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  #1  
Old 05-22-2015, 12:34 PM
mikemech mikemech is offline
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Default Ejection charge

I'm building a rear ejection rocket, 24mm MMT in a BT-80 airframe. The motor tube is 300mm long and the airframe tube is 310mm long. The front of the motor tube butts against a bulkhead. The front centering ring is 30mm from the front of the motor tube.
I need to find out if the ejection charges on LPR BP motors are all the same. During ejection testing do I need to use 24mm motors or can I use far cheaper A8-3s?
Also posted in TRF.
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Old 05-22-2015, 01:02 PM
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tbzep tbzep is offline
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Quote:
Originally Posted by mikemech
I'm building a rear ejection rocket, 24mm MMT in a BT-80 airframe. The motor tube is 300mm long and the airframe tube is 310mm long. The front of the motor tube butts against a bulkhead. The front centering ring is 30mm from the front of the motor tube.
I need to find out if the ejection charges on LPR BP motors are all the same. During ejection testing do I need to use 24mm motors or can I use far cheaper A8-3s?
Also posted in TRF.

They aren't the same. In fact, you can take 10 motors of the same designation, such as a D12-5 and have anything from a light ejection to a shotgun ejection. We'd like them to be consistent and so would Estes, but they aren't. I've had 13mm motors blow out body tubes and 24mm motors not even blow half the clay off.
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Old 05-22-2015, 01:18 PM
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Rich Holmes Rich Holmes is offline
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Yes, ejection pressures vary a lot. But you can in principle look at averages and extremes over many motors.

The article “Ejection Pressure Measurements” by Janice and Harold Larson in the Jan/Feb 2007 Sport Rocketry includes the following Editor’s note:

Quote:
The post by “doctor_dynasoar” quotes Ed Brown at Estes regarding the ejection charge sizes for Estes motors: 13 mm motors, 0.4 grain; 18mm motors, 0.6 grain; 24mm motors, 1.0 grain.

HOWEVER, my understanding is that the units here are wrong: should be GRAMS not GRAINS.

In any case, 24mm ejection charges are nominally almost twice as large as 18mm ones.
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Old 05-22-2015, 10:24 PM
mikemech mikemech is offline
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Thanks, guys. That's pretty much what I anticipated. I'll find and read that Sport
Rocketry article and do the calculations. Then I'll just stick an A8-3 in and light it. If it ejects, good. If not, try a C-11. If that ejectes, good. If not, redesign, rebuild, or use more talc or dry graphite. Yeah, I'll do that.
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Old 05-23-2015, 09:19 AM
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Doug Sams Doug Sams is offline
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Quote:
Originally Posted by mikemech
Thanks, guys. That's pretty much what I anticipated. I'll find and read that Sport
Rocketry article and do the calculations. Then I'll just stick an A8-3 in and light it. If it ejects, good. If not, try a C-11. If that ejects, good. If not, redesign, rebuild, or use more talc or dry graphite. Yeah, I'll do that.
<whisper voice>You might could sweeten the ejection charge a tad to ensure you get the laundry out. You can put a pinch of 4F on top of the ejection cap, and place a ball of wadding on top of that to hold it in. I doubt anybody over here would make a federal case out of using this trick, but be careful who you share this with - you don't wanna attract the interest of the internet cops </whisper>

Doug

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Old 05-23-2015, 03:59 PM
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ghrocketman ghrocketman is offline
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+1 to Doug's idea.
I however would use a disc of masking tape instead of wadding to cover the loose FFF or FFFF powder.
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Old 05-25-2015, 11:07 PM
mikemech mikemech is offline
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A8-3 worked fine. Shot the MMT ~2m in the air.
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