G-whiz is a LMR/MPR/ low end HPR 3FNC/Payload design with one good idea at each end. Arrr! Blimey! It be intended t' be a testbed for an as-yet unbuilt rocket called I-whish ('cause I wish I could afford t' build it now), 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, ya bilge rat, me bucko, size, and material, shiver me timbers, shiver me timbers, me bucko, 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, and is epoxied t' t' MMT, t' airframe tube, and t' forward and aft centerin' rings for a rock-solid motor mount. Avast!

Double-wall payload construction means: (1) A cozy, ya bilge rat, me bucko, 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, matey, shiver me timbers, matey, requirin' less than if it were attached at t' base, and safer than if it were cast into t' tip. Ahoy! (4) T' real reason I did it, though, arrr, me bucko, was t' adapt t' an electrical stagin' arrangement in a project I call Hop on Pop -- which, when it flies, will launch a tiny dartlike sustainer powered by an Apogee B2-9. Ahoy!

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

G-whiz is nay related t' t' Pratt Hobbies altimeter called G-wiz, arrr, though it may someday use one. Arrr! Avast, me bucko, me proud beauty!

Fin Material Selection: My fins are made from 1/16" fiberglass printed circuit board (PCB) material, me bucko, which I bought from an electronics surplus outlet. Avast, me proud beauty! 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. Arrr! T' other basic option is 1/8" aircraft plywood. Avast! Pylon and Fairin' Cores referred t' below can be o' any material but must be at least as thick as t' fin. Ahoy! Avast, me proud beauty! Blimey! (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.

Remainin' parts list:
LOC 2.14" tube, 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, 1.14" in 2.14", consists o' 7.75" MMT(will be used), me bucko, 6" coupler and two centerin' rings
LOC motor mount adapter kit, 1.52" in 2.14", matey, consists o' MMT (not used), matey, matey, 6" coupler and two centerin' rings
Two 3/16" thread eyebolts, arrr, 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! tube
One LOC 2.14" blow-molded white plastic nose cone
1/4" x 6" launch lug
Six , 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. T-nuts for motor retention, and oval head screws t' fit
Recovery harness and approx. Arrr! Begad! 20" hemispherical or 26" flat cloth parachute
One sheet any color Trim Monokote for edge reinforcement and nose cone retention

Five minute epoxy, yellow glue, ya bilge rat, and/or C/A for quick assembly; 15 min. Blimey! Blimey! or 30 min. Arrr! Blimey! 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.

From t' two 2.14" o.d. Ya scallywag! couplers cut three pieces: 1/2", me bucko, arrr, 3" and 4" long (to match prototype; the 4" piece could just as easily be full 6"). Avast! Epoxy t' two 1.14" centerin' rings flush with one end of the 1/2" and 3" pieces, matey, 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. Begad! Stack all six pylon shells, me hearties, ya bilge rat, clamp together, me bucko, matey, me hearties, and drill t' two holes shown on the pattern, me hearties, usin' a 1/16" drill. Aye aye! Aye aye! T' 2mm screws will thread through these but nay slide through.

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

Mark t' main booster tube for three fins. Well, blow me down! Make an additional mark clockwise from each mark by t' thickness of the assembled pylons. Arrr! Arrr! Mark 1" and 3.75" from t' aft end o' t' tube. Aye aye! Ahoy! Usin' t' metal angle as a guide/ruler and a very sharp hobby knife, matey, cut t' three slots, which will each be 2.75" long, me hearties, either 3/16" or 1/4" wide, and 1" from t' aft end. Avast, me proud beauty! Clean up t' edges o' t' cut slots, me hearties, especially inside. Ya scallywag! Ya scallywag! 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. Ahoy! 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! Aye aye! Push t' coupler down against t' pylons from t' front. Avast! Test-fit the 2.14" i.d. x 7.75" motor mount tube through t' pylons t' and through t' centerin' ring, arrr, 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. Avast! Carve or sand until this be t' case. Ya scallywag! T' short coupler should be flush with the aft end o' t' body tube. Avast! Remove everythin' (except t' 3" coupler may be left inside t' forward part o' the tube; move it several inches out o' position). Ya scallywag! Note and mark which pylon fits which slot; they are unlikely t' be identical. Begad! Begad!

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

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

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, me bucko, me hearties, alignin' it carefully. Arrr! 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, pausin' when it is a half-inch from the pylon. Avast, me proud beauty! Use t' stick t' spread epoxy generously on t' face o' t' centerin' ring, 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.

Again usin' t' fast-curin' epoxy (C/A if you prefer) tack t' other two pylons in place. Ya scallywag! 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. Begad! Ya scallywag! 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! Adjust everythin' for best possible alignment and allow it t' cure. Blimey!

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

When all fillets are firm, 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. Ahoy! Arrr! Blimey! Allow t' cure.

(4) Shock Cord Mount. Aye aye!

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

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. One of the eyebolts goes in t' center hole. Put a nut on t' eyebolt, then t' disk, then a washer, ya bilge rat, matey, then another nut. Ya scallywag! Blimey! Coat the nuts, bolt threads, matey, and washer with epoxy and cinch down t' nuts on both sides o' t' bulkhead. Blimey! 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. Arrr! Arrr! Fillet t' edges o' t' bulkhead with slow-cure epoxy. Avast, me proud beauty! Arrr!

(5) Fin Fairings

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

Test-fit t' fins and be sure each fin will sit firmly in one o' t' pylons. Avast, me proud beauty! Mark them, if necessary, ya bilge rat, shiver me timbers, t' assign one particular fin t' each pylon. Well, blow me down!

With a fin in place, ya bilge rat, test-fit a pylon t' t' front o' it. Blimey! T' tabs on t' fairin' shells should just touch the tabs on t' pylon shells. Ahoy! Trim or sand as necessary for best fit. Well, 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. Blimey! Aye aye! When dry, remove t' fin and fillet t' fairin' and fin pylon tabs with a single application o' epoxy. Begad! Begad! At this point I also worked a coat o' epoxy into t' entire surface o' t' fairing, especially t' leadin' edge. Allow t' cure. Aye aye! Begad!

Re-install each fin and mark with a pencil where t' screw holes will be. Arrr! Blimey! Ya scallywag! Blimey! Drill slightly larger holes in the fins: I used a 3/32" bit. Aye aye! Blimey! Now you can test fit t' nuts and bolts. Aye aye! Blimey!

(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, me hearties, with the aft end about 2" forward o' t' tips o' t' fin fairings. Begad! Since t' CP (as drawn) is near t' forward tip o' the fairings, t' rocket will be stable if t' balance point (CG) is within t' length o' t' launch lug. Avast! If you prefer, you can use two short lengths o' launch lug, matey, 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, arrr, shiver me timbers, leavin' about 2" o' shoulder. Begad! Clean up t' cut end and sand the edge slightly rounded on t' outside. Well, blow me down! T' payload core tube will extend up into t' hollow nose cone. Blimey! Avast, me proud beauty!

(8) Payload Tube Assembly

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

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

c. Assembly: Epoxy t' 1.52" tube into t' coupler and rin' assembly, arrr, so t' aft ends are flush. T' hole in t' tube must be betwixt t' rings inside t' coupler. Aye aye! Begad! When epoxy is set, apply generous fillets in t' recesses at each end o' t' coupler, one at a time. Avast! 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. Well, blow me down! Well, blow me down! 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. Bulkhead: Prepare a 1.52" bulkhead with eyebolt. Begad! Blimey! Well, blow me down! Blimey! Blimey! Blimey! In t' prototype, this bulkhead is made from an aluminum "silver dollar" token from Reno, shiver me timbers, 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. T' recess allows any hardware -- such as quicklinks -- used t' attach t' shock cord t' be out o' t' way. Hint: If you plan t' use a quicklink, me bucko, it will be easier t' install if t' eyebolt is about 3/8" off center with t' hole facin' center. Aye aye! Blimey! Well, blow me down! Blimey! Blimey! Blimey!

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

T' nose cone will be fitted over t' core tube and plugged into t' outer tube. Aye aye! It can be secured with screws through t' outer tube if you wish, but I used plastic electrical tape t' secure it. Ahoy! Blimey! T' protect t' finish, I applied a trim stripe o' Trim Monokote t' t' forward end o' t' payload. Ahoy! Well, shiver me timbers, shiver me timbers, actually, shiver me timbers, matey, 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, arrr, or t' motor can be taped t' t' aft end o' t' MMT. I have done both on this rocket. Aye aye!

(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, shiver me timbers, btw). Three flights, however, confirm that a 20" hemi cloth chute is adequate and does fit. Well, blow me down!

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

a. Well, blow me down! Blimey! 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. Begad! Blimey! 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. Well, blow me down! Ahoy! Blimey! This will move both the CP and CG forward, me hearties, shiver me timbers, me hearties, but will nay hurt t' design overall, ya bilge rat, me hearties, and would facilitate use o' t' 29/240 hardware. Well, blow me down! Aye aye! Blimey!

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

c. Blimey! Inside bulkhead and shock cord mount: T' bulkhead in t' prototype be part o' a baffle arrangement that did not work. Ya scallywag! Arrr! T' centered eyebolt works well with me elastic shock cord. Blimey! 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.

d. Avast, me proud beauty! Arrr! 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. Aye aye!

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. Blimey! Avast! It had a different set o' fins and t' 1.52" nose cone, ya bilge rat, matey, which made it look quite different. Ahoy! 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! Ya scallywag!
(the benefit o' reconfigurable fins - ed.)