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Maximum Volume for Ejection Charge of Various Motors

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Suborbital Maniac

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Anyone know how I can estimate the maximum safe volume for the ejection charge of various motors ? Specifically, I'm looking at the Estes E to Aerotech F range but it would be good to have a chart for others.....

Making a scaled-up Big Bertha and are pushing what my gut says is safe for reliable ejection but I'd like something more quantitative than my gut !
 

Rex R

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based on what I have found, Estes 24mm & AT 24mm rms motors 'come' with 0.5g of BP which (if I did my math right) ought to provide 15psi to a space of 83 cubic inches (12 inches of 2.6" diameter tube).what size bird were you thinking of?
Rex
 

Suborbital Maniac

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I'm looking at 36" of BT-80.....less 4" for motor mount....32" of 2.6" tube.
 

manixFan

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There are many on-line and downloadable apps to help you figure out ejection charges. Max volume is very subjective. You also need to take into account things like weight of the nosecone, shear pins, etc.

I witnessed a flyer destroy a beautiful upscale Mosquito because he did not augment the ejection charge of the motor. He had turned a nosecone out of solid wood so it was very heavy. The built-in ejection charge had plenty of powder to pressurize a 'normal rocket' but it clearly did not have enough power to fully eject the his nosecone. I've also seen it happen when a lot of weight had to be added to the nosecone for CG balance. That extra weight requires more force to get moving. In your case it's possible you'd need to add nose weight to an upscale Big Bertha if those big fins are heavy enough (and considering how much of the fin is behind the body tube) and you are using a light nosecone. Also take into account that the parachute will be forced back against the motor in flight unless you use a stuffer tube to keep it forward.

I believe some of the 29mm AT motors come with 1 gm bp charges. For example a quick calculation (using Rocket Calc on iOS) shows that 1 gram will produce 15psi in a 3" x 18" body tube section. That is about 107 lbs of force against the nosecone and will shear 3 2-56 nylon screws. How that translates to your setup only you can determine.

One way I like to look at things is this: What are the odds I'll regret increasing the ejection charge size compared to leaving it alone?

Good luck,


Tony
 

Suborbital Maniac

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Problem is, these are BP and single use composite motors....I don't control the ejection charge ! If I did, I wouldn't have an issue but since they are fixed, I'm stuck with what I've got. Question is: what have I got ?
 

Rex R

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couple items of note, the Estes 'super big bertha' had an airframe 29" long and a 10" long motor mount tube, both items combined reduced the required charge size to .5g (which is the amount in a D motor). AT 24mm motors come with 0.5g of BP (IIRC 29mm motors come with 0.75g). you will need a stuffer tube to reduce the amount of space to be pressurized, your parachute only needs 10 - 12" of tube.
Rex
 

DavidMcCann

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this is the ejection on a Shadow from a D12-3, pretty robust. 39" of body tube, and about 29" of stuffer tube, ballpark.
 

Suborbital Maniac

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The question is largely an academic one; an exercise in the science of design. The body is comprised of two 18" sections so we will simply have it split in the middle but I wanted to demonstrate to the boy that this wasn't just on a whim. Of course, he's my kid so he's a smartass and said "prove it, old man !"

Since you're not supposed to smack kids anymore, I'm trying to prove it !

Frankly, I'm rather surprised that motor manufacturers don't provide figures for maximum recommended volumes for their ejection charges.
 
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