DesignQues: Couplers for boosters - rule of thumb?

Hi all,
I am designing a booster for a BT70 sized rocket, and am wondering if there is a 'rule of thumb' for the length of the coupler into the sustainer?

From doing some research, it appears to be 3/4 - 7/8 of the diameter, at least for fiberglass. This is a 2.5- 2.625" length for a 3" fiberglass BT. I am building paper rocket, standard Estes BT.

I plan to use/make several boosters in the future and wondered what people are doing, irregardless of direct ignition or airstart on sustainer.

PS> Happy New Year to you all. OH, time to redo signature for this years stats. Flown 3, Lost 0, Tree'd 0.

*bump* since I'm curious too.

Personally I like just over 1 caliber in length. Doing this now here on a kit that utilizes Estes-type components: https://www.rocketryforum.com/showthread.php?131300-TLP-Nike-Ajax-Two-Stage-Conversion.

+1 for 1 cal or more

Are we talking black powder staging or composite?

For black powder, I've built a few, and I haven't really gone with caliber. I've just gone with what is long enough that it won't wiggle or slip off if I shake the rocket by hand, wiggling it back and forth. Not that I'm an expert - I'm just going by what has seemed to not fail on me so far. I've always made sure it's snug enough it won't slip off too easily, but not so tight it won't blow off when the sustainer ignites. This means that on some couplers I've had to sand them down a bit, and on some I've had to build them up.

I am building a rocket now I'd like to add a booster to, but I don't know that I have enough space to feel comfortable with. It's definitely not a full body tube diameter from the aft of the tube to the aft centering ring on the sustainer. I guess I could build the booster and see how it fits, and if it's too loose, I won't bother launching it with a booster.

But I have never tried it with composite, so I couldn't tell you about that.

You can shorten the coupler for the outside tube (somewhat) if you also couple the motor tubes (or flash tube).