Kevin do you have more detail on how the entire system worked? What held the bottom of the boosters?
Look at the first photo in
this thread and you'll see the rear of the main portion of the rocket. We cut a piece of plywood into "scallops", which cradled the rear of each booster. Small blocks where the booster rests would work, as well.
We then used a strip of thin brass between the rear of the booster and the rear of the main portion of the rocket. The brass was cut in the field with a pair of scissors (yes, it's that thin) and held in place via a wood screw into each portion.
When the separation charge fired, it pushed the top of the booster out and away from the rocket. As it rotated, the brass would tear, allowing the booster to fall free.
One thing we didn't want was two charges, with the risk of one charge firing while the other doesn't, resulting in a booster that's hung up. We wanted a system where the booster either separated, or it didn't -- nothing in between.
In addition, the recovery timer within each booster was built such that it wouldn't begin its countdown unless it detected a separation from the main. That way, if a booster separation charge failed, we wouldn't have a booster pop its recovery system while still attached.
The brass strip may seem light, but all it did was hold the rear of the booster in place -- the booster pin (1" Al rod) bore all the weight. With the motors canted in towards the CG (to prevent stability problems in case booster didn't come up to pressure at the same time), the boosters had a tendency to hold themselves in place, anyway.
On the first flight, due to a motor ignition timing problem, we actually lifted the 400+ lb rocket with six of the boosters. That gives you an idea of the strength of the booster attachment.
I'm actually working on a "mini" version of the Delta III, which will use an almost identical booster attachment method. In this case, though, everything will be significantly scaled down, as my boosters are only going to be 54mm, so 1" Aluminum rod would be horrific overkill!
I do need to get more photos posted, and better descriptions.
It's a method that's flight-proven and has worked beautifully. Our boosters each held an L motor, and the central motor was a P, to give you an idea of the forces involved.
-Kevin