Wednesday, May 28, 2008

Pogo progress (update)

I've started on the Pogo PMC. Progress to date includes:
  • I attached the two body sections and built the 2 main wings.
  • I ground the front of the body out and fitted a length of 24mm tubing, which fits very nicely.
  • I've designed and built the nose assembly.
  • I build a RockSim model.
Knowing that I was going to have to add nose weight, I decided to replace the plastic propeller shaft, gears and retainer rings. I used a 3/16" x 2.5" eyebolt as the shaft. The propeller hubs are spaced and held in place with nuts. The loop of the eyebolt is inside a piece of a used 24mm motor case epoxied to a metal washer. The case forms the nose's shoulder and the washer both aligns the eyebolt and keeps the nose from slipping into the tube. This design weighs a couple of ounces and provides ample room for lead shot if required. It also allows the propeller sections to spin freely on boost.

I modeled the body of the Pogo as a simple tube and the protruding cockpit section as a single thick fin. The propellers themselves are not accounted for. The model says the GP is right where the leading edge of the dorsal fin meets the body. (rest of this paragraph is updated) At first, the model was hosed up. It turned out, I had polycarbonate selected for the body instead of polystyrene. This shouldn't corrupt the model but for some reason RockSim didn't compute a component mass. I changed it over and the sim runs. I changed it back and it did compute a mass. ????? Next, I added an over-ride for mass and CG using actual values from TRF user Micromeister. His cutout-derived side view LCP is 5.3125" from the tip and the top view LCP is 5.5" from the tip. Rocksim computes the CP as 5.88 from the tip. I'm on the right track.

The main open item is whether to go with a central 13mm mount and 2 outboard 13mm JATOs, or to go with a central 18mm motor and two 13mm JATOs. The 13mm mount will fit better and provide more room for 'chutes. Here's how various motors stack up. The three A10s should handle the job nicely, but I haven't decided.

Motor(s) Total impulse Max impulse Avg Impulse Burn

C6 8.8 14.1 4.7 1.9
C5 9.1 21.9 5.3 1.7
A10 2 12.6 2.4 0.9
3xA10 6 37.8 7.2 0.9