Aft closure lip

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Jeff

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Why is it that the Rouse-Tech aft closures, even on 75MMmotors, barely (1/16th inch maybe?) contact the motor tube hardware (Aeropack retainers in my case). In other words, it seems to me, the untrained amateur, that the entire thrust of the motor is pushing up on a very small circumference of retainer. The lip of the aft closure could be much wider and I am wondering why it isn't. Is this the reason I have seen some motors shoot right through the motor tube and CATO the rocket?
 

falingtrea

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I would say that the amount of force being put on the retention ring of the motor is much lower than the internal pressure. The enclosure should be designed to take both forces without failure. In fact, if motor test stands are using the thrust ring to transmit the motor force to the test stand, then the integrity of the thrust ring has already been tested under worst case conditions. Also,a thrust ring made of aluminum should be as strong as the molded or glued on thrust rings on some single use motors.
 

bobkrech

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The thrust loads developed by a motor that could crush a body tube simply aren't that high. The highest average thrusts for a given motor size is shown below (data from https://www.thrustcurve.org ).

29 mm - 80 pounds over 3.6 linear inches = 22 pounds/inch or 360 psi max load stress
38 mm - 350 pounds over 4.7 linear inches = 75 pounds/inch or 1200 psi max load stress
54 mm - 600 pounds over 6.7 linear inches = 90 pounds/inch or 1440 psi max load stress
75 mm - 800 pounds over 9.3 linear inches = 86 pounds/inch or 1380 psi max load stress
98 mm - 1000 pounds over 12.1 linear inches = 83 pounds/inch or 1330 psi max load stress

The yield strength of most 1/16" wall BT is above these values (typically about 2000 psi, so most 1/16" fiberboard will carry the motor thrust load just fine and phenolic and fiberglass tube strengths far exceed this.

https://www.rocketmaterials.org/datastore/tubes/Axial/

Bob
 
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Jeff

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Bob - I am not talking about CRUSHING the tube I am curious as to whether the motor could SLIP up the tube due to the fact that there is very little aft closure actually in contact with the retainer/end of tube, and I am wondering if this is in fact a valid concern and why the aft closure lip does not extend outward further.
 

troj

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Bob - I am not talking about CRUSHING the tube I am curious as to whether the motor could SLIP up the tube due to the fact that there is very little aft closure actually in contact with the retainer/end of tube, and I am wondering if this is in fact a valid concern and why the aft closure lip does not extend outward further.
I can honestly say that in all the flights I've seen, I've never seen a motor do what you describe.

I've seen failures where the motor went up through the motor mount, and in every case, they were one of two situations -- 1) the motor was in an adapter and the adapter failed such that the motor was able to move up the motor mount, or 2) the motor itself failed and the aft closure let go.

-Kevin
 

bobkrech

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Bob - I am not talking about CRUSHING the tube I am curious as to whether the motor could SLIP up the tube due to the fact that there is very little aft closure actually in contact with the retainer/end of tube, and I am wondering if this is in fact a valid concern and why the aft closure lip does not extend outward further.
In theory you could fail the motor tube in buckling, making it into a bellows like structure, but the failure you're worried about can't happen. Somewhere along the tube there is a ring fixturing the motor tube. If somehow the motor got inside the motor tube by expaning the motor tube, or buckled it, the motor still couldn't get past the ring because of the flange width and the fact the glued tube going throught the ring has no where to expand to and won't move.

The 6061-T6 aluminum used for the motor closures and the retainer flanges has a yield and shear strength in excess of 30,000 psi so it's not going to budge at a stress of of 1500 psi.

Bob
 
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