• Nose Cone: Estes Reliant/Vikin' BT-20
• Nose Weight: 1/2 oz. clay
• Body Tube: QCR 18mm, me hearties, arrr, 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, me bucko, 1/8"-1/4" dia., me bucko, matey, matey, 6" long

Observations: Evan Ross' entry last DesCon reminded me o' some speculative calculations I'd done earlier and laid aside. Ya scallywag! I had wondered what was t' minimum number o' conventional fins that would stabilize a rocket, but I took a different approach. Yes, me bucko, you can make a two-finned rocket, or even a finless rocket, me bucko, matey, matey, 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, but on rockets more than a few inches long, they become unwieldy. I suspected thar was also a weight penalty. Begad! A rin' fin could arguably be called a single fin, me bucko, but because in profile, me hearties, one side is "behind" t' other, me bucko, I didn't see how it could readily be compared with conventional fins. Ahoy! Three fins obviously work, 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. Aye aye! Ya scallywag! Four-finned rockets have two pairs o' fins perpendicular t' each other, me bucko, so in t' direction one pair presents minimum stabilizin' surface t' t' airstream, me bucko, matey, t' other presents its maximum surface. Avast, me proud beauty! Blimey! On a three-finned rocket, in t' direction one fin presents minimum surface, shiver me timbers, shiver me timbers, t' other two fins combine vectored forces t' make up for it. Two fins opposite each other can't do this. Avast! So, if a stable two-finnned rocket is possible, matey, 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, position t' fins so that, if you turn t' rocket so t' least fin area is visible, me bucko, you have t' most visible area left. Ya scallywag! If you put t' fins opposite each other, and turn t' rocket so t' least fin area is visible, me bucko, you are lookin' at a fin edge-on, ya bilge rat, for practically zero area. As you reduce t' angle betwixt t' fins, t' minimum area increases t' a point, me hearties, then decreases until, as t' fins become nearly parallel, me hearties, t' minimum area again approaches zero. Begad! I calculated that t' "maximum minimum" area would be found when t' angle betwixt t' fins, ya bilge rat, theta, ya bilge rat, me bucko, was such that cos theta=2 sin theta. Well, blow me down! So theta should equal t' arc cosine o' twice t' sine o' theta, arrr, or 53 degrees. Arrr! (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. Arrr! I used a long body tube, me bucko, shiver me timbers, plenty o' nose weight, matey, and big enough fins t' make t' rocket stable by t' most conservative estimate, shiver me timbers, arrr, t' cardboard cutout method. In a proof o' concept model, a marginally stable rocket wouldn't prove much. Avast, me proud beauty! Ya scallywag! 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. Ya scallywag! I used a 24" length o' 18 mm body tube, me bucko, ya bilge rat, because that was t' longest piece I had on hand. I used a long NC-20 nose cone t' allow plenty o' room for nose weight. I added half an ounce o' clay, matey, which almost completely filled t' nose cone, me hearties, then I glued t' base in place with plastic model cement. Ya scallywag! 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, and found that t' distance betwixt t' tips o' t' fins just happened t' equal their span -- 3"! Blimey! I glued one fin in place, ya bilge rat, then used an extra piece o' 3" wide balsa sheet t' set t' correct spacing. Avast, me proud beauty! Blimey! "Hmm," I thought, "53 degrees is awfully close t' 60. Blimey! Blimey! Ahoy! Blimey! 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, ya bilge rat, and, I hoped, protect them from damage. Arrr! I cut 48" o' 130 lb. test Keelhaul®©™. Then I made a hole with a toothpick just inside t' angle o' t' root edge and t' trailin' edge. Arrr! Aye aye! I threaded t' Keelhaul®©™ through t' hole, arrr, and used CA t' tack one end t' t' fin fillet just behind t' leadin' edge. When this was dry, arrr, I pulled t' cord taut along t' fillets on both sides o' t' fin, and CA'd it down. Well, blow me 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. I tied t' other end o' t' shock cord t' t' nose cone, matey, me bucko, and tied a swivel t' t' cord near t' nose end for a parachute. Begad! 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 be complete. Avast, me proud beauty! Blimey! Now I needed a highly visible finish. Avast! Blimey! I colored it with Magnum 44 permanent markers -- body and one fin red, for visibility, arrr, arrr, and t' other fin black, matey, so I could easily see if t' rocket spun on its way up. Arrr! 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. Blimey! I did a swin' test, arrr, and it appeared quite stable. Ya scallywag! Just t' be on t' safe side, me bucko, (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. I did brin' me wife, so I'd have another witness t' confirm that t' rocket had made a stable flight. Begad! Avast! I had already been appointed RSO o' t' next club launch, shiver me timbers, and I wanted t' fly t' model there. Blimey! I anticipated havin' some difficulty convincin' certain members that I should be allowed t' fly it.

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

T' next weekend be our club's big meet. Avast! After I'd gotten all me competition flights in, ya bilge rat, arrr, I prepped TFNK and brought it t' t' safety check-in. 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. Ahoy! "A rocket can't be stable without at least three fins!" he said. Ya scallywag! I reminded him that some aerodynamic experts had insisted that t' WAC-Corporal couldn't be stable because it didn't have four fins, arrr, until someone pointed out that arrows are stable with only three feathers. Arrr! Arrr! (Handbook o' Model Rocketry, 6th ed. Avast! p. Ahoy! 154) I heard another old-timer mutter, "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, shiver me timbers, so he said that it couldn't fly because t' grain wasn't parallel t' t' leadin' edge o' t' fins. That's nay in t' safety code. It hadn't been a problem on t' earlier flight. Arrr! It was only a rather strong suggestion in t' Handbook, me hearties, (6th ed. Avast, me proud beauty! Begad! p. Avast, arrr, me proud beauty! 52) but without t' force o' law, ya bilge rat, and no evidence t' support it.

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

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