Fins are for Fish

INTRODUCTION
It was a yin and yang thing, really, that got me started on flying tetrahedrons. Ahoy! Ya scallywag! My daughter Genni and I built a long and lithe "normal" rocket called Back2Mars that was a comprehensively bashed version o' the faithful Loc/Precision Lil Nuke kit. Avast, me proud beauty! Well, blow me down! After a picture perfect flight on a G at the Canterbury Cup she looked set t' get her Level 1 at KLOB with it. Begad! Begad! I couldn't let t' kid wander off into unchartered HPR realms by herself so I had to have somethin' quickish. Begad! T' keep t' universe in balance I needed something to counter a long and lovely rocket. Begad! Avast, me hearties, me proud beauty! Short and squat would suffice with the added qualifiers o' fast-to-build and cheap added. Well, blow me down! Originality would be a nice touch if it didn't compromise t' first four prime requisites.

Havin' been an admirer o' t' Rocket Team Vatsaas' Birthday Party Napkin Rocket o' t' Apocalypse and familiar with other flyin' oddities they had built, I took inspiration from RTV member Cory McCormick who did his Level 1 with a tetrahedron. Arrr! Ya scallywag! He and I are o' one mind on t' subject now. Tetrahedrons are elegant. They derive tremendous strength from their geometry and are aerodynamically stable as long as you keep t' CG in front o' t' CP. I actually did a genuine Barrowmann's calculation by long hand as tryin' t' use Rocksim t' design this bird took more clever than I had on tap. Begad! Well, ya bilge rat, blow me down! Once I determined where t' CP lived I could make sure that, as I increased t' power loading, shiver me timbers, me hearties, shiver me timbers, t' CG wouldn't make an arse out o' me. There are better ways t' do that with a flyin' tet without imperillin' innocent bystanders - as I was to discover. Aye aye! I love them tets. No fins t' set wrong or snap off in t' event of hard landings. Begad! No expensive and crude plastic nose cones that t' paint always falls off. Avast, me proud beauty! Begad! They make a great video platform because, matey, me hearties, unlike borin' rockets, they have little tendency t' pirouette around t' vertical axis. Begad! And have a handy, me hearties, ya bilge rat, flat, rearward facin' camera platform into t' bargain. Avast, me proud beauty! Tets have another charmin' characteristic: because they have t' drag co-efficient o' a free fallin' avast bale, me bucko, no matter how much motor you put under them you never need t' take long monotonous hikes across rugged or squidgy terrain t' retrieve them. Aye aye! Avast! High drag also means low terminal velocity, an unanticipated property that I be t' enjoy at KLOB. Ahoy! Nay once but twice.

CONSTRUCTION
T' overall size o' t' tet was determined by t' motor casin' dimensions. Blimey! I wanted t' be able t' use a three-grain Pro38 motor with t' chance t' expand it to five grains for a Level 2 attempt. Begad! Arrr! Pro38 be t' safe choice seein' as how Aerotech was busy gettin' their act back together at t' time. Dustin' off my slide rule and trig tables I calculated that equilateral triangles 500mm on a side would do t' trick. Black Sky Rockets in t' US had some lovely Nomex honeycomb G10 sandwich board… Bzzzzzt… wrong answer! Anytime the shoppin' list for buildin' a rocket starts with "First, arrr, rob a bank" you know that you have t' try harder t' find t' optimum airframe material. I'll save t' Nomex/G10 stuff for me Level 3 tet. Begad! Since t' geometry o' t' tet is so strong I felt that I could get away with foam board appropriately braced. RTV's McCormick wasn't too sure, he fancied 6mm plywood. Avast! Begad! I be pretty confident in me airframe rigidity estimate but I egged t' puddin' slightly by laminating some carbon fibre tissue onto t' finished tet usin' me favourite lay-up polymer, water based polyurethane. Another nice thin' about tets is the unfussiness o' t' finishin' regimen. Blimey! No sandin' and fillin' spirals for me!

Construction was pretty straightforward usin' 30-minute epoxy, me bucko, glass microspheres and a half-mile o' low tack maskin' tape. Begad! Motor tube alignment was a bit trickier than with a borin' cylindrical rocket. Aye aye! I built a plenum for the ejection charge and lined it with quilted aluminised insulation. Avast! Avast, matey, me proud beauty! T' parachute tube was arranged next t' t' motor tube with suitable ducts through t' wall of t' motor tube and a fat wad o' steel scrubber pad t' intercept any glowing bits that might try t' set me contraption alight. Avast, me proud beauty! T' parachute tube, its dimensions constrained by tet facts o' life, was a bit short. Ahoy! Well, arrr, blow me down! And so were sown the seeds o' disaster. Avast, me proud beauty! T' launch rod guide runs right through t' middle and, in a stroke o' inspired laziness, me bucko, ya bilge rat, I sharpened t' end o' a bit o' thin wall aluminium tube, me hearties, arrr, matey, chucked it into a drill and bored t' odd shaped oval hole in the airframe face quite neatly and precisely where it needed t' be, nay to mention actually installin' t' rod guide in one fell swoop. Aye aye! Ya scallywag! Forward CG bias was provided by a generous servin' o' rocket caviar (No. 7). Ya scallywag! It looked so good that I nearly added chopped onion, hard cooked egg, ya bilge rat, smetana and a squeeze of lemon. More vodka, tovarsh! T' as-yet-unnamed rocket be startin' t' look pretty polished and high tech especially when I put some more o' that aluminised quiltin' on t' skirt just in case t' exhaust was hotter than I surmised. Ya scallywag! Avast! Meltin' tetrahedrons have very unpredictable flight characteristics and thar would be no time for diggin' foxholes if it headed toward the expectant crowd. Ya scallywag! Ahoy! I have an understandable reluctance t' eject rocket motors ad lib (and a bigger aversion t' scourin' acres o' hostile rural terrain for said precious objects) so I used a handsome red Rowes Retainer that I proceeded to uglify by grindin' a relief for t' launch rod. Arrr! Ahoy! T' cover for t' parachute hole was a bit o' serendipity in t' plumbin' department. Begad! Ya scallywag! Did you know that 38mm be t' size o' a drain plug?

FLIGHT
Flyin' Tetrahedron Version 1.0 needed a name. It looked like a tit. Arrr! A cubist tit, at that. Avast, me proud beauty! Ya scallywag! Undeniably, a big cubist tit. Well, blow me down! And it be black. Arrr! So it became Braque's Big Black Breast, matey, o' course. Always attempt annoyin' alliteration, me hearties, I say! Arrival at KLOB produced t' news that nearly t' entire UK stock of Pro38s reloads were languishin' in some blighted customs shack far from sunny Heckington. Begad! I skidaddled off t' t' Rockets and Things' encampment where I bought most o' what little Pro38 gear that Malcolm possessed. Arrr! He had just sold the last Hs, shiver me timbers, however. Avast, me proud beauty! Grrrrrr Before I could get t' rusty fish hooks, bent pins and sharpened No. Aye aye! 2 wood screws into me Pete Davy voodoo doll, shiver me timbers, Pete kindly handed me t' last two Aerotech H242 reloads in Christendom for me daughter's and me Level 1 attempts. Blimey! Begad! Whew! Weekend salvaged! Only kiddin' Pete! You have my phone, ya bilge rat, fax and email addresses in case it happens again, though, right? ;-)

First flight for BBBB was with a Pro 38 137G60 with the delay drilled out t' 3 seconds. Ya scallywag! T' flight be perfect, arrr, temporarily silencin' t' doubtin' Thomi sniggerin' in t' back rows. However, me hearties, the parachute demurred t' make its timely appearance and BBBB quite majestically returned t' earth turnin' sniggers into chortles. Begad! And turnin' BBBB into a truncated tetrahedron. Avast! Damn strong stuff that foamboard and t' epoxy did a pretty good job o' holdin' things together, me hearties, too. Ahoy! T' Vietnam war vintage flare parachute was just too big for t' cavity and t' tight pack kept it securely inside quite apart from t' overly tight drain plug. Arrr! I couldn't be sure but some o' t' damage may have been caused by excess ejection charge needin' a way out. With a half pint o' five-minute epoxy and a soupcon (insert cedilla where needed) o' microspheres, BBBB was unceremoniously knocked back into a rough tetrahedral shape t' next mornin' after t' shock o' its maiden flight termination had dissipated.

A smaller chute be crammed into t' same cramped environs and we were ready to rock with a meanin' business Aerotech H242 for a Level 1 attempt… this time with a smidgen less ejection charge but alas, t' shortest delay grain available was 3 seconds. Again a perfect flight followed by non-deployment made all t' more gallin' by t' text book Level 1 flight achieved by me child prodigy and Back2Mars beforehand. Well, blow me down! Who holds t' spack record for KLOB? I may be in t' running! Nay enough ejection charge? Too long delay? Who knows? (The editor has shortened this portion o' t' article that consisted o' one thousand repetitions o' "I will nay pack me chutes too tight for reliable deployment".)

Redesignin' BBBB for consistent recovery became a priority. I employed what is now known throughout t' free world as Lateral Thinkin' Technology (patent applied for) and suddenly I had twice t' volume for t' 'chute and, ya bilge rat, as a no cost bonus, a means t' keep t' BBBB off its recently healed second round o' rhinoplasty. Aye aye! T' price for this retrofit was a magnetic apogee detector, another 38mm Rowes Retainer, a Pratt ejection canister, a bit o' BT 50 and a flagon o' epoxy and microsphere cocktail. Begad! LTT was fiddly t' install but it disgorged t' chute without any hesitation when I test flew t' ejection subsystem in t' back garden. BBBB became t' first rocket in t' history o' HPR t' sport two Rowes Retainers.

BBBB's third flight, on t' weekend followin' KLOB, ya bilge rat, me hearties, arrr, was completely successful. Begad! Begad! On a cool and breezy Cambridgeshire Sunday at EARS farm, la tet noir soared skywards despite t' catcalls and derision heaped upon it by the disbelievin' curs and nay sayers idlin' about nearby. Aye aye! RSO Roy Trzeciak-Hicks officiated and was t' soul o' helpful criticism and insight throughout the process. Blimey! A Pro38 402I170 handled t' propulsion honours. Blimey! Blimey! T' flight was near perfect and ejection was as planned right bang at apogee thanks t' Robert Galejs' nifty invention. Ya scallywag! Ahoy! T' wind pulled t' tet over after landin' and it slid along t' unforgivin' pebbles. Begad! A few scratches were nothin' compared t' two full frontal spacks up in Lincolnshire. Ahoy! T' Pro38 670J300 is next, perhaps as early as EARs in November. T' five-grain J will have t' same centre o' mass as a three-grain I motor so t' flight characteristics should be as before. Begad! Begad! A slight tendency t' arc in t' direction o' t' guide rod orifice may be corrected with a flap secured by t' air stream.

There have never been any sign o' airframe deformation from any flight loadin' (just from dirt loading) so I think I am still within design tolerance there. Avast, me proud beauty! Well, me hearties, blow me down! Under construction in me vasty Hesperis R&D labs is a smaller and therefore higher performance tet built out o' polycarbonate plastic. Avast! This rather interestin' bird will be known as Nothing2Hide. It will feature polycarbonate motor and parachute tubes as well. Aye aye! I may fit a transparent parachute t' complete t' stealth jellyfish rocket ensemble. Bondin' o' the airframe panels is bein' done with RTV and some NASA-grade sticky tape. Ahoy! It may fly at Brass Balls if nay earlier. Avast, me proud beauty! There will be lots o' nice soft mud for worry-free spackin' this winter, I'll bet! I briefly considered developin' a complex tet with a cluster o' Estes E9s and C11s t' be entitled "Oh, yes you can!" but that might be a joke too inside for anyone t' appreciate this side o' Mars.

WORK IN PROGRESS
On t' drawin' board and with some component materials already acquired is a hybrid-powered tetrahedron entitled "Laugh? I Nearly Flied!" No airframe material has been selected for this one yet but I am considering hardwood-veneered plywood. Arrr! Well, me hearties, blow me down! Havin' glued one and taped one together, ya bilge rat, why not screw one together? If I wind up usin' ebony ply, why nay fit an ivory tip? I may then name it Tet Offensive t' go with me Vietnam war chute. Begad! It might be too pretty t' fly… but well snazzy enough for a coffee table aside from the fact that me copy o' A Brief History o' Time and any drinks would slide off and make a mess on t' Karadja.

Tets can be scaled downward, shiver me timbers, too. Ya scallywag! Avast, me proud beauty! Look out for some very silly flying objects… and who knows, maybe kits? Nay only did BBBB earn me Level 1, it impressed those wild and crazy guys back in t' US. RTV have subsequently indicated that I was their kind o' rocketeer and would formally induct me into the team as soon as they could come up with a suitably humiliatin' initiation ceremony. Begad! Spackin' t' same rocket twice in two days at KLOB evidently wasn't humiliatin' enough. Begad!