a 300g booster seems to be a good weight to a folded 1m compound sling.Whatcha think? Also that automatic release seems to me that could have problems not releasing,or releasing at the slightest slack of the cord. Of course it would have to be adjusted,but still..the biggest problem with this whole compound sling would be an automatic release,the rest is a piece of cake(finding the ammo booster weight ratio for ideal energy transfer)

Who would want a 300g booster with a 200g projectile? that's 500g+ ,it would be heavy and very inefficient. A 300g booster and a 50g projectile would be much more suitable. Although the ideal weights and their ratios will have to be tested.  
Chuck,what weights were those guys using,and with what degree of success?

I came to this party late.  A few pages back there were several links to a video of a working model of this 'compound sling' but the none of the links work for me.  They all lead to a 404 error 'page not found'?  Is there anyplace else that the video's are still available?
 
I'm also curious how the release issue was solved to allow controlled release at just the right point in the compound arc?
 
-Modified: Never mind on the release I found links to the pictures... but still no video.
 
-namuh1

I noticed that in the photos and posts that I was able to read the release was 'automatic' by virtue of the energy storage weight sliding to hit a release hook at the apex of the arc of the secondary pouch.  I read a lot about needing to tune this to release at the right place.  It seems that this would make accuracy very challenging  since the release could not be 'adjusted' in realtime for variations in the spin up by the thrower.  I haven't tried any of this obviously and since I'm so new to this sport, I hope not to sound off and offend anyone, but may I suggest an alternate that might give control to the release.
 
Attach a second string to the sliding weight and adjust the length so the weight is restrained only a fraction of an inch from the release hook.  The separation would have to be enough to ensure that the weight didn't hit the hook prematurely due to stretch of the restraining cord.  The other end of the restraining cord would be knotted and held in the fingers just like the release side of a conventional sling.  When the thrower times his/her throw and senses the pouch reaching the chosen release point the secondary restraint/release cord is released and the weight slides the fraction of an inch and releases the actual release cord of the pouch.
 
It might take some getting used to the lag from the release of the restraint cord to the release of the actual pouch release cord but I think it would not be much different from the lag in the firing of a flintlock rifle in compensating for the pan flash to actual ignition... something every flintlock shooter does in shooting at a moving target.  
 
If this is an unworkable suggestion, for some reason I haven't seen in my brief time looking at this problem please forgive my intrusion, but my first career was as a theoretical physicist and I'm fascinated by these kind of problems.
 
-namuh1
 
Modified: After looking deeper I'm afraid it would not work

Your explanation is precisely the conclusion I reached after opening my mouth when I shouldn't have.  At first glance I didn't think about the fact that the ballast is released when the pouch is released.  I was only thinking about the release of the secondary.  After further observation, I saw that the real impulse of energy came from the sliding of the ballast in much larger ratio than it's presence in the first place... exactly as you recounted.  I'm not sure that I follow your comment that the hook will have already released by the time the ballast hits the hook?  I'll have to make some calculations regarding the event before I make any more mistakes but this is one of the most fascinating problems in multiple body motion I've encountered, not to mention the fun of making one of these things and getting it to work!
 
-namuh1

  Great idea.  I don't know if this has anything to do with it, but I have made and seen bullroarers on the end of a pole, you don't swing the pole but you kind of rotate it in a sort of butter churning motion, it works kind of the way a screwdriver works increasing the torque with a longer handle.  A bullroarer on the on the end of a stick needs only a very short cord to make it work but it needs a long one without it.  I suppose you could try the same thing with a sling and there would be different ways to release it, including a hook that it slips off when you tilt it or run the cord through a ring down to the hand hold and let go of it.

I'm not exactly sure I understand your proposal about a slip knot, kid.
 
To answer your question, the original purpose of the compound sling was to sling lighter weight projectiles with a higher speed than is normally possible with a sling. Potential Energy is stored in the ballast and transferred to the projectile as Kinetic Energy during the second, automatic release. The compound sling works just like a staff sling at the moment of release, but the complicated booster mass mechanism makes it so that the second sling moves faster than a staff sling ever could.
 
C_A did a great job of explaining the benefits, and operating principles of the compound sling.
Quote from curious_aardvark on Mar 25^th, 2007, 8:30am:

 
Quote from curious_aardvark on Mar 27^th, 2007, 6:02am:

I'm working on the mechanics to understand what's going on here. For the sake of endless repetition I see the breakdown this way:
 
With a static ballast, i.e. one that is fixed and does not slide, the effect is like an inefficient staff sling.  Why? Because the ballast is not transferring its energy to anything but simply acting as a 'stabilizer' against which the secondary retaining string rotates.  Without the ballast in this configuration the rotational energy imparted to the pouch at the end of the secondary retainer would be absorbed by flexion in the primary string thus subtracting from the transfer of energy that a stiff staff would impart.  With the ballast and to the degree that the ballast resists flexion in the primary string because of its mass, the system works like a staff sling where the transfer of energy is increased only because of the longer moment arm created by the primary string added to the slingers arm.
 
Since this whole problem is about rotational velocity:
 
The sliding ballast system works in a very very different manner.  Here the ballast actually does store energy that is transferred to the pouch on release of the pouch from the hand.  It does so because it 'effectively' pulls the secondary retention string shorter as it slides out to the end of its travel as the pouch at the end of the secondary retention string (along which the ballast is sliding) spins up around the center of mass of the ballast.  This is conservation of momentum of the pouch which is seen every day in school yards.  On the spinning platforms that the kids push into circular motion and then jump on the kids can substantially increase the rotational velocity by moving their bodies closer to the center of the spinning platform.  It may seem backwards to the spinning pouch but it is the same mechanically.  As the radius of the secondary retaining string is shortened by the sliding ballast, the momentum of the pouch is kept constant by the pouch increasing it rotational velocity exactly as the originators of this design wanted.  You can test this simply by tying a rock to a string and passing the string through a sewing thimble.  Then whirl the string/rock combination over your head helicopter style but by holding the thimble as the center of rotation and the end of the string in the other hand to keep it from flying off.  Then while the rock is spinning around, pull the string shorter through the thimble and observe that the rock moves faster the shorter you pull the string.  You will also feel why the mass of the ballast is important because your hand holding the thimble will feel quite some force to resist being pulled out of line as you shorten the string through it.
 
In truth, this is a simplification of the real problem.  For example there are second order effects that do contribute to the problem.  The fixed ballast does, in fact, interact to some degree with the energy given to the pouch because its mass must resist, by inertia, being displaced by the rotating mass of the pouch pulling on it, but I believe that this is a second order effect and contributes little to (and may even detract from) the pouches energy compared to the increase in moment arm achieved by the long strings.  This would be a terrific problem to assign to a graduate physics class in mechanics, better even than the classic dragster problem.
 
My skill with differential equations is 40 years old so I've asked a friend, more competent than I am, to work out the actual mechanics so I'm hoping to have a more complete and accurate description of the problem in a few weeks.  In the meantime, I've GOT to try making one of these things!