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Whistle motor teardown due to frequent catos


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Hello pyros

i decided to tear down one of my whistle motor because it would always cato even on a short spindle. Propellant used was 68:28:2:2 which are kclo4:KHP:vaseline:BHT


what i found is this. Is this what you called glassing due to over exceeded pressure on the fuel grain. I suspect this to be the culprit of my frequent catos. Spindle length used was at 1.3in on a 2oz and 1.5 in 4oz.


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From what i see here would be the opposite, although i have to mention that i am not familiar with KHP the fuel looks quite coarse to me .If i would compare that to mine which mire look like a table candle surface-wise. But that aside.. i think you might look at the wrong place here to begin with. The fuel grain itself tells you less about what migth be wrong that your  motor tube. Which is at the same time the number one culprit in such a case. I have unrolled over a hundred whistle motors while i was in the process of dublicating nept like tubes. The tube tells you basically everything you need to know.

If you are using nept tubes, its unlikely the tube´s fault, but to narrow it further down one would need more details .

Hope that helped ;)

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I wonder about the coarse look of the grain too. And what are the brownish patches? It looks like a spiral crack in the grain on the piece on the left, and (maybe) another on the piece on the right? I'm assuming the propellant grain fell apart along the increment boundary lines? What was the loading pressure used? Can we see a picture of the spindle?

I got a little curious about KHP again yesterday since seeing this and decided to whip up a small amount to try. As usual, I thought I had it 'all figured out', lol. My batch was 

68- PP

28- KHP

 2- airfloat charcoal

 2.5- water, spritzed and screened in

It burned and whistled fairly well pressed with no core in a 3/4" ID tube. I made a small rocket with Wolters extreme(ly weak) whistle tooling, 4oz. size. with a 20" stick, it flew fairly well but was very raspy on takeoff. I pressed it to 1500 pounds on the PtoF gauge on an arbor press. There were compression wrinkles on the outside of the tube. I have found the raspiness of benzoate whistles on takeoff to be caused by pieces of the core blowing off, which I have recorded happening. I assume the same is happening with the KHP whistle. KHP itself is very fluffy and makes a crumbly propellant grain, unlike salicylate whistle. My salicylate grains look like a candle too.

My water experiment was not the magical solution I'd hoped for, but it did work. I'm not suggesting anybody else do it. I'm going to try some other ideas to attempt to get a more solid grain with KHP whistle.

Sometimes a few of us have had whistle propellant grains that were crumbly and cracked at the first increment (too much pressure?), and it helped to press a small amount of BP as the first and last increments to 'lock' the whistle grain in place. This was with salicylate whistle though. 

Florida Pyro has done a few experiments with KHP whistle and made a few YouTube videos. You might check him out. He answers questions in the comments as well. 

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Well i think the reason for the course grain is that the propellant wasnt fine enough but all component went thru a blade milling process. Also, the brown patches were residue of my other propellant mainly benzo with red iron oxide. And yes propellant grain fell apart along the increment boundary while im trying to unroll the tube ( no pressure exerted it just fell of)


Well as for the pressing force or pressure, im pressing in at 1555psi. I dont know if my math was right but heres my computaion. 

bottle jack diameter: 0.9inches

rammer area: 0.1319in2 for 12in OD 6in ID

req pressure: 7500psi in the grain

pressure = force/area

7500x0.1319=989lbs of force

989lbs/0.6361in2=1555 psi


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