How much space between charges and motor in DD fincan?

I just finished a Nike Smoke intended to earn my L1. It hasn't even flown yet, and now I'm looking for a DD rocket that could work for an L2 cert. The MAC Zodiac 3" is looking attractive. Mike's a great guy, even wanted to talk with me by phone to make sure things come out right. The fincan is usually 27" long, but he's happy to cut it to any length.


The shock cord will be Onebadhawk 11/32" tubular Kevlar 25' long with three loops. Drogue chute is TFR 15". All will be Nomex-covered. There is 5.25" between the ends of the charge canisters and the top of the CTI K600-WH motor. Is this enough room? The fincan in this image was extended from 27" to 29", but could be 30" and keep the Zodiac lines.

Thanks for your help!

For the sake of clarity - I'm aware that it's necessary to adjust the BP charge for the volume and shear pins. My question may seem lame, but I'm a good R/C modeler new to HPR. I simply don't know if the top of the Y harness, the tubular Kevlar, chute, swivel and quick-release will fit in 37 cubic inches (5.25" x 3"). Another inch in length results in 44 in^3.

Well, the math says a 25' cylinder 11/32" diameter is 28 in^3. Hmmm, that's 3/4 of the volume and assumes it's tightly packed. If the fincan tube is 30" long, it's 28 out of 44 in^3. That sounds reasonable. 31 sounds safe.

Has anyone used a setup like this and have experience with how much room is used by the cord?

I'd ask for the extra inches. That is really tight. Some of the shock cord could go down around the motor, but that could create a tangle hazard. It may be doable, but .... The extra inches will also increase stability. The CP is pretty far forward with those smallish fins.

Rockethound said:

There is 5.25" between the ends of the charge canisters and the top of the CTI K600-WH motor. !

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Clearly the answer here is that you have 5 more inches of space for motor

Personally, I think you're probably going a bit too big on the kevlar. The length is ok, but something smaller in diameter would be fine.

I've done the "push the kevlar down around the motor with a stick" for an extra large motor. It worked, but I wouldn't recommend it in normal cases. In that particular instance, I probably unpacked/repacked the chute, shock cord, etc for a good hour before I got it to where I wanted it.

Have you also considered how you'll attach a tracker? Usually I put mine somewhere on the drogue line.

edit: Not sure how the Mac Zodiac is set up, but you might also want to offset the switch band a little bit, to move the av bay upwards.. might buy you another 0.5- 1" in space?

Personally, I think you're probably going a bit too big on the kevlar. The length is ok, but something smaller in diameter would be fine.

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Thanks to everyone. All the advice is good, and this particular suggestion is very helpful. Thanks for recognizing the missing knowledge. Let me see how this works out.

Rockethound said:

Thanks to everyone. All the advice is good, and this particular suggestion is very helpful. Thanks for recognizing the missing knowledge. Let me see how this works out.

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There's nothing "wrong" with going with bigger kevlar- it's just probably a waste in this case.

There's nothing "wrong" with going with bigger kevlar- it's just probably a waste in this case.

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That's the part I didn't know. What a difference 3/16" Kevlar makes!


The volumes for the Kevlar are now appropriate for their raw volume plus packing space. There's room for the Y harness from the MM. This is at 30" fincan length rather than 29".

Re the tracker - Mike has a nifty NC accessory to hold trackers that I'm considering.

djs, saw you L2'd on a DS2.6 - that's the other rocket in the running. Funny - our L1 / L2 timing would work out about the same by the calendar, and I'm planning an H115DM for L1. Sparky matters!

Thank you!

Rockethound said:

djs, saw you L2'd on a DS2.6 - that's the other rocket in the running.

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2.6" darkstar with a 54 motor mount. It's almost minimum diameter. Lots of fast, high flights on it until I blew it up a few weeks ago on an EX motor.

Uh... DavidMcCann blew up his 3" Zodiac on an EX motor. Small world again.

Rockethound said:

Well, the math says a 25' cylinder 11/32" diameter is 28 in^3.

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Kevlar is not a solid cylinder. It is hollow and compresses. The volume is much less.

Rockethound said:

Uh... DavidMcCann blew up his 3" Zodiac on an EX motor. Small world again.

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The hazards of experimental rocket motors, if everything isn't just right its boom or road flare.

Kevlar is not a solid cylinder. It is hollow and compresses. The volume is much less.

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While building skills transfer from R/C to HPR, there is so much new knowledge that only comes from years of HPR experience. Thanks for your input!

I think you'll be fine with 5.25". I burrito wrap a 15" TFR drogue and 20' of 1/4" Kevlar in a 6" Nomex protector and it almost gets lost inside my 3" diameter tubes.