01-12-2009, 08:11 PM
Just another BAR!
Join Date: Mar 2005
Updated Design - Predator Advanced BGM-18M
Originally Posted by CPMcGraw
I agree, that C6-5 is the optimum motor in your list, and I would not recommend using any of the others. They are either at the edge of, or deep into, chute-shred territory. Getting about 670' on a C6-5 is good, though.
One thing I have to caution you on is the guide length. You call out a 36" x 1/8" rod, which is standard. However, you need to look at the flight summary. Double-click the line on the simulations screen where you flew the C6-5, and start reading through. You'll get to an area where it says "Launch Guide Data". RockSim says this design doesn't reach safe flight V until it's 38" up the rod. You're 2" short. This rocket needs a 48" x 3/16" rod using that motor, so that you get a full 38" of guide length. The extra won't hurt it.
Try simulating instead with the D13W-7. This is a composite D13 White Lightning. It jumps to velocity in only 20", reaches 1581', and has a Dv of just 5 FPS. Here is your optimum motor! Of course, it also has a Max V of 435 FPS, and an acceleration of 614 FPS/S. Really quick, but I think the TTW construction will hold up.
Good work for a RS9 first!
Thanks for the feedback. I haven't got much experience with composite motors. Are 18mm composites readily available?
I tweaked and updated the design and found another black powder engine that seems to work. The Quest B6-4 (B6Q-4 in Rocksim) has both a higher total impulse and average thrust numbers than the Estes B6-4.
I also increased the launch rod length to 48". The tweaked design reaches safe minimum launch velocity using both the Quest B6-4 and Estes C6-5.
Name: Predator BGM-18M
Diameter: 1.64" (ST-16)
Fin Span: 4.498"
Weight: 2.8098 oz
Engine Size: 18mm
Launch Guide Length: 48" x 1/8"
Motor......Max Altitude......Dep. Velocity
B6Q-4.........282.92 ft.........12.37 ft/s
C6-5..........668.50 ft..........1.88 ft/s