38 mm CTI Motor Grain Interface Shape

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wighty44

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While working with a CTI 38 mm I303 BS motor one of the grains slid out of the insulator. To my surprise I saw that the end that interfaced with the forward grain was not the same shape - the fwd end was somewhat concave and the aft end was flat. I guess I assumed that all end surfaces of motor grains were flat, but clearly that is not the case.

A few questions come to mind:
  1. What is the reason for the shape difference?
  2. What happens if the grain is installed backwards?
  3. Is this configuration peculiar to the motor composition or grain diameter?
 
It is part of the manufacturing process. It allows a bit of space between grains for uniform ignition. You will find most of the 24 thru 54mm loads are like that. You probably wouldn't notice anything if put in backwards. You should have one flat end and one concave end facing each other.
 
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CTI provided me with this reply to the question today that aligns with Jim's note above, "The concave shape ensures that the grain faces will light instantaneously when the motor is lit. All our Pro24-29-38-54 grains have this shape - example drawing"
 
In other words, when one grain face is concave, you don't need a grain spacer.....

Bob
 
Bob, I assume you mean an o-ring inserted between grains? I have only seen those in 75 mm and 98.
 
Correct. Not all other manufacturer use spaces or concave grain surfaces. Without either, the motor deflagration time will be variable.

Bob
 
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