I suppose even throwing a stone with your hand involves some friction - You could see the palm as the retention cord, and the fingers as the release cord. They "pinch" the projectile until the release, upon which the fingers fly forward to allow the stone to leave the hand. It's possible to get a rifle spin on a stone when throwing this way (This is how American footballs are thrown more accurately, apparently.) So, I would say that in order for a sling to be frictionless, it would either (A) have to be a fundamentally different kind of throw, or (B) made from a hypothetical frictionless material... But maybe not made from it entirely, or the whole sling would fly out of your hand.
Sarosh wrote on Jul 2
nd, 2020 at 3:41am:
A string attached to a weight is the closest we can get to frictionless. it feels weaker than a regular sling. it is similar to the y-sling on the regard that the force is felt mainly on the release cord and it's not equal between release and retention.
Another thing that could come close to frictionless is essentially a rigid pole with a cup or bowl on the end. My reasoning is that it would be like tossing a ball in the air with an open palm. The motion would be essentially a quick thrust downrange, and the projectile would fly free - I guess that would be moving away from the sling, however, and more toward the catapult (Or maybe a javelin thrower?)
I feel like CA's idea with the electromagnet has the most merit when it comes to improving the single-cord design, but I wonder if it would be possible to create a mechanism built into the projectile that could do the same thing? Perhaps you could use a wire in a tube (Like they use in brake mechanisms on bicycles), attaching to some kind of hook or catch inside the projectile, and, when the wire is pulled (perhaps with some kind of trigger), lets the projectile free?