Madcow DX3 2.6" - Centering Ring Spacing

[Spitfire222]

I have a quick and simple question I was hoping to get some advice with. I'm currently building a Madcow DX3, cardboard 2.6" version.

The instructions indicate that the forward centering ring should be 2" from the end of the motor tube (Image 1). This is further forward than the forward edge of the fin tangs, so there will be no contact between the fins and the forward centering ring (Image 2). Am I interpreting the instructions correctly?

If so, is there any reason it would be a bad idea to move the forward centering ring back so that it contacts the fin tangs? (Image 3). This would create that pocket that sandwiches the fins between both of the centering rings, but the forward centering ring would be quite far back along the motor tube. Or should I simply not worry about the lack of joint between the fin tangs and the forward centering ring?

Image below for (hopefully) clarification. Thanks in advance for any help!

 

[markg]

It's fine either way. The TTW part of the fin will be pushing against the slot in the BT so the joint between the CR and the fin isn't really necessary.

Personally, I'd sandwich the TTW (Through The Wall) portion of the fins tight between the two centering rings. I usually inject internal fillets after gluing in the MMT, CRs & Fins. Having the fins tight against the CRs keeps the epoxy in the right spot.

cheers - mark

 

[shawn_rocket]

I agree with @markg. But what I have done in that situation is get another centering ring. Sandwich the fin with one, and put the other at the top of the MMT. Reduces the volume you need to pressurize for deployment.

 

[Spitfire222]

Thank you for the replies. I have a tendency to overbuild, so I'd prefer to avoid a third centering ring. I guess my main concerns were:

1. That much of the motor tube unsupported, and,
2. An aft CG shift, though I guess on a rocket of this size and weight, it's probably negligible (and I would simulate the change in OR).

Either way, good to know that I wasn't mis-reading the instructions! I might stick to them to keep things simpler, instead of complicating things like I usually do.

 

[Spitfire222]

I figure I'll keep my DX3 questions in this thread.

Any recommendations/advice on how to connect the kevlar shock cord to the nylon shock cord? My usual method on LPR builds is to tie small loops at the end and feed on loop through the other (as in image #2). I always add a dab of glue to the knot and put heat shrink tubing for added security. I just wanted to double check if this is still an acceptable way to tie the shock cords together, or if there is a better way for the thicker cords used in this MPR build.

 

[markg]

I'm not a knot expert, but I prefer to use a double fisherman's knot to secure two lines together like this. I skip the glue but usually add heat shrink tubing on MPR sized builds.

https://www.animatedknots.com/double-fishermans-bend-knot
Again, I'm knot an expert. However, from the look of your knot above, I'd be mildly concerned about the kevlar knot opening up.

cheers - mark

 

[Spitfire222]

I'm not a knot expert, but I prefer to use a double fisherman's knot to secure two lines together like this. I skip the glue but usually add heat shrink tubing on MPR sized builds.

https://www.animatedknots.com/double-fishermans-bend-knot
Again, I'm knot an expert. However, from the look of your knot above, I'd be mildly concerned about the kevlar knot opening up.

cheers - mark

Thank you! I know very little about knots, so I will give this one a try. And for the record, the knot in the kevlar in the photos was for demonstrations purposes only, it definitely would not hold on a real ejection!

 

[Spitfire222]

Hi everyone, me again with one more question about my DX3. I downloaded an OR model, and overrode the weight with the finished empty weight I measured. Now, I'm wanting to add CG and CP decals to my model for any future RSO inspections.

I added the heaviest motor system I have right now (a CTI Pro 29mm case, two spacers, and a 1-grain propellant), and balanced the rocket to find the aft-most CG. It's just about 2.9" forward of the CP that OR says, so with a 2.6" diameter I'm just a little over 1 cal of stability.

My question is, should I place the CG decal on the rocket at the aft-most location, with the knowledge that I'll probably mostly fly with AT single-use motors, and therefore the CG will be further forward than the decal? Is it possible that an RSO would flag my rocket since the CG decal shows barely 1 cal of stability?

I guess I'm asking in general what's the correct process for CG measuring/marking on rockets? I want to make sure I get it correct as I get into higher powered and heavier rockets. Thanks in advance for any help!

CG and CP decals at their current location (aft-most CG)


OR simulation showing different CG than I actually measured (with the correct, heaviest motor I have, mentioned earlier)

 

[dhbarr]

I normally mark the CP but not the CG, since as you mention it changes. Instead, I just write it on the flight card and demonstrate it at inspection if necessary.

 

[Spitfire222]

I normally mark the CP but not the CG, since as you mention it changes. Instead, I just write it on the flight card and demonstrate it at inspection if necessary.

Thank you, that makes sense and keeps things simple!