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High-Altitude Ballooning

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We use small aluminum "carabiners" to connect the various payloads together (actually they are key chains - not to be used for rock climbing!!!).  We have used steel locking chain links from Home Depot, but they weigh a lot.

During NBS-07-07 we lucked out because one of the carabiners actually broke. It was holding the bottommost payload, which was lying only a few feet away from the rest of payloads. We are not sure when the carabiner broke, but we think it must have been during on landing. It appears that the carabiner snagged some sagebrush during landing, causing the 'biner to fail and separate that payload from the rest of the payloads.

After that experience we decided to take a closer look at the construction of the carabiners. Unlike real rock climbing carabiners, the screw gate is not actually attached to both ends of the opening. If you pry the carabiner open, the gate simply slips off the unthreaded end (while staying attached to threaded end).

We decided to test a variety of carabiners to see which ones are strong enough to use on future missions. We used a digital force gage to measure the maxium load the carabiner can hold before opening. All of the carabiners were aluminum, except for the stainless steel ones as noted below. The photo below shows the results of the tests from lowest to highest failure load (from left to right):

  1. 2.125 inch orange (Bison Designs Versa-Link, $1.30 rom 19 lbs
  2. 2.625 inch magenta (Bison Designs Versa-Link, $1.47 from 20 lbs
  3. 2.625 inch green: 25 lbs (repeat test of  #2)
  4. 2.125 inch purple (Versa-Link, $2.50 from REI): 28 lbs (similar construction to test #1)
  5. 1.5 inch stainless steel ($1.50 from 55 lbs to faiure
  6. 1.5 inch stainless steel carabiner: 30 lbs (to open), 60 lbs (to failure) (repeat of test #5)
  7. 1.5 inch greeen (Versa-Link, $1.75 from REI): 90 lbs
  8. 1.5 inch red: 92 lbs (repeat of test #7)

From our tests it appears that the larger the carabiner, the lower the failure load. This makes sense from a strain standpoint becuae the strain required to open the carabiner would be lower for a larger size. It also appears that the location of the gate makes a difference. If the gate is closer to the "small" end of the carabiner, it seems to have a higher failure load. Again, this makes sense from a strain point of view. This is probably well documented somewhere for acutal rock climbing carabiners.

Although the stainless steel carabiners failed at a fairly low load (30 lbs), the failure mode was good. Whereas the aluminum carabiners fail suddenly (the carabiner "pops open"), the stainless steel carabiners slowly yeild open as the load is increased (typical of steel yeilding). It took about 30 lbs to open the steel carabiners wide enough for a string to pass through and about double that to open it to the point shown in the photo.

We have also found zipties to get brittle in cold temperatures, so we do not use them either.

UPDATE: 9/30/08

While shopping at Walmart, we came across a package of aluminum carabiners (sold in pairs) for less than $2. The nice thing about these locking rings is that both sides of the gate are threaded. This means the threads have to physically fail in order for the whole structure to fail. We tested it closed and it supported 100 lbs (the maximum rating of the load cell used in all the tests) without failure, more than any other carabiner tested. We then opened (unscrewed) the gate and tested it again. The photo below shows what 100 lbs of force did. The carabiner bent but did not break.

At 2.9 grams each, these seem to be a good balance between weight, cost and strength. The come in both red and green.

 walmart carabiner

Conclusions: The Walmart screwlocks are more than adequate for our use.