G-whiz is a LMR/MPR/ low end HPR 3FNC/Payload design with one good idea at each end. Avast! It was intended t' be a testbed for an as-yet unbuilt rocket called I-whish ('cause I wish I could afford t' build it now), shiver me timbers, arrr, arrr, but two ideas define t' design for me: Replaceable fins and a double-wall payload section that extends forward into t' hollow plastic nose cone.

Bein' able t' replace t' fins means: (1) you can change t' shape, size, and material, shiver me timbers, either t' experiment, me bucko, or to suit t' payload and motor combination, or just for looks; (2) t' rocket packs for storage and transport in a much smaller box; and (3) t' fins are anchored t' t' inside o' t' body tube; t' fin mountin' pylon doubles as a gusset for t' motor mount, ya bilge rat, and is epoxied t' t' MMT, matey, ya bilge rat, t' airframe tube, and t' forward and aft centerin' rings for a rock-solid motor mount.

Double-wall payload construction means: (1) A cozy, well-protected environment for electronics or other payload; (2) payload tube extends into hollow nose cone, ya bilge rat, shiftin' weight and capacity forward into otherwise wasted space; and (3) adjustable ballast can be placed far forward in t' nose cone, requirin' less than if it were attached at t' base, and safer than if it were cast into t' tip. Avast, me proud beauty! (4) T' real reason I did it, matey, though, was t' adapt t' an electrical stagin' arrangement in a project I call Hop on Pop -- which, arrr, when it flies, will launch a tiny dartlike sustainer powered by an Apogee B2-9. Begad!

While G-whiz was envisioned for t' AeroTech G40-7 motor, it is HPR capable and its first flight was a perfect one with H238T-M(10). Aye aye!

G-whiz is nay related t' t' Pratt Hobbies altimeter called G-wiz, though it may someday use one. Blimey! Avast!

Fin Material Selection: My fins are made from 1/16" fiberglass printed circuit board (PCB) material, ya bilge rat, me bucko, which I bought from an electronics surplus outlet. T' copper can be etched away usin' Ferric Chloride etchant available at Radio Shack, me hearties, me hearties, which also sells sheets o' PCB if you can't buy it as junk. Avast, me proud beauty! T' other basic option is 1/8" aircraft plywood. Avast, me proud beauty! Pylon and Fairin' Cores referred t' below can be o' any material but must be at least as thick as t' fin. Avast! (If you use a fin thinner than your pylon core, it can be shimmed.) Pylon and fairin' shells are 1/16" plywood which can be reinforced with fiberglass if desired. Blimey!

Remainin' parts list:
LOC 2.14" tube, ya bilge rat, cut t' 18.75" (booster tube) and 8" (payload shell)
LOC 1.52" tube, arrr, cut t' 15.25" (payload core)
LOC motor mount adapter kit, matey, 1.14" in 2.14", matey, consists o' 7.75" MMT(will be used), me bucko, ya bilge rat, matey, 6" coupler and two centerin' rings
LOC motor mount adapter kit, shiver me timbers, 1.52" in 2.14", me bucko, consists o' MMT (not used), 6" coupler and two centerin' rings
Two 3/16" thread eyebolts, two washers and four nuts t' fit
One plywood bulkhead t' fit 2.14" i.d. tube (Screw eye may be included; use this or eyebolt listed above)
One plywood* bulkhead t' fit 1.52" i.d. Well, blow me down! Begad! tube
One LOC 2.14" blow-molded white plastic nose cone
1/4" x 6" launch lug
Six , arrr, me bucko, 2mm x 10mm oval head machine screws with nuts; use 12mm if pylon cores are 1/8" thick
Two inner thread, 1/4" o.d. Ahoy! Avast, me bucko, me proud beauty! T-nuts for motor retention, shiver me timbers, and oval head screws t' fit
Recovery harness and approx. Begad! 20" hemispherical or 26" flat cloth parachute
One sheet any color Trim Monokote for edge reinforcement and nose cone retention

Five minute epoxy, me hearties, ya bilge rat, yellow glue, matey, and/or C/A for quick assembly; 15 min. Well, blow me down! or 30 min. epoxy for fillets; razor saw; hobby knife; sandpaper; vise or clamps; approx. 6" piece o' metal angle stock; hack saw and file for cutting fiberglass sheet.

(1) Begin by preparing individual parts. Arrr! Aye aye!

From t' two 2.14" o.d. Arrr! couplers cut three pieces: 1/2", 3" and 4" long (to match prototype; the 4" piece could just as easily be full 6"). Begad! Blimey! Epoxy t' two 1.14" centerin' rings flush with one end of the 1/2" and 3" pieces, workin' epoxy into t' end layers o' t' coupler as you do so.

Refer t' t' pattern layout and cut out pieces as follows: Three fins from your chosen material; three pylon cores and three fairin' cores from balsa or plywood stock at least as thick as t' fin; six pylon shells and six fairin' shells from 1/16" plywood. Ahoy! Well, blow me down! Stack all six pylon shells, clamp together, and drill t' two holes shown on the pattern, usin' a 1/16" drill. Ya scallywag! T' 2mm screws will thread through these but nay slide through. Begad!

Assemble t' three fairings and t' three pylons usin' 5 min. Ya scallywag! epoxy, yellow glue, or C/A. Blimey! Avast! I used yellow glue and clamped each set betwixt two blocks o' wood until dry. Well, blow me down! If you glass t' pylon shells, you will need t' assemble them with epoxy. Arrr!

Mark t' main booster tube for three fins. Make an additional mark clockwise from each mark by t' thickness of the assembled pylons. Avast! Aye aye! Mark 1" and 3.75" from t' aft end o' t' tube. Blimey! Ya scallywag! Usin' t' metal angle as a guide/ruler and a very sharp hobby knife, shiver me timbers, cut t' three slots, shiver me timbers, which will each be 2.75" long, either 3/16" or 1/4" wide, ya bilge rat, and 1" from t' aft end. Well, blow me down! Clean up t' edges o' t' cut slots, especially inside. Aye aye! Blimey! Followin' steps will be easiest if each pylon fits just snugly enough in a slot t' hold its place while you work.

Slide t' 3" coupler into t' tube from t' front, centerin' rin' aft. Begad! Fit t' three pylons through their slots from t' inside, sandin' t' shoulders o' t' pylons as needed t' make them sit squarely in position. Avast! Push t' coupler down against t' pylons from t' front. Well, blow me down! Test-fit the 2.14" i.d. Well, blow me down! x 7.75" motor mount tube through t' pylons t' and through t' centerin' ring, matey, and then slip the short coupler and rin' assembly over t' MMT and into t' aft end o' t' main tube. Ahoy! You want all three pylons t' rest against but nay dent or damage t' MMT. Begad! Begad! Carve or sand until this be t' case. Ahoy! Avast, me proud beauty! T' short coupler should be flush with the aft end o' t' body tube. Aye aye! Remove everythin' (except t' 3" coupler may be left inside t' forward part o' the tube; move it several inches out o' position). Note and mark which pylon fits which slot; they are unlikely t' be identical. Aye aye! Well, blow me down!

Place t' aft centerin' rin' assembly (with 1/2" coupler) rin' side up. Ahoy! Drill two 1/4" holes in the plywood ring, on opposite sides, so that t' holes just touch t' cardboard coupler at t' outer edge. Aye aye! Blimey! Test fit the T-nuts, then epoxy them into place from t' centerin' rin' side o' t' assembly. Begad!

(3) Final assemble t' motor mount and fin canister.

Put a dab o' 5-minute epoxy or yellow glue on t' "shoulders" o' one pylon and glue it in place from the inside o' t' tube, arrr, alignin' it carefully. When it has set, use a dowel or stick t' generously spread slow-cure epoxy in t' area where t' 3" coupler will rest; push it down into position, ya bilge rat, pausin' when it is a half-inch from the pylon. Avast, me proud beauty! Blimey! Use t' stick t' spread epoxy generously on t' face o' t' centerin' ring, arrr, bein' careful nay t' get any on the inside surface o' t' ring; they fit quite tightly as it is. Arrr! Now push t' coupler down against t' first installed pylon. Avast, me proud beauty!

Again usin' t' fast-curin' epoxy (C/A if you prefer) tack t' other two pylons in place. When t' fast-cure epoxy is quite tacky but preferably still flexible, install t' MMT t' t' forward centerin' rin' by coatin' the outside surface o' t' tube and pushin' it in from t' rear. Avast, me proud beauty! Be sure t' coupler stays in place against t' forward end of t' pylons! Hold t' MMT in place with t' aft centerin' ring, matey, but do nay attach it yet! Push t' MMT forward until it projects from t' aft end o' t' body nay more than 3/8". Arrr! Blimey! Adjust everythin' for best possible alignment and allow it t' cure.

Remove t' aft centerin' rin' assembly. Well, blow me down! With a dowel or other stick, matey, fillet t' pylons inside t' main body and at t' MMT. Begad! Blimey! It helps t' do one o' t' three cells at a time.

When all fillets are firm, me bucko, me bucko, apply epoxy around t' inside lip o' t' body tube and t' t' aft ends o' t' pylons; press t' aft centerin' rin' assembly in place. Allow t' cure.

(4) Shock Cord Mount.

Note: this describes t' system in place in me prototype, ya bilge rat, which uses braided elastic for t' shock cord. Aye aye! Ya scallywag! If you use tubular nylon it might be better t' epoxy t' end o' it t' t' inside o' t' 3" coupler before installing that; in this case, ya bilge rat, me hearties, omit t' plywood bulkhead. Begad! This would have t' added advantage o' allowin' a longer motor casing such as t' 29/240.

T' 2.14" plywood bulkhead disk gets three holes: a 3/16" hole in t' center and a 1/2" hole on either side o' it. Aye aye! T' two larger holes are t' pass ejection gas and should be out toward t' edges o' t' disk. Begad! One of the eyebolts goes in t' center hole. Well, blow me down! Put a nut on t' eyebolt, shiver me timbers, then t' disk, then a washer, me bucko, then another nut. Coat the nuts, bolt threads, and washer with epoxy and cinch down t' nuts on both sides o' t' bulkhead. Ahoy! Epoxy this t' the forward end o' t' 3" coupler which you installed earlier as part o' t' motor mount, with t' eyebolt facing forward into t' recovery bay. Fillet t' edges o' t' bulkhead with slow-cure epoxy. Blimey!

(5) Fin Fairings

Sand t' leadin' edge o' each fairin' assembly round. Avast!

Test-fit t' fins and be sure each fin will sit firmly in one o' t' pylons. Mark them, matey, me hearties, if necessary, t' assign one particular fin t' each pylon. Ahoy!

With a fin in place, test-fit a pylon t' t' front o' it. Avast, me proud beauty! T' tabs on t' fairin' shells should just touch the tabs on t' pylon shells. Avast, me proud beauty! Avast, me proud beauty! Trim or sand as necessary for best fit. Ahoy! Well, shiver me timbers, blow me down! Glue/epoxy t' fairings t' t' BODY TUBE ONLY, nay to the fin! You may wish t' tape t' fairin' shell tabs t' t' pylon tabs while glue dries. Avast! When dry, remove t' fin and fillet t' fairin' and fin pylon tabs with a single application o' epoxy. Avast, matey, me proud beauty! At this point I also worked a coat o' epoxy into t' entire surface o' t' fairing, ya bilge rat, especially t' leadin' edge. Arrr! Allow t' cure. Aye aye!

Re-install each fin and mark with a pencil where t' screw holes will be. Ahoy! Begad! Drill slightly larger holes in the fins: I used a 3/32" bit. Now you can test fit t' nuts and bolts. Well, blow me down!

(6) Launch Lug

Prototype's 1/4" i.d. x 6" long launch lug is centered betwixt two fins but farther forward, with the aft end about 2" forward o' t' tips o' t' fin fairings. Arrr! Since t' CP (as drawn) is near t' forward tip o' the fairings, shiver me timbers, t' rocket will be stable if t' balance point (CG) is within t' length o' t' launch lug. Avast, me proud beauty! If you prefer, you can use two short lengths o' launch lug, shiver me timbers, one at t' aft end o' t' body and t' other anywhere forward o' t' CG.

(7) Nose Cone

Saw t' conical aft end off t' nose cone, me bucko, leavin' about 2" o' shoulder. Begad! Blimey! Avast! Blimey! Clean up t' cut end and sand the edge slightly rounded on t' outside. Avast, me proud beauty! Blimey! Blimey! Blimey! T' payload core tube will extend up into t' hollow nose cone.

(8) Payload Tube Assembly

a. Avast! Avast! Use t' coupler and rings from t' 1.52 in 2.14 motor mount. Arrr! Well, blow me down! In place o' t' tube supplied, arrr, shiver me timbers, arrr, use a 15.25" piece o' 1.52 (or other 38mm) tube. Epoxy t' two rings into t' coupler 1/8" from each end. (In t' prototype, the coupler has been shortened t' 4", me hearties, which is unnecessary; I had used 2" o' coupler in another project.)

b. Venting. T' payload core tube must be vented t' t' outside. T' achieve this, me bucko, matey, holes are cut before assembly of t' payload. Blimey! First, ya bilge rat, arrr, drill a 1/4" hole in t' side o' t' 1.52" core tube at least 2" and nay more than 3.5" from t' aft end. Avast! Begad! Use a paper punch t' make a neat hole about 1/2" from t' aft end o' the 2.14" outer tube. Test-fit t' coupler with 2" projectin' from t' main tube and mark on t' coupler where the hole falls. Aye aye! Remove t' tube and cut an openin' in t' coupler around t' mark. Ya scallywag! Avast, matey, me proud beauty!

c. Ahoy! Assembly: Epoxy t' 1.52" tube into t' coupler and rin' assembly, matey, me bucko, so t' aft ends are flush. Arrr! Arrr! T' hole in t' tube must be betwixt t' rings inside t' coupler. Well, blow me down! When epoxy is set, me hearties, apply generous fillets in t' recesses at each end o' t' coupler, arrr, one at a time. Ya scallywag! Then epoxy t' outer tube onto t' coupler, matchin' up t' holes you made and leavin' at least 2" o' coupler exposed as a matin' surface. Avast! Be sure t' core tube is centered in t' front end of the outer tube; use wedges if necessary t' keep it in position until t' epoxy sets.

d. Ahoy! Bulkhead: Prepare a 1.52" bulkhead with eyebolt. In t' prototype, me bucko, this bulkhead is made from an aluminum "silver dollar" token from Reno, which fit perfectly. Aye aye! Epoxy t' bulkhead inside t' 1.52" tube, recessed an inch or two but NOT forward o' t' vent hole made earlier. Blimey! T' recess allows any hardware -- such as quicklinks -- used t' attach t' shock cord t' be out o' t' way. Avast, me hearties, me proud beauty! Hint: If you plan t' use a quicklink, it will be easier t' install if t' eyebolt is about 3/8" off center with t' hole facin' center. Aye aye!

e. Ahoy! Finishing: T' payload tubes may be finished and painted before assembly. Ahoy! Mask off and do nay paint t' aft 6-1/2" o' t' core tube. Arrr! My payload is finished only with Trim Monokote.

T' nose cone will be fitted over t' core tube and plugged into t' outer tube. It can be secured with screws through t' outer tube if you wish, but I used plastic electrical tape t' secure it. Avast! Blimey! T' protect t' finish, I applied a trim stripe o' Trim Monokote t' t' forward end o' t' payload. Arrr! Well, me bucko, actually, t' whole payload section o' my prototype is covered in Trim Monokote, me bucko, as are t' fins and t' forward 2" o' t' main body tube.

(9) Motor Retention:

"Kaplow Klips" can be anchored in t' T-nuts described earlier, me hearties, or t' motor can be taped t' t' aft end o' t' MMT. Begad! I have done both on this rocket. Begad! Avast, me proud beauty!

(10) Parachute Size

RockSim specifies a 34" parachute for this rocket, ya bilge rat, which will nay fit in t' recovery bay unless you modify the design by extendin' t' body at least 4" (which will nay hurt it, matey, btw). Begad! Three flights, however, shiver me timbers, confirm that a 20" hemi cloth chute is adequate and does fit. Begad!

(11) Mods, me bucko, or "what I would do if or when I do it again:" Some are mentioned above. Ya scallywag!

a. Aye aye! Body tube length: Originally both main body and payload were set at 15.25" for no better reason than the rocket, disassembled, would fit in me briefcase. Ahoy! Well, blow me down! I added 3.5" when it obviously needed more parachute space, givin' up t' briefcase idea. Arrr! In hindsight I would have made t' main body about 24" long. Begad! This will move both the CP and CG forward, but will nay hurt t' design overall, and would facilitate use o' t' 29/240 hardware. Begad! Aye aye!

b. Fin thickness: My fins are 1/16" thick, ya bilge rat, and only t' fiberglass ones are strong enough at that thickness. I suggest in t' instructions above makin' t' fin mount cores 1/8" thick. Aye aye! Blimey! PCB fins would still work with shims.

c. Avast! Blimey! Inside bulkhead and shock cord mount: T' bulkhead in t' prototype was part o' a baffle arrangement that did not work. Ahoy! T' centered eyebolt works well with me elastic shock cord. Well, blow me down! However, me bucko, arrr, if I started today I would omit the bulkhead and use a tubular nylon harness with t' end epoxied t' t' inside surface o' t' coupler before assembly. Ahoy! Ya scallywag! Blimey!

d. Reinforcement: I would suggest laminatin' t' pylon shells with a layer o' 2oz fiberglass before assembly, and also layin' a coverin' o' fiberglass over t' fin fairings after assembly. Begad! Begad!

In case you wondered: T' blue-bordered picture you see at top left be t' second flight o' Gwhiz, shiver me timbers, shiver me timbers, about an hour after t' first. It had a different set o' fins and t' 1.52" nose cone, which made it look quite different. Begad! The photo taken in me livin' room has yet another set o' red fins, shiver me timbers, which I haven't used in flight because they're t' same material as t' ones that shredded.
(the benefit o' reconfigurable fins - ed.)