Lessons Learned:

Kosdon 38mm Thrust Ring Failure

This page documents what I feel is an incompatibility between the Kosdon (clamp-style)Thrust Ring and the PML Positive Motor Retention System (PRS-54). The photograph at right shows the 38mm and 54mm clamp-on thrust rings. The diagram below it shows the Positive Motor Retention System securing an Aerotech single-use style motor.

The Kosdon Reloadable Motor casing has some great advantages. It is much less prone to forward seal failures. Every motor goes together the same way. They are very easy to assemble.

There is one design disadvantage (at least in the early 38mm casing) - the smooth motor casing with no ridge to keep the motor from passing through the rocket. If you don't build an Estes style engine block into your design, you need to use a thrust ring. This is a ring that clamps on the outside of your motor casing using a set screw. The early versions of the Kosdon case relied entirely on the clamping action of the ring to hold it in place - there are no grooves of any kind for the ring to hold on to.

And hold on is exactly what it did NOT do for me. (I once had a Norwegian Elkhound that got loose in my mother's back yard and ate an entire suet ball she had left out there for the birds. The Kosdon motor passed through my rocket at nearly the same velocity as that suet ball went through the dog...)

Click Here for Downloadable video.

There may have been a number of contributing factors, but I believe that the major cause of this failure was the combination of the old design Kosdon Thrust Ring and the PML Positive Motor Retention System (PRS-54).

This diagram documents my theory as to why the PML Positive Motor Retention System is incompatible with Kosdon reloadable motors:

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THE HYPOTHESIS:

Because the nozzle of the Kosdon motor is recessed from the end of the casing, the diameter of the aperture in the motor retention plate was less than the diameter of the plume of flame. As a result the blast plate became red hot, and this heat was transferred to the thrust ring. The thrust ring expanded as its temperature increased, causing the ring to fail.

Check out these photos of the Motor Retention Plate after the flight. You can tell it got extremely hot - it is charred, distorted, and one edge of the aperture is even melted away. This ring was in pristine condition prior to the flight - perfectly flat with a circular opening.

A motor retention plate suitable for use on a Kosdon system - if that's the right solution - would have a much wider aperture. The PML plate is fine with an Aerotech motor because the nozzle is flush with the plate. With the Kosdon motor, it was as if I had applied a blow torch to the retainer.

I don't know if I could have predicted the superheating of the motor retention plate before the flight. But I will tell you that I was using a borrowed casing, and the only time the Kosdon motor was inserted into the rocket was about ten minutes before the launch. If it was my hardware and I had assembled everything ahead of time I could possibly have predicted the impending disaster (although, quite frankly, I doubt it).

It is also worthy to note that the owner of the casing flew it in his own rocket just before he handed it to me. In that flight the chute failed to deploy and the rocket landed hard. The jolt could have loosened the thrust ring slightly, and to my knowledge the set screw was not tightened between flights.

What's the lesson?

1. Make sure your hardware is compatible with your rocket design. - This is especially important if you are looking at the rocket/hardware combination for the first time immediately prior to launch. There is a reason that NASA runs all those simulations....
   
   
2. Don't take for granted that any component of your hardware is in proper working order. - I could kick myself for not tightening the Thrust Ring set screw.
   
   
3. Don't try to do too much on launch day. - Since I was attempting a Level Two certification, the only flight I had scheduled for this day was this one. What I didn't realize is that the hardware owner was also going to fly with the casing. By the time I finally had the casing in my hands, I was rushing to get the flight in before the flight line closed. Given the amount of time actually available, I was essentially out of time.
   
   
4. Don't be afraid to scrub your launch - The day was windy, I had to wait for the casing, I was nearly out of time - there were a number of occasions that day that I had determined it wasn't a good day to launch. Ever wish you had trusted your instincts and waited for another day? It shouldn't matter that you've worked hard to get to this launch, or that friends made a long drive to see your flights. Wait until the conditions are more favorable. Sure, you'll be disappointed - but that's nothing compared to seeing your prized rocket reduced to shrapnel.

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