But last week in Utah, I learned something useful: how to make a 40m rappel with a single 70m rope.
The basic idea is that, instead of rapping on both ends of your rope, as normal, you fix one end with enough rope to reach the ground, and let the other end function as a pull cord. You can then extend the pull-end with anything handy: slings, cordalette, belts, quickdraws, cams, wires, even jackets, t-shirts, pants, whatever. Since the pull end is not load-bearing, anything at your disposal can function to extend your rappelling range.
I'm sure most of you are having no trouble picturing the setup, but for those that haven't tried something like this before, here's some steps:
In order to to determine the length of the rap, and even if you have to use the extendo-rap at all, have one member of the party (let's call him Scott) single-line rap first on the full length of the rope (70m). Once Scott's on the ground (or to the next station), he can go off rap, but should hold on to the end of the rope. The second, still at the higher station (let's call him Blake), should then pull up rope until it comes tight on Scott. Now Blake knows that he has exactly enough rope to reach Scott.
Using the method pictured below, Blake then fixes the rap line. As you can see in the photo, this method allows Blake to rap on the longer end, and use the shorter end as a pull-cord to retrieve the rope.
Blake then begins rapping down the longer line, making sure to hold on to the pull-end (you can clip it through a QD on the back of your harness). When Blake reaches the end of the pull-end, he begins to extend it with any available material (slings, pants, etc) until he reaches the ground. Once on the ground, pull the pants and you'll retrieve the rope!
IMPORTANT CAVEAT! This is a slick maneuver, but not one that you'd want to rely on for multi-pitch rappels in adventurous settings. There is a huge potential problem: if you use the extendo-rap to reach a hanging belay, and then have a snag pulling (the knot gets caught) you could potentially find yourself in a situation where your entire rope is out of reach, and all you have is a few meters of slings (or pants!) with which to work. So, I would only recommend using the extendo-rappel when making you're last rap to the ground, or if the pull is super-clean.
Oh Benjamin Gratz Brown, what a different place the country might be if you'd been Vice President instead of that scoundrel Henry Wilson!