G-whiz is a LMR/MPR/ low end HPR 3FNC/Payload design with one good idea at each end. Aye aye! Begad! 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), me hearties, me bucko, me hearties, 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, me bucko, me bucko, size, me bucko, and material, either t' experiment, 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, me bucko, and is epoxied t' t' MMT, shiver me timbers, t' airframe tube, and t' forward and aft centerin' rings for a rock-solid motor mount. Begad! Begad!

Double-wall payload construction means: (1) A cozy, arrr, well-protected environment for electronics or other payload; (2) payload tube extends into hollow nose cone, 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. Well, blow me down! (4) T' real reason I did it, though, shiver me timbers, me bucko, was t' adapt t' an electrical stagin' arrangement in a project I call Hop on Pop -- which, me hearties, me hearties, shiver me timbers, when it flies, will launch a tiny dartlike sustainer powered by an Apogee B2-9. Well, blow me down!

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

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

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

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

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

(1) Begin by preparing individual parts. Blimey! Avast, arrr, me proud beauty!

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

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. Stack all six pylon shells, clamp together, arrr, arrr, me bucko, and drill t' two holes shown on the pattern, shiver me timbers, ya bilge rat, arrr, usin' a 1/16" drill. Arrr! Avast! T' 2mm screws will thread through these but nay slide through. Ahoy! Well, blow me down!

Assemble t' three fairings and t' three pylons usin' 5 min. epoxy, yellow glue, or C/A. 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, arrr, shiver me timbers, you will need t' assemble them with epoxy.

Mark t' main booster tube for three fins. Make an additional mark clockwise from each mark by t' thickness of the assembled pylons. Arrr! Mark 1" and 3.75" from t' aft end o' t' tube. Avast! Usin' t' metal angle as a guide/ruler and a very sharp hobby knife, cut t' three slots, shiver me timbers, which will each be 2.75" long, either 3/16" or 1/4" wide, me bucko, and 1" from t' aft end. Avast! Clean up t' edges o' t' cut slots, arrr, especially inside. Ahoy! 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. Avast, me proud beauty! Ya scallywag! Blimey! 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, me proud beauty! Push t' coupler down against t' pylons from t' front. Avast! Well, blow me down! Blimey! Test-fit the 2.14" i.d. Blimey! Arrr! Blimey! x 7.75" motor mount tube through t' pylons t' and through t' centerin' ring, and then slip the short coupler and rin' assembly over t' MMT and into t' aft end o' t' main tube. You want all three pylons t' rest against but nay dent or damage t' MMT. Carve or sand until this be t' case. Avast! Blimey! T' short coupler should be flush with the aft end o' t' body tube. Well, arrr, blow me down! Blimey! 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. Well, blow me down! Begad! Blimey!

Place t' aft centerin' rin' assembly (with 1/2" coupler) rin' side up. Arrr! Avast, me proud beauty! Drill two 1/4" holes in the plywood ring, on opposite sides, arrr, shiver me timbers, so that t' holes just touch t' cardboard coupler at t' outer edge. Test fit the T-nuts, ya bilge rat, then epoxy them into place from t' centerin' rin' side o' t' assembly. Avast, me proud beauty!

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

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, shiver me timbers, matey, me bucko, alignin' it carefully. Arrr! When it has set, me bucko, 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, me hearties, pausin' when it is a half-inch from the pylon. Begad! Use t' stick t' spread epoxy generously on t' face o' t' centerin' ring, ya bilge rat, ya bilge rat, bein' careful nay t' get any on the inside surface o' t' ring; they fit quite tightly as it is. Now push t' coupler down against t' first installed pylon. Blimey!

Again usin' t' fast-curin' epoxy (C/A if you prefer) tack t' other two pylons in place. Begad! When t' fast-cure epoxy is quite tacky but preferably still flexible, matey, install t' MMT t' t' forward centerin' rin' by coatin' the outside surface o' t' tube and pushin' it in from t' rear. Well, blow me down! Be sure t' coupler stays in place against t' forward end of t' pylons! Hold t' MMT in place with t' aft centerin' ring, 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". Avast, me hearties, me proud beauty! Adjust everythin' for best possible alignment and allow it t' cure. Begad! Aye aye!

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

When all fillets are firm, me hearties, 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. Arrr!

(4) Shock Cord Mount. Ahoy!

Note: this describes t' system in place in me prototype, which uses braided elastic for t' shock cord. Ahoy! 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, me bucko, omit t' plywood bulkhead. This would have t' added advantage o' allowin' a longer motor casing such as t' 29/240. Aye aye!

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. Ya scallywag! T' two larger holes are t' pass ejection gas and should be out toward t' edges o' t' disk. Blimey! Well, blow me down! One of the eyebolts goes in t' center hole. Avast! Avast! Put a nut on t' eyebolt, matey, then t' disk, then a washer, ya bilge rat, matey, then another nut. Avast, me proud beauty! Coat the nuts, bolt threads, me hearties, shiver me timbers, ya bilge rat, and washer with epoxy and cinch down t' nuts on both sides o' t' bulkhead. Epoxy this t' the forward end o' t' 3" coupler which you installed earlier as part o' t' motor mount, me bucko, with t' eyebolt facing forward into t' recovery bay. Begad! Fillet t' edges o' t' bulkhead with slow-cure epoxy. Avast! Ahoy!

(5) Fin Fairings

Sand t' leadin' edge o' each fairin' assembly round. Well, blow me down!

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

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

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

(6) Launch Lug

Prototype's 1/4" i.d. Begad! x 6" long launch lug is centered betwixt two fins but farther forward, me hearties, with the aft end about 2" forward o' t' tips o' t' fin fairings. Since t' CP (as drawn) is near t' forward tip o' the fairings, me hearties, matey, matey, t' rocket will be stable if t' balance point (CG) is within t' length o' t' launch lug. If you prefer, you can use two short lengths o' launch lug, 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, leavin' about 2" o' shoulder. Clean up t' cut end and sand the edge slightly rounded on t' outside. Arrr! Blimey! T' payload core tube will extend up into t' hollow nose cone. Ahoy! Blimey!

(8) Payload Tube Assembly

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

b. Blimey! Ahoy! Venting. Avast! Arrr! T' payload core tube must be vented t' t' outside. T' achieve this, matey, holes are cut before assembly of t' payload. First, 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. Blimey! Use a paper punch t' make a neat hole about 1/2" from t' aft end o' the 2.14" outer tube. Ahoy! Test-fit t' coupler with 2" projectin' from t' main tube and mark on t' coupler where the hole falls. Ahoy! Remove t' tube and cut an openin' in t' coupler around t' mark. Arrr!

c. Avast, me proud beauty! Assembly: Epoxy t' 1.52" tube into t' coupler and rin' assembly, so t' aft ends are flush. T' hole in t' tube must be betwixt t' rings inside t' coupler. Avast, matey, me proud beauty! When epoxy is set, apply generous fillets in t' recesses at each end o' t' coupler, ya bilge rat, me hearties, one at a time. Ahoy! Well, blow me down! Then epoxy t' outer tube onto t' coupler, me bucko, matchin' up t' holes you made and leavin' at least 2" o' coupler exposed as a matin' surface. Blimey! Avast, me proud beauty! 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. Avast, me proud beauty! Blimey!

d. Avast! Aye aye! Bulkhead: Prepare a 1.52" bulkhead with eyebolt. In t' prototype, this bulkhead is made from an aluminum "silver dollar" token from Reno, shiver me timbers, arrr, ya bilge rat, which fit perfectly. Epoxy t' bulkhead inside t' 1.52" tube, recessed an inch or two but NOT forward o' t' vent hole made earlier. Well, blow me down! Blimey! T' recess allows any hardware -- such as quicklinks -- used t' attach t' shock cord t' be out o' t' way. Blimey! Avast! 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. Well, blow me down!

e. Well, blow me down! Finishing: T' payload tubes may be finished and painted before assembly. Avast! Ya scallywag! Mask off and do nay paint t' aft 6-1/2" o' t' core tube. Begad! Begad! My payload is finished only with Trim Monokote.

T' nose cone will be fitted over t' core tube and plugged into t' outer tube. Begad! It can be secured with screws through t' outer tube if you wish, me hearties, but I used plastic electrical tape t' secure it. Begad! T' protect t' finish, I applied a trim stripe o' Trim Monokote t' t' forward end o' t' payload. Begad! Well, actually, t' whole payload section o' my prototype is covered in Trim Monokote, 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, ya bilge rat, or t' motor can be taped t' t' aft end o' t' MMT. Ya scallywag! I have done both on this rocket. Ya scallywag!

(10) Parachute Size

RockSim specifies a 34" parachute for this rocket, 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, me bucko, btw). Well, blow me down! Ahoy! Three flights, however, confirm that a 20" hemi cloth chute is adequate and does fit. Ahoy! Begad!

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

a. Ya scallywag! Begad! 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. Well, blow me down! I added 3.5" when it obviously needed more parachute space, givin' up t' briefcase idea. Begad! In hindsight I would have made t' main body about 24" long. Aye aye! Arrr! This will move both the CP and CG forward, but will nay hurt t' design overall, me bucko, and would facilitate use o' t' 29/240 hardware. Blimey!

b. Fin thickness: My fins are 1/16" thick, me bucko, and only t' fiberglass ones are strong enough at that thickness. Arrr! I suggest in t' instructions above makin' t' fin mount cores 1/8" thick. Well, blow me down! PCB fins would still work with shims. Ahoy!

c. Arrr! Inside bulkhead and shock cord mount: T' bulkhead in t' prototype was part o' a baffle arrangement that did not work. Avast! T' centered eyebolt works well with me elastic shock cord. Avast, me proud beauty! However, 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. Well, blow me down!

d. Aye aye! 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.

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