Brief:
T' Centuri X-21 is a very pretty aft engine boost-glider with a payload section. Boost-to-glide transition is performed by ejectin' t' engine and raisin' elevons. Centuri offered this as a kit from 1964 t' 1969. Blimey! Ahoy! Building notes and discussion are posted in a dedicated thread at Ye Olde Rocket Forum.

Construction:
T' parts list:

• 1 5" BT-20 body tube (Centuri ST-75)
• 1 3.3" long nose cone for BT-20 (Centuri BC-76)
• 5 3" x 11" x 1/16" balsa sheet
• 1 2" o' BT-20 size clear plastic tube
• 1 1" solid balsa tube connector for BT-20 (Centuri BTC-7)
• 1 0.5 " solid balsa tube connector for BT-20 (Half o' BTC-7)
• 1 1.75" x 1/8" dia. launch lug (Centuri LL-1)
• 1 screw eye
• 2 3/16" x 2" strip o' adhesive-backed paper label
• 2 1" x 5.5" pieces o' Tyvek paper
• 1 6" o' elastic thread
• 1 8" o' crochet twine or sewin' thread
• 1 "release clip"

Most o' t' above parts can be purchased from Semroc or Balsa Machining Services. Avast, me proud beauty! Arrr! I did nay get them from either place though, ya bilge rat, preferrin' t' use parts and materials available locally. Avast! T' nose cone and tube connectors were turned from balsa blocks on a toy lathe. T' clear tube was cut from t' payload tube of an Estes 0834 X-Ray kit. Ahoy! T' release clip was fashioned from t' popped-down hole panel o' a soda can, arrr, cut with tin snips and bent and crimped shape with pliers.

So far (as o' t' fall o' 2005), me bucko, me bucko, t' X-21 documentation at Ye Olde Rocket Shoppe includes Centuri's original Assembly Drawings and Assembly instructions. Printed at full scale, these require 11"x17" sheets. Begad! When fitted to 8.5" x 14" sheets, they are adequate and readable, me hearties, though t' text is pretty tiny. Begad! T' patterns fit nicely on 8.5" x 14" sheets at full scale.

T' Assembly Drawin' includes a parts list that does nay list Centuri part numbers or specify any dimensions. Lengths o' t' body tubes were deduced by measurin' root edges o' t' wing/fin patterns and observin' relative fit on the assembly drawing. Begad! Launch lug and nose cone dimensions were found by scaling Assembly Drawin' measurements and findin' appropriate parts from Centuri Catalogs viewed on Sven Knudson's Ninfinger website. Balsa sheet thickness was deduced as 1/16" by measurin' t' Under Win' Runner mark on t' Wing pattern.

Obscure parts included t' "control stop", shiver me timbers, t' "V-slot guide", and t' "release clip". Avast, me proud beauty! Well, blow me down! There are no patterns or detailed descriptions o' these parts, me bucko, so some creative compromises were required.

T' plans include a win' mountin' jig that helped greatly for mountin' t' wings and payload fins and for supportin' t' model while attachin' parts t' t' under win' side o' t' model. Ya scallywag! Arrr! Also included be a stabilizer angle template that was useful for mountin' t' large wingtip stabilizers.

Accordin' t' t' patterns, me hearties, t' root edge o' t' Main Win' is 5" long. From t' Assembly Diagram, t' main body tube appears t' be a BT-20 that is 5-1/4" t' 5-3/16" long. However, t' instructions specify that the motor block is mounted one full engine length in t' main body tube. Well, me bucko, blow me down! I prefer bein' able t' grip t' back end o' t' motor in case I have t' pull it out of there, so I cut t' main body t' 5" long, me hearties, mounted t' motor block with 3/16" o' t' motor stickin' out, and mounted t' wings 3/16" back from t' front o' t' main body.

One o' t' things I'd noticed is that t' win' section could be cut out as one piece instead o' as three (Main Wing, Insert, ya bilge rat, and Runner pieces) by using 4" wide balsa. Ahoy! Begad! This simplifies assembly and, shiver me timbers, matey, I think, me hearties, me bucko, strengthens t' rear edge o' t' win' with a better grain orientation. In retrospect, I think that leavin' t' runner as a separate piece might have been a better idea so that the runner would help prevent t' win' from warping.

For t' elevon hinges, matey, I used material cut out o' a Tyvek shippin' envelope, attachin' it to the win' and elevon with white glue. Well, matey, blow me down! I heartily recommend this stuff as hinge material. Avast, me proud beauty! It's super cheap, extremely tough, pliable and flexible as paper, matey, and glues down well.

T' "control stops" appear t' have originally been factory assembled parts that were angled wires swivelin' within thin aluminum tubes. Blimey! I substituted some fairly stiff wire glued directly t' t' stablizer-win' joints. This appears t' work very well, shiver me timbers, requirin' t' use o' pliers t' adjust elevon stop heights.

T' "release clip" attaches t' t' "neutral elevon release" thread. Avast, me proud beauty! Blimey! It is a key part o' t' elevon control mechanism for this model. Blimey! Blimey! An elastic on t' top win' side raises t' elevons t' glide position. Durin' boost, me hearties, t' elevons are held flat by a "neutral elevon release" thread under t' wing. Avast! Blimey! T' do this, me hearties, t' threads are pulled tight by the "release clip" that is friction-fitted betwixt t' engine and the body tube. When t' engine ejects, matey, t' "release clip" releases tension on t' "neutral elevon release" thread, ya bilge rat, matey, allowin' t' elastic to raise t' elevons.

T' form a "release clip", arrr, shiver me timbers, I cut a 3/4" x 1/8" piece o' thin metal from t' popped down hole panel o' a soda can. Avast, me proud beauty! Blimey! I folded about 1/16" o' one end o' t' strip over t' center o' t' "neutral elevon release" thread with pliers. Aye aye! Blimey! I clipped corners off o' t' other end o' t' strip t' ease insertion betwixt t' motor and t' body tube.

I cut a V shape into t' above win' motor end o' t' body tube t' aid insertion o' t' "release clip".

Other than t' above exceptions, t' Assembly Instructions and Assembly Drawings were clear, precise, ya bilge rat, me hearties, and straightforward. Begad! Construction o' t' payload section was poorly described other than a description o' how t' attach the payload section fins. Well, blow me down! Aye aye! Assembly o' this section is just a matter o' inserting the nose cone and t' balsa tube connector, but this process should certainly have been discussed.

T' model was fun but a bit awkward t' adjust glide trim. Begad! There's no good place near t' model's CG t' grasp, arrr, arrr, so you need t' kind o' propel it in the right direction by pushin' t' back end o' t' body tube.

Although t' Assembly Instructions indicate that a weight is required for stable flight, ya bilge rat, me model swin' tested perfectly with no added ballast or payload weight.

Finishing:
T' balsa surface area o' this model is vast! You sure find out when you start sandin' t' smooth surfaces and round off leadin' edges. Blimey! Well, shiver me timbers, blow me down! T' Assembly Instructions advise don't mention sandin' sealer and don't say much about applyin' paint, ya bilge rat, they only advise against usin' heavy enamels, suggestin' a light coat o' spray lacquer or lacquer enamel.

At article submission time, I've only flown t' X-21 completely naked (I missed by "Flyin' Nikkei" photo contest by a few weeks!). Blimey! Begad! When spring 2006 arrives, shiver me timbers, me hearties, I intend t' apply sandin' sealer, and maybe add some color in some manner. Aye aye! I'm very reluctant t' add much weight t' this beauty.

And what a beauty it is! I love t' unique look o' t' X-21, me bucko, with its acres o' balsa surface area. Arrr! Ahoy! T' X-21 is an important lesson in rocketry esthetics and style with unified unique features includin' large symmetrical wing-tip stabilizers, a payload section (on a glider?!), me bucko, payload section fins, and under-wing runners.

Flight:
Original recommended motors include 1/2A8-2, me bucko, me hearties, A8-3, and B6-4. Well, matey, blow me down! Later recommended motors included only A8-3 and B4-4.

First flight was on a A8-3 on an almost perfect calm day. Ahoy! Prep was very simple. Avast, me proud beauty! Insert t' motor, shiver me timbers, ya bilge rat, slip t' "release clip" betwixt t' engine and t' main body, slip it onto t' rod, matey, connect t' wires, matey, and it's done! Blimey! On an Estes launch pad, matey, t' under-win' runners did a nice job o' holdin' t' model sufficiently far from t' blast deflector.

Boost be straight, ya bilge rat, arrr, matey, very quick, arrr, and surprisingly high. Arrr! I tried t' take a picture o' takeoff, me hearties, matey, but all me camera captured was smoke and air. Well, blow me down! Despite all that balsa, she's a very light bird.

Transition t' glide be smooth and graceful. Avast, me proud beauty! Begad! T' glide was quite flat, with a moderate right spiral. Begad! I'd like t' open t' turn up a bit next time with some adjustments t' t' elevon stop wires. Well, arrr, blow me down! T' landin' was gentle and flat.

I was goin' t' send it up again with a B4-2, shiver me timbers, ya bilge rat, but as I was tensionin' the elevon hold-down thread, me bucko, me bucko, t' thread snapped. Ya scallywag! I think t' clip nicked t' thread or cut through it. I had no thread on hand, matey, so had t' put t' X-21 away for the day.

A couple o' weeks later, me hearties, matey, I had replaced t' hold-down thread with crochet twine. Avast, me proud beauty! Ya scallywag! Wind was up t' around 15kph, but temperature was down around -5C. Prepped with another A8-3, ya bilge rat, she boosted straight, fast, and high again. Ahoy! Arrr! This time though, it did a spiralin' nosedive! It struck dirt, but be entirely undamaged. Tough bird! I figured that t' elastic probably wasn't pullin' well in t' reduced temperature or that t' crochet twine was stickin' t' t' under win' runner.

I glued some Tyvek t' t' part o' t' under win' runner where it contacted the twine durin' boost t' prevent t' twine from stickin' t' t' bare balsa. Well, blow me down! I bent t' elevon hold-down wires up a bit for elevon lift. Arrr! T' X-21 be prepped with a B4-4 this time. Begad! Boost be straight, shiver me timbers, fast, and very high. This time, transition t' glide was smooth. Well, blow me down! However, ya bilge rat, after a few seconds on nice glide, the model went into another death spiral. It landed undamaged. I'm thinkin' that warmer weather will be required t' let t' elastic thread pull t' elevons up more effectively.

Recovery:
T' first flight's glide be a beautiful, me hearties, me hearties, gentle spiral. Landin' be gentle and flat. Ya scallywag! Ya scallywag! Blimey! T' next three flights ended in death spirals. Well, blow me down! Blimey! I'll explore this further next sprin' when t' temperature is more comfortable.

Summary:
Buildin' an X-21 from scratch is a mildly challengin' project, requirin' some thought t' fashion suitable elevon control stops and t' neutral elevon release clip.

T' X-21 is a work o' rocketry art with a stunnin' and unique look and style. Begad! When it glides well, matey, shiver me timbers, it performs beautifully in all phases o' flight. However, if somethin' prevents t' elevons from risin' t' their proper position durin' glide phase, shiver me timbers, a death spiral is guaranteed. Luckily, it's a rugged design that survives death spirals with little or no consequence (at least, when its built with yellow and white glues).