Hi DD -
There'a probably as many theories/opinions as to the best way to do this as there are rocketeers trying!
My project is based on a composite airframe I bought from a local guy on ROL. I am making some minor changes to the structure and reengineering the electronics and recovery to suit my own views. It's about 17' tall and is a 6" diameter design made from filament wound 3/32" FG tubing and 3/16" G10 fins, glassed/CFed to the BT's. The booster is a slip fit (with an 8" long coupler) into the sustainer. Both sections have dual deploy with redundant electronics. It has already flown at least once that I am aware of.
I am planning on using drag separation with dual timers in the upper stage, one controlling a separation charge and the other used for sustainer ignition. The sustainer ignition timer(s) will be activated by a trip wire attached to the booster (actually a slip fit 2-pin connector). The booster also has a redundant pair of altimeters (AltACC2C and MAWD) to handle deployment and a single timer (TBD) for redundancy for the separation charge. I have chosen drag separation to obtain maximum altitude; the separation charges are "just in case". An M2400 or N2000 makes a mess of the coupler on ignition.
Why do you say "no drag sep allowed?
Have not found a timer that can sense vertical orientation before firing (i.e. for sustainer ignition). I want to use redundant electronics and the budget won't allow the use of a dual flight computer setup that might do this, though it's not a bad idea.
As far as abort mode, in my opinion the conditions that would cause an abort will be impossible to see above 10-15K anyway; barring a CATO on the pad, or a shred (unlikely on the booster), you'd be reacting too late if using R/C.
The sustainer will have one full boat flight computer (to get accelerometer data, baro data, recovery event timing, and recovery GPS telemetry) and a simple backup altimeter for recovery only. I'll also have dual Walson trackers (one for booster and sustainer) in addition to the realtime GPS data on the sustainer.
This is about as simple as I think I can safely get. Biggest fear is getting the timing wrong on sustainer ignition, or too slow a speed on separation, causing an off-vertical trajectory. I'll be very conservative on the ignition delay (still running sims) rather than going for the last 1K' or so.
But I am very open to comment and suggestions - this is still new territory to me.