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Need help calculating ejection pressure

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Dane Ronnow

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I'm getting ready to start assembling this rocket, and I'm a little nervous about whether or not the ejection charge from Aerotech F and G motors will be enough to push everything out the top. And as I'm trying to figure this out (looking at various BP ejection charge calculators) I realize that I don't know if the tube length measurement is from the forward CR to the bottom of the parachute package, to the bottom of the nose cone, or all the way to the top end of the body tube.

The body tube is 2.6" diameter (lightweight BT-80). The motor tube, which is 7" long, is going to be cut down to 4.125", with the forward CR flush with the end.

Distance from the forward CR to the bottom of the parachute package is 14". To the bottom of the nose cone, 21". To the top of the tube, 24" (3" shoulder on the cone).

I'm working under the assumption that the three Aerotech motors I'll be using (F67, G74, G80) have a .7 gram black powder charge.

If the distance is to the bottom of the parachute package, the ejection pressure is 19 psi.
If the distance is to the bottom of the nose cone, pressure is a little over 12.
If the distance is to the top of the tube, pressure is just under 11.

If the pressure is not sufficient, I can leave the motor tube at 7" long, and glue a third ring flush with the end. If I need more length than that, I'd have to figure out how to extend the 7" tube. (I've already epoxied the ejection retainer base the to the tube, as well as the forward and aft CRs.)

BTW, the nose cone is a loose fit.

I'm attaching the .ork file if that helps. Any and all input would be greatly appreciated.
 

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Nytrunner

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Distance is forward centering ring to base of nosecone. That's the surface that'll be pushed outwards.

You have a 2.6ish diameter tube, so that's 5.3 in^2 of push area. 12 psi on that area is over 60lbs of ejection force, so I think you're good.

What ejection calculator did you use?

Keep in mind thise single use motors also worked for Estes Pro Series II kits like the partizon which was 50" long and 2.5 diameter.
 

shawn_rocket

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I'm getting ready to start assembling this rocket, and I'm a little nervous about whether or not the ejection charge from Aerotech F and G motors will be enough to push everything out the top. And as I'm trying to figure this out (looking at various BP ejection charge calculators) I realize that I don't know if the tube length measurement is from the forward CR to the bottom of the parachute package, to the bottom of the nose cone, or all the way to the top end of the body tube.

The body tube is 2.6" diameter (lightweight BT-80). The motor tube, which is 7" long, is going to be cut down to 4.125", with the forward CR flush with the end.

Distance from the forward CR to the bottom of the parachute package is 14". To the bottom of the nose cone, 21". To the top of the tube, 24" (3" shoulder on the cone).

I'm working under the assumption that the three Aerotech motors I'll be using (F67, G74, G80) have a .7 gram black powder charge.

If the distance is to the bottom of the parachute package, the ejection pressure is 19 psi.
If the distance is to the bottom of the nose cone, pressure is a little over 12.
If the distance is to the top of the tube, pressure is just under 11.

If the pressure is not sufficient, I can leave the motor tube at 7" long, and glue a third ring flush with the end. If I need more length than that, I'd have to figure out how to extend the 7" tube. (I've already epoxied the ejection retainer base the to the tube, as well as the forward and aft CRs.)

BTW, the nose cone is a loose fit.

I'm attaching the .ork file if that helps. Any and all input would be greatly appreciated.
Tighten the fit of the nosecone, and ground test .7g. More info about how I "learned" how to test this:
https://www.rocketryforum.com/threads/ground-testing-ejection-charge-for-mpr-motor-deploy.162007
 

Dane Ronnow

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Tighten the fit of the nosecone
What?! My brain just did this really amazing triple back flip with a half-gainer. Two seconds after I read that line, I thought about reloading ammunition for high-power, long-range target shooting, and this is what hit me—there needs to be some resistance against venting in order for pressure to increase sufficiently. Is that what I'm missing when I think the nose cone should be a loose fit?

The link you posted was interesting. I always pictured ground-testing BP charges as relevant only to those who fly RMS, dual deployment stuff. I guess I could load a charge in the motor tube and block the back end with a spent casing, then light it off. Trouble is, I have to build the rocket to reach that point, and once that's done, I've burned the bridge that would be extending the motor tube and adding a third CR to decrease internal volume, if that's what's needed.

But the link was helpful. As an aside, I had to chuckle when I read about the blow test. That's what I always did with rockets when I was a kid. Now that I have COPD . . . (And if I ask my wife to blow the rocket she'll kill me.)
 

Nytrunner

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That sounds right.

They say an optimum fit is if you can grab the top (separating) piece, and the bottom doesn't immediately fall off. But if you start to shake/jostle it, the bottom part will start to slide off.
 
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