Manufacturer: | Scratch |
While perusing the China Lake Weapons Digest a few months ago, I happened across this photo of an extremely cool looking bomb, the Mk81 "Snakeye". Low flying aircraft can safely deploy Snakeye bombs as the open "fins" retard the bomb's descent as the aircraft escapes the bomb's blast radius. This design has become a testbed for larger versions I plan on building in the future, and to experiment with 'chuteless and piston recovery systems. Additional images of the completed rocket can be found here. |
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Mk81 Snakeye Finished Length: 14" Finished weight: 4.2oz (no motor installed) Main Airframe: BT60 Motor Mount: 18mm Recommended engines: B6-2, C6-3 Parts List: Construction Piston
Drill several 1/8" vent holes in the two of the centering rings. Sand the outer circumference of the two vented rings until they slide easily inside a BT60 tube. The inner rings should fit snugly (but not too tight) on the 8" BT20 motor mount tube. Now, saturate the inside surface of the entire 8" BT20 piston/motor mount tube with thin CA for strength. When dry, drill/cut/dremel 3/16" vent holes around the entire piston/motor tube starting at 5" from one end. Use 5 minute epoxy to affix one ring 1.75" from the "short" end of motor tube, and the next ring at the very end. Use generous epoxy fillets on both rings. Set the piston/motor mount aside. |
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Airframe Take the third (unvented) centering ring and sand the inside until it slides easily around the free "long" end of the piston/motor mount. This centering ring should fit snugly inside a BT60 tube. Epoxy the centering ring about 1/4" inside one end of one of the 4.75" lengths of BT60 tube. Fillet generously with 5 minute epoxy. Piston/Airframe Coupling Inside Hinge Cut four 3/16" lengths from the Evergreen 1/8" bar. Drill a 1/16" hole through the center of one side of each. Epoxy them one of the BT20 to BT50 Estes (green) centering rings. Fillet as best as you can, but be sure the epoxy does not obstruct the holes. This hinge assembly may not seem very strong, but if you build it carefully, (use long curing epoxy) it will stand up to the opening and drag forces on descent. You might want to cut a bass or ply "silhouette" to "sandwich" the hinge assembly. Hinge Attachment Now, take the other centering ring and glue it 1/2" from the end of the piston/motor tube. Epoxy the four R/C aileron hinges (as shown) to the end of the tube. Make sure the hinged portion of the hinges do not get any epoxy on them (it's easiest if they hang off the end. Don't try to "flex" the hinges to follow the curvature of the tube, as they will tend to bind. Use generous epoxy fillets here as well. Once the epoxy has cured, you should wrap the attached portion of the hinges with electrical or duct tape to ensure they remain attached to the motor tube. Install the motor thrust ring. Airbrakes Bend the four 1/2" lengths of 1/16" brass or music wire into a loop hinge (as shown below). Cut a slit into the center of the four 1/2" x 2.5" strips of 1/16" basswood. Slip the wire loops into the slit and turn them at a 90 degree angle to "lock" them into place. Epoxy these basswood reinforcing strips to the inside of each of the airbrakes, making sure to keep 1/2" towards the narrow end free of epoxy (you'll want to slip the free end of the nylon R/C hinge into this space between the basswood and BT60 tube when you attach them to the motor tube)
Attaching the Airbrakes While the epoxy is curing, take the 5" x 1" 1/16" basswood and cut four fins to the following shape. Once the epoxy on the hinges has cured, epoxy or CA one fin along one edge of each airbrake. Final Assembly Glue the rear end of the launch lug flush with the rear end of the forward portion of the airframe. Insert a motor, and add clay nose weight to until it balances at the C.G. shown in the diagram above. I had to use about an ounce of clay. (See! I told you not to glue the nose cone on! :-) Prepping for Flight Friction fit a B6-2 or C6-3 motor and let 'er rip! Flight Report Additional flight information and photos can be found here. |
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