Hmm, I guess I should jump in here somewhere and do some of flying ****...
BTW- I haven't been real active the forum lately becasue of my studies...5 honors classes...including a double Science class to help my college aplication to PSU for Aerospace engineering... anyway...
Now, about staging, there are two definite ways to go with this. I, personally, have used PML's intersatge system because it eliminates having to change the design of my Thunder N Lightning kit. It works well in most cases and you don't have very many complexities with it. Unfortunately, because the pieces don't seperate, drag is increased and optimal flight performance is reduced. Also, in the event of a lawndarted booster (this happened to T&L a month ago) the electronics WILL vaporize!
The second route would be my perferred method in a scratch built kit. This involves an electronics bay in the fincan of the sustainer that lights the sustainer. Theoretically, with a few modifications, the interstage unit from PML could be inverted and placed into the fincan coupler with relative ease. Or, the bay could be built into the side of the fincan or something similar. In this case the way to go, in my opinion, is to put the staging electronics in the sustainer's fin can coupler, the deploy electronics in the typical bay in the sustainer and another set of deploy electronics in a bay in the booster (where the interstage would normally be) that deploys its chute, instead of motor based ejection. I say this because, with high altitude flights, the booster can reach 5000 and it would be good to have dual deplyment on the booster. Some say this is impossible, but it isn't using today's technology...the tether. Use a tether as your deployment method for the booster so that the length of the tube required for the recovery section is minimal. Also, in the event of a motor malfunction in the booster, you can still recover it intacted.
Dave
PS: If you go with electronics for staging in the sustainer, then the carbon fiber rod idea is a good one.