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. Begad! Blimey! Earlier in t' month I had launched his Lil’ Nuke on an H128 and it vanished. Aye aye! I told him we could buy a new one or he could design a rocket. 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. T' construction is straight forward, shiver me timbers, basic, shiver me timbers, and sturdy. Ya scallywag! T' rocket is designed t' fly on an G64 through J350. Ahoy! Ya scallywag! 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. Ya scallywag! Markin' t' BT is easy. Aye aye! Tape 2 sheets o' notebook paper together lengthwise t' make one sheet about 14 inches long. Avast! From t' 8 1/2 inch edge, measure two marks; One at 107mm and one at 214mm. Wrap this long sheet around t' BT so t' edges line up perfectly around t' circumference. Aye aye! Now transfer t' marks, includin' t' paper edge t' t' BT. Avast! 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. I taped t' angle t' t' BT and then used a very sharp x-acto t' make t' tube slots. 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, blow me down! Aye aye! Blimey! We were usin' a set o' Minie Magg fins from a kit o' leftover parts. Ya scallywag! Blimey! (Crash repair) T' fins are locked into t' centerin' rings and are very sturdy. Avast! First slide t' CR’s into t' BT and make marks through t' fin slots onto t' edge o' t' CR’s. Blimey! Well, blow me down! Blimey! Also, make an index mark on t' BT and both CR’s so they can be oriented t' same way in t' future. Well, blow me down! Avast! Blimey! On t' upper CR, ya bilge rat, 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. Well, blow me down! wide at 1 inch from t' top o' t' tab. Ya scallywag! Ya scallywag! Blimey! (see photo)

Now, me hearties, bond t' upper CR into t' BT. Begad! Well, matey, blow me down! I used Elmer's Yellow Wood Glue for all t' CR bonds. Begad! Glue t' fins t' t' upper CR t' locate it evenly and hold it in place while t' upper surface fillet dries. Aye aye! Well, blow me down! 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. Apply a small amount o' CA t' t' wood surface and in t' hole. Ahoy! Seat t' "T-nuts", ya bilge rat, 6/32 x 1/4, into t' upper surface o' t' CR.

T' fins require internal fillets for strength. Ya scallywag! Begad! (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. Aye aye! Bond these strips t' t' fins and BT t' form a structural fillet. Begad! When this is complete t' MMT and aft CR can be installed. Arrr! Ahoy! Cut slots in t' aft CR t' capture t' fin tabs. Arrr! T' slots need t' be as deep as t' fin tab sticks through t' BT wall at t' very bottom. Avast! Well, blow me down! Smear glue on t' inside edge o' t' motor hole on t' upper CR and on t' MMT near t' top. 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. Arrr! Avast, me proud beauty! Push t' MMT through t' upper CR. Blimey! Seat t' lower CR into t' BT until it hits t' fins. Blimey! (DO NOT ENGAGE THE SLOTS) Push t' MMT up until it’s only protrudin' down 1/4 inch from t' lower CR. Aye aye! Begad! Let t' rocket dry upside down, arrr, this will form t' fillet on t' upper CR, lower side. (If you put on enough glue)

Now t' aft CR can be removed with a wire hook in t' fin slot hole. Blimey! Avast! Smear glue on t' MMT, BT, me bucko, ya bilge rat, fin tabs and some on both edges (inner and outer) o' t' aft CR. Slide t' aft CR into place, engagin' t' fin tabs in t' slots. Let t' rocket stand upright t' form fillets on t' upper surface o' t' aft CR. Avast, me proud beauty! Later, 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. Ahoy! Well, blow me down! If a stock CR is used a block plate must be cut t' plug t' big center hole. Ahoy! Plywood is bonded over t' hole for this. Aye aye! (see photo). 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. Begad! Another method is t' use a tube coupler installed above t' bulkhead t' keep it from de-bondin' under stress. Aye aye! 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. Begad! Bond t' bulkhead into t' BT 6" down from t' top. Begad! Blimey! 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 hearties, stock no. Well, blow me down! Ya scallywag! 472315 purchased on-line from REI for t' shock cord. Avast! Ya scallywag! 12 - 20 feet is about right. Begad! Attach it t' t' lower anchor with a quick-link. Blimey! 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. Avast, me proud beauty! (see photo) You want an open ended NC with as long as shoulder as possible. Avast, me proud beauty! Drill several (20-30) 1/16" holes in t' tip end o' t' NC 1/2" apart from one another. All these holes should be within 2 1/4" from t' tip. Avast! 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. Ahoy! Ya scallywag! Mix up 3 ounces o' resin. Arrr! Avast, me proud beauty! Pour a small amount down into t' tip and swirl it around t' completely wet t' tip area and fill all t' little holes. Aye aye! Well, shiver me timbers, blow me down! Now, 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! Drop it into t' resin and secure it straight up and down. Ya scallywag! Blimey! Some paper lightly formed around it will do. Ahoy! Begad! Blimey! This is where t' shock cord will attach with a quick-link.

ENGINE

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

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

Oh yeah, me bucko, don't forget t' launch lug!

Finished Rocket



LAUNCH REPORT:

Wednesday, shiver me timbers, November 18th, 1998
Snohomish River Valley, shiver me timbers, Everett, matey, WA
55 degrees f, shiver me timbers, gentle southerly breeze o' 5 knots
Noonish

T' weather be goin' t' cooperate and Eric had only a half day o' school. Blimey! 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 was off with a roar. Perfect boost with no weather cock t' about 800 feet. T' 4 sec delay was right on as t' chute was expelled just as t' rocket reached apogee and tipped over t' horizontal. Begad! Blimey! It came down as expected with t' nose cone touchin' down first in t' soft dirt. Avast, me proud beauty! Begad! Blimey! It landed squarely on t' base so t' fins are nay goin' t' be prone t' damage. It's just beggin' for=more power. Aye aye! Blimey! 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. Aye aye! PRICE
     
Body Tube, me hearties, shiver me timbers, BT 3.90 (21 1/2"L) LOC Precision $6.32
Fin Stock, shiver me timbers, 1/8 Ply Sheet (10 x 14) Hobby Store $7.00
Nose Cone, arrr, matey, PNC 3.90 LOC Precision $12.56
Motor Mount Tube, 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, matey, shiver me timbers, 9/16" (20 feet) REI (stock #472315) $4.40
Parachute (28") LP28 LOC Precision $8.29
Launch Lug, ya bilge rat, arrr, me bucko, 1/2" LL.50 LOC Precision $0.50
"T" Nuts 1/4 x 6/32 (2) Hardware Store $0.67
Motor Adapter, shiver me timbers, MMA-2 LOC Precision $2.59
Motor Hooks, ya bilge rat, 10ga bare copper wire Hardware Store $0.28
    Total $52.63

comment Post a Comment