TheAviator
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I was discussing with a friend at NARAM-51 the prudence of using proper knots on rocket recovery systems, and I figure sharing this could prevent some needless losses. From reading and experience, I know that a knot can weaken a rope/shock cord by upwards of 60% of its original breaking strength. Listed below are knots I have found that have a rather high efficiency (some as low as 20% strength loss) and have experience using in my rockets. For some of the more difficult knots I have listed easier alternatives that have slightly less efficiency.
If anyone has any suggestions/better ideas, please let me know.
Standing Ends of a shock line/shroud line
Standard: Two-half hitches, multiple overhand knots. Efficiency: 50%. Easy to tie, may slip, may be hard to untie.
Improved (Option 1): Figure-Eight Loop, follow-through method. Efficiency: 80%. This knot is used in climbing and does not slip. May be difficult to untie if heavily loaded.
Improved (Option 2): Double Bowline. Efficiency: 75%. Also used in climbing and does not slip. Easier to untie than the figure-eight loop if heavily loaded, but not quite as efficient. Harder to tie than a figure-eight loop.
Mid-line attachment point (parachute, tracking smoke charge, etc)
Standard: Overhand on a bight. Efficiency: <50%. Easy to tie, hard to untie after loaded.
Improved: Figure-Eight Loop on a bight. See above for discussion.
Best: Butterfly knot. Efficiency: 75%. While theoretically less efficient than a figure-eight loop, the Butterfly can take the three-way (both standing ends and the bight) loading better than a figure-eight loop, which is designed to take only a two-way load (loop and standing end). Can be easily shifted up and down the line or untied. Very difficult to master.
If anyone has any suggestions/better ideas, please let me know.
Standing Ends of a shock line/shroud line
Standard: Two-half hitches, multiple overhand knots. Efficiency: 50%. Easy to tie, may slip, may be hard to untie.
Improved (Option 1): Figure-Eight Loop, follow-through method. Efficiency: 80%. This knot is used in climbing and does not slip. May be difficult to untie if heavily loaded.
Improved (Option 2): Double Bowline. Efficiency: 75%. Also used in climbing and does not slip. Easier to untie than the figure-eight loop if heavily loaded, but not quite as efficient. Harder to tie than a figure-eight loop.
Mid-line attachment point (parachute, tracking smoke charge, etc)
Standard: Overhand on a bight. Efficiency: <50%. Easy to tie, hard to untie after loaded.
Improved: Figure-Eight Loop on a bight. See above for discussion.
Best: Butterfly knot. Efficiency: 75%. While theoretically less efficient than a figure-eight loop, the Butterfly can take the three-way (both standing ends and the bight) loading better than a figure-eight loop, which is designed to take only a two-way load (loop and standing end). Can be easily shifted up and down the line or untied. Very difficult to master.