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.  

Parts List:
1X - BT60 "Tomcat" nosecone
1X - 8" length of BT20
2X - 4.75" length of BT-60 tubing
2X - "18mm to 24mm" Estes (green) centering rings
3x - 3/32" basswood or plywood "BT20 to BT60" centering rings
4X - 2.25" lengths of brass or music wire
4X - 1/2" lengths of brass or music wire
4X - 1/2" x 2.5" strips of 1/16" basswood
4x - 1/2" wide nylon R/C aircraft aileron hinges
1X - 1" length of 1/8" launch lug
1X - 1/8" x 1/8" x 12" evergreen styrene "bar"
1X - BT20 motor thrust ring
4X - 1" x 5" x 1/16" basswood

Construction

Piston
Begin by cutting three centering rings from 3/32" basswood (or plywood) I traced the proper inner and outer diameters from a "BT20 to BT60" cardboard centering ring from the Estes centering ring pack. 

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. 

Piston/Airframe Coupling
After the epoxy has cured, you are ready to attach the piston assembly to the airframe. Slide the piston all the way into the BT60 tube. Guide the long end of the piston tube through the centering ring. The piston should slide easily. You may need to sand the area where you drilled vent holes in the piston/motor tube so it can easily slide through the airframe centering ring. Place a wrap or two of masking tape around the nose cone shoulder and slide the nose cone on. It should fit (very) snugly so as not to come off at motor ejection. You may also want to tape it on the outside before flight. If you glue it on, you will not be able to clean the inside of the airframe after each flight.

Inside Hinge
You are now ready to build the "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
With the piston/motor mount inside the airframe, you are now ready two attach two sets of hinges. Sand the inside of the hinge assembly, and slide it onto the piston. It needs to slide freely, but should not be too loose.

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
You're now ready to build the airbrakes. Take the remaining 4.75" length of BT60 and cut it lengthwise into four equal pieces. Trim each piece as shown below. (cut along the dashed lines)

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
Test fit your airbrakes to the main airframe. There should not be a significant gap between the airbrakes. You may need to trim them, or sand the rear centering ring to get them to fit properly. Once you're satisfied with the fit, epoxy the nylon hinges (one at a time using 5 minute epoxy) to each of 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
Bend the ends of each of the four pieces of brass or music wire into a small "U" shape. Attach the wire to the forward and rear hinge on each airbrake. Be sure that each airbrake slides to the full open and closed position easily. Slightly crimp the ends of the wire with a pair of needle nose pliers so they will not come off during flight.

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
Close each of the airbrakes and slip them under the "lip" of the forward airframe. Remember to tape the nose cone to the airframe!

Friction fit a B6-2 or C6-3 motor and let 'er rip!

Flight Report
The Mk81 has flown four times so far with in-flight damage on the first three flights. I strengthened the construction several times, and if the instructions and materials shown here are used, the Mk81 will hold up well. Flights on B6-2's are a few hundred feet at best, but it allows you to watch ascent and deployment in detail. Descent is on the fast side (don't fly over asphalt!) This rocket is fun to fly at club meets and gets a great reaction every time.

Additional flight information and photos can be found here.