non-symmetric coupler

I am building the Adventurer 2.2

I would like to use the 54mm 3D printed sled, which has a location for a rotary switch on one end, in the 7" coupler with switch band. However, to line up the switch with the switchband would require biasing the switchband to one side, rather than in the middle of the coupler. This leaves 2" of effective coupler for the payload body tube and 4" of effective coupler in the booster tube.

I will secure the 2" of coupler in the payload tube with rivets or screws. Is this enough length/strength to secure the payload tube to coupler? I am thinking it will be fine, but looking for any other thoughts/ideas.

Of course, I could not use the switchband at all for access holes, as I have done several times before. Or, shorten the length of the sled.

Here is the resulting configuration and relative placement of parts:

The general rule of thumb is one caliber of coupler into each side of the BT. With a 54mm tube, the 2 inches of coupler into the payload is pretty close to that "one caliber". I would go with that format, just make sure the ends of your tubes are very square and the switch band is on the coupler square. The less misalignment the better.

Couple of other ideas:

- Shorten the sled and fit the altimeter on one side & the battery on the other. Or,

- Cut the sled, and move the cut-off to the other side of the switch mount. Hold the parts together with nuts on the threaded rod.

Rivets and screws both work fine, I'm not even sure what other options there are. If you go with rivets, invest in a pair of flush cut dykes https://www.amazon.com/gp/product/B00FZPDG1K/?tag=skimlinks_replacement-20, they make removing rivets and shear pins a breeze.

I've done that when I had a lot more space in the bottom tube than in the payload bay and I needed more AV bay room. It works fine.

Buckeye said:

I will secure the 2" of coupler in the payload tube with rivets or screws.

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The asymmetric coupler is actually a good thing in this case. If one end is secured tightly, it will be a much more rigid connection, so it makes sense to dedicate more coupling length to the loose end. Also, don't forget that the 1 caliber rule of thumb is an old rule, that it is used across the board. This includes softer and less precise tubing material like cardboard. FG couplers are less sensitive in this regard, so slightly below 1 caliber shouldn't spell disaster.

The only minor drawback is the location of the coupler in the middle of the Adventurer, where bending moments are the highest. This is one the reasons why the longer coupler is a good idea in this kit. But again, your securing of the short end negates this concern.

Reinhard

My Punisher & Wildman both have offset switch bands since they both haves HED deploy. Never had any issues with either.

Handeman said:

The general rule of thumb is one caliber of coupler into each side of the BT. With a 54mm tube, the 2 inches of coupler into the payload is pretty close to that "one caliber". I would go with that format, just make sure the ends of your tubes are very square and the switch band is on the coupler square. The less misalignment the better.

Click to expand...

Thanks. Yep, forgot about the 1 caliber rule of thumb.

Dad Man Walking said:

- Cut the sled, and move the cut-off to the other side of the switch mount. Hold the parts together with nuts on the threaded rod.

Click to expand...

Hey, I like this idea. Make a custom sled layout, just like my homebrewed designs.

timbucktoo said:

My Punisher & Wildman both have offset switch bands since they both haves HED deploy. Never had any issues with either.

View attachment 337488

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If I understand HED correctly, the short length of coupler is supporting a nose cone and some recovery gear. I think a midbody coupler as more bending moment to deal with, hence my concern.

One solution is to simply not use the switch band. I use the printed sleds a lot, and because of the exact same issue you have, I don't use a switch band with the shorter AV bays. I drill a 1/4" hole through the booster/coupler about 2 inches (depending on the length of the sled) below the center of the coupler for access to the switch.

Asking for a friend :blush: The one caliber rule; is that derived from equal lengths of tubing being connected and the bending forces created by pressures on the fartherest ends translating along the body tube? If you're adding a 3" piece of BT to an existing body of a BT80 or 100 (for altimeter and camera), then one caliber of surface contact is equal in length to the added piece and affects the nose cone fit and moves the CG. Or would 1/2 caliber suffice in a card board BT80 or 100 where the coupler is actually a part of an equivalent and extra BT cut and then glued within the BT?
Always learning...