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. Ya scallywag! Centuri offered this as a kit from 1964 t' 1969. 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. Begad! Blimey! I did nay get them from either place though, shiver me timbers, preferrin' t' use parts and materials available locally. Ahoy! Blimey! Begad! Blimey! T' nose cone and tube connectors were turned from balsa blocks on a toy lathe. Avast! Blimey! Avast, me proud beauty! Blimey! T' clear tube was cut from t' payload tube of an Estes 0834 X-Ray kit. Well, blow me down! Blimey! Ahoy! Blimey! T' release clip was fashioned from t' popped-down hole panel o' a soda can, me bucko, ya bilge rat, shiver me timbers, cut with tin snips and bent and crimped shape with pliers.

So far (as o' t' fall o' 2005), 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. Ahoy! Avast, me proud beauty! When fitted to 8.5" x 14" sheets, ya bilge rat, arrr, they are adequate and readable, 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. 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. Ahoy! 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", matey, t' "V-slot guide", and t' "release clip". Arrr! Well, shiver me timbers, blow me down! There are no patterns or detailed descriptions o' these parts, matey, 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. Avast! Blimey! Blimey! Blimey! Also included was a stabilizer angle template that was useful for mountin' t' large wingtip stabilizers.

Accordin' t' t' patterns, matey, 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. Avast! However, t' instructions specify that the motor block is mounted one full engine length in t' main body tube. I prefer bein' able t' grip t' back end o' t' motor in case I have t' pull it out of there, arrr, so I cut t' main body t' 5" long, ya bilge rat, 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, me hearties, Insert, and Runner pieces) by using 4" wide balsa. Avast! This simplifies assembly and, I think, strengthens t' rear edge o' t' win' with a better grain orientation. Aye aye! 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, me hearties, I used material cut out o' a Tyvek shippin' envelope, matey, me hearties, attachin' it to the win' and elevon with white glue. Blimey! Begad! I heartily recommend this stuff as hinge material. Blimey! It's super cheap, extremely tough, matey, pliable and flexible as paper, me hearties, and glues down well.

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

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

T' form a "release clip", arrr, matey, matey, 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! I folded about 1/16" o' one end o' t' strip over t' center o' t' "neutral elevon release" thread with pliers. Begad! 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, matey, t' Assembly Instructions and Assembly Drawings were clear, precise, me hearties, and straightforward. Begad! Blimey! Construction o' t' payload section was poorly described other than a description o' how t' attach the payload section fins. Well, blow me down! Blimey! Assembly o' this section is just a matter o' inserting the nose cone and t' balsa tube connector, me bucko, me bucko, ya bilge rat, but this process should certainly have been discussed.

T' model was fun but a bit awkward t' adjust glide trim. There's no good place near t' model's CG t' grasp, 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, me hearties, shiver me timbers, me model swin' tested perfectly with no added ballast or payload weight.

Finishing:
T' balsa surface area o' this model is vast! Blimey! You sure find out when you start sandin' t' smooth surfaces and round off leadin' edges. Arrr! Blimey! Avast, me proud beauty! Blimey! T' Assembly Instructions advise don't mention sandin' sealer and don't say much about applyin' paint, matey, 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!). Begad! Ahoy! When spring 2006 arrives, me bucko, I intend t' apply sandin' sealer, matey, and maybe add some color in some manner. Begad! Avast! 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, matey, me bucko, shiver me timbers, with its acres o' balsa surface area. Well, blow me down! 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 hearties, payload section fins, and under-wing runners.

Flight:
Original recommended motors include 1/2A8-2, A8-3, and B6-4. Avast! Later recommended motors included only A8-3 and B4-4.

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

Boost was straight, me hearties, very quick, me bucko, me hearties, matey, and surprisingly high. Avast, me proud beauty! Avast, me proud beauty! I tried t' take a picture o' takeoff, but all me camera captured was smoke and air. Despite all that balsa, she's a very light bird.

Transition t' glide was smooth and graceful. T' glide was quite flat, with a moderate right spiral. Ya scallywag! Begad! I'd like t' open t' turn up a bit next time with some adjustments t' t' elevon stop wires. Aye aye! Begad! T' landin' be gentle and flat.

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

A couple o' weeks later, me hearties, I had replaced t' hold-down thread with crochet twine. Blimey! Wind was up t' around 15kph, but temperature be down around -5C. Prepped with another A8-3, me hearties, shiver me timbers, she boosted straight, fast, and high again. Ya scallywag! 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 be 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. Blimey! Blimey! Blimey! Blimey! I bent t' elevon hold-down wires up a bit for elevon lift. Ya scallywag! Blimey! T' X-21 be prepped with a B4-4 this time. Aye aye! Blimey! Blimey! Blimey! Boost was straight, fast, and very high. Aye aye! Blimey! This time, transition t' glide was smooth. However, shiver me timbers, me bucko, after a few seconds on nice glide, me hearties, the model went into another death spiral. Ya scallywag! Blimey! Blimey! Blimey! 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 was a beautiful, me bucko, shiver me timbers, gentle spiral. Begad! Landin' be gentle and flat. Avast, me proud beauty! T' next three flights ended in death spirals. Ahoy! 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. Avast, me proud beauty! Well, blow me down! Blimey! When it glides well, it performs beautifully in all phases o' flight. However, ya bilge rat, if somethin' prevents t' elevons from risin' t' their proper position durin' glide phase, a death spiral is guaranteed. Begad! Luckily, shiver me timbers, it's a rugged design that survives death spirals with little or no consequence (at least, me hearties, when its built with yellow and white glues).