Notes on Cap Plug Shell

WARNING AND DISCLAIMER:  If you are underage, then consult with your parents or guardians before attempting any of this.  You are on your own - I'm not responsible for your actions or harm you may bring to others because of your actions.  Making the items described below  can result in injury or death to you or people in your vicinity. Some things mentioned here may be illegal to make in your city, county, state, or country so check the laws that apply to you before you attempt anything described here. These notes are not complete on purpose. If you are reading them and new to pyrotechnics, then you are making a mistake. Stop now - this page is not for you. Get a beginning book on fireworks (see Skylighter or American Fireworks News (very quick shipping)  for a start) and read up. You can't make any of this work without more information so read up or join a club or ask someone to help you.

Additional warning about this project: My ground tests show that cap plug headers go *everywhere* if the header is ignited on the ground. The burst is strong and I had one test of a 2 3/8 shell that covered a 200' diameter circle. If they don't function.... yuck. I advise anyone that makes them to do thorough ground tests and put them on a rocket they know will launch. Even if using paper - I am not one who thinks paper is any safer in the short term - a shell that bursts without igniting the cap plugs will leave a mess.  Be careful and pick up all duds.  The yellow vinyl plugs shown here have good visibility and are easy to spot. 

Use a reliable motor. I use significantly detuned BP motors that I can trust. Even with that, the rocket gods will probably intervene one day and I'll have a cleanup to do.

Above is a mess of 1" vinyl cap plugs that will be
inserts. You can also use the 1.5" variety, too, as
long as the I.D. is between around .218 to .223
Kinda looks like Macaroni and Cheese. 
I like Macaroni and Cheese.

Above is a completed shell loaded with cap
plugs and mounted on a 3/4" motor.

This is a note on a Cap Plug shell meant to fly on a one pound (3/4") motor. This is NOT my design but was inspired by a master pyro who wishes to remain anonymous. I have found that I cannot exactly duplicate his version.  This is my way of skinning the cat.

The following pictures and captions show one way.  You may wish to do it more traditionally.  This is a rocket header so it does not need to be pasted in. Pasting primarily provides fireproofing but we can make the header fireproof enough for a rocket launch without all that extra weight.  

We will use vinyl cap plugs for the inserts.  These are available online and through group buys.  One location for them is Argon they sell the VC218 yellow cap plug for about $20 a thousand as of this writing.  These plugs are about 1" in length.  Another convenient size is the VC223-1500BL cap plug which is a black 1.5" version. It is $26 per thousand.  Any firm offering cap plugs should have the kind needed.  The inside diameter should be .218 to about .223 - the plug should friction fit the time fuse. The length should be 1" to 1.5".

Motor Configuration

The total weight of this shell will be about 160 grams - which can be lifted with most 3/4" BP motors.  The BP motor can be nozzled or nozzleless so see the rocket section on how to build one. The one used for this shell was 3/4", 4" spindle with a 3/8" base and a 1% taper. It used a clay nozzle and a clay bulkhead. The fuel was RP equivalent (see composition page). The delay was 3/4" of the same fuel with 20% Ti added. The dimensions of the spindle are similar to the LWS motors produced by Steve La Duke and his tooling set would work fine.  If a 'standard' BP spindle is used, then the fuel will need to be slowed down by adding charcoal - start with about 5%.  I use two sticks on each motor - one is 5/16 x 32 and the other is 1/4 x 18.  For slightly heavier shells (200 grams or so), I increase the 18" stick slightly.  For lighter shells, I decrease it or remove it. For instance, for the five times report shell which is less than 100 grams, I remove the 18" stick entirely.

While BP is probably the most complimentary to this shell, just about any 3/4" motor would lift it.  Whistle, red/green mag, strobe or any of the variants could be used.

We'll make a lightweight shell case first. 

Start by cutting out some end plugs (2 for each shell)
If you use the former shown below, you can use a
2.5" hole cutter to make some 2 3/8" plugs out of
1/8" particle board.

Optionally, you can make some cover pieces for the
ends of the shell. These are cosmetic but are nice

The plastic tube is a 2" conduit. It's outside diameter is
2 3/8". The hole saw (above this) cuts a plug that is
just about right for this combination.  The paper is
50 lb Kraft 5.5" wide and about 15" long - enough for
two wraps around the former.

Wrap the paper around the former and either tape
or glue it.  I like to glue it since it makes a nicer
looking shell.

After pasting the paper, drop one of the end plugs in
Snip the paper and form a rose. Open up the rose so
you can see all the petals and add a dab of paste to
each petal and to the end plug
Fold together
Press on a board or flat surface and you get something
like the above
Optionally, you can add a bit more paste and put on
the cover. This makes a nice looking shell case.
Put a spare plug on each end of the work and
squeeze it to make sure the glue and paper are in good
contact.  Leave the pressure on for just a few minutes
else you may end up permanently gluing the spare plug
onto the end. ;-}
While you are at it, make up a few.

If you want to put the bottom shot on the outside of
the shell, you will need to put a hole in the end to
receive the fuse.

You can make up a few of these at the same time and
save a bit of preparation work


Now let's make the inner chipboard liners

Cut some 2.5" strips of chipboard.  You can use
commercial stuff or cereal boxes or other sources.

Mist the cut pieces lightly so they will form more easily

Wrap them around your case former. Just keep
wrapping them until you have made up several.

Once they are wrapped, put a piece of tape on them
to hold them tightly.

Slide them off the former and let them dry.

For a single shell, cut enough to make a 15" length
(two layers inside the shell)

Let's make the inserts

We're going to fill some vinyl cap plugs and put a bit
of time fuse in them.  To keep the timing close, black
match is folded over the top and either tied or fastened
in some way - above you see vinyl bands from a
spare cap plug used as holders.  You can also tie them
with string. In all cases, super glue or PVC cement
is used to hold the fuse in place and to hold the string
or vinyl band in place.

Make up 20 or 30 grams of your favorite salute mix.
If you use flash, be sure to use a low ignition point
flash.  'Standard' 70/30 will not reliably ignite.  See the
first video at the end of this page for an example. Hot
BP is pretty reliable and easy to use. I recommend
using it until you get this shell working.

Gather 15 cap plugs together and fill them with your
salute mix

Once full, tilt the cap plugs to a 45 degree angle and
shake them to get them to an even fill line

They will look like this. There should be enough room
in the plug to insert the time fuse about 1/4".
 Use the remaining mix for the bottom shot (see below)

Another way to fill the tubes is with a funnel. Above left
is a candle snuffer that was modified for duty. Members
on Passfire have had good luck with disposable ear exam
cones (Otoscope Specula) - above right. Ask your
doctor for one. The largest size (4 mm) seems best.

Cut off 15 1" pieces of time fuse. You should cut them
as accurately as possible. If you don't have a time fuse
cutter, you can make one easily enough.  See here.

You should prime the time fuse. It holds the powder
train in and also helps with ignition. However, the prime
should be uniform on each fuse so timing isn't affected.
I shake my NC bottle and use the lid to wet the end of
the fuse.

Next,  dip the fuse into mill dust that has 10% of a
fine metal added. Aluminum, MgAl, or silicon all work.
I do both ends of the fuse.

Here is the finished article. The prime essentially widens
the powder core face on cheap fuse and also gives an
extra spurt when passing fire to the burst. The trick is to
make the prime thin and consistent from fuse to fuse. That
way the timing will be affected equally on all shots

Super glue the time fuse by putting a drop on it and
twisting it a few times inside the vinyl cap plug. Insert
the fuse about 1/4"

To make the vinyl bands, just snip them with some
anvil cutters. I like the Craftsman Handi-Cut ones.
You can also just tie them with a bit of fine wax string.

Sometimes the vinyl bands are a little stiff to get on.
Try warming them and stretching them a little

Add the match across the top, put a drop of super glue
on the band/tie and store the completed insert in a
convenient place while the super glue fully dries

Tying black match can be tedious. Click on the above
picture for a somewhat obvious hint on how to make
it easier.


Now the bottom shot

Cut a piece of time fuse and punch it at 1.75". Be sure
to cross match the fuse with some good black match

Put a plug in a 3/4" x 2" tube and epoxy the back of it.
It is important to securely epoxy the ends so make a
1/4" fillet at least

Fill bottom shot and put in fuse and end disk. Fill
this with epoxy being sure to make a good thick
seal. Note the generous recess in the bottom shot.
Be careful to keep the time fuse close to the end
of the tube. If it sticks up too far, you will have
trouble closing the shell.

If you decide to put the bottom shot on the outside of
the shell, you can do so at this time. This one is glued
on with epoxy

If you put the bottom shot on the outside, then the
fuse will be sticking through like this. You'll have to
finagle that black match up into the center of the shell
so you can add the inserts. This is a good time to fill the
area around the fuse with hot glue just to secure it

Fill the shell casing

Put the bottom shot in an empty shell case if it is not
mounted on the outside of the shell.

Add the fifteen inserts

If you are having a hard time keeping the cap plugs
in place, try using a 1" piece of chipboard as a
'spring' to hold them tight. Just leave the chipboard
in the shell and fill it with BP hulls. If you decide to
make this bigger shell (2 7/8" or so) then remember
it will weigh about 210 grams or so.

The 23 insert shell on the left, with a bottom shot, takes 3
grams of slow flash booster to break it nicely. It is made the
same way as the 15 insert shell except there are two fuse
lengths.  3/4" and 1". Probably the fuse length should have
been 3/4" and 1 1/4". The stick was 42" long past the motor.
Click on the above link to see it fly.

Fill to the top of the chipboard liner with BP coated
rice hulls
. Press it down firmly with your fingers. You
may wish to add a couple of grams of  slow flash or
whistle to the burst at this time.  My advice is to go
with just BP coated rice hulls for the first shells and
see how it works for you. Having said that - I used
1.6 grams of slow flash in my last shell. ;-}

Insert a wad of black match into a 1/4" hole in an
end disk. We will spike the shell soon and we need
a good anchor for the string. It makes our life easier
if we use a small 1/4" tube or spolette tube to hold
the black match.

Put the end disk onto the shell.

Snip around the shell paper so you can make another

Glue the rose - similar to the way you did the bottom
of the shell

Fold the glued tabs over and make it look nice

You can add a clean end disk - same as we did on
the bottom - to make it look even nicer.

Cut a hole in an end cap and clamp things together for
a few minutes to get the glue to hold tight

Spike the shell

Get some spiking twine - if you can't get hemp
or flax, then use some good cotton butcher's string that
is 6 ply or better. You want cotton and not an
artificial fiber or waxed string - it needs to shrink after it is
wetted with paste. 24 lb twine is best for this shell.

Wrap about 20 feet of twine between two bolts on
your work bench. I like about four or five feet spread
between the bolts.

Rub some wheat flour glue on the twine

Start wrapping the vertical spikes on the shell. If
you break the string while spiking, just tie the
broken ends together and keep on truckin.

For this shell, I use 16 vertical spikes with 28 lb
string. This is done with something called 'off center'
spiking - you can see how it works in the picture.
You use the fuse as the pivot to change directions
with the string wrap

Once the vertical spikes are done, roll the shell on its
side and do the horizontal wraps.  For this shell, I like
about 3/8" between the horizontal wraps. Don't be
afraid to pull the strings tight!  The shell should be rock
hard when you are done spiking. If it is soft, you need
to re-spike it.

Once wrapped, slather a bit of paste over the strings
and let it dry.

If you put the bottom shot on the outside, then it
should look something like this

Click on the above image and see our rocket fly.  Not
perfect but heck... it was fun! This video was changed a
few times while developing the shell.  The shell in this
video is the 'outside bottom shot' one seen in the picture
above. The bottom shot itself was whistle (I had some to
get rid of)