I was faced with a desperate situation. A dozen D12-3 motors presented themselves, nearly free, and I wanted to fly them. What rocket to use? Hmm . . . I didn't have anything. Most D powered rockets are still going pretty fast after three seconds. What I needed was something big, light, and fairly fat. Here's what I came up with. This rocket hits apogee at exactly three seconds, only 300 feet up. It's perfect for schoolyard demonstration flights, with its big slow takeoffs. Give it a catchy paint job, and it's sure to be a hit.
The parts listed below are all from Balsa Machining Services.
See this page for fin and ring patterns.
Instead of BMS parts, you can use Estes tubes, which are 18" long, joining them with couplers. I got away with a single 18" BT-50 motor mount/stuffer tube. This puts the forward centering ring inside a coupler, so use these patterns instead. If you use Estes parts, be aware that payload coupler should ideally be longer than the standard Estes coupler: you should glue it into the payload section with well over half protruding. Careful with the alignment.
This version of the Rocksim file uses 18" long tubes.
Glue bulkhead inside payload compartment coupler at one end. Drill and install screw eye into the bulkhead. Glue payload compartment coupler into payload compartment, leaving 2" sticking out. Optionally glue nose cone in place. Rig the recovery gear.
If your stuffer tube is short, the ejection charge of a D engine won't push much out. So here's what you do. The final wrap of shroud lines around your chute will be a vertical wrap, so that the chute is suspended just below the bulkhead. This way, as soon as the payload section is out, it hauls the chute first thing.
With a D12-3, it takes off pretty slowly, so I use a five-foot long, quarter-inch diameter rod. Other motors possibilities: