While perusin' t' China Lake Weapons Digest a few months ago, shiver me timbers, me bucko, I happened across this photo o' an extremely cool lookin' bomb, t' Mk81 "Snakeye". Arrr! Blimey! Low flyin' aircraft can safely deploy Snakeye bombs as t' open "fins" retard t' bomb's descent as t' aircraft escapes t' bomb's blast radius.

This design has become a testbed for larger versions I plan on buildin' in t' future, and t' experiment with 'chuteless and piston recovery systems. Additional images o' t' completed rocket can be found here. Avast!  

Parts List:
1X - BT60 "Tomcat" nosecone
1X - 8" length o' BT20
2X - 4.75" length o' BT-60 tubing
2X - "18mm t' 24mm" Estes (green) centerin' rings
3x - 3/32" basswood or plywood "BT20 t' BT60" centerin' rings
4X - 2.25" lengths o' brass or music wire
4X - 1/2" lengths o' brass or music wire
4X - 1/2" x 2.5" strips o' 1/16" basswood
4x - 1/2" wide nylon R/C aircraft aileron hinges
1X - 1" length o' 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 cuttin' three centerin' rings from 3/32" basswood (or plywood) I traced t' proper inner and outer diameters from a "BT20 t' BT60" cardboard centerin' rin' from t' Estes centerin' rin' pack. 

Drill several 1/8" vent holes in t' two o' t' centerin' rings. Begad! Sand t' outer circumference o' t' two vented rings until they slide easily inside a BT60 tube. Begad! T' inner rings should fit snugly (but nay too tight) on t' 8" BT20 motor mount tube. 

Now, saturate t' inside surface o' t' entire 8" BT20 piston/motor mount tube with thin CA for strength. Begad! Blimey! When dry, arrr, drill/cut/dremel 3/16" vent holes around t' entire piston/motor tube startin' at 5" from one end. Aye aye! Begad! Blimey! Use 5 minute epoxy t' affix one rin' 1.75" from t' "short" end o' motor tube, ya bilge rat, and t' next rin' at t' very end. Use generous epoxy fillets on both rings. Set t' piston/motor mount aside. 

Piston/Airframe Coupling
After t' epoxy has cured, you are ready t' attach t' piston assembly t' t' airframe. Aye aye! Well, blow me down! Slide t' piston all t' way into t' BT60 tube. Avast! Guide t' long end o' t' piston tube through t' centerin' ring. Aye aye! T' piston should slide easily. Aye aye! You may need t' sand t' area where you drilled vent holes in t' piston/motor tube so it can easily slide through t' airframe centerin' ring. Aye aye! Place a wrap or two o' maskin' tape around t' nose cone shoulder and slide t' nose cone on. Ahoy! It should fit (very) snugly so as nay t' come off at motor ejection. You may also want t' tape it on t' outside before flight. If you glue it on, matey, you will nay be able t' clean t' inside o' t' airframe after each flight.

Inside Hinge
You are now ready t' build t' "inside" hinge

Cut four 3/16" lengths from t' Evergreen 1/8" bar. Drill a 1/16" hole through t' center o' one side o' each. Epoxy them one o' t' BT20 t' BT50 Estes (green) centerin' rings. Begad! Fillet as best as you can, arrr, me bucko, but be sure t' epoxy does nay obstruct t' holes.

This hinge assembly may nay seem very strong, shiver me timbers, but if you build it carefully, matey, (use long curin' epoxy) it will stand up t' t' openin' and drag forces on descent. Begad! Begad! You might want t' cut a bass or ply "silhouette" t' "sandwich" t' hinge assembly.

Hinge Attachment
With t' piston/motor mount inside t' airframe, you are now ready two attach two sets o' hinges. Blimey! Well, matey, blow me down! Sand t' inside o' t' hinge assembly, and slide it onto t' piston. It needs t' slide freely, but should nay be too loose.

Now, take t' other centerin' rin' and glue it 1/2" from t' end o' t' piston/motor tube. Aye aye! Epoxy t' four R/C aileron hinges (as shown) t' t' end o' t' tube.

Make sure t' hinged portion o' t' hinges do nay get any epoxy on them (it's easiest if they hang off t' end. Don't try t' "flex" t' hinges t' follow t' curvature o' t' tube, as they will tend t' bind. Aye aye! Avast! Use generous epoxy fillets here as well.

Once t' epoxy has cured, arrr, you should wrap t' attached portion o' t' hinges with electrical or duct tape t' ensure they remain attached t' t' motor tube. Begad! Install t' motor thrust ring.

Airbrakes
You're now ready t' build t' airbrakes. Arrr! Take t' remainin' 4.75" length o' BT60 and cut it lengthwise into four equal pieces. Arrr! Aye aye! Trim each piece as shown below. (cut along t' dashed lines)

Bend t' four 1/2" lengths o' 1/16" brass or music wire into a loop hinge (as shown below). Begad! Cut a slit into t' center o' t' four 1/2" x 2.5" strips o' 1/16" basswood. Avast, me proud beauty! Avast! Slip t' wire loops into t' slit and turn them at a 90 degree angle t' "lock" them into place. Epoxy these basswood reinforcin' strips t' t' inside o' each o' t' airbrakes, arrr, shiver me timbers, makin' sure t' keep 1/2" towards t' narrow end free o' epoxy (you'll want t' slip t' free end o' t' nylon R/C hinge into this space betwixt t' basswood and BT60 tube when you attach them t' t' motor tube)

Attachin' t' Airbrakes
Test fit your airbrakes t' t' main airframe. Arrr! There should nay be a significant gap betwixt t' airbrakes. Avast! You may need t' trim them, arrr, me bucko, or sand t' aft centerin' rin' t' get them t' fit properly. Begad! Blimey! Once you're satisfied with t' fit, shiver me timbers, epoxy t' nylon hinges (one at a time usin' 5 minute epoxy) t' each o' t' airbrakes.

While t' epoxy is curing, ya bilge rat, take t' 5" x 1" 1/16" basswood and cut four fins t' t' followin' shape.

Once t' epoxy on t' hinges has cured, ya bilge rat, epoxy or CA one fin along one edge o' each airbrake.

Final Assembly
Bend t' ends o' each o' t' four pieces o' brass or music wire into a small "U" shape. Begad! Attach t' wire t' t' forward and aft hinge on each airbrake. Avast, me proud beauty! Be sure that each airbrake slides t' t' full open and closed position easily. Slightly crimp t' ends o' t' wire with a pair o' needle nose pliers so they will nay come off durin' flight.

Glue t' aft end o' t' launch lug flush with t' aft end o' t' forward portion o' t' airframe.

Insert a motor, and add clay nose weight t' until it balances at t' C.G. shown in t' diagram above. I had t' use about an ounce o' clay. Blimey! Begad! (See! I told you nay t' glue t' nose cone on! :-)

Preppin' for Flight
Close each o' t' airbrakes and slip them under t' "lip" o' t' forward airframe. Blimey! Remember t' tape t' nose cone t' t' airframe!

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

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

Additional flight information and photos can be found here.