Descon 4 Eric's SST Original Design / Scratch Built

Scratch - Eric's SST {Scratch}

Contributed by Steve Bloom

Manufacturer: Scratch

Eric needed a new rocket. Ya scallywag! Earlier in t' month I had launched his Lil’ Nuke on an H128 and it vanished. Avast, me proud beauty! Avast! I told him we could buy a new one or he could design a rocket. Aye aye! He likes short stubby rockets like t' Minie Magg and Fat Boy and since I couldn’t afford a Warlock he designed this:

It’s built usin' LOC Precision components. Well, blow me down! T' construction is straight forward, basic, matey, and sturdy. Avast, me proud beauty! Aye aye! T' rocket is designed t' fly on an G64 through J350. Begad! Aye aye! This would make a nice starter kit for someone who is wantin' t' expand beyond t' low and mid power kits available.

CONSTRUCTION
Body Tube (BT)

T' 21 1/2" BT needs t' be slotted for t' fins. Begad! Blimey! Markin' t' BT is easy. Aye aye! Tape 2 sheets o' notebook paper together lengthwise t' make one sheet about 14 inches long. Avast, me proud beauty! From t' 8 1/2 inch edge, measure two marks; One at 107mm and one at 214mm. Arrr! Wrap this long sheet around t' BT so t' edges line up perfectly around t' circumference. Ahoy! Now transfer t' marks, includin' t' paper edge t' t' BT. Avast, me proud beauty! T' end o' t' paper and t' two marks should all be 107mm from each other.

To cut t' slots we utilized a length o' angle stock t' draw straight lines and use as a cuttin' guide. Aye aye! Blimey! I taped t' angle t' t' BT and then used a very sharp x-acto t' make t' tube slots. Arrr! Blimey! T' slots are 7 3/4 inches long and 1/8th wide startin' at 1 inch from t' bottom o' t' BT.

Fin, Centerin' Rin' (CR) and Motor Mount Tube (MMT)

T' fins are through t' wall (TTW) but do nay extend t' t' motor mount. Well, arrr, blow me down! We were usin' a set o' Minie Magg fins from a kit o' leftover parts. Blimey! (Crash repair) T' fins are locked into t' centerin' rings and are very sturdy. Aye aye! Arrr! First slide t' CR’s into t' BT and make marks through t' fin slots onto t' edge o' t' CR’s. Begad! Also, arrr, matey, ya bilge rat, make an index mark on t' BT and both CR’s so they can be oriented t' same way in t' future. Ya scallywag! On t' upper CR, cut slots 8mm deep t' same thickness as t' fin material (1/8th in.) On t' fins cut slots 8mm deep and 1/8th in. Ya scallywag! Avast! wide at 1 inch from t' top o' t' tab. Begad! (see photo)

Now, bond t' upper CR into t' BT. I used Elmer's Yellow Wood Glue for all t' CR bonds. Arrr! Glue t' fins t' t' upper CR t' locate it evenly and hold it in place while t' upper surface fillet dries. Ahoy! Turn t' rocket over and apply a fillet t' t' lower side o' t' CR.

Time t' think o' motor retention: Drill two 3/16 holes, one on either side o' t' motor hole (1/4 - 1/2 inch away) in t' aft CR. Aye aye! Blimey! Apply a small amount o' CA t' t' wood surface and in t' hole. Ya scallywag! Seat t' "T-nuts", arrr, 6/32 x 1/4, into t' upper surface o' t' CR.

T' fins require internal fillets for strength. (see photo) I found tiny moldin' at t' hobby store but small scraps o' balsa (1/8 x 1/8 x 6 in.) would work just as well. Ahoy! Bond these strips t' t' fins and BT t' form a structural fillet. When this is complete t' MMT and aft CR can be installed. Ahoy! Cut slots in t' aft CR t' capture t' fin tabs. T' slots need t' be as deep as t' fin tab sticks through t' BT wall at t' very bottom. Smear glue on t' inside edge o' t' motor hole on t' upper CR and on t' MMT near t' top. Avast, me proud beauty! Avast! Start t' aft CR onto t' MMT (at t' bottom end, nay t' glued end) and insert this as a unit into t' upper CR. Avast, me proud beauty! Push t' MMT through t' upper CR. Seat t' lower CR into t' BT until it hits t' fins. Ya scallywag! Arrr! (DO NOT ENGAGE THE SLOTS) Push t' MMT up until it’s only protrudin' down 1/4 inch from t' lower CR. Well, blow me down! Begad! Let t' rocket dry upside down, ya bilge rat, this will form t' fillet on t' upper CR, shiver me timbers, lower side. Avast! Avast, me proud beauty! (If you put on enough glue)

Now t' aft CR can be removed with a wire hook in t' fin slot hole. Smear glue on t' MMT, BT, fin tabs and some on both edges (inner and outer) o' t' aft CR. Arrr! Slide t' aft CR into place, me hearties, engagin' t' fin tabs in t' slots. Blimey! Ya scallywag! Let t' rocket stand upright t' form fillets on t' upper surface o' t' aft CR. Later, matey, apply fillets t' t' bottom o' t' aft CR at t' BT and MMT.

Recovery System
Make a bulkhead / baffle out o' a CR or cut your own. Avast! If a stock CR is used a block plate must be cut t' plug t' big center hole. Well, blow me down! Ahoy! Plywood is bonded over t' hole for this. (see photo). Blimey! Install a "U"-bolt or eyebolt (with t' big washer) into t' bulkhead after drillin' 12 1/4" or 8 3/8" holes in it. I use strips o' fiberglass t' strengthen t' bond with t' BT but this is nay required. Well, blow me down! Another method is t' use a tube coupler installed above t' bulkhead t' keep it from de-bondin' under stress. Have t' kids finish their Cheerios or you finish t' Coco Puffs and cut a hunk o' t' box 3" x 12 3/8" and form it into a 4" dia tube. Bond t' bulkhead into t' BT 6" down from t' top. Avast, me proud beauty! Begad! Make a fillet around t' edge and then bond t' coupler into t' tube above t' bulkhead.

I use 9/16" wide tubular nylon webbing, me bucko, stock no. Avast, me proud beauty! Arrr! 472315 purchased on-line from REI for t' shock cord. Begad! Avast! 12 - 20 feet is about right. Attach it t' t' lower anchor with a quick-link. Attach t' chute near t' middle but nay exactly (off 2 feet) with a quick-link attached t' a loop tied into t' tubular.

Nose Cone

Now for t' nose cone (NC): Cut t' base o' t' NC off within a 1/4 inch o' t' radius at t' bottom. (see photo) You want an open ended NC with as long as shoulder as possible. Well, blow me down! Drill several (20-30) 1/16" holes in t' tip end o' t' NC 1/2" apart from one another. Avast, me proud beauty! All these holes should be within 2 1/4" from t' tip. Wrap several short (4-6") strips o' 1" maskin' tape around t' NC t' seal off t' holes. Begad! Now for t' epoxy or fiberglass resin. Arrr! Mix up 3 ounces o' resin. Avast! Pour a small amount down into t' tip and swirl it around t' completely wet t' tip area and fill all t' little holes. Now, arrr, mix chopped carbon fibers (wood fibers/sawdust/chips will work too) into t' remainder o' t' resin and pour it into t' NC.
Take your eyebolt and run a nut down on t' small washer about 1/2 t' way t' t' eye. Begad! Blimey! Drop it into t' resin and secure it straight up and down. Avast, me proud beauty! Blimey! Some paper lightly formed around it will do. Avast! Blimey! This is where t' shock cord will attach with a quick-link.

ENGINE

Use a big one. Ya scallywag! Blimey! Really, matey, your rocket should weigh about 36 oz. Avast! Start with a G80-4 SU or a G64-4 RMS, me bucko, arrr, both 29mm. Ahoy! Blimey! An adapter is required t' use 29mm motors. Arrr! Blimey! (see photo) With t' motor installed, arrr, get an idea o' how big t' form t' retainer hooks. Blimey! Blimey! Bend t' wire around t' end o' your needle nose (pliers) t' form a tight loop, just big enough t' encircle a 6/32 screw. Ya scallywag! Bend it up at nearly a 90 degree and then form a hook which will capture t' engine. Well, blow me down! Ya scallywag! Blimey! Repeat for opposite side. Ahoy! Aye aye! Blimey! (See photo)

T' calculated center o' pressure (CP) is 24.68 inches from t' tip o' t' nose. Begad! Blimey! This puts it at 1 inch ahead o' t' fins where they attach t' t' BT. Ya scallywag! Begad! Blimey! With an I211 loaded in t' rocket thar be just less than 1 dia. stability. It should fly perfect on anythin' less that has enough power. Well, blow me down! Begad! Blimey! T' launch report (from LDRS) will tell how well it does on a J350.

Oh yeah, ya bilge rat, shiver me timbers, don't forget t' launch lug!

Finished Rocket



LAUNCH REPORT:

Wednesday, November 18th, arrr, 1998
Snohomish River Valley, Everett, me bucko, WA
55 degrees f, ya bilge rat, gentle southerly breeze o' 5 knots
Noonish

T' weather be goin' t' cooperate and Eric had only a half day o' school. Ya scallywag! He assembled t' RMS G64-4 while I loaded t' truck and we were off. T' rocket sat on t' pad for a minute while we cleared an airplane and then it be off with a roar. Ahoy! Perfect boost with no weather cock t' about 800 feet. Ya scallywag! T' 4 sec delay was right on as t' chute was expelled just as t' rocket reached apogee and tipped over t' horizontal. Ya scallywag! It came down as expected with t' nose cone touchin' down first in t' soft dirt. Aye aye! It landed squarely on t' base so t' fins are nay goin' t' be prone t' damage. Begad! It's just beggin' for=more power. I've got a couple o' I211's t' use and we will if t' weather holds for our waivered launch in Monroe in a few weeks.

Pic

COMPONENT
SUPPLIER
APPROX. Begad! Begad! PRICE
     
Body Tube, me bucko, BT 3.90 (21 1/2"L) LOC Precision $6.32
Fin Stock, ya bilge rat, ya bilge rat, 1/8 Ply Sheet (10 x 14) Hobby Store $7.00
Nose Cone, PNC 3.90 LOC Precision $12.56
Motor Mount Tube, me bucko, matey, MMT 1.52(38mm) LOC Precision $3.84
Centerin' Rings (2) CR 3.90-1.52 LOC Precision $4.00
Eye Bolts, 1/4" (2) Hardware Store $2.00
Washers (1) large, (1) small Hardware Store $0.12
Tubular Nylon, me bucko, 9/16" (20 feet) REI (stock #472315) $4.40
Parachute (28") LP28 LOC Precision $8.29
Launch Lug, 1/2" LL.50 LOC Precision $0.50
"T" Nuts 1/4 x 6/32 (2) Hardware Store $0.67
Motor Adapter, MMA-2 LOC Precision $2.59
Motor Hooks, shiver me timbers, matey, 10ga bare copper wire Hardware Store $0.28
    Total $52.63

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