Scratch Twin Jet TR5 Fighter Original Design / Scratch Built

Scratch - Twin Jet TR5 Fighter {Scratch}

Contributed by Matt Gillard

Manufacturer: Scratch

Twin Jet TR5Brief:
This is me second attempt at buildin' a rocket t' match a picture. Well, arrr, blow me down! Blimey! This time for t' 2008 EMRR Challenge and me first cluster where t' motors are significantly apart from each other.

Construction:
Parts include:

  • 1 launch lug for 3mm (1/8 inch) rod
  • 2 parachutes (Estes 12 inch)
  • 2 3-foot lengths o' knicker elastic
  • 2 Snap swivels
  • 2 18mm engine block
  • Keelhaul®©™thread
  • 2 Body tube 8.5 inches in length
  • 2 18mm motor mounts
  • 3/16 inch balsa sheet
  • 3/8 inch balsa sheet, shiver me timbers, matey, 3 by 8 inches
  • 2 cones shaped nose cones
  • 4 Motor mount centerin' rings
  • Polymorph
  • Green floral foam block

Twin Jet TR5

To build t' rocket I used:

  • Bostik glue
  • White PVA glue
  • Sandin' sealer
  • Modelin' filler
  • Model knife
  • Aluminum oxide sand paper
  • Primer
  • Spray paint --window grey
  • Spray varnish
  • Future floor polish
  • CA gel glue

T' motor mounts were constructed form two old Estes motor mount kits in t' standard fashion.

After measurin' t' photo from t' web site, matey, me bucko, t' body tube came out t' be a BT-55 that be 8.5 inches long. Arrr! Blimey! T' nose cone was a NC55AC (or as near t' AC as you can get).

T' motor mount was glued into place after a 16-inch length o' Keelhaul®©™was first added. Begad! 3 feet o' knicker elastic was then added t' t' Keelhaul®©™, and a snap swivel was added t' that. Arrr! Two 1.5 square inch swatches o' Nomex® heat shield be then added t' t' shock cords.

Given t' model looks as if t' CP is goin' t' be high and with two aft motors, me bucko, a significant amount o' modelin' clay was added t' both nose cones. Avast, me hearties, me proud beauty! T' nose cones were then added t' t' shock cords. Well, blow me down! A twelve-inch parachute be added t' each tube.

T' central platform be made from 3/8 inch balsa measurin' 3 by 9.5 inches. This be sanded t' profile at t' front and then sealed. T' two body tubes were then added t' either side. Blimey! Aye aye! Heavy fillets o' PVA glue were then added.

Again from measurement from t' picture, arrr, arrr, t' 4 fins were made usin' quite stiff 3/16 inch balsa. Ahoy! T' upper fins were cut from a triangle measurin' 1-5/8 by 6-3/4; inches. Begad! T' fins were then sealed and glued into place on t' body tubes. Avast! I filleted t' fins three times. Begad! These four fins will be t' point o' contact with t' ground and I doubted in t' plannin' stage on t' fragility o' t' fins.

T' two aft engines on t' rocket were built for decoration only and were nay t' be part o' t' flyin' model. Arrr! Avast, me proud beauty! This was because these engines were where t' motors for t' flyin' model were goin' t' be in me original plan but this would have lowered t' CG t' low and would have been out o' scale.

Twin Jet TR5 T' build t' aft engines I used polymorph, arrr, me bucko, a thermo softenin' plastic that can be made very pliable at temperatures around hot water (70-80 degrees C) but hardens at room temperature. Avast, me proud beauty! T' plastic when set can be sanded, me hearties, drilled, painted, etc. Its density is similar t' t' plastic used by Estes for its nose cones. T' polymorph be molded around a 16mm balsa rod, which proved t' be more difficult than I imagined. Arrr! T' polymorph cools and goes from clear and colorless t' a white plastic. Aye aye! Avast, arrr, me proud beauty! This was then sanded into shape and then painted silver gray. Ya scallywag! Ya scallywag! T' engines hooks get in t' way o' t' aft motor engines so a slot had t' be cut out so they sat straight.

One o' t' design flaws o' this model be t' canopy. Aye aye! Avast! Its shape will create lift and given its size if its construction is made o' anythin' with a density o' balsa or higher, me bucko, it could cause t' center o' gravity t' come out o' line with t' center o' thrust.

After spendin' time makin' a few hollow cardboard canopies--which failed. I decided t' use green floral block. Begad! Avast, arrr, me proud beauty! Nay bein' a flower arranger, shiver me timbers, I purchased some from a craft shop. Floral block has a very low density and cuts very easily. Within minute a canopy be cut. Avast, me proud beauty! Aye aye! T' canopy be glued with PVA t' t' rocket.

Twin Jet TR5 T' floral block is porous and would give a bilge-suckin' finish. I thought that a few coats o' thick sealer would help fill in t' gaps. Begad! Well, shiver me timbers, blow me down! As t' solvents in t' sealer evaporated, matey, shiver me timbers, t' sealer coat shrank and peeled a layer off t' floral block. Avast! T' canopy was ruined. Another canopy was cut and glued into place. Avast! Ahoy! This time layers o' PVA glue was added t' t' floral block until it was smooth--this took 6 days. Blimey! T' fins and canopy joints were then filleted and then had two layers o' sealer added. Aye aye! After sandin' with fine sandpaper t' whole rocket had 3 layers o' primer, me hearties, sandin' betwixt coats.

Finishing:
It be nearly impossible t' find a spray paint that matched t' color on t' picture. In t' end I went for window gray. Begad! Three coats were added, sandin' with fine sandpaper betwixt coats.

Twin Jet TR5 T' details were added usin' a black ink pen for t' lines and black stickers for t' windows on t' canopy. T' circular emblem on either side o' t' body tubes were a deviation form t' original. Ahoy! Begad! I used t' EMRR "Write A Review" circular emblem, matey, which I thought was a nice touch! T' rocket was then sprayed with clear varnish and then two coats o' finish was applied.

 

Flight and Recovery:
I had lots o' concerns about this rocket, me hearties, ya bilge rat, it looks as if it would arch on boost. T' motors are far apart and Estes rocket motors are nay always exactly t' same, me hearties, which would give asymmetric trust. T' fins are t' first point o' contact with t' ground, and t' rocket is rather heavy for 18mm motors.

Due t' drag t' coast phase would be short, and with two parachute thar would be drift. Begad! Blimey! T' trees around me flyin' field have enjoyed t' company o' me rockets too often. Aye aye! Avast, matey, me proud beauty! Blimey! T' overcome this I decided upon two different delays for each tube, ya bilge rat, shiver me timbers, t' first short delay would deploy a 12 inch chute, and t' second delay would deploy a 24 inch chute 4 seconds later. Well, blow me down! Avast, me proud beauty! Blimey! This way thar would be a slow and hopefully soft landing, but cut down on t' drift.

Twin Jet TR5 Each parachute was protected by two swatches o' Nomex cloth, but a few sheets o' Estes waddin' were used as extra protection. Avast, me proud beauty! A standard Estes 3mm rod be used.

For t' first flight, matey, two Estes B motors were used, me hearties, me hearties, shiver me timbers, a B6-2, and a B6-6. Begad! Blimey! There was a wind o' about 5 miles per hour. Avast, me proud beauty! Blimey! This be a nervous moment, with a rocket that broke so many design rules a lot could go wrong. Ahoy! Blimey! I was expectin' an arched boost, with a possible power prang. However, matey, both motors lit simultaneously and t' rocket lifted at a decent rate.

T' flight be underpowered and gave a lower altitude, wobbly flight. Arrr! After t' burn phase she almost glided in (a very long 2 second delay). T' first eject went off about 30 foot from t' ground, which was just enough t' slow her down. T' second ejection went off on t' ground. Ahoy! Fortunately, me bucko, only minor damage occurred with one fin poppin' off, arrr, which I expected. There was some charrin' t' another aft fin.

Flight two: After a quick re-gluing, about 1 hour later she was prepped again and ready for flight, this time powered by a C6-3 and a C6-7. She ripped off t' pad and climbed t' a good height o' about 800 feet. Avast, me proud beauty! There was quite a lot o' barrel rollin' towards t' end o' t' boost phase, which increased durin' t' coast phase. Aye aye! She must have done 7 complete rolls. Arrr! Avast, me proud beauty! T' boost had a slight arch t' t' right side o' t' rocket, me hearties, which I put down t' unequal thrust from t' motors. Ahoy! Avast! Ejection was bang on apogee. She drifted over t' small strip wood by t' side o' t' field, and then t' second parachute deployed. Arrr! With t' rocket gods smilin' on me, t' rocket slowed down enough for it t' just drift clear t' wood. Begad! Avast! I however did nay fare so well, runnin' through t' wood, lookin' up at t' descendin' rocket, I managed t' find a trench that was dug durin' t' Second World War. Avast, me proud beauty! Ironically, me hearties, ya bilge rat, I had had a group o' cadets re-dig this trench out only a few months earlier. Begad! Avast, me proud beauty! There was no damage t' either t' rocket or t' me (except pride --which comes before a fall).

I prepped t' third flight t' same as I did for flight 2. Ahoy! Blimey! Arrr! Blimey! This flight was straighter and had a lot less roll. Ahoy! Blimey! T' wind be near calm by now. After ejection at apogee, t' rocket descended smartly until t' second parachute deployed, matey, and another soft damage free landin' be observed.

Summary:
I am pleased with t' overall rocket. Arrr! She flies well and is a bit different from a standard cluster.

comment Post a Comment