I agreeTo create friction for descent, the rappel rack is approaching ideal in my opinion. Modulation of friction is easily managed to accommodate wildly different conditions. They work well on very supple ropes and ropes so stiff you can barely knot them. Dry ropes, wet ropes, ropes covered in mud. Single rope and double rope. Thin ropes and fat ropes. Slick rope and furry rope. Short rappels and rappels of over 1/2 mile. Little climbers and 2 person loads. I see the big challenge to be creating a device with great descent characteristics which instantly and effortlessly converts to negligible friction when moved upward.