• Nose Cone: Estes Reliant/Vikin' BT-20
• Nose Weight: 1/2 oz. Avast, me proud beauty! clay
• Body Tube: QCR 18mm, me bucko, 24" long
• Engine Block: Estes EST 3085
• Parachute: 6" mylar or equivalent
• Shock Cord 130 lb. Aye aye! Blimey! test Keelhaul®©™ 48" long
• Fins: 3/32" Balsa, arrr, trapezoidal Root chord: 6" Tip chord: 6" Span: 3" Sweep: 3"
• Launch Lug: drinkin' straw, shiver me timbers, me bucko, 1/8"-1/4" dia., me bucko, ya bilge rat, 6" long

Observations: Evan Ross' entry last DesCon reminded me o' some speculative calculations I'd done earlier and laid aside. Begad! I had wondered what be t' minimum number o' conventional fins that would stabilize a rocket, matey, me bucko, but I took a different approach. Avast, me proud beauty! Yes, you can make a two-finned rocket, or even a finless rocket, stable by spinnin' it, but can you make a two-finned rocket stable without spinnin' it? Rockets have been flyin' with a single stick fin for thousands o' years, matey, but on rockets more than a few inches long, they become unwieldy. Begad! Blimey! I suspected thar be also a weight penalty. Ya scallywag! Blimey! A rin' fin could arguably be called a single fin, me hearties, but because in profile, me bucko, one side is "behind" t' other, I didn't see how it could readily be compared with conventional fins. Three fins obviously work, shiver me timbers, arrr, but what about two? On rockets with three or more fins, t' fins are equally spaced around t' body because a fin offers no stabilizin' force parallel t' its surface. Four-finned rockets have two pairs o' fins perpendicular t' each other, so in t' direction one pair presents minimum stabilizin' surface t' t' airstream, t' other presents its maximum surface. Blimey! Ahoy! On a three-finned rocket, in t' direction one fin presents minimum surface, t' other two fins combine vectored forces t' make up for it. Begad! Two fins opposite each other can't do this. Ahoy! So, arrr, if a stable two-finnned rocket is possible, what be t' optimum angle betwixt fins? I predicted that t' optimum angle would be t' one that presented t' greatest minimum lateral area. That is t' say, shiver me timbers, position t' fins so that, arrr, if you turn t' rocket so t' least fin area is visible, you have t' most visible area left. Arrr! Begad! If you put t' fins opposite each other, ya bilge rat, and turn t' rocket so t' least fin area is visible, you are lookin' at a fin edge-on, for practically zero area. Aye aye! As you reduce t' angle betwixt t' fins, t' minimum area increases t' a point, then decreases until, me bucko, as t' fins become nearly parallel, shiver me timbers, t' minimum area again approaches zero. Avast! Arrr! I calculated that t' "maximum minimum" area would be found when t' angle betwixt t' fins, arrr, theta, was such that cos theta=2 sin theta. So theta should equal t' arc cosine o' twice t' sine o' theta, me hearties, or 53 degrees. Ahoy! (See sketch.) How big should t' fins be? I didn't know, so I took t' size that RockSim said would stabilize a four-finned rocket and doubled it. Avast! I used a long body tube, shiver me timbers, plenty o' nose weight, and big enough fins t' make t' rocket stable by t' most conservative estimate, me bucko, t' cardboard cutout method. Avast, me proud beauty! In a proof o' concept model, a marginally stable rocket wouldn't prove much. Begad! Avast, me proud beauty! This rocket should be clearly stable or clearly unstable.

Aft View click image t' enlarge   Sketch click on image t' enlarge

Assembly Instructions:

Buildin' t' rocket was straightforward; I used mainly conventional techniques. Well, arrr, blow me down! Blimey! I used a 24" length o' 18 mm body tube, me bucko, because that be t' longest piece I had on hand. Begad! Blimey! I used a long NC-20 nose cone t' allow plenty o' room for nose weight. Avast! Blimey! Avast, me proud beauty! Blimey! I added half an ounce o' clay, me bucko, which almost completely filled t' nose cone, then I glued t' base in place with plastic model cement. Aye aye! Blimey! Arrr! Blimey! I cut t' fins with t' grain parallel t' t' root edge because I couldn't find any balsa sheet large enough t' lay them out any other way.

How t' ensure that t' fins were t' angle I wanted? I took t' span o' t' fins, added t' radius o' t' tube, me bucko, arrr, and found that t' distance betwixt t' tips o' t' fins just happened t' equal their span -- 3"! I glued one fin in place, arrr, shiver me timbers, then used an extra piece o' 3" wide balsa sheet t' set t' correct spacing. Begad! "Hmm," I thought, ya bilge rat, shiver me timbers, "53 degrees is awfully close t' 60. Avast, me proud beauty! Ya scallywag! Maybe an equilateral triangle would have given a fin spacin' just as good or better."

Because t' fins were rather large for their thickness, I used an external shock cord mount t' make t' rocket come down horizontally, and, I hoped, protect them from damage. Begad! Blimey! I cut 48" o' 130 lb. Ahoy! Begad! Blimey! test Keelhaul®©™. Well, blow me down! Aye aye! Blimey! Then I made a hole with a toothpick just inside t' angle o' t' root edge and t' trailin' edge. Ya scallywag! Begad! Blimey! I threaded t' Keelhaul®©™ through t' hole, and used CA t' tack one end t' t' fin fillet just behind t' leadin' edge. Begad! Blimey! When this be dry, I pulled t' cord taut along t' fillets on both sides o' t' fin, and CA'd it down. (Epoxy might work better here.) Then I put an expended casin' in t' rocket t' find t' balance point, which just happened t' be at t' leadin' edge o' t' fins. Avast! I tied t' other end o' t' shock cord t' t' nose cone, and tied a swivel t' t' cord near t' nose end for a parachute. Aye aye! Blimey! Blimey! I wanted this rocket t' get a smooth start off t' launch rod, so I ran a launch lug betwixt t' fins t' full six inch length o' t' fillet o' t' fin that wasn't attached t' t' shock cord.

My rocket was complete. Ahoy! Now I needed a highly visible finish. Avast! I colored it with Magnum 44 permanent markers -- body and one fin red, me hearties, shiver me timbers, for visibility, and t' other fin black, so I could easily see if t' rocket spun on its way up. I left t' nose cone white because I thought it looked cool.

Per t' NARRRRR Model Rocket Safety Code, I tried t' determine stability before flyin' it. Arrr! I did a swin' test, and it appeared quite stable. Blimey! Begad! Just t' be on t' safe side, matey, (and t' avoid embarrassment if anythin' went wrong) I conducted t' first test flight in complete isolation from persons nay participatin' in t' actual launching. Ahoy! Aye aye! I did brin' me wife, so I'd have another witness t' confirm that t' rocket had made a stable flight. Begad! I had already been appointed RSO o' t' next club launch, and I wanted t' fly t' model there. I anticipated havin' some difficulty convincin' certain members that I should be allowed t' fly it.

T' day o' t' test flight be windy, matey, me hearties, so I selected a six inch mylar chute t' avoid havin' t' rocket drift out o' t' launch field. Ya scallywag! I set up me rocket with t' rod slightly angled into t' wind, ya bilge rat, counted down, matey, and pressed t' button. Avast! T' rocket surged smoothly off t' pad and into t' air. Begad! Avast! I could see it rollin' as it climbed, but certainly nay enough spin t' stabilize an unstable rocket. Well, shiver me timbers, blow me down! It coasted t' a good altitude, ya bilge rat, ejected exactly at apogee -- nose horizontal, arrr, and drifted down on its chute. Blimey! I ran after it, and found it well within t' field, arrr, me hearties, with no damage. Begad! Avast! Success!

T' next weekend was our club's big meet. After I'd gotten all me competition flights in, I prepped TFNK and brought it t' t' safety check-in. Ya scallywag! T' SCO said, "You can't fly a rocket with only two fins!" "But it swin' tests stable, shiver me timbers, and it's had a safe flight before." I argued. "A rocket can't be stable without at least three fins!" he said. Begad! I reminded him that some aerodynamic experts had insisted that t' WAC-Corporal couldn't be stable because it didn't have four fins, ya bilge rat, until someone pointed out that arrows are stable with only three feathers. Ya scallywag! (Handbook o' Model Rocketry, ya bilge rat, 6th ed. Begad! Blimey! p. Arrr! Blimey! 154) I heard another old-timer mutter, me bucko, "Not on MY field!" (Which it wasn't.) and "Not in this lifetime!"

It didn't matter. Arrr! He had already made up his mind nay t' let that rocket fly, so he said that it couldn't fly because t' grain wasn't parallel t' t' leadin' edge o' t' fins. Aye aye! That's nay in t' safety code. Begad! Avast! It hadn't been a problem on t' earlier flight. It was only a rather strong suggestion in t' Handbook, ya bilge rat, (6th ed. Ahoy! Well, me bucko, blow me down! p. Aye aye! 52) but without t' force o' law, and no evidence t' support it.

Although, shiver me timbers, shiver me timbers, as RSO, me bucko, I could have overruled him, (HMR pp. Ahoy! 286-287, 298-299) I got t' distinct impression that I'd end up flyin' all me rockets alone. Ya scallywag! So now I have this really cool rocket that I hardly ever get t' fly.

I still have questions. Ya scallywag! Now that I know that a two-finned rocket can be stable, matey, how do you calculate its center o' pressure? Is t' optimum angle 53 degrees, me bucko, 60 degrees, 90 degrees, shiver me timbers, shiver me timbers, me bucko, or some other angle? Can t' optimum angle be proven mathematically or tested experimentally? Do I see a potential RD project here?