coupler length rules of thumb

We all know 1.0 calibers of stability as a rough rule of thumb. How about coupler length? I have a fairly high power rocket that, due payload and avionics, might benefit from being a little short in the coupler department. How low should I go to stay safe?

I would think 50 to 75 percent of the diameter would be minimum.....sticking out that is. My 4 inch model uses a coupler I think 6 inches long.......

normally I use 2x tube dia. for couplers that move.
Rex

If it's really a hot high power, pushing hard on hammer motors, you need longer.
Your main failure point will be joint at coupler, where the bending moment from wind shear or any such stresses will be "pushing" on that joint sideways.
Longer the coupler, the more those stress will be spread out and dissipated on the airframe. [lever arm]
The shorter the coupler, you risk snapping off the top of fin-can.

Are you using a vent band?
Then it should be even longer, not shorter.

My humble opinion, based on past experience.........

38mm = 6in coupler
54mm = 7in coupler
3in = 9 in coupler
4in = 12 in coupler

deduct vent band width if not using.
Then many kits ,use 2 couplers and a section of airframe between for av-bays, due to paper coming in 6in couplers. [or 4in in smaller size diam.]

I don't get your question as normally when dealing with payload & avionics .... a longer coupler has more room for stuff.
What am I missing?
How are you benefitting from shorter?

Obviously there are many bays out there much shorter, such as the Raven for 38's....only 2 inch. I have lost 2 rockets due to shearing off the top of my fin can at Max-Q using such a short coupler.

You must fit the proper length coupler to your project, based on maximum thrust your largest motor you intend to use, & stresses it can create on your airframe. This will also depend on WHAT the coupler & what the airframe, is made out of...i.e. paper, phenolic, glass, carbon, metal.
It is also imperative you have a very tight, smooth fit for high power, high stress flights or bad things will happen.:dark:

All that being said, if just sport flying high-power, one can "get away" with shorter couplers.

By the way:

1 caliper of stability is a very loose interoperation of stability, and will bite you if followed for many designs.
Short fat rockets , long skinny ones, high mach flights....all have their own ...different requirements to be clear.

After all.....it IS rocket science....lol.

Thanks all!

blackjack2564 said:

If it's really a hot high power, pushing hard on hammer motors, you need longer.
Your main failure point will be joint at coupler, where the bending moment from wind shear or any such stresses will be "pushing" on that joint sideways.
Longer the coupler, the more those stress will be spread out and dissipated on the airframe. [lever arm]
The shorter the coupler, you risk snapping off the top of fin-can.

Are you using a vent band?
Then it should be even longer, not shorter.

My humble opinion, based on past experience.........

38mm = 6in coupler
54mm = 7in coupler
3in = 9 in coupler
4in = 12 in coupler

deduct vent band width if not using.
Then many kits ,use 2 couplers and a section of airframe between for av-bays, due to paper coming in 6in couplers. [or 4in in smaller size diam.]

Click to expand...

Wow, those are long. I do not want to take chances here, though, and I am dealing with some real hot motors, so I might custom order to your specifications.

I don't get your question as normally when dealing with payload & avionics .... a longer coupler has more room for stuff.
What am I missing?
How are you benefitting from shorter?

Click to expand...

Oh that part is simple, this is for single board computers and cansats in sealed payload outside the avbay! Every inch of body tube I fill with avbay is one less inch for other use.

Obviously there are many bays out there much shorter, such as the Raven for 38's....only 2 inch. I have lost 2 rockets due to shearing off the top of my fin can at Max-Q using such a short coupler.

You must fit the proper length coupler to your project, based on maximum thrust your largest motor you intend to use, & stresses it can create on your airframe. This will also depend on WHAT the coupler & what the airframe, is made out of...i.e. paper, phenolic, glass, carbon, metal.
It is also imperative you have a very tight, smooth fit for high power, high stress flights or bad things will happen.:dark:

All that being said, if just sport flying high-power, one can "get away" with shorter couplers.

Click to expand...

Yes, and I bet the forces will be a bit lower on Punisher style head end couplers. All this explanation helps a ton, though, and I will certainly take your advice into account when I start cutting up tubes.

By the way:

1 caliper of stability is a very loose interoperation of stability, and will bite you if followed for many designs.
Short fat rockets , long skinny ones, high mach flights....all have their own ...different requirements to be clear.

After all.....it IS rocket science....lol.

Click to expand...

Definitely!

Those are interesting rules of thumbs...just pulled out my Estes Leviathan, still building it, and for the 3" diameter rocket, they provided a 4" long coupler...

Well, a Leviathan is never approaching mach 2 or lifting 10lb loads, which is what I am trying to do here. Nice rocket though, mine is a ton of fun!

The_Lone_Beagle said:

Those are interesting rules of thumbs...just pulled out my Estes Leviathan, still building it, and for the 3" diameter rocket, they provided a 4" long coupler...

Click to expand...

Please bear in mind, we are talking about "high" power rockets,generally fiberglass or fiberglassed paper, low & mid-power go by another set of rules. They don't experience the large loads of force found in high power. So couplers are MUCH smaller in length.

With high power, comes more weight. More weight = more stress on components.

Also if you eliminate the payload tube entirely & go with a "Punisher" type DD [i.e. nose cone deploy on top of fin can].....you can reduce the size of coupler/shoulder and get away with it. You have reduced the "lever arm" effect.

Once again...."this IS rocket science" and one must figure the needs of an individual project & motors being used to propel it.

From what I read about Can-Stats, they have pretty much standard design criteria, you might want to read up/research that before designing on your own. Google is your friend.