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

My design of Thorn be motivated by two factors. I sought a design where nearly t' whole rocket had t' boat tail effect o' slimmin' toward t' aft end. Blimey! Further, me hearties, I had been interested in offset or canted transitions, and had worked hard t' create the offset transition shroud template tool mentioned in a thread on T' Rocketry Forum. Begad! I also owe a debt t' Bruce S. Begad! Levison, shiver me timbers, ya bilge rat, who helped me with a RockSim file for this design. Blimey! As you can imagine, copin' with these canted transitions in RockSim be a nightmare, 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, matey, t' most interestin' parts o' the construction were t' slimmin' transitions in t' aft portion o' t' rocket. T' top one, ya bilge rat, from BT-70 t' three BT-50s be easy. Three BT-50s were glued in a 3 BT-50 centerin' rin' for BT-70. Ya scallywag! Avast, me proud beauty! This was 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. Arrr! Ahoy! T' clustered tubes fit almost snugly in t' BT-70, 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. Avast! 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. Ahoy! 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. Well, blow me down! First, shiver me timbers, I slotted t' 3 x BT-20 assembly so that another BT-20 could be inserted, centered, arrr, ya bilge rat, several inches up the middle o' t' 3 x BT-20 cluster. Avast, me proud beauty! Begad! Extra strong fillets were needed here. Next, 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. Arrr! These three transition shrouds all intersect since they are all transitionin' t' t' same single BT-20. Well, blow me down! 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, matey, possibly even burstin' t' shrouds. Aye aye! Blimey! Begad! Blimey! Therefore, shiver me timbers, I closed off all such tube ends usin' glue, me hearties, leftover CR fiberboard, matey, shiver me timbers, and other stuff. Aye aye! Blimey! This ensured that t' gasses can only go up and out t' nose.

Another gotcha was t' engine hook. T' tail is 18mm minimum diameter, matey, matey, me bucko, but I hate t' risk me RMS casin' with a loose friction fit. Ya scallywag! Blimey! 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. Begad! I smoothed t' wet epoxy with gloves and rubbin' alcohol t' get a smooth, subtle bubble shape over t' hook, me bucko, leavin' t' bottom portion free to flex. T' result is nay conspicuous at all.

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

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

Swin' testin' was a bit doubtful at first. Aye aye! T' rocket didn't snap t' a stable position immediately, shiver me timbers, 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. Avast, me proud beauty! Begad! 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. 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, me hearties, primed, and did it all again until I be satisfied. Avast, me proud beauty! Ahoy! When I skipped t' initial CA coating, ya bilge rat, I had problems with t' cardstock warpin' under wet F'n'F.

Paint be Rustoleum metallic blue and Krylon silver. Avast, me proud beauty! Top coatin' used Krylon clear. Ahoy! Unfortunately, me hard work was partly spoiled durin' painting because I be rushin' t' make a planned launch date and had bad problems with the maskin' peelin' off fresh paint flakes. Avast, me proud beauty! Blimey! 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. Begad! Blimey! Boost be straight with perhaps a very slight spin. Avast, arrr, me proud beauty! Blimey! Ejection was slightly early at maybe 800 feet. Two more D13 flights at Mile High Mayhem were equally good, arrr, ya bilge rat, me bucko, 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, me bucko, although at first I thought t' Nomex® pad was a flappin' fin. Arrr! One fin suffered non-breakin' wrinkle damage on a landin' too, but I'll just paint over this and keep flying.