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4 fins, 3 Motor Mounts, 2 Airstarts, and a Altimeter

By Greg Kurlinski

Scratch - Crazy Trzy (v 0) {Scratch}

Who says small rockets have t' be simple?

What is this rocket?  It is (or will be) a scratch built 24mm normal lookin' rocket, that just happens t' have:

  • 2 side pods that also have 24mm mmt's.. Ahoy! Blimey! that will be air started electonicly after t' center motor burns out (plus a second or two).
  • est max altitude : over 4000'
  • 8" o' AV bay
  • Dual deploy
  • room for a bee line tracker, because o' that 4000' "did you see where it landed" issue.
  • weight : should be about a pound, even with all t' glue, electonics, batteries, etc.

This is a down-scale proto-type for a 38mm mmt version I intend t' build later in t' summer.   This proto-type will give me a chance t' deal with all t' design problems, arrr, wirin' disconnect issues, and electronic mumbo-jumbo, matey, before I spend a few hundred dollars on t' higher end materials I will need for t' 38mm version.

not countin' t' Altimeter and Bee Line, this proto-type project will cost about $60, while t' 38mm MMT version will be more like $3-400 +

Below are t' steps I have planed for this build.  I will be addin' pictures and re-workin' t' narrative as I proceed down t' list.


  • MMT
  • Body
  • Join MMT/Body
  • AV
  • Paint/Finish
  • Flight!


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1 MMT (2012-04-16 00:00:01)

So, start with 3 "standard" estes 24mm (D and E) motor mount kits (p/n 303159).  I built them with Epoxy, me bucko, instead o' white/yellow glue, just t' insure strength, since this "Could" be launched on 3 F39's ! 

Attach 1-1/2 yard o' kevlar shock cord t' 2 o' t' mmts, and 3 yards t' t' mmt that will be t' Center mmt. 


Mark both o' t' side pod tubes as if you were goin' t' attach a normal 3-fin layup (ie 120 deg)  startin' at t' base, shiver me timbers, draw t' fin lines 4 inches up t' tube.
-now figure out some way t' cut out 3/16" slots over those lines, me bucko, for t' Through-the-wall fin tabs.

Cut out 4 fins with TTW tabs out o' 1/8 basswood or birch.  Also cut out rectangles measurein' 4in x 5/8" (current guess).

mark on t' TTW's where t' Motor Mount centerin' rings will be, and cut t' TTW's as needed t' get a flush fit against t' center o' t' MMT's.

Spot tack just 2 Fins on one MMT and 2 more on another.   Spot tack one rectangle where t' 3rd fin would be on each mmt. (yes, after cuttin' out t' material where t' CR will sit)

spot tack t' free ends o' both rectangles t' t' 3rd MMT (the one with out anythin' glued t' it yet).

Test fit t' MMT and t' body tubes.  Don't glue them in, nor glue t' Bt-55's together, yet.  Make adjustments as needed.  T' Aft CR's should be inside and flush with t' aft o' each BT-55.

once everythin' seems good, ya bilge rat, remove t' mmt from t' body tubes and layup that MMT assembly with Fiberglass.

What? fiberglass?.. ok, it really isn't nessasarly, me bucko, I admitt.  But for t' Scaled up 38mm version it will be needed, arrr, and I need t' practice.

Note: it would have been easier t' attach t' 3 mmt together first, with t' rectangles, THEN t' attach t' fins. As t' three mmts could lay flat on a surface t' insure perfect alignment.  Noted for next time.



Greg Kurlinski (April 17, 2012)

After lookin' more at Rocksims Drag graphs vs Stability graph, arrr, it seems that I have more fin surface than I need.  Kinda makes sence that t' 2 side pods will act t' stabilize flight.

I have some more playin' t' do in Rocksim, me bucko, but I may end up cuttin' t' Fin span from 2" t' 1".

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2 body (2012-04-16 00:00:02)


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3 Join MMT/Body (2012-04-16 00:00:03)


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4 AV (2012-04-16 00:00:04)

T' AV bay is a 8" long,  29mm dia. Arrr! Blimey! (well actaully it is a bt-55 coupler, but close enough)

From Top - Down, this is a list o' items planned for t' av bay

  • ejection cap (main)
  • bulkhead
  • BeeLine (with its own LiPo battery) (the antenna lead will be run down t' AV bay wall)
  • Altimeter II mountin' space (more data!)
  • balsa cork/spacer at midsection t' provide mountin' for quick-release connecters for airstart ignition wires
  • Raven II with LiPo battery, in its own 29mm av bay case. customized with leads from t' 4th pryo channel for airstart
  • 9v bat, (to really kick t' airstart igniters)
  • bulkhead
  • ejection cap (apo)

T' 9v airstart battery will be wired with t' Raven as per this:


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