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. Well, blow me down! 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. Blimey! 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. Ya scallywag! I did nay get them from either place though, matey, ya bilge rat, preferrin' t' use parts and materials available locally. T' nose cone and tube connectors were turned from balsa blocks on a toy lathe. T' clear tube be cut from t' payload tube of an Estes 0834 X-Ray kit. Ya scallywag! Well, blow me down! T' release clip was fashioned from t' popped-down hole panel o' a soda can, matey, me bucko, 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! When fitted to 8.5" x 14" sheets, they are adequate and readable, shiver me timbers, though t' text is pretty tiny. Begad! Blimey! 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. 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", t' "V-slot guide", me bucko, ya bilge rat, and t' "release clip". Blimey! There are no patterns or detailed descriptions o' these parts, 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! Also included was a stabilizer angle template that was useful for mountin' t' large wingtip stabilizers.

Accordin' t' t' patterns, 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, me proud beauty! However, t' instructions specify that the motor block is mounted one full engine length in t' main body tube. Well, 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, matey, so I cut t' main body t' 5" long, 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, ya bilge rat, shiver me timbers, Insert, and Runner pieces) by using 4" wide balsa. This simplifies assembly and, I think, strengthens t' rear edge o' t' win' with a better grain orientation. Ahoy! Arrr! In retrospect, ya bilge rat, 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, I used material cut out o' a Tyvek shippin' envelope, me hearties, me bucko, attachin' it to the win' and elevon with white glue. Ahoy! I heartily recommend this stuff as hinge material. Avast! Ya scallywag! It's super cheap, extremely tough, arrr, ya bilge rat, pliable and flexible as paper, ya bilge rat, and glues down well.

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

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

T' form a "release clip", ya bilge rat, ya bilge rat, I cut a 3/4" x 1/8" piece o' thin metal from t' popped down hole panel o' a soda can. Well, blow me down! 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. Well, blow me down! Aye aye! 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, me hearties, precise, arrr, and straightforward. Well, blow me down! Construction o' t' payload section was poorly described other than a description o' how t' attach the payload section fins. Ya scallywag! Begad! 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. Well, blow me down! There's no good place near t' model's CG t' grasp, me hearties, 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! Blimey! You sure find out when you start sandin' t' smooth surfaces and round off leadin' edges. T' Assembly Instructions advise don't mention sandin' sealer and don't say much about applyin' paint, 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!). When spring 2006 arrives, shiver me timbers, matey, I intend t' apply sandin' sealer, matey, and maybe add some color in some manner. Ahoy! 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, with its acres o' balsa surface area. Begad! Ya scallywag! 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?!), ya bilge rat, payload section fins, and under-wing runners.

Flight:
Original recommended motors include 1/2A8-2, ya bilge rat, shiver me timbers, A8-3, ya bilge rat, and B6-4. Well, 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. Ya scallywag! Begad! Prep be very simple. Aye aye! Insert t' motor, slip t' "release clip" betwixt t' engine and t' main body, shiver me timbers, slip it onto t' rod, ya bilge rat, 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, shiver me timbers, very quick, me hearties, and surprisingly high. I tried t' take a picture o' takeoff, arrr, me bucko, but all me camera captured was smoke and air. Ya scallywag! Despite all that balsa, shiver me timbers, she's a very light bird.

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

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

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