Length of Couplers?

[sailmike]

Is there any rule of thumb for the length of couplers and the shoulders of nosecones? It should be long enough to hold the two tubes together during launch, but how long is long enough?

Thanks a lot,
Mike

 

[doug_man_sams]

Is there any rule of thumb for the length of couplers....? It should be long enough to hold the two tubes together during launch, but how long is long enough?

It depends. But a good rule is two calibers. That lets you insert one caliber into each tube being joined. That said, I have some rockets with rather long sections. These will impart greater bending moments on the couplers, so in those cases, I like to have up to two calibers inserted in each tube which implies a coupler four calibers long.

OTOH, with model rocket boosters, a half caliber of insertion is usually more than adequate, so a total length of one caliber will do. One caveat: with short couplers, the tubes being joined must be cut perfectly square at the mating ends. Otherwise, you'll likely get a crook at the joint. A long coupler can overcome this, but there's usually not enough space for that - the centering rings on the motor tubes limit the coupler lengths. So make sure your tubes are cut square in this case.


Doug


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[jcsalem]

Like Doug, two calibers (i.e., two times the airframe diameter) has been a very reliable coupler length for me.

Like with most rocketry construction, try prototyping the connection before gluing. The joint should be very stable and not wobble at all.

Jim

 

[powderburner]

I don't know if these lengths are "optimum" but they have worked for one or two rocket designs...

The coupler for Estes BT-60 (which has a 1.6 inch O.D.) is 1.5 inches long

The coupler for Estes BT-50 (which has an O.D. just under 1 inch) is 1.0 inches long

The coupler for Estes BT-20 (O.D. just under 3/4 inch) is 3/4 inches long

The pattern I see here is coupler length = approx 1 diam

In most cases that I can remember, the instructions prepared by Estes recommend gluing half of the coupler on each side of the joint. That seems to work OK.

I am not sure that the length of a NC shoulder is so clear-cut. I have seen lengths equal to anywhere from half to one full diam. This is also going to depend on how much bending moment there is on the joint, how strong the local BT is (can you reinforce with a fiberglass wrap, or by soaking the BT with CA?), and how strong/stiff the shoulder materials are (can you reinforce the shoulder by soaking with CA?). Rockets with a long payload section might need a stronger shoulder at the base than simpler rockets with only a plain NC. And the longer the shoulder, the heavier the rocket; you get less speed, less altitude, more stability.

 

[sailmike]

That's what I suspected. Thanks guys!

Mike