Scratch FGM-148 Javelin Original Design / Scratch Built

Scratch - FGM-148 Javelin {Scratch}

Contributed by Todd Mullin

Manufacturer: Scratch
(by Todd Mullin - 04/16/08)

scratch_fgm148javelinBrief:
In search o' a subject for t' unkitted scale model portion o' t' 2008 EMRR Challenge, I stopped by me favorite missile scale data web site. Ya scallywag! Avast! There is a HUGE arsenal o' missiles thar t' choose from but I settled on a design that I've been eyein' for several years, me hearties, t' Javelin. Ya scallywag! For more background information on this missile, me bucko, check out this web site. Arrr! Begad! One o' t' things that I liked about t' Javelin is it's small size. Ya scallywag! Ya scallywag! I be intrigued by t' thought of buildin' a full scale version o' it! T' downside t' that thought is that t' real missiles have an outside diameter of 5" which is nay a standard rocket tube size. My decision then was t' build a 80% scale model rather than oversizin' it t' a 110% version. Ya scallywag! Ya scallywag! For this scale I decided t' try and use at least a 29mm motor mount because in several of t' pictures o' real Javelins, ya bilge rat, it appears t' have a greenish flame which I wanted t' reproduce with t' newly released Aerotech Mojave Green motors.

Construction:
T' parts for t' Javelin are:

  • 32.625" 98mm tube
  • 4" 98mm tube coupler
  • 1/4" Eye bolt
  • Small quick connect
  • 48" long 1/2" tubular nylon shock cord
  • (2) 24" nylon parachutes
  • (2) 98mm x 29mm x 1/4" plywood centerin' rings
  • 8 - Front rins
  • 4 - Rear fins
  • 12" 29mm motor tube
  • (2) 8-32 T-nuts
  • (2) mirror clips
  • (2) 8-32 x 1" bolts
  • nose cone - formed with 7 ounce fiberglass fabric.

I "built" t' Javelin in Rocksim and was pleasantly surprised that t' design be stable without extra nose weight. Ya scallywag! Begad! Normally real missiles are designed t' be marginally stable in t' real world so that their active guidance systems can turn them smartly as their target moves. Ahoy! Rocksim's user interface is intuitive and easy t' use, even for a relative novice at simulation software. Arrr! I knew that I would be startin' with a 4" airframe, shiver me timbers, so I scaled the nose cone and fins t' 80% o' t' real missile. After I had t' appearance o' t' design correct, me hearties, ya bilge rat, I added the "guts" t' make it a flight worthy model. Well, ya bilge rat, blow me down! I then printed out t' fin patterns from Rocksim.

I built t' semi-hemispherical nose cone usin' t' technique in this featured tip. Avast! After buildin' t' nose, me hearties, I collected t' rest o' t' parts for t' build and cut out the fins. Well, blow me down! I carefully marked and cut t' fin slots into t' tube, all twelve o' them. Begad! Ya scallywag! I epoxied on t' front motor mount centerin' rin' but left t' aft one loose. Avast, me bucko, me proud beauty! Next, ya bilge rat, matey, I glued t' motor mount into t' airframe with t' aft ring holdin' t' end o' t' motor tube centered bein' careful nay t' block t' fin slots. Begad! Since I had put maskin' tape tabs onto t' aft ring, ya bilge rat, me hearties, I was able t' pull it out once I had epoxied in all o' t' fins. Begad! This allowed me t' be able to apply internal fillets t' t' fins t' greatly increase their strength. Well, blow me down! Begad! Luckily, arrr, me bucko, me hearties, I be able t' do all t' fillets in one shot on t' inside. Blimey! Avast! At this point, shiver me timbers, I added in t-nuts t' t' aft rin' for motor retention. Ahoy! Ahoy! Next came t' insanely tedious part on this rocket - lots and lots o' little tiny fillets.

After doin' fillets for many days, I finally finished off t' body o' t' rocket by epoxyin' in t' tail centering rin' and addin' 1/4" launch lugs. All that remained was t' attach t' recovery system.

Finishing:
I filled in some slight chips in t' wood o' t' fins with wood putty and then sprayed t' entire model with a super heavy coat o' Kilz fillin' primer. Blimey! Avast! Next came lots o' sanding. Avast, me proud beauty! I started t' finished paint scheme with black on the fins, nose and airframe. Avast! Next was t' white on t' nose and tail and finally silver on t' main body with some masking to produce t' roll patterns seen on t' missile. Well, blow me down! There seem t' be several distinctly different sets o' markin' on the Javelins but I chose t' one from t' photos I was able t' find on t' internet for t' "A" model.

scratch_fgm148javelin_launchFlight:
Workin' with Rocksim, I decided that I be goin' t' need t' work with smaller than expected motors for flyin' here on San Diego's Fiesta Island. Begad! My original plan was t' use t' Aerotech H250G but that would make t' flight MUCH t' high for a field surrounded by water. Ya scallywag! I decided that I would start off with t' Aerotech F40-4 and if that went well, I'd try t' G71R-7. Begad! T' G76G will be released in about a month, me hearties, me bucko, so I will probably try that one or t' H250G at t' next Tripoli San Diego desert launch.

I loaded up t' F40 reload and inserted it into t' rocket, me hearties, arrr, me bucko, tightenin' down t' mirror clips for motor retention. I packed t' parachute with several handfuls o' dog barf waddin' and off I went t' t' pads. Arrr! After loadin' up the Javelin, me bucko, I returned t' t' launch control panel and waited in t' queue for me turn t' launch. Ahoy! When it's time came, the Javelin shot o' t' pad like it's namesake. Ahoy! T' Rocksim numbers said that t' rocket would only reach 524 feet on the F40, but t' thrust t' weight ratio looked safe. Ahoy! T' actual flight seemed t' fly much higher than what I expected from the sims, matey, though this is only a gut feelin' as thar be no altimeter bay or flight data that I'm basin' this on.

T' second flight, I loaded up t' G71R-7 and let it rip! T' red flame looked great with t' rocket, even though it wasn't quite t' right color. Avast, me proud beauty! Avast! T' flight was nice and straight but instead o' t' normal "bonus delay" on the flight, it was only t' about 3-4 seconds o' delay when t' ejection charge went off.

Recovery:
For t' first flight, I decided that I wanted t' get t' rocket down as smartly as possible and nay risk puttin' it in t' water. Begad! Avast, me proud beauty! Blimey! I loaded up just a single 24" parachute. Ya scallywag! Rocksim said that this would come in at about 30 fps. Faster than recommended, but it would be landin' on sand so I decided t' risk it. Ahoy! Blimey! T' ejection charge deployed the parachute perfectly and t' rocket made what looked like a safe descent. T' rockets landed a touch hard and had one of the aft fins crack slightly and delaminate one o' t' plys in t' plywood.

For t' second flight, I added a second 24" parachute. Since t' ejection charge blew early, I be concerned about zipperin' but after a much slower landin' I be pleased t' see that t' Javelin had nay been damaged.

Summary:
T' Javelin has long been one o' t' missiles that I wanted t' build a model of. Arrr! I think that Rocksim be a valuable tool in me development o' t' Javelin. Begad! More than likely, without t' simulations, I would have added unnecessary nose weight t' t' rocket. Avast, me proud beauty! Bein' able t' have t' program print out t' fin patterns with t' correct fin tab lengths was also very helpful.

T' Javelin caught t' attention o' many o' t' other fliers at t' range t' day that I flew it. Ya scallywag! Lots o' people have heard o' t' missile, me bucko, but nay many people seem t' recognize it. Ahoy! Many asked if it was goin' t' helicopter recover for some reason as well. Ahoy! Avast, me proud beauty! This rocket is a great flier and is a fairly simple scale build that goes way beyond 3FNC!

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