Stage coupler length?

I guess this is as good a section to post this as any. when designing a low- mid-power rocket based on Estes-style paper tubes, how long of a (male part) coupler do you use? That is, how much goes into the upper stage? Is it based on a certain percentage of diameter? I've never really worried about it, but I've got a new design that uses a long booster coupled to a long upper, and I want to make sure they stay together (until staging).

sooner.boomer said:

I guess this is as good a section to post this as any. when designing a low- mid-power rocket based on Estes-style paper tubes, how long of a (male part) coupler do you use? That is, how much goes into the upper stage? Is it based on a certain percentage of diameter? I've never really worried about it, but I've got a new design that uses a long booster coupled to a long upper, and I want to make sure they stay together (until staging).

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Generally, you want one caliber inserted into each section making the coupler two calibers in length. For example, with BT-50, the diameter (caliber) is about 1", so you want the coupler about 2 inches long.

For typical short boosters, you can get by with less, a coupler about 1 caliber in length with half inserted into each section.

For joining long sections together, I prefer two calibers inserted into each section for an overall coupler length of four calibers.

Doug

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If for 2 stageing, I like to keep it small, like 1/4 - 1/2 inch coupler

plano-doug said:

Generally, you want one caliber inserted into each section making the coupler two calibers in length. For example, with BT-50, the diameter (caliber) is about 1", so you want the coupler about 2 inches long.

For typical short boosters, you can get by with less, a coupler about 1 caliber in length with half inserted into each section.

For joining long sections together, I prefer two calibers inserted into each section for an overall coupler length of four calibers.

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This is all good 'rule-of-thumb' advice, but I would also point out that it makes some difference how well the ends of the body tube are cut. If they are cleanly cut and properly square, the ends of the body tube will behave pretty well all by themselves and you might not need as much coupler overlapping the joint. If your body tube ends are badly cut and uneven you may need more coupler length to keep the joint straight and working properly.

If you are designing a staging joint (that will come apart to separate the stages) and using the traditional Estes staging system with motors taped together, I think you can get by with a bit less coupler because the motors are basically holding the stages together (and straight). If your staging joint is for motors using gap staging I think you will want a bit more coupler length because the coupler is the joint, and to give a few extra hundredths of a second for upper stage ignition to take place.

One more thought---the approach apparently used by Estes is to use coupler length equivalent to approx one full body diameter. They have designed one or two kits, and rocketry hobbyests have successfully launched a few of their kits. IIRC, the usual Estes approach is to seat half of the coupler on each side of the joint. Seems to work just fine....

powderburner said:

This is all good 'rule-of-thumb' advice, but I would also point out that it makes some difference how well the ends of the body tube are cut. If they are cleanly cut and properly square, the ends of the body tube will behave pretty well all by themselves and you might not need as much coupler overlapping the joint. If your body tube ends are badly cut and uneven you may need more coupler length to keep the joint straight and working properly.

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Ours is a hobby of rules of thumb

I agree wholeheartedly that poorly cut tubes need longer couplers to maintain alignment. Furthermore, short couplers are more likely to be installed cockeyed (off axis).

I have found, with accumulated experience (and a nice sanding station to square up the tube ends) that I can get by with shorter staging couplers that I used to use. I also avoid using yellow glue when installing couplers. I usually mix up a small batch of epoxy - it gives me more working time to get the coupler positioned just right.

Doug

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