Motor Eaters
After seeing Dave Schaefer fly his 6-motor cluster/stager, I decided I needed to build one myself.  The BT-70 based (2.2x) upscale Midget at left was the outcome.  Called the Thridget, it will burn three 18mm motors per stage per flight. 

While I was at it, I finally built a booster for my older BT-70 upscale Midget 70 (right).  When I first built it, I thought a single D12-powered booster would work, but after seeing how heavy it was, I shelved the idea for a booster until the inspiration for the clustered booster (clooster?) hit me. 

The other rocket in the center has been over five years in the making.  It uses my interchangeable motor mount system, and will burn as many as four motors at a time.  The bottom section is heavy, and uses a Pringles can under all that paint.

 
Here's the business end of the Thridget70.  Each booster motor has its own fin and motor tube enabling them to fall away separately in case the sustainers don't all light at the same time.  It requires extra tracking eyes, but gives more reliable staging.

 
In the sustainer, the motors will be retained via friction-fit for staged flights, but the 2-56 all-thread in the center allows a nut and washer to be used as retention for single-stage flights.

The silver paint turned out shiny but easy-to-mar.  I applied a clear coat to protect it, but alas, it dulled the finish.  Silver is a PITA.

 

Even with the dulling effect of the clear coat (yes, I used gloss clear) it still looks pretty good.  It will be great with B and/or C motors in the booster and sustainer.  Need to run some sims to see what it will do with A10-0s and/or A8-5s.  Besides needing boosters powerful enough to ensure adequate rod speed, it needs to have enough overall impulse to achieve survivable altitude even if one of the booster or sustainer motors doesn't light.

No matter what the combination, with six motors per flight, the Thridget is a motor eater.

 
The new Midget-70 booster has a 24mm center mount staging to the 24m sustainer.  The three outboards are 18mm.  Rocksim shows, with B6 or C6 outboards, that the center motor can be a C11-0, but I plan to use a D12-0 on its first flight.  Its bigger thrust spike will be more forgiving in case one of the outboards doesn't light.  A burn string will be attached to the center motor.

 
To reduce mass and place the outboards closer to the center line, the booster steps down to 1" motor tube aft of the forward BT-60 section that mates to the sustainer.

The outboards are mounted in the fin roots and thus provide some reinforcing strength.  The forward ends are beveled for aesthetics and drag.

Low weight fiberglass reinforces the fins.  The 'glass spans from one fin, across the valley, and onto the adjacent fin making the fins stronger AND securing them more firmly to to the center tube. 

 
Here's a shot of the Midget-70's new red fin.  A wind gust at the last launch resulted in a hard landing that snapped it off.

With four booster motors plus a sustainer, when flying on a full stack, the Midget-70 is definitely a motor eater.

 
Here's the Core3, named in honor of my son, Cory.  The silver turned out really shiny.  After the dismal results clear coating the Thridget, I'll forego the clear coat and take my chances on the marring.

The Core3 uses four different motor mounts. The three below are shared with the Core3's twin, the Elena Lou.

The mount below was built just for the Core3.

A pair of A10-PT motors along with a pair of B6 or C6 motors makes the Core3 a motor eater. But this combo should get it off the rod with authority while keeping it low enough for small field flying.

And for big fields, there are lots of options with the 24mm and 29mm mounts.

 
December 17, 2005