Fins are for Fish

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

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

CONSTRUCTION
The overall size of the tet was determined by the motor casing dimensions. I wanted to be able to use a three-grain Pro38 motor with the chance to expand it to five grains for a Level 2 attempt. Pro38 was the safe choice seeing as how Aerotech was busy getting their act back together at the time. Dusting off my slide rule and trig tables I calculated that equilateral triangles 500mm on a side would do the trick. Black Sky Rockets in the US had some lovely Nomex honeycomb G10 sandwich board… Bzzzzzt… wrong answer! Anytime the shopping list for building a rocket starts with "First, rob a bank" you know that you have to try harder to find the optimum airframe material. I'll save the Nomex/G10 stuff for my Level 3 tet. Since the geometry of the 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. I was pretty confident in my airframe rigidity estimate but I egged the pudding slightly by laminating some carbon fibre tissue onto the finished tet using my favourite lay-up polymer, water based polyurethane. Another nice thing about tets is the unfussiness of the finishing regimen. No sanding and filling spirals for me!

Construction was pretty straightforward using 30-minute epoxy, glass microspheres and a half-mile of low tack masking tape. Motor tube alignment was a bit trickier than with a boring cylindrical rocket. I built a plenum for the ejection charge and lined it with quilted aluminised insulation. The parachute tube was arranged next to the motor tube with suitable ducts through the wall of the motor tube and a fat wad of steel scrubber pad to intercept any glowing bits that might try to set my contraption alight. The parachute tube, its dimensions constrained by tet facts of life, was a bit short. And so were sown the seeds of disaster. The launch rod guide runs right through the middle and, in a stroke of inspired laziness, I sharpened the end of a bit of thin wall aluminium tube, chucked it into a drill and bored the odd shaped oval hole in the airframe face quite neatly and precisely where it needed to be, not to mention actually installing the rod guide in one fell swoop. Forward CG bias was provided by a generous serving of rocket caviar (No. 7). It looked so good that I nearly added chopped onion, hard cooked egg, smetana and a squeeze of lemon. More vodka, tovarsh! The as-yet-unnamed rocket was starting to look pretty polished and high tech especially when I put some more of that aluminised quilting on the skirt just in case the exhaust was hotter than I surmised. Melting tetrahedrons have very unpredictable flight characteristics and there would be no time for digging foxholes if it headed toward the expectant crowd. I have an understandable reluctance to eject rocket motors ad lib (and a bigger aversion to scouring acres of hostile rural terrain for said precious objects) so I used a handsome red Rowes Retainer that I proceeded to uglify by grinding a relief for the launch rod. The cover for the parachute hole was a bit of serendipity in the plumbing department. Did you know that 38mm is the size of a drain plug?

FLIGHT
Flying Tetrahedron Version 1.0 needed a name. It looked like a tit. A cubist tit, at that. Undeniably, a big cubist tit. And it was black. So it became Braque's Big Black Breast, of course. Always attempt annoying alliteration, I say! Arrival at KLOB produced the news that nearly the entire UK stock of Pro38s reloads were languishing in some blighted customs shack far from sunny Heckington. I skidaddled off to the Rockets and Things' encampment where I bought most of what little Pro38 gear that Malcolm possessed. He had just sold the last Hs, however. Grrrrrr Before I could get the rusty fish hooks, bent pins and sharpened No. 2 wood screws into my Pete Davy voodoo doll, Pete kindly handed me the last two Aerotech H242 reloads in Christendom for my daughter's and my Level 1 attempts. Whew! Weekend salvaged! Only kidding Pete! You have my phone, 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 to 3 seconds. The flight was perfect, temporarily silencing the doubting Thomi sniggering in the back rows. However, the parachute demurred to make its timely appearance and BBBB quite majestically returned to earth turning sniggers into chortles. And turning BBBB into a truncated tetrahedron. Damn strong stuff that foamboard and the epoxy did a pretty good job of holding things together, too. The Vietnam war vintage flare parachute was just too big for the cavity and the tight pack kept it securely inside quite apart from the overly tight drain plug. I couldn't be sure but some of the damage may have been caused by excess ejection charge needing a way out. With a half pint of five-minute epoxy and a soupcon (insert cedilla where needed) of microspheres, BBBB was unceremoniously knocked back into a rough tetrahedral shape the next morning after the shock of its maiden flight termination had dissipated.

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

Redesigning BBBB for consistent recovery became a priority. I employed what is now known throughout the free world as Lateral Thinking Technology (patent applied for) and suddenly I had twice the volume for the 'chute and, as a no cost bonus, a means to keep the BBBB off its recently healed second round of rhinoplasty. The price for this retrofit was a magnetic apogee detector, another 38mm Rowes Retainer, a Pratt ejection canister, a bit of BT 50 and a flagon of epoxy and microsphere cocktail. LTT was fiddly to install but it disgorged the chute without any hesitation when I test flew the ejection subsystem in the back garden. BBBB became the first rocket in the history of HPR to sport two Rowes Retainers.

BBBB's third flight, on the weekend following KLOB, was completely successful. On a cool and breezy Cambridgeshire Sunday at EARS farm, la tet noir soared skywards despite the catcalls and derision heaped upon it by the disbelieving curs and nay sayers idling about nearby. RSO Roy Trzeciak-Hicks officiated and was the soul of helpful criticism and insight throughout the process. A Pro38 402I170 handled the propulsion honours. The flight was near perfect and ejection was as planned right bang at apogee thanks to Robert Galejs' nifty invention. The wind pulled the tet over after landing and it slid along the unforgiving pebbles. A few scratches were nothing compared to two full frontal spacks up in Lincolnshire. The Pro38 670J300 is next, perhaps as early as EARs in November. The five-grain J will have the same centre of mass as a three-grain I motor so the flight characteristics should be as before. A slight tendency to arc in the direction of the guide rod orifice may be corrected with a flap secured by the air stream.

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

WORK IN PROGRESS
On the drawing 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. Having glued one and taped one together, why not screw one together? If I wind up using ebony ply, why not fit an ivory tip? I may then name it Tet Offensive to go with my Vietnam war chute. It might be too pretty to fly… but well snazzy enough for a coffee table aside from the fact that my copy of A Brief History of Time and any drinks would slide off and make a mess on the Karadja.

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