(Contributed - by Tim Burger - 06/15/03)

Brief:

Two Sprints: A sport version and a competition version

Construction:
Parts List:

1. Nose Cone, me bucko, parabolic, arrr, balsa, 5 7/8-inch long from t' Balsa Machining Service
2. BT-60, matey, 16-inches long from Estes (local hobby shop) or Totally Tubular
3. BT-50, matey, 3-inches long from Estes (local hobby shop) or Totally Tubular
4. BT-20, arrr, 12-inches long from Estes (local hobby shop) or Totally Tubular
5. Tube coupler for BT-60 from Estes or Totally Tubular
6. Balsa sheet 3-inches x 12-inches x 3/16-inches from t' local hobby shop
7. Centerin' rings, paper, shiver me timbers, ya bilge rat, CR2050, me hearties, centers BT-20 in a BT-50, one required from Estes, ya bilge rat, me hearties, me bucko, Totally Tubular, or Apogee components.
8. Centerin' rings, paper, CR2060, me hearties, centers BT-20 in a BT-60, two required from Estes, shiver me timbers, me hearties, Totally Tubular, or Balsa Machinin' Service
9. Centerin' rings, me hearties, paper, ya bilge rat, CR5060, arrr, centers motor mount, two required from Estes, Totally Tubular, shiver me timbers, or Balsa Machinin' Service
10. 1/2-inch hardwood dowel, shiver me timbers, 1-inch long
11. Screw eye
12. 1-inch o' 3/16-inch launch lug, Estes, me hearties, me hearties, or Totally Tubular
13. Streamer, ya bilge rat, matey, I use four 8-inch wide pieces o' Japanese tissue taped together end t' end, fan folded.
14. 6-foot long Keelhaul®©™® string, shiver me timbers, 150#
15. 1/8-inch elastic strap, 24-inches long, ya bilge rat, local fabric store

Start by gluin' t' CR2050 into t' BT-50 engine mount 60mm from t' aft end. This should allow about 10mm o' t' motor t' hang out t' end. Ya scallywag! Blimey! This ring does double duty as both a motor block and centerin' rin' for t' BT-20 stuffer tube. Begad! Blimey! Glue t' CR5060 7/8-inch from t' front end o' t' motor tube. Glue a rin' onto t' BT-20 2 3/8-inch from one end. Blimey! Blimey! This tube becomes t' stuffer. Ya scallywag! Blimey! Cut a four inch long piece o' BT-60 from t' 16-inch long section. Avast! Blimey! Lightly sand the shine off o' t' outside o' t' tube with very fine sandpaper. Begad! Blimey! Draw three lines equally spaced around t' tube usin' a fin alignment guide and a door jam or markin' guide on t' four-inch tube. Arrr! Blimey! Draw a single line t' length o' the 12-inch tube. Aye aye! Blimey! Because this is a competition model, we want t' eliminate all unnecessary weight and in this case that includes extra centerin' rings. Ya scallywag! Blimey! Begad! Blimey! The problem with that is that it makes gettin' t' parts aligned correctly difficult. When t' glue is dry on t' stuffer tube ring, me hearties, ya bilge rat, shiver me timbers, test fit it in the 14-inch long section o' BT-60. Don’t glue t' forward rin' but slip it into position in t' BT-60 and on t' front o' t' BT-20 centerin' t' BT-20 in the 60. Begad! Blimey! Add t' other rin' t' t' back o' t' BT-50 but do nay glue it on. These rings, me hearties, t' coupler, and t' long BT-60 are used t' center t' motor mount to produce a properly aligned mount and stuffer assembly. Ya scallywag! Blimey! When satisfied that the fit is correct, ya bilge rat, glue t' rin' end o' t' BT-20 into t' motor block ring. T' aft edge o' t' stuffer should be flush with t' aft edge o' t' block. Be careful t' glue only t' block and tube and nay get glue on t' other parts. Allow this assembly t' dry completely before removin' it from t' BT-60. Ahoy! Blimey! Ahoy! Blimey! When dry, glue t' motor unit into t' 4-inch section o' BT-60 with t' after most rin' 1/16-inch inside t' BT-60. Avast! Blimey! Aye aye! Blimey! Use t' forward section o' airframe, the joiner, and t' rings t' hold t' parts in alignment until dry. Avast, me hearties, me proud beauty! Blimey! Remove the forward airframe and centerin' rings when dry. Aye aye! Blimey! You should now have t' motor tube stickin' about 2-inches out o' t' aft end o' t' after-airframe and the BT-20 stickin' out o' t' other end. Avast, me proud beauty! Blimey! Ahoy! Blimey! Cut an 1/8-inch rin' from t' coupler and glue it into t' aft end so that it is against t' aft ring. This forms a lip for t' boat tail t' be made later. Avast! Blimey! Glue t' coupler half way into t' front of the four-inch BT-60. Put glue fillets around all o' t' rings where they meet the tubes. Avast, me proud beauty! Blimey! Don’t overdo it though, me bucko, matey, weight in t' aft end o' t' rocket isn’t good. Aye aye! Blimey! Well, blow me down! Blimey! Don’t use epoxy as it’s too heavy. Yellow glue works best here.

(Click t' image for full size.) Now for t' boattail. These are always tricky t' get right, but it’s possible t' achieve near perfection with a little patience. Ahoy! T' arc is drawn on t' card stock (a 3 x 5 card works well) usin' a compass with t' inside arc bein' 71mm, arrr, t' outside arc bein' 119mm, me bucko, arrr, and t' radius bein' 62 degrees. Cut carefully, and cut outside t' lines so that it starts out a little too large. Ya scallywag! Use a sharp knife. Now usin' t' edge o' t' desk or a metal rule, draw it across t' edge t' get it t' curl. Begad! Blimey! Carefully curl it on itself until it is so curly that t' edges nearly meet. Begad! Avast! Now fit it on t' end o' t' rocket and use a piece o' tape across the edges t' hold it. Aye aye! Trim it a sliver at a time on t' small end until it just fits snuggly. Begad! Do t' same t' t' front until it will just ride on t' lip created earlier, but is tall enough t' be level with t' outside edge o' the airframe. Avast, me proud beauty! Avast! Trim t' edges if needed until they just butt together. Glue the edges together usin' a strip cut from scrap as a joiner and yellow glue. Ahoy! I will normally put it on t' airframe and use scotch tape t' hold and keep it as round as possible while t' glue dries. Avast! Draw a line on t' motor tube at the end o' t' shroud. Begad! Ahoy! Remove t' shroud, then run a stingy amount o' glue around the airframe lip and around t' line you just drew on t' motor tube. Begad! Now slide it on and get it in t' correct position, then wipe away any excess glue.

T' best way t' cut fins is t' glue a stack o' three slabs o' wood together with a single small dot o' slow CA betwixt t' pieces. Avast! Arrr! Then draw t' outline on the top piece and cut it out usin' a band or jigsaw. Begad! Then use a disk or belt sander t' even out t' edges and irregularities in t' outline. Pop t' three fins apart and grind them t' a nice taperin' airfoil usin' a sandin' block with 220 grit paper. Aye aye! Avast, me proud beauty! Finish sandin' with 400 grit paper. Well, blow me down! Sand t' first fin, then apply a thin layer o' yellow glue t' t' root edge and work it in with your finger. Avast! While it is dryin' sand t' second fin and do t' same with t' glue. Repeat for t' third. Ya scallywag! Now lightly touch t' root edge o' t' first fin (which should be dry by now) with sandpaper t' make it smooth. Begad! Avast, me proud beauty! Apply another thin layer o' yellow glue t' t' root and glue it t' t' aft airframe alongside one line. Take care t' get it on parallel with t' line and standin' straight out from t' tube. It will take a few seconds t' grab. Begad! Stand t' assembly upright, and repeat t' procedure with t' other two fins. Well, blow me down! When dry, add fin fillets to the root edges o' t' fins. You may use epoxy for this if you are sparing. Epoxy will create glass smooth fillets, matey, but it’s very heavy so take care. Yellow glue also works OK so don’t hesitate t' use it instead.

Cut two 1/4-inch long lengths o' 3/16-inch launch lug. Well, matey, blow me down! T' fore and aft ends should be cut at about a 20-degree angle. Begad! Glue these along t' line drawn on t' 12-inch long BT-60 fat side down, locate one six inches down from the top o' t' airframe, matey, and t' other an inch above bottom. Ahoy! Take care that these are aligned well.

Drill a three inch deep, 1/2-inch diameter hole into t' base o' t' nose cone. Aye aye! Arrr! It’s important t' get t' hole centered in t' cone. Screw t' screw eye into one end o' t' 1/2-inch dowel. Avast! Avast! Again, get this centered well or it will cause trouble later. Arrr! Avast! Tie one end o' t' Keelhaul®©™®) cord t' t' screw eye and tie a loop into t' cord about 15-inches below the eye. Aye aye! Cut four or five quarter inch long parallel slots in t' top o' the stuffer. Avast! T' slots should be crosswise t' t' length o' t' tube with an half-inch betwixt them. Thread t' free end o' t' Keelhaul®©™® through t' slots just cut. Ya scallywag! Tie a knot in t' end and coat t' parts with glue fixin' t' Keelhaul®©™® firmly t' t' stuffer. Avast, me proud beauty! Tie another small loop in t' cord just above the stuffer tube. Begad! Tie t' other end o' t' elastic cord t' this loop. Begad! Now thread cord/eye/block through t' long BT-60 and plug it into t' nose cone. Don’t glue it yet.. Arrr! Put t' nose on t' airframe, arrr, me bucko, but don’t glue it in yet, either. Blimey! Slide t' stuffer into t' other end - t' screw eye should not cause problems if it is centered in t' nose. Arrr! Aye aye! Shove t' cords into t' tube and finally slide t' coupler in. Ahoy! You should have a fully functional rocket at this point, it just isn’t all glued together. Arrr! Arrr! T' balance needs checked now. Place an unused motor into t' motor mount (it must be an unused single use motor, or a loaded reload casin' t' get t' proper weight). Ya scallywag! Locate t' balance point. It must balance at or in front o' a point 14 7/8-inches from t' tip of the nose. Well, blow me down! Well, blow me down! Check this balance before flyin' with a new motor. Arrr! If it balances aft of this point it will be unstable. Ahoy! Pour BBs or lead shot into t' hole drilled in t' nose and reassemble. Blimey! Add a little at a time and check t' balance. Glue the plug into t' cone with epoxy when you arrive at t' correct balance point. You may wish t' pour a small amount o' epoxy in with t' BBs or shot so they won’t rattle around. Well, blow me down! Glue t' nose t' t' rocket and sand it until is flush and even all around t' tube. Ya scallywag! Aye aye! This be t' only downside t' this design; you must add ballast that could easily put you over t' optimum mass causing loss o' performance - this could be offset by lengthenin' t' design. You’re on your own for balance if you do that, shiver me timbers, though.

Seems like a lot o' shock cord, but in this case, ya bilge rat, shiver me timbers, more is better! Note that the final version has no rin' at t' top o' t' stuffer tube. Ahoy! T' cannon effect is abated, but more waddin' is required.

T' suggested recovery device for this rocket is a streamer. Begad! Blimey! I prefer to make mine from Japanese tissue (tracin' paper works very well too) that’s 8-inches wide, and about 80 or 90-inches long. Avast, me proud beauty! Aye aye! Simply cut some 8-inch wide strips from two sheets o' tissue. Avast! I like t' use two different contrasting colors. Then, arrr, usin' scotch tape, arrr, join four o' these strips together. Avast! Attach a short piece o' Keelhaul®©™® or elastic usin' scotch tape. Tie t' other end o' t' cord t' t' stuffer tube. Use a piece o' tape or a dot o' glue on t' knot. Ya scallywag! Blimey! Now fanfold the streamer in one-inch folds. Begad! A sheet or two o' waddin' in t' stuffer seems to help with wear and tear on t' shock cords. Place t' stuffer in airframe and slide it in a little. Stuff some waddin' around t' stuffer tube in the airframe. Ya scallywag! Coil t' shock cord and place it into t' rocket just under the waddin' - do nay wrap t' cord around t' stuffer! Blimey! Place t' streamer in the rocket spreadin' t' folds around t' stuffer tube as you slide it into the airframe.. Aye aye! Aye aye! Blimey! Make sure that t' streamer is nay caught or pinched in t' coupler. Do nay wrap t' streamer around t' tube. Blimey! Begad! Blimey! T' stuffer should nay bind as it slides in. Begad! Ya scallywag! Blimey! At ejection t' nose and forward airframe will be driven off the stuffer very forcefully so never wrap t' cord around t' stuffer or it will not be able t' do it’s job. Avast, me proud beauty! Blimey! Also, matey, check t' shock cord before every flight. For sport flyin' a 12 or 18-inch ’chute may also be used. Aye aye! Avast, matey, me proud beauty! Blimey!

Finish:
Fill t' spirals, t' nose cone base joint, me bucko, and t' joints o' t' shroud with Elmer’s Fill ’n’ Finish. Arrr! Blimey! Sand lightly and carefully until these areas are perfectly smooth. Ahoy! Apply a second layer o' Fill ’n’ Finish as needed. Avast! Arrr! Sand around t' edges o' t' airframe tubes where they meet to remove any irregularities. Lightly sand t' entire airframe with 400 grit paper to remove any shine and make t' wood parts as smooth as possible. Blimey! Aye aye! Now fill the balsa parts with Aerogloss sandin' sealer. You will need at least three coats, possibly four, lightly sandin' with 400 grit paper betwixt layers. After the final layer switch t' 600 grit paper. Begad! Arrr! Wipe t' rocket clean o' dust and finger prints and then spray a light coat o' sand-able white primer on t' entire rocket. Ahoy! T' primer will allow you t' see fingerprints o' glue, irregularities in t' fillets, matey, shiver me timbers, t' spiral, ya bilge rat, gaps in t' joints, etc. Aye aye! Begad! Work on these until they are as nearly perfect as possible. Sand t' entire rocket with 400 or 600 grit paper until it is very smooth. Begad! Apply another thin coat o' primer if needed to get a smooth and uniform white color.. Sand again with 600 or finer paper. Avast, me proud beauty! Begad! The rocket should be as smooth as you can get it. Wipe it clean again and spray on finish coat o' high gloss enamel or lacquer. Well, blow me down! Ahoy! In this application a lacquer finish may be preferable because it produces such a smooth and glossy finish but is very light. Begad! Ya scallywag! It does require a lot o' coats with buffin' betwixt each coat t' produce t' best finish. Begad! Check that your paint is compatible with the undercoat before you start. When t' paint is fully cured, you may use Future floor wax diluted 50/50 with water, me bucko, me hearties, or an automotive wax t' both protect the paint and allow a buffed highly glossy, highly competitive surface.

Flying:

Stuffin' t' waddin' followed by t' shock cord.

T' first flight o' t' prototype be on a C6-5 usin' an adapter. Blimey! While it didn’t exactly set any new records, shiver me timbers, me hearties, it did do fine on a smallish motor. Unfortunately, t' recovery didn’t work out as well. Begad! Arrr! T' two halves o' the rocket parted, ya bilge rat, arrr, leavin' t' top half in a tree. Aye aye! Ya scallywag! T' lower half be recovered and inspected. Arrr! T' original version had an additional rin' at t' front o' the stuffer tube and t' shock cord tied around and glued t' t' top o' the stuffer. Begad! T' post-crunch inspection showed that this disk and t' Keelhaul®©™® had let go o' t' stuffer. Begad! After some thought, it occurred t' me that t' top paper rin' was causin' too good a seal and creatin' a piston effect that we could do without. Ahoy! T' second flight with a C11-5, this time without the uppermost ring, and with a better attachment o' t' shock cord was quite a bit better. T' C11 performed admirably, ya bilge rat, carryin' t' rocket t' a very respectable altitude, and t' ejection charge deployed t' streamer right at apogee. The third flight, on another C11, shiver me timbers, saw t' ejection charge blow t' nose off and the fin can out, matey, but t' streamer stayed inside t' airframe. Begad! Ya scallywag! Guess I didn’t get t' nose glued in so well t' second time around. Avast! Begad! Despite this, matey, matey, I felt that the bugs are more or less ironed out.

T' competition version with an Aerotech E15-7.

It should fly well on these motors: C11-5, me bucko, matey, matey, D12-5 or 7, me bucko, matey, me hearties, E15-7, shiver me timbers, me hearties, arrr, and if you have t' temerity t' fly it with RMS hardware D9-4 or 7, D15-7, ya bilge rat, E18-7, matey, E28-10, F24-10, ya bilge rat, and F39-9.

A sport version was also made at t' same time, arrr, me hearties, with a conventional recovery out t' front rather than rearward and with a motor mount that will handle an E9. Begad! It also sported basswood fins, ya bilge rat, that are tough as nails so it can be recovered fast (read that hard) without tearin' it up easily. Begad! Begad! That rocket flew quite nicely, me hearties, arrr, shiver me timbers, and was flown a lot since it was easier t' prep, but was lost t' t' trees.

Partin' comments:
This was a fun rocket t' build; more than t' common three fins and a nose cone with some interestin' permutations involved in gettin' a fully aligned motor mount with as few parts as possible. Less weight in t' tail equates t' less weight in t' nose for ballast! Some fine tunin' o' t' length could be done to produce a lighter model; trackin' powder could then be used t' brin' it up to the optimum weight, me bucko, ya bilge rat, but would be jettisoned at ejection makin' t' rocket lighter for better hang time for duration events.

T' rocket be test flown on a small field a few times with low power motors t' work out t' ejection system. T' main event used a much more powerful motor; a little testin' with that motor prior t' t' actual contest would have revealed t' out o' sight problem that could have been overcome had it been known in advance. Ya scallywag! Arrr! Live and learn!