nartreb wrote:I'm sure you're right about the basic crossed-biners setup shown above, but there's a variation I'd like to try. Imagine if the 'biners in the photo were twice as long, there'd be a lot less friction and you'd need a lot more braking force. Well you can simulate 'biners of arbitrary length by constructing a chain of 'biners.
Normally, additional 'biners are added to the chain so you can thread the rope through all of them, increasing friction. (You can also increase friction by adding more crossing 'biners.) But it should be possible to thread the rope through, say, half of the "holes", creating the equivalent of the long 'biner described in my second sentence. The danger is that if your'e not careful, you could allow the chain to bend in a way that removes all friction entirely, but I think that can be avoided.
There is a fundamental problem with trying to rappel with a bodyweight of tension on the rope below the device, as long as the device is dependent on the
tension in the rope! Even if you could invent a device that gives you a friction low enough to descend, how are you going to effectively modulate that friction enough to do a safe rappel?
If you have a body weight of tension in the rope below your device, you will have no way to change the angle the rope makes as it leaves the device. The rope is always going to be straight down, unless you can lift your partner's weight with one hand.
But even more importantly, this is going to be a device that allows you to rappel with a bodyweight of tension in the rope below the device. But if 200 pounds of tension in the rope
below this theoretical device is just enough to let you rappel at a safe speed, how much tension is it going to take in order for you to stop? 300 pounds? 400 pounds? It's less than safe if you have to add 100 or 200 pounds of tension with your brake hand just to stop. And remember, if your partner's weight is on the rope below, you're not going to be able to wrap it behind your back or around your leg or anything like that to create the friction.
And then what happens when your partner reaches the ground and unloads the rope? Now you're on a device that requires 200 pounds of tension for a safe rappel speed and well over 200 pounds for stopping, and you have only your brake hand and some body-part-wrapping techniques to do that.
So, no way. The only thing that is going to work is something that does not depend on the amount of tension in the rope or the angle the rope makes as it exits the device. The descenders shown earlier would be an example, I think. But I don't know if those devices are going to be fast enough to justify their use over just rappelling one-at-a-time with a standard device.