A post mortom of a wrecked Estes High Flyer XL

Discussion in 'Low Power Rocketry (LPR)' started by Tom, Oct 29, 2019.

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  1. Oct 29, 2019 #1

    Tom

    Tom

    Tom

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    I will make it brief. I saw a few threads where the use of Kevlar line for shock cord was discussed.

    In those discussions it was stated that the cord can be used without any 'stretchy' shock cord as long as it is at least 3 times the body tube length.

    I had built the now demised HF XL with a Kevlar shock cord a little more than 4x the body tube length.

    It flew great however at deployment the plastic parachute ripped right off leaving the shroud lines with the knots still intact attached to the nose cone.

    I believe the failure was due to too short of a delay of 4 seconds and the system deployed while it was still doing around 80mph. The resulting stress ripped off the parachute, and zippered the tube about an inch. Also the snap swivel holding the altimeter was bent.

    If the bent swivel was from the landing or the ejection impact is un known. But it musta been one hell of a jerk!

    I 'think' that if a section of elastic cord had been included it may have been enough to avoid the catastrophic failure.

    Yes choosing a longer delay would also have probably averted the event. But the elastic gives some backup in case of such a thing. Delay charges are notoriously erratic from what I understand.

    I will be including some stretchy part to my shock cords from now on.

    Thanks for bearing with me :D
     
  2. Oct 30, 2019 #2

    tsai

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    In my experience, nothing would save the typical plastic chute in this scenario. When a parachute inflates at speed, the amount of stress created is pretty significant, with the majority of it going to the shroud lines.
     
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  3. Oct 30, 2019 #3

    dhbarr

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    I'm too lazy to look up all the variables, but 18in of elastic yields about a rubberband of leeway. Plus or minus the slowest second or two, sure.

    But for the little ones I do kevlar in the tube and attach the rubber to the end of that, typically just doubling the original length.
     

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