Simple, easy to mass-produce, and the engine is the rear: Volksrocket - "The People's Rocket".
Here's a photo of one of the boilerplate models in an experimental 3-stage configuration. I also have the RockSim 4.0 design file. A two-stage version can be constructed from a single 18" length of BT20 tubing cut into 3 equal lengths. The boosters and fin canister are constructed identically*. The fins are clipped-delta with a 45-degree leading edge which makes for very economical grain layout.
As you will see, the RockSim file contains a 3-stage model. Just for giggles I modeled an additional booster identical to the 1st (which in turn is identical to the fin canister)... and it appears to be stable w/o modifications! However, I'll want to do extensive testing with lots of motor combinations before endorsing the design.
*Note: The only differences between fin canister and booster is that the booster has port holes for the gap staging and the fin canister has launch lugs. Otherwise, the tube lengths, fin plan, and motor block are identical.
[More detailed construction instructions will appear soon.]
Matt Stum - NAR#73280 Level 2