA) is aligned exactly with RM. This keeps the motor in the exact same place as a larger motor would be. It does require a spacer because the shoulder ring on the adapter (green) is wider than the motor shoulder (roughly 1/2" and 1/4"). A washer along would retain the motor, but the spacer keeps the it from shooting aft when the ejection charge goes off.
To avoid the need for a spacer, one could move the inner motor tube back 1/4" or so. This is diagram c). The back of the motor shoulder then corresponds to the back of the out motor adapter tube and shoulder. A flat washer (41mm by 25mm or so) can then keep the motor secure for ejection. The shift is small - about 6mm for my adapter kit. Apogee sells an adapter kit and Tim van Milligan put a little scalloped wooden washer in the kit to allow retention, apparently using drawing c arrangment.
Diagram d) shows how my instructions suggested making it. If one does this the inner motor tube sticks out so far that the 38mm motor retainer can't work without a very complex "top hat" retainer. Or duct tape. And the motor is moved aft about 5/8", changing both the nozzle position and the CG a little bit. If the instructions would have worked I never would have dove into this!
Also, my adapter kit doesn't have a washer. I'd probably buy the Apogee product based upon completeness.
For now, I'm going with the arrangement in diagram b. The best idea for a spacer I've found is a copper tubing union for a 1" tube, sawn to be about 7mm long. This with a 1-5/8 fender washer, drilled out to 1 inch ID. One might try CPVC for the spacer but motors are supposed to be allowed to get to 200°C, and CPVC has a service temperature limit of 200°C.
Comment or ignore as the Spirit moves you.
So in diagram a), I show a standard 29mm motor, with a shoulder on the aft end to serve as a thrust ring. RM indicates the registration line of the shoulder. This is where the aft of the motor tube meets the motor and RM sets the position of the motor and motor nozzle.
As I'm fairly new to high powered (or medium powered here) rockets, I'd be a bit reticent to create an adapter that moves the motor forward or aft. I figure that the rocket designer knew where they wanted the motor, and put it there. So in diagram b), I've put the adapter registration line (RA) is aligned exactly with RM. This keeps the motor in the exact same place as a larger motor would be. It does require a spacer because the shoulder ring on the adapter (green) is wider than the motor shoulder (roughly 1/2" and 1/4"). A washer along would retain the motor, but the spacer keeps the it from shooting aft when the ejection charge goes off.
To avoid the need for a spacer, one could move the inner motor tube back 1/4" or so. This is diagram c). The back of the motor shoulder then corresponds to the back of the out motor adapter tube and shoulder. A flat washer (41mm by 25mm or so) can then keep the motor secure for ejection. The shift is small - about 6mm for my adapter kit. Apogee sells an adapter kit and Tim van Milligan put a little scalloped wooden washer in the kit to allow retention, apparently using drawing c arrangment.
Diagram d) shows how my instructions suggested making it. If one does this the inner motor tube sticks out so far that the 38mm motor retainer can't work without a very complex "top hat" retainer. Or duct tape. And the motor is moved aft about 5/8", changing both the nozzle position and the CG a little bit. If the instructions would have worked I never would have dove into this!
Also, my adapter kit doesn't have a washer. I'd probably buy the Apogee product based upon completeness.
For now, I'm going with the arrangement in diagram b. The best idea for a spacer I've found is a copper tubing union for a 1" tube, sawn to be about 7mm long. This with a 1-5/8 fender washer, drilled out to 1 inch ID. One might try CPVC for the spacer but motors are supposed to be allowed to get to 200°C, and CPVC has a service temperature limit of 200°C.
Comment or ignore as the Spirit moves you.
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