Venting ejection charge, how to do it?

Hello, I have a cluster rocket and I want to vent the ejection from two of my motors. The rocket has three tubes, with one motor in each tube. The rocket will be dual deploy, and I want to deploy the parachutes in those tubes electronically.

The tube is BT-60, and I've capped it off about 2" above the top of the motor with a plywood bulkhead and coupler. Since the volume is quite small (about 4 cubic inches), it will develop a lot of pressure. I imagine I'll need quite large holes to adequately vent the ejection charge and prevent blowing out the thin wall BT-60. Does anyone have any suggestions for determining how large the venting holes need to be?

I should add that I'll be using 24mm BP motors, so D12s and C11s....

Why vent. Just remove the ejection charge and plug it.

Don't think you can modify BP motors, I don't think that's legal.

That was my concern, I considered plugging a "-0" motor, but that seems to be a grey area. I also don't see the D12-P anymore, I remember that used to be a motor.

1.Use booster motors without an ejection charge. If your club/meet rules allow ejection of spent engine casings then no vent needed. If not, then:
2. Booster motors with a slotted vent going into the valley between the three tubes in the center of the rocket. Leave the bottom of the valley open, use engine hooks for retention. OR
3. Open vent to the outside through the BT-60 tube. Less appealing visually.
It would be a good idea to have at least one of the motors with an ejection charge opening into the main body tube as a backup in case the electronics fail. Use a delay that will fire well past apogee.

Vent between the mmt tubes, since you are not using the ejection charges use different length delays so they go off sequentially. No centering rings needed for the mmts.

Whichever method you choose I'd suggest a static test using a mock up that closely simulates the configuration you have.

Use motors without a delay (-0) rather than trying to vent ejection charges. The NCR Cluster Duck vents 6 of the 7 motors. The motors are 18mm and built stock they vent through 1/4" holes at the top of each motor tube. I didn't like the amount of damage they were taking, so in my second one I cut the whole top off the motor tubes so they vent through a 1/2 gap between the tops of the motor tubes and an epoxy-coated bulkhead. I still didn't like all the soot so I now plug the tops of the motors with epoxy and launch from the mid-powered pads.

kuririn said:

3. Open vent to the outside through the BT-60 tube. Less appealing visually.
It would be a good idea to have at least one of the motors with an ejection charge opening into the main body tube as a backup in case the electronics fail. Use a delay that will fire well past apogee.

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Yes, thanks for the reply. That is my plan, I'm just unsure how large the vent should be to prevent over-pressurization of the thin wall tube. I'll ground start the center motor, and use that for motor ejection at apogee. That'll ensure that some recovery deploys. The outboard motors will be electronically air started, and I want to deploy dual parachutes electronically out of the side tubes.

rharshberger said:

Vent between the mmt tubes, since you are not using the ejection charges use different length delays so they go off sequentially. No centering rings needed for the mmts.

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I'm not sure I'm able to visualize this. Is this venting downwards?

lakeroadster said:

I'd suggest a static test using a mock up that closely simulates the configuration you have.

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That's probably the smart thing that I oughta do.

Dugway said:

Use motors without a delay (-0) rather than trying to vent ejection charges. The NCR Cluster Duck vents 6 of the 7 motors. The motors are 18mm and built stock they vent through 1/4" holes at the top of each motor tube. I didn't like the amount of damage they were taking, so in my second one I cut the whole top off the motor tubes so they vent through a 1/2 gap between the tops of the motor tubes and an epoxy-coated bulkhead. I still didn't like all the soot so I now plug the tops of the motors with epoxy and launch from the mid-powered pads.

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Interesting that 1/4" vents were enough to prevent over-pressurization. That's encouraging.

Thanks.

Dugway said:

Use motors without a delay (-0) rather than trying to vent ejection charges. The NCR Cluster Duck vents 6 of the 7 motors. The motors are 18mm and built stock they vent through 1/4" holes at the top of each motor tube. I didn't like the amount of damage they were taking, so in my second one I cut the whole top off the motor tubes so they vent through a 1/2 gap between the tops of the motor tubes and an epoxy-coated bulkhead. I still didn't like all the soot so I now plug the tops of the motors with epoxy and launch from the mid-powered pads.

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All of this is good advice. Clair knows what he’s talking about.

Joe

Yes venting between the motor tubes is downward, another option is to find -P motors (plugged).

rharshberger said:

Yes venting between the motor tubes is downward, another option is to find -P motors (plugged).

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Ok, thanks, I can see it now. It seems that the D12-P has disappeared, I wonder if Estes discontinued it.

Well, for what it is worth, I found that Animal Motor Works has a nice stock of D12-P motors.

I also tested the venting of the ejection charge on a test mock-up of the side boosters. Turns out that two 1/2" holes vents the pressure just fine, no damage to the BT-60 tube.

Static test, on a Workmate.. well done sir.

Bore of a D motor = 0.70 in, area = 0.384 sq. in.

Area of (2) 1/2" holes = 0.393 sq. in.

I know... there's more to it than simple area calculation. But with any given airframe that can handle the flight stresses of a given motor, it makes sense that as long as the vent hole area is equal to or greater than the bore of the LPR motor, the airframe will not be damaged.

I still think the motor vendors should supply it's users with data though on the force their motors produce during ejection. Then the rocket designer could calculate the internal pressure generated.

To improve the chances your body tube will take the heat, use a tube coupler to give it a thicker wall... You might also look into making some StarLite fire retardant to help keep things cool in there.

lakeroadster said:

Static test, on a Workmate.. well done sir.

Bore of a D motor = 0.70 in, area = 0.384 sq. in.

Area of (2) 1/2" holes = 0.393 sq. in.

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Yeah, they say "multi-purpose"

And yup, that's how I picked two 1/2" holes, it seemed safe. But then I had visions of the ejection charge blowing up the side pods, but turns out to be a non-issue. This opens up more motor options, and one less thing to worry about.

K'Tesh said:

To improve the chances your body tube will take the heat, use a tube coupler to give it a thicker wall... You might also look into making some StarLite fire retardant to help keep things cool in there.

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Thanks for the suggestion, I haven't heard of StarLine fire retardant, so that is good to know about. I do have a coupler on the inside there, so it is mostly double thickness, but it doesn't extend all the way down to the motor mount.

craaaazy