Originally Posted by dwmzmm
I ordered and received my Birdie from the 1969 Estes catalog (page 128) and still have that model. Most recent launch using the 13 mm engines (with streamer taped to the engine to allow fit in the 18 mm engine body tube).
Your posting just inspired something: It would be possible to clone the original (18 mm diameter, 1.75" long, Series III "Shorty" motor powered) Birdie, and fly it on 13 mm motors without modifying the original-design airframe at all (ditto for the Semroc eRockets.biz "Retro-Repro" Centuri Lil' Herc [see: http://www.erockets.biz/semroc-flyi...hercules-kv-14/
], the original version of which also used [Centuri's] "Shorty" 18 mm motors). Here's how:
Rather like how Estes have their red plastic, lengthwise-split two-piece motor adapter sleeves (for using 13 mm motors in 18 mm motor mounts [and minimum-diameter 18 mm motor rockets, 18 mm motors in 24 mm tubes, etc.), the "ex-Shorty 18 mm motor" kits (now modified to accommodate 13 mm motors) could use their original "Shorty 18 mm motor" mounts, with a *tri-sector*-split 13 mm motor mount, and:
Especially in motor-ejecting, "Shorty" motor models such as the original Birdie (and the original Centuri Lil' Hercules: http://www.ninfinger.org/rockets/ca...i65/65cen6.html
), a 13 mm motor mount (of the same "envelope dimensions"--the diameter and length--of an 18 mm [0.69"] diameter x 44.45 mm [1.75"] long "Shorty" motor), with centering discs (or centering rings) glued to the ends of its BT-5 tube, could be built. Also:
It could be arranged in different ways. It could be left in one piece (and it could have either a 13 mm motor clip [with a thrust ring glued in ahead of it, for greater durability], or it could utilize just a thrust ring, with the 13 mm motor being friction-fitted into the BT-5 tube using masking tape [which is best applied at an angle down the slightly narrower front end of the motor, as it permits gradual tightening, easier removal after flight, and avoids "buggering up" the rear edge of the motor mount tube]). This one-piece, 13 mm motor-to-18 mm "Shorty" motor adapter mount could descend, after ejecting itself out of the rocket, under a streamer (either directly affixed to the BT-5 tube, or affixed to it via a short length of cotton or Kevlar line). *OR*...
A more challenging--(like the "second version" [or second paint scheme, requiring more colors and/or more complicated masking] that many Centuri kits included provision and instructions for), but fascinating to build, prep, and watch descend after ejecting itself--version of this motor adapter mount could be built thus:
The motor adapter mount could be built just as described above, but it wouldn't need a 13 mm motor clip, just *two* thrust rings (one ahead of the motor as before, and one directly ^behind^ it [as is often the case with the first stages of minimum-diameter, multi-stage model rockets]). How is the 13 mm motor installed (and removed), then? Easily...by inserting and removing it -sideways-. After the motor adapter mount is built (leaving out the two thrust rings until the last step), it is cut lengthwise into three equal, 120-degree sectors (as viewed from either the front or rear of the mount). If necessary (to maintain the strength and stiffness of each sector-piece), one wrap of thin paper, such as computer printer paper, could be glued around the outside of the BT-5 tube, then allowed to dry before the mount is cut into three sectors. Then:
The three now-separate sectors could be "tied" together, by affixing short--maybe 3" - 4" or so (short enough to easily fit, when folded, between the BT-5 tube and the inner wall of the rocket's BT-20 [or ST-7] motor mount [or body] tube)--lengths of plastic streamer material between them. (Metallized, perhaps color-tinted [red, gold, blue, green, purple, silver, etc.] mylar streamer material would be particularly eye-catching, in the air and on the ground, and some gauges of it are thinner--and fold up more compactly--than the Day-Glo Orange plastic flagging tape that is frequently used as streamer material.) If desired, the streamer material could be affixed between the adapter mount sectors via short lengths of cotton carpet thread or thin Kevlar line (the thicker, sometimes color-dyed Kevlar *thread* [Apogee Rockets has it in Burgundy color] is strong, heat-resistant, easy to tie, and rolls or folds up very compactly)--and in addition:
Only two lengths of streamer material--at a minimum--would be needed to "tie" together the three motor adapter mount sectors (with one sector in the middle, and the other two at either end of the "daisy chain," with the two lengths of streamer material in between the three sectors. But if a slower descent rate was desired (less than, or the same as--or nearly so--that of the rocket itself, say [the Birdie falls pretty fast, but some Lil' Hercs actually enter a fast, flat glide, or drift relatively slowly down while tumbling, depending on how they're built and ballasted, and on the winds on a given day]), one or two extra, "outboard" lengths of streamer material (particularly the thinner-gauge, metallized mylar type, which folds up more compactly) could be attached to the "outer" adapter mount sectors. Now for the last assembly step:
The two BT-5 size thrust rings would be glued into the front and rear ends of one of the motor adapter mount's BT-5 tubing sectors (they ^could^ be glued into two different sectors, but that would complicate the 13 mm motor's installation and its retention in flight [in order to be "NAR-kosher/halal" for NAR-sanctioned contests and sport launches, ^nothing^ can fall free, unsupported by a parachute or streamer, unless it is gliding instead of falling]). Any of the three streamer-"tied" motor adapter mount sectors could hold the motor, which would fit (without "play," but not tightly) between its fore-and-aft thrust rings (selecting a sector at one end of the sectors-and-streamer material "daisy chain" would, I conjecture, cause more flapping, and generate more drag [and thus a slower descent] than would selecting the middle sector to hold the motor), and:
Before launch, the 13 mm motor would simply be slipped into the thrust rings-equipped sector, before its igniter (and igniter plug, if used; otherwise, the old-fashioned tamped-in wadding ball [and/or masking tape "patch," pressed into the nozzle to hold the igniter in place]) was installed, protruding forward into the motor's nozzle, through the opening in the rear thrust ring. To ensure that the motor casing wouldn't fall free after ejection:
A short length of thread--Kevlar thread, if desired--could be wrapped once around the middle of the motor (and around its sector's portion of the BT-5 tube), then tied (with the knot--an easily-untied granny knot, or a shoe lace "bow-tie" knot, would do--on top of the BT-5 tubing sector, so that it wouldn't interfere with the fit of the motor). A single, full or partial wrap of cellophane tape (or masking tape), applied in the same way as the thread, would work just as well, or even better, without interfering with the 13 mm motor's fit in the assembled motor adapter mount. And then:
With the motor tied (or taped) in place in the thrust rings-equipped sector (and with its igniter installed), the other two motor mount sectors would be assembled around the motor, the streamer material pieces (and their interconnecting cotton carpet thread or Kevlar thread lines, if utilized) would be folded up to fit between the (assembled) tri-sectored BT-5 tube and the rocket's inner tubing wall, and the whole assembly would be slid into the rear of the rocket, just as an 18 mm Series III "Shorty" motor would be. The subsequent normal pre-launch and launch checklist steps (the version for tumble-recovery model rockets; Estes--and possibly also Centuri--printed "generic" checklist variants for all of the main types of recovery system [featherweight, tumble, streamer, parachute, helicopter, and glide recovery]) would be followed from this point onward, and:
The models--either of these, the original-design Birdie or Lil' Hercules--would lift off and climb just as they did/would have under the power of 18 mm Series III "Shorty" motors. At ejection, the motor adapter mount would--like a "Shorty" motor--jettison itself rearward, and the now-lightened rocket would arc over and begin falling (the Birdie), or would start tumbling--or would perhaps enter a fast, shallow glide--moving almost horizontally, but hopefully circling as well (the Lil' Hercules). The motor adapter mount, meanwhile, would separate into its three sectors, all held together by the lengths of streamer material (and perhaps also lengths of cotton or Kevlar thread), and begin a relatively slow, fluttering descent (either faster than, slower than, or at about the same velocity as the separately-descending rocket, depending on several voluntarily-chosen--and random--factors).