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  #1  
Old 07-28-2023, 04:48 PM
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Arogen Arogen is offline
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Default Motor Retainers (BT-20 and BT-50) Estes compatible

You can mix and match these parts with the Estes produced ones, so if you lost a cap you can print and use one from here as a replacement.

https://www.printables.com/model/51...stes-compatible
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  #2  
Old 07-28-2023, 11:43 PM
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Quote:
Originally Posted by Arogen
You can mix and match these parts with the Estes produced ones, so if you lost a cap you can print and use one from here as a replacement.

https://www.printables.com/model/51...stes-compatible

The Estes retainers all seem like clunky overkills to me. Can they be made much thinner and still be reliable?


Bill
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  #3  
Old 07-29-2023, 01:13 AM
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Quote:
Originally Posted by Bill
The Estes retainers all seem like clunky overkills to me. Can they be made much thinner and still be reliable?


Bill

Probably, but my objective was to make them compatible with what Estes sells so you can replace a lost piece. That makes them inherit the 'clunkiness' of the original.
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Old 07-29-2023, 04:58 AM
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While plastic is not a very dense material, a motor retainer does add mass to the wrong end of a rocket.

What percentage of infill are you using? How light is enough?

After looking at your linked page, I will use your terminology.

1. For functionality compatibility with the Estes parts, it is essential that the engaging threaded part be practically the same. The length of threaded parts of the base and the cap may possibly be shortened to lessen mass. How short is still good enough to remain effective?

2. The thickness (in the fore to aft dimension) of the lip of the base which butts up against the end of the motor tube does not have to be very substantial; it only serves as an aid when gluing the base to the tube. A thinner lip allows the cap to be shortened by a corresponding amount.

3. The thickness of the end of the cap only needs to be enough to keep the motor from kicking through when the ejection charge goes off. Can it be made much thinner?

4. Instead of the raised lines on the outside of the cap, some kind of knurling can be used for grip when screwing and unscrewing the cap.

5. How much can the walls of the cap be reduced while maintaining sufficient strength?

Items #4 and #5 reduces the outside diameter of the cap, possibly making retainers more practical to use with clustered motors.


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  #5  
Old 07-29-2023, 08:45 AM
SolarYellow SolarYellow is offline
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Quote:
Originally Posted by Bill
The Estes retainers all seem like clunky overkills to me. Can they be made much thinner and still be reliable?


Bill


I have a resin printer and a placeholder project plan to design threaded retainers that are much slimmer than the Estes retainers. Target will be to make 24mm no larger than BT-55 and 29mm no larger than BT-60. Should be doable. No guarantee a filament printer will be able to make the parts reliably.

I've had a mishap with an Estes retainer when I put it in a lathe to lighten it up. Thinned down the cap in the Z direction a little too much. Super-secure at room temperature, but after one launch, I picked up the rocket and found no motor in it. Not an empty, but no motor at all. The cap was securely screwed on and the flange that should have held the motor in was intact, looking just like it should with all the exhaust residue and burning on it. Solid, no way for the motor to get past it. I can only imagine that the cap was softened by heat during flight, had enough flex to allow the motor to eject through it when the charge fired, and returned to its non-deformed shape before cooling during descent. Fortunately, the pressure required to do that was also enough to get the laundry out. So I had a perfectly nominal flight, except for the D12-7 somewhere out there on Lucerne.

Last edited by SolarYellow : 07-29-2023 at 10:02 AM.
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  #6  
Old 07-29-2023, 10:16 AM
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Quote:
Originally Posted by Bill
The Estes retainers all seem like clunky overkills to me. Can they be made much thinner and still be reliable?


Bill


While kind of pricey, the Rocketarium 18mm aluminum retainers are very nice, and not too bulky. I use these on models I want to fly using the Aerotech 18/20 RMS, and welcome the added security compared to an engine hook. They are a perfect fit.
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  #7  
Old 07-29-2023, 02:23 PM
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I don't think there is any part of it that is so thick as to need infill. The threads are a match and I can easily swap parts between printed and bought. They screw together smoothly and securely. The end of the cap is 2mm thick, I'm not sure if thinner would be a good idea. The lip between the end of the tube and the bottom of the base is 3mm but for compatibility the total length of the base can't be shorter than the original or an original cap will be too long to use with it. For the diameter you have to take into account how deep the threads cut. Making the cap shorter would be OK. it would still work with a Estes base. I haven't weighed them, but I printed mine in PLA+ and they feel lighter to my hand than the originals.

Minimizing the weight would require abandoning Estes compatibility. Then you can use a thread that doesn't cut as deep and reduce diameter, and shorten the length of the cap and the base.
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