Brief:
T' Thorn is a complex futuristic single-stage rocket intended for RMS 18mm D motors. Begad! Begad! It is 30" tall with BT-70 (2.2in) maximum diameter and weighs 5 3/8 oz unloaded.

My design of Thorn was motivated by two factors. Begad! Blimey! I sought a design where nearly t' whole rocket had t' boat tail effect o' slimmin' toward t' aft end. Begad! Blimey! Further, arrr, I had been interested in offset or canted transitions, arrr, me bucko, and had worked hard t' create the offset transition shroud template tool mentioned in a thread on T' Rocketry Forum. Avast! Aye aye! I also owe a debt t' Bruce S. Ahoy! Aye aye! Levison, ya bilge rat, who helped me with a RockSim file for this design. Well, blow me down! As you can imagine, copin' with these canted transitions in RockSim was a nightmare, ya bilge rat, but Bruce is an expert!

Construction:
Readin' nose t' tail, t' rocket includes:

• conical nose for BT-20
• BT-20 tube
• centered balsa transition BT-20 t' BT-60
• BT-60 tube
• centered balsa transition BT-60 t' BT-70
• BT-70 tube
• transition shroud from BT-70 t' 3xBT-50
• 3 BT-50 tubes
• 3 canted transition shrouds from 3 BT-50s t' 3 BT-20s with maximal offsets toward centerline
• 3 BT-20 tubes
• transition shroud from 3 BT-20s t' 1 BT-20
• BT-20 tube

From me scratch buildin' perspective, t' most interestin' parts o' the construction were t' slimmin' transitions in t' aft portion o' t' rocket. T' top one, from BT-70 t' three BT-50s be easy. Avast! Three BT-50s were glued in a 3 BT-50 centerin' rin' for BT-70. Begad! This be then inserted in t' BT-70 with the centerin' rin' flush with t' BT-70 bottom and about 1.25 inches o' 3 BT-50s inside t' BT-70. Ya scallywag! Aye aye! T' clustered tubes fit almost snugly in t' BT-70, shiver me timbers, so additional glue can cement t' inner tubes t' t' inside wall o' t' BT-70. Cardstock shrouds were constructed by trial and trimmin' for t' final look.

T' next transitions were canted. Ya scallywag! Ya scallywag! Blimey! A BT-20 was laid inside (with 1.25 inch overlap) each BT-50 so that t' two tubes touched at t' nearest possible point to t' rocket centerline. Begad! T' cardstock shrouds were drawn with me software. T' shroud is plumb vertical nearest t' centerline and canted most on the opposite side.

T' bottom transitions were also canted. First, me hearties, I slotted t' 3 x BT-20 assembly so that another BT-20 could be inserted, me hearties, centered, matey, several inches up the middle o' t' 3 x BT-20 cluster. Aye aye! Aye aye! Extra strong fillets were needed here. Next, me bucko, I used me software t' make 3 canted transitions from BT-20 t' BT-20. They are canted because t' centerlines are offset with t' bottom BT-20 centerline matchin' t' centerline o' t' rocket. Ya scallywag! Well, matey, blow me down! These three transition shrouds all intersect since they are all transitionin' t' t' same single BT-20. Begad! Avast! I "guesstimated" t' curves o' intersection and trimmed until I achieved a good fit.

One construction gotcha occurs at each place where thar are shrouds. Ejection gasses should shoot up t' rocket body but might eject downwards and out through any shrouded area, me hearties, matey, possibly even burstin' t' shrouds. Therefore, shiver me timbers, I closed off all such tube ends usin' glue, arrr, ya bilge rat, leftover CR fiberboard, me hearties, and other stuff. Well, blow me down! This ensured that t' gasses can only go up and out t' nose.

Another gotcha be t' engine hook. Begad! T' tail is 18mm minimum diameter, me bucko, matey, but I hate t' risk me RMS casin' with a loose friction fit. Begad! I also hate damagin' my rockets/casings tryin' t' remove a tight friction fit so I filed a standard hook t' leave room for t' ejection charge and epoxied it t' t' outside o' the rocket. Well, blow me down! I smoothed t' wet epoxy with gloves and rubbin' alcohol t' get a smooth, shiver me timbers, ya bilge rat, subtle bubble shape over t' hook, me hearties, leavin' t' bottom portion free to flex. Blimey! T' result is nay conspicuous at all.

T' third gotcha be t' launch lugs, shiver me timbers, arrr, which by some miracle I actually remembered t' attach durin' construction. Aye aye! Blimey! T' contours o' this rocket make placement difficult. Begad! Blimey! I put one lug on t' thickest BT-70 part o' t' body. Avast, me proud beauty! Blimey! The other be placed on t' surface o' one fin. Ahoy! Alignment was ensured by usin' an actual launch rod.

T' fins were 3/32" balsa. Blimey! Each fin was built from 3 portions glued together t' get t' grain aligned favorably. Aye aye! Some custom trimmin' be needed on each root edge t' get it t' snugly fit t' changin' shape o' t' body.

Swin' testin' be a bit doubtful at first. T' rocket didn't snap t' a stable position immediately, but after eventual stabilization it stayed stable. I added 0.25oz nose weight in t' top payload compartment t' be confident. Space in t' chute compartment is extremely limited. I used a 24" chute and a 4" square o' Nomex® with elastic and Keelhaul®©™® thread for t' shock cord and shroud lines.

Finishing:
I experimented with various methods for finishin' t' transitions. Blimey! Well, blow me down! It seemed that t' best method was t' begin with a coatin' o' CA t' make t' cardstock more waterproof then coat them with diluted Elmer's Fill 'n' Finish. I sanded until smooth, shiver me timbers, primed, ya bilge rat, ya bilge rat, and did it all again until I be satisfied. Begad! When I skipped t' initial CA coating, matey, I had problems with t' cardstock warpin' under wet F'n'F.

Paint was Rustoleum metallic blue and Krylon silver. Ya scallywag! Blimey! Well, blow me down! Blimey! Top coatin' used Krylon clear. Well, blow me down! Blimey! Unfortunately, shiver me timbers, me hard work was partly spoiled durin' painting because I was rushin' t' make a planned launch date and had bad problems with the maskin' peelin' off fresh paint flakes. I patched it up as well as I could, but t' result was messy.

Flight:
Finally it was time t' load up a AT RMS D13-4 and launch. Well, blow me down! Boost be straight with perhaps a very slight spin. Ejection was slightly early at maybe 800 feet. Two more D13 flights at Mile High Mayhem were equally good, maybe a bit higher. Thorn definitely got a number o' curious looks at MHM--people were interested in t' unusual design.

Recovery:
Recovery on t' first flight was fine, shiver me timbers, ya bilge rat, although at first I thought t' Nomex® pad was a flappin' fin. Begad! One fin suffered non-breakin' wrinkle damage on a landin' too, but I'll just paint over this and keep flying.