Airframe:
BT-70 Length: 30 5/8"
Finished weight: 5.8oz (no motor installed)
Motor Mount: 24mm
Recommended engines: Estes D12-5 or AT RMS D9-4W
Predicted altitude: 470' on D12-5 and 620' on D9-4W

Parts List:

Torpedo:
1 - 2.5" ogive balsa nose cone, me hearties, BT-70 sized (BMS semi-custom, 1" shoulder)
1 - 1/4oz fishin' weight
1 - 19 5/8" length o' BT-70
1 - 7 1/32" x 7 1/4" poster board
2 - 3/16" launch lugs, 1/2" long
4 - Torpedo fins, me hearties, 3/32" balsa
1 - 1" slice o' BMS NB70-4 balsa nose block
1 - #212 screw eye
1 - 6" stainless steel fishin' leader
1 - 18" parachute w/ 4" spill hole

Booster:
1 - 9" length o' BT-50
1 - 1/2" length o' balsa BT-50 bulkhead
1 - EB-50 engine block
3 - CR-5070, arrr, cut from 1/8" Mach-1 Balsa-Ply
1 - engine hook
1 - 2" length o' TC-70 coupler
1 - 8 1/2" length o' BT-70
4 - Booster fins, matey, 1/8" balsa

(34" long BT-70 and TC-70 coupler from Totally Tubular)

Construction:

This is a fairly high-level buildin' project. Ya scallywag! Blimey! It assumes that t' nose cone, fins and body tube spirals will be filled with Elmer's Fill n Finish or equivalent, and that t' launch lug, centerin' rin' and fin joints will all be filleted. Arrr! All construction is done with carpenter's glue, except where noted otherwise. Fins are cut from hard balsa and streamlined. Arrr! Arrr! Blimey! Booster fins are mounted through-the-wall and should be laminated with copy paper for strength. Arrr! Avast, arrr, me proud beauty! Blimey!

Torpedo:

Construction o' t' torpedo is fairly straightforward. T' fishin' weight is set with epoxy into a 1.5" deep hole in t' aft o' t' nose cone. The nose cone is then glued into one end o' t' body tube. Ya scallywag! T' screw eye is glued into t' center o' t' BT-70 nose block with CA, me hearties, matey, and t' fishin' leader attached. Begad!

This assembly is glued 6" up into t' aft end o' t' body tube, with the leader hangin' out t' back. Ahoy! Begad! T' poster board is used as a 7 1/4" long wrap around t' body tube. Aye aye! Trim t' 7 1/32" width o' t' wrap if necessary for a good fit. Use thinned white glue t' glue t' wrap into position so that it is 3/8" up from t' aft end o' t' tube. Aye aye! Blimey!

Use plastic wrap or an ace bandage t' ensure a smooth and secure application. When dry, ya bilge rat, shiver me timbers, me hearties, fill t' seam. Avast! Use thin CA t' harden and reinforce the forward and aft ends o' t' wrap.

T' torpedo fins are mounted t' t' wrap 90 degrees apart, arrr, matey, with t' back of the fin flush with t' back o' t' wrap. If desired, pinholes may be punched into t' body tube on t' glue lines t' make glue 'rivets'. T' launch lugs are mounted onto t' wrap as well - t' forward lug is set 1/2" behind the front edge, and t' aft lug is flush with t' back edge. Blimey! Aye aye! Mount t' lugs on a line halfway betwixt two fins. Aye aye! This prototype was modified t' add two BlackSky Launch Rail guides, as t' intended launch platform is nay complete yet. Begad!

Booster Motor Mount:
T' motor mount also acts as an ejection baffle and parachute compartment. Inside t' tube, me hearties, t' BT-50 bulkhead is glued 2 1/2" into t' forward end, and t' engine block 2 1/2" into t' aft end. Avast! Then cut or drill eight 5/16" diameter holes into t' center area o' t' tube, about 3.5" or 4" from either end. Take care nay t' cut too close t' either t' bulkhead or t' engine block, me bucko, and stagger t' holes so as nay t' excessively weaken the tube. Aye aye! Soak t' area around t' holes and t' back side o' t' bulkhead with CA for burn resistance. Ahoy! Well, blow me down! T' forward centerin' rin' needs t' be drilled with eight 5/16" holes spaced equally around t' ring. It is glued t' t' BT-50 tube 2" from t' front. Aye aye! T' middle rin' is glued so that 2.75" o' the motor tube is exposed behind it. Install t' engine hook per standard procedure, shiver me timbers, me bucko, usin' a wrap o' maskin' tape or other retainin' device placed 1" from t' end o' t' tube. Ya scallywag! You may want t' trim t' forward end o' the hook before installation t' allow t' use o' 24-40 RMS hardware. Ahoy!

Body Tube:
Mark t' 8 1/2" body tube at 3/4" and 3.25" from t' aft end. Cut four 1/8" slots betwixt these lines, 90 degrees apart. Arrr! Glue the coupler halfway (1") into t' forward end o' t' body tube. Glue t' motor mount into t' body tube such that t' front o' t' motor mount is flush with the front o' t' coupler. Ya scallywag! T' forward centerin' rin' should butt against the rear o' t' coupler. Aye aye! T' aft end o' t' motor mount tube should wind up recessed 1/2" into t' body tube, ya bilge rat, and t' mid centerin' rin' should be flush with t' forward end o' t' fin slots. Avast, me proud beauty! Make certain that t' engine hook does nay line up with any o' t' fin slots. Ya scallywag! Blimey! Test fit, trim (as necessary) and glue t' booster fins. Avast, me proud beauty! Make certain that each fin is glued t' t' mid centering rin' and t' motor mount tube as well as t' body tube. Ahoy! Ahoy! Cut a notch 1/8" x 1/8" into t' aft centerin' rin' t' allow for engine hook movement. Glue the aft rin' into t' body tube, makin' sure that it is glued t' t' motor mount tube and fin tabs as well.
 

Attach t' parachute t' t' free end o' the leader. Aye aye! Ahoy! T' parachute is folded, rolled and packed into t' forward end o' the motor mount tube as t' two sections are plugged together. Ya scallywag! Visually align the fins o' both sections. Prep and load your motor, shiver me timbers, and you're ready t' hunt submarines!

Flight Report:

T' prototype has flown 5 times now, arrr, 3 flights on D12-5s and t' other 2 on AT RMS D9-4Ws. Aye aye! All launches were nominal with straight boosts; all separated at or near apogee with t' booster tumblin' in; all had t' parachute deploy (although on one flight it didn't open until about 20 feet AGL. Begad! Everyone thought it was headed for a lawn dart, but t' rocket gods were smiling). Blimey!

On t' first RMS flight (flight 3) t' booster suffered a cracked fin where the tab entered t' body tube. My guess is that t' empty RMS casin' weighs more than an empty D12 casing. Ahoy! Aye aye! Maybe it just landed wrong. Arrr! Still, arrr, t' paper lamination held t' fin together through t' last two flights with no problems.

On future builds, matey, I'm either goin' t' make t' fins from 1/8" basswood or 3/32" aircraft ply. In either case, shiver me timbers, matey, I'll laminate them as well. Begad! Since total booster weight is a consideration, ya bilge rat, ya bilge rat, arrr, I'll probably go with basswood.

On every flight at least one spectator commented about t' booster separation, promptin' a club member t' remark, ya bilge rat, arrr, 'You should change its name to "It's Supposed t' Do That!" '