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MD 4 Inch Level 3

By Mark H

Preliminary Design

This rocket will be a 4 inch 4 fin minimum diameter rocket built for L3 certification.  A 98/75 mm adapter will be used t' fly 75 mm motors, as shown.

Leve l3 75



 OR 3D rendering



 D Plots

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Fins (2014-03-01 20:50:05)

T' fins were cut from 1/8 inch 5 ply Baltic Birch.  T' fin span is 5.5 inches, with a 6 inch root.  Here I have match sanded t' fins usin' a sandin' block.  T' fins are taped together usin' double sided carpet tape.

Fins sanded


T' fins will be laminated with three layers o' carbon.  Two layers o' 5.7 oz plain weave, and one 9 oz. unidirectional layer. Aye aye! Here be t' unidirectional carbon.  T' white lines are t' backin' glue that holds t' unidirectional strands together.  T' backin' material runs perpendicular t' t' carbon strands.


Carbon uni

 T' uni layer will go on first.  One layer o' t' plain weave is cut on a 45 degree bias, and will go on second.  The third layer is plain weave and will go on in t' 90/ 0 degree orientation.  A six ounce layer o' fiberglas will be placed on top.

Here is one fin with a vacuum applied after laminatin' with the carbon pieces.  I am usin' Aeropoxy with 60 minute hardener (3660).

vacuum bagging fin


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Fins Removed from Bag (2014-03-02 14:21:50)

Here are t' fins after vacuum baggin' all four o' them.   Notice t' lines from t' backin' on t' unidirectional carbon pieces showin' through.

The layer o' 6 oz. fiberglass on top o' t' carbon will be mostly sanded away when I sand t' fins smooth.


Fins Out of Bag

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Body Tube Lamination with Carbon Fiber (2014-03-07 21:21:28)

Here be t' 34 inch long LOC precision tube after t' outer layers o' paper have been removed t' reduce t' diameter.  T' tube has been wetted out with Aerpoxy resin, prior t' layin' down t' carbon fiber.


LOC tube


I wet out t' first foot or so o' t' carbon cloth before startin' it on t' tube.  T' rest o' t' cloth gets  wet out with t' spreader as I turn t' tube.  I measured the carbon so it was long enough for 3 wraps around t' tube plus an extra 3 inches.  T' extra gets cut off.


I cut t' carbon, arrr, on one o' t' lines drawn on t' carbon, after t' third wrap passed t' startin' line that I had marked on t' coupler protudin' from t' tube.  There was probably about an extra 1/2 inch o' overlap, after cuttin' off t' excess..


carbon 3

Finally, A layer o' peel ply (actually it's porous teflon coated release film from Aircraft Spruce) is wrapped around t' carbon.  T' peel ply is wetted out with epoxy usin' t' Bondo spreader as it is wrapped around t' tube, t' same as with t' carbon layer. Nay shown is a layer o' 2 oz satin weave fiberglass that I added before puttin' on t' release film.  T' purpose o' t' satin weave is t' make sure t' carbon weave is thoroughly filled, by applyin' this extra thin FG layer on top.  T' fiberglass layer will be sanded off.


 Peel Ply

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Cured Body Tube (2014-03-17 23:03:39)

T' main body tube after removin' t' release film:


Cured Tube

There are some voids from air pockets under t' teflon release, but really they're not not too bad.  I should be able t' sand them out.

Close up



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Payload Tube (2014-03-20 20:51:07)

T' CF layup on t' payload tube went pretty much t' same as t' main body tube.  T' payload tube is 22 inches long. 

I decided the main tube was nay quite as stiff as I'd like with three layers, so for t' payload tube I went with a 4 layer lay-up.

I will add a 4th layer t' t' other tube later. Below are pictures o' t' payload tube.  I trimmed t' edges o' t' cloth 

more closely and carefully this time and also taped t' edges with maskin' tape t' control t' edges and t' keep t' shape from distorting. 

T' results were a little better than t' main tube, ya bilge rat, but doin' two tubes in a row tends t' produce better results t' second time.

Payload Tube


Payload 2

Payload 3



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Sandin' Fins (2014-03-21 22:09:58)

I sanded t' fins back t' shape usin' 60 grit paper and a sandin' block. 


Sanding Fins

Fins 2


Fins Sanded 2

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Addin' Extra Layer o' Carbon (2014-03-22 20:51:21)

I sanded t' main body tube in preparation for addin' a 4th layer o' carbon fiber.  This just involved sandin' off t' veil layer o' fibergalss by hand with 60 grit paper till I hit t' carbon fiber.  T' white patches are fiberglass that was left on t' tube.

More Carbon


End of tube

I used Cotronics 4461 for this layer, me hearties, because I had some leftover from another build that needed t' be used.

Tube coated liberally with epoxy before applyin' t' carbon fiber.

Coated with Epxoy

Added CF layer.  I cut off t' extra so thar was hardly any overlap.

CF layer # 4

I added another layer o' 2 oz fiberglass on top t' carbon.  Then I added the peel ply followed by breather material, before placin' in t' bag.  I used a 55 gallon lawn and leaf bag from Home Depot. 

Here I should have put t' vacuum valve at t' end o' t' tube on t' coupler instead o' in t' middle o' t' tube.  I just decided t' leave it there. 

I capped t' ends o' t' tube with solid plywood bulkheads t' keep t' vacuum from suckin' t' bag into t' tube.

In the bag



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Out o' t' Bag (2014-03-23 20:16:42)

Results o' t' vacuum bag CF layup. Ahoy! T' fiberglass layer had some wrinkles, but t' carbon fiber underneath looks good.  T' fiberglass layer will be sanded off again, then t' fins will go on.


 BT w 4th layer

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Fins on Rocket (2014-03-25 19:01:14)

I made a fin alignment guide by recyclin' some cardboard pieces from me daughter's science project (conductivity o' Play Doh) that be headed for t' trash. 

I printed out an alignment template usin' VCP (, taped it t' t' cardboard and then cut out t' fin pattern. 

I tacked t' fins on with 5 minute epoxy.  Before puttin' t' fins on I sanded t' bottom 12 inches or so o' t' body tube with 100 grit paper.  I still need t' sand t' rest o' t' tube.

Fin Template

Fin Jig 2

Nothin' like a late March snowstorm. 

4 Fins on

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Fillets (2014-03-26 22:21:46)

I made t' fillets usin' Proline 4500 epoxy, shiver me timbers, and a short piece o' 29 mm tubin' for shapin' t' fillets.


Proline 4500


Fillets 2

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Laminatin' t' Fin Section (2014-03-29 12:50:30)

For t' fin section, I planned t' laminate with two layers o' t' same 5.7 ounce plain weave carbon fiber, tip-to-tip. 

First I traced t' fin pattern from t' rocket with a pencil and notebook paper.

Trace of fin pattern

Two layers will go on.  T' first layer will be cut on a 45 degree bias.  T' top layer will be a normal 0/90 dgree orientation.

Here I am ready t' cut a layer with a 45 degree bias. 

45 degree layer

T' fin section was first coated with Aeropoxy.  Before hand, ya bilge rat, I had already sanded t' fillets, and wiped the lamination area down with denatured alcohol.

This photo is actually t' second o' 4 sections bein' laminated.  Before placing the CF on t' fins, I wet out t' CF by spreadin' t' epoxy with t' CF

placed on wax paper.  Parchment paper works too.

Aeropoxy on fin can

This be t' second layer laid down over t' 45 deg layer.

2nd layer

Two ounce veil layer o' fiberglass has been placed on top.

2 oz FG layer


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Vacuum Baggin' Fin Section (2014-03-29 13:11:17)

Here are a few pictures o' t' vacuum bagging.  One layer o' Teflon coated peel ply.  One layer o' absorbin' material. 

Then put inside a Home Depot trash bag.

VB prep

Hey Jessie

It's in t' bag.  This bag is probably too small for t' job. With t' 34 inch length o' tube it is difficult t' tie a knot in t' bag

and keep t' bag loose enough on t' fin section too.  But this method is easier than gum tape and standard baggin' material.

In da bag

This actually took about 2 and 1/2 hours.  Though t' workin' time on t' Aeropoxy is only 60 minutes with 3660 hardener,  I mixed a new 25 gram batch prior t' laminating for each section. 

Even at two hours Aeropoxy is still pretty workable.  My basement temperature be in t' low 60s when I did this, so that probably extended t' workin' time too. 

(Aeropoxy specs. on working and cure times are made at 77 degrees F)



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Out o' t' Bag (2014-03-30 18:38:06)

A few pics o' t' results.  T' only problem was some minor bunchin' o' t' FG layer along t' fillets in a couple spots. Ya scallywag! It should sand out easily.

Fin can

FIn can 2

I cut off t' excess with a Dremel (actually a Black and Decker rotary tool).  T' rest I will sand by hand.

fin can 3

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Thrust Plate (2014-04-02 21:00:29)

In order t' fly a 75 mm motor, I needed t' create a thrust plate with an OD that be t' same diameter

as t' airframe, and an ID t' same diameter as a 75 mm motor case.  I used two hole saws, a 4 1/8 "

and 2 3/4 " t' cut two 1/2 inch thick rings for t' thrust plate.  T' 4 1/8" saw made a perfect 4.0 "

OD that matched t' airframe.  T' 2 3/4" rin' was undersized, me hearties, so I used a Dremel t' open up t' ID after

cuttin' t' ring.  A 2 7/8"  saw would probably have been perfect but Home Depot did nay carry one. 

I tried a 3 inch saw and t' hole on t' ID was a little too big.


Thrust Plate 1


T' max length case I can fit in this rocket is a 26 inch long 75/5120.  A single 1/2 inch thick rin' would have sufficed.

T' reason for two 1/2 inch thick rings is t' push t' motor farther aft in order t' have more recovery space. 


Thrust plate

TP 3

Additionally, a removable 75 motor tube with centerin' rings is used as an adapter. 



T' motor is retained by securin' t' a forward bulkhead.

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Electronics Bay (2014-04-05 07:03:03)

I'll be usin' a Madcow electronics bay for this rocket.  Actually I'll be re-usin' t' bulkheads from another four inch rocket which

are interchangeable.  T' coupler will be new and permanently epoxied in t' payload tube, me hearties, with both ends bein' removable. 

While t' e-bay coupler is thicker than a regular coupler, I felt it needed some reinforcin' so I laminated t' inside with 3 layers o' carbon fiber. 

I had planned t' use 4 layers, but I be usin' 30 minute Z poxy laminatin' resin and it began t' thicken before I got to the fourth layer,

so I abandoned t' fourth layer.


carbon pieces cut

Spread epoxy.

spread expoxy

Roll CF layers into a cylinder.  T' diameter needs t' be a good bit smaller than t' ID o' t' coupler t' get it in t' tube easily without grabbing.

roll cf clayers

Place into coupler and expand outward.  I used a 4 inch nose cone tip t' expand t' roll outward once in t' coupler.

Place CF inside coupler

I used another 50 gallon trash bag for t' bagging.  A short summary o' t' method I used is as follows:

One layer o' peel ply is placed on top t' carbon, followed by one layer o' breather, then place in bag.

I placed t' coupler inside t' bag and then pulled t' bag back through t' inside o' t' coupler, before applyin' t' vacuum. 

I think John Coker pioneered this method o' vacuum baggin' t' inside o' a tube - it is shown on his website (link t' come).

Drawing vacuum

Vac applied



Tape Removed:

Tape Removed

I also laminated t' exterior o' t' coupler tube with one layer o' fiberglass t' increase t' diameter because t' coupler was a loose fit in t' body tube.

av bay assembled


ebay end 

Inside t' e-bay.  One Raven 2 altimeter with power perch and a Perfectflight Stratologger.


 electronic jam




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shear pin testing (2014-04-14 20:57:31)

I tested my shear pins for strength as shown in t' picture below.  With t' nose cone pinned with a single pin,

I pushed down on t' payload tube till t' nose cone separated, and watched t' scale for t' force required t' break t' pin.

T' pins I tested were Evergreen styrene rod with 0.0625 and 0.080 inch diameter.  I repeated t' test 5 times for each size rod

and got consistent results.   T' 0.0625 inch rod sheared in t' 20-25lb range and t' 0.080 inch rod sheared at about 30-35 lbs. 

For t' payload tube/main deployment I plan t' use 3 x  0.080 rods t' give about 90 lbs. Avast, me proud beauty! minimum protection against separation at apogee,

which works out t' be about 90 gees, since t' nosecone and internals weigh about 1 lb.  For t' drogue side/ lower airframe joint,

I chose t' use 3 o' t' 0.0625 inch rods as pins, ya bilge rat, providin' a minimum o' 60 lbs. resistance t' separation force after burnout.  Actually my

calculations, based on mass and drag coefficients o' each half show that t' lower half is nay subject t' drag separation, so this is

just for added insurance, me bucko, or peace o' mind.

shear pins

For ejection charge testin' on t' main, 1.5 grams was a little weak, 1.75 was  good, me bucko, and 2 grams was maybe a little too much. 

For t' drogue testin' 1.5 grams provided pretty decent separation. For an actual flight, for t' main, I plan t' use 1.75 on the

primary charge and 2 grams on t' backup charge. On t' drogue side, ya bilge rat, I plan t' use 1.5 grams and 2 grams as backup.

Here is a shot o' t' 2 gram test o' t' main deployment.

Charge Testing


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Finish (2014-04-15 17:57:49)

For t' finish,I applied a thin coat o' epoxy.  After t' first coat, I sanded with 180 grit, me bucko, then re-applied a second coat.

I tried t' remove as much epoxy as possible while it be wet,  goin' around t' circumference with t' spreader scrapin' it off. 

I could probably do this a couple more times with finer sand paper, t' get a more polished look, but it looks halfway decent as is.

epoxy finish


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Trackin' Electronics (2014-04-15 20:58:49)

A couple pics o' t' 900 MHz tracker.  Tracker design by Derek on TRF.

900 mhz 2



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Almost Launch (2014-04-15 21:10:33)

So I took t' rocket t' Red Glare 16 for t' L3 cert flight. I got thar a little late on Saturday,

and couldn't get it t' t' pad in time due t' a number o' things (e.g. Begad! pickin' up t' motor,

findin' me L3CC, goin' over t' rocket with him, gettin' t' grains stuck in t' liner and a couple others).

So I decided t' attempt it on Sunday. Here are a few pics before takin' it t' t' pad.


Aerotech M1500 G:

M1500 G


display pic 1

display pic 3

On Sunday, I had it ready t' go t' t' pad around 10:00 am.  While Saturday was perfect with winds around 5 mph,

Sunday t' winds were probably in t' 10 - 15 mph range with gusts even higher, me hearties, and increasin' as t' day progressed,

but I loaded t' rocket on t' pad anyway.

On Pad

As me rocket sat on t' pad, we watched another flight that went t' about 8,000 ft land out by t' road.  Given t' unfavorable

winds and wind direction, me hearties, and t' fact that me rocket was goin' to reach about 12,000 ft, my L3CC commented that I

had a good chance o' landin' in t' trees.  I concluded t' same, matey, and decided t' scratch t' flight and wait for another day.

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Mission accomplished (2014-07-05 20:39:35)

L3 flight was successful at URRF II.







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