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What is the simplest setup for rappelling that is fully redundant?

The most common system seems to be rappelling off of both strands of a rope passing through two rappel rings on separate anchors. While providing the option to recover the rope by pulling on one end, that setup still leaves the rope as a single point of failure.

Assuming the highest degree of redundancy is required, would it not be more safe to tie two figure-eight knots on two bights, connecting each to a separate anchor, and then rappelling off of those two strands? Thus if one of the anchors fail, or even if one part of the rope fails, the other strand is still fully intact.

Any specific reason this setup is not more common?

  • Why are you concerned about having the rope as a single point of failure? Ropes don't break, though they can be cut by sharp edges. This has happened when a lead fall caused the rope to hit a sharp edge with force. However, when rappelling, the rope does not move so this is very unlikely. The only way the rope could really be cut is if it took a direct hit by a falling rock which is also quite unlikely. – Qudit Jul 8 at 18:39
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    How would you get the rope down if you tied it to the anchor?? – endolith Jul 8 at 18:43
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Strictly speaking, the system you propose (fix each strand of the rope to one anchor, rappel with the same device) is not fully redundant as you still have the device, carabiner and your harness as single points of failure.

If you disregard that point and simply want redundancy against rope failure, then, yes, fixing the strands to their own anchors is more redundant than the standard way.

The reason that this setup is not more common is really simple: In most cases, the goal of rappelling (at least in sports and alpine climbing) is to arrive at the bottom of the rock/pitch and retrieving the rope. Either to climb another route, or to continue the descent. However, you can't retrieve the rope if it is fixed to the anchor with one or two knots! Also, rope failure during rappels is really rare as the load is much smaller than e.g. during lead climbing falls.

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    One small comment, the device is usually backed up by a friction hitch (e.g. prussik) either above or below it, but yes you are correct that the harness is still a failure point. Anyway, your answer makes sense to me :) – Yuval Adam Jul 8 at 14:14

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