softlaun.txt

           Additional instructions for Soft Drink Can Launcher

This file is intended to accompany the file softlaun.gif, which
graphically details the device described herein.

Overview:
~~~~~~~~~
This is a very short "mortar" constructed for the express purpose of
launching soft drink cans straight up into the air.  It is crude, prone to
rupture during use, and consumes rather inordinate amounts of Pyrodex. 
But it sure is fun, and it makes an incredible amount of noise when it
goes off!

Construction Materials:
~~~~~~~~~~~~~~~~~~~~~~~
- Large coffee can, empty, no lid
- Concrete mix, admixture, water, etc. (driveway repair mix okay)
- Steel reinforcing wire
- Full soft drink can (non-carbonated, if possible)
- Paper or thin flexible plastic sheeting
- Tape
- Vaseline
- Drill and drill bit to fit fuze wire

Construction Procedures:
~~~~~~~~~~~~~~~~~~~~~~~~
Tape either paper or plastic around the soft drink can, enclosing the
bottom of it as smoothly and consistently as possible.  Try to avoid any
kinds of seams, and make a fairly tight fit with whatever size can you
intend to launch the most.  This wrapped-up can will be used to form the
inside of the barrel (I refer to this as the barrel mandrel), so its
diameter is important.  Some cans are slightly different diameters, so
make sure you choose one that is similar to the types of cans you intend
to launch.  When wrapping the can, you may leave the top part open, but
the bottom must be sealed up smoothly.

When you have smoothly wrapped the full soft drink can in paper or
plastic, coat the sides of it with a smooth, thin layer of vaseline.  This
will ensure easy removal after the concrete has set.

Construct a basket out of the steel wire.  This basket will sit inside the
coffee can and be encased in the concrete, providing tensile
reinforcement.  Concrete is strong for compressive uses, but this is a
tensile use, and the addition of a reinforcing basket will greatly extend
the life and maximum loads of this mortar.

If you are unsure how close to space the wires of this basket, then
experiment, or ask someone who has experience with reinforcing concrete. 
As a rule of thumb, the more reinforcing wire you have, the stronger it
will be, but only to a point.  I would recommend a mesh of no closer than
1/2", and optimally I would recommend spacing the wires 1" (one inch)
apart from each other.

The inner portion of the basket must be large enough to handle the
displacement of the soft drink can barrel.  Test this by putting the
reinforcing basket into the coffee can, then holding the barrel mandrel
approximately where it will go.  Adjust the shape of the reinforcing
basket to accommodate the barrel mandrel.

When you are sure of the shape of your reinforcing basket, then tape it in
place inside the coffee can.  This is only to hold it in place while the
concrete is poured.

Mix the concrete as directed.  The stronger concrete you have the better,
but it's still probably going to end up blowing up after some amount of
use.  :)  It is better to err on the side of a little extra moisture in
the concrete mixture, to aid in the removal of air bubbles.

Pour the concrete into the coffee can, allowing it to flow smoothly around
the reinforcing basket.  Do not fill the coffee can yet!  When it is close
to full, press the barrel mandrel into the wet concrete right in the
center.  Seat the mandrel to the required depth for your barrel length. 
Add any more concrete mix you might need to fill the coffee can
completely.

Once everything is poured and the mandrel is straight and true (vertical),
shake the can, even banging it on a hard surface (this is where a non-
carbonated soda can comes in handy!) or rapping on the side of it with
your hands to try and settle the concrete and remove any air bubbles that
might weaken the structure.

After the concrete has completely set (anywhere up to a week or so),
carefully pull the barrel mandrel out and inspect your work.  It may help
to rotate the mandrel as you withdraw it from the concrete.  You should
have a perfectly-shaped shallow mortar made out of reinforced concrete.

If you wish, you may wipe the vaseline out of the barrel.  It is not
necessary to remove the vaseline, as it will burn off within a few
launches.  Before using this mortar, it is important to allow the concrete
to fully set.  Depending on the brand of concrete, this could require up
to a full month!  Don't jump the gun (pun intended), because incompletely
set concrete is far weaker, and will easily explode during launch.

When the concrete has fully set, use the drill bit to drill a flash hole. 
Measure your barrel "depth" carefully and drill straight in from the side,
right into the very bottom part of the combustion chamber.  Choose a drill
bit that is approximately the diameter of the fuzewire you intend to use,
and drill carefully to make sure it goes in straight.  A drill press or
drill jig may be helpful to drill accurately.

Launch Materials:
~~~~~~~~~~~~~~~~~
- Pyrodex
- Green hobby fuze
- Soft drink can
- Some type of load/filler for can
- Match or lighter

Launch Procedures:
~~~~~~~~~~~~~~~~~~
Carefully review the safety notes contained at the bottom of this document
before you attempt a launch.

Choose an area of soft grass or dirt to place the mortar.  It is best not
to place it on a hard surface, since there is nothing to absorb the launch
shock, and this will greatly reduce the useful life of the mortar.  If you
must place it on a hard surface, then place it on a stack of old
newspapers or a telephone book to help cushion the launch shock.

A keen feature of this device is that you will be launching the contents
of a soda can.  When starting out, you may want to just launch an empty
aluminum can, as this is very lightweight and will be a good first test of
your mortar-building skills.  Later you may want to add some water or sand
to the can to increase its weight, and therefore its ultimate range.

When you have selected and filled up your projectile, slip it into the
barrel to ensure a good fit.  Slightly loose is fine (less than 1/10th of
an inch diameter), but make sure it does NOT bind or stick firmly on the
way down to seat against the bottom of the barrel.  When you are satisfied
with the smooth operation of the projectile within the barrel, remove the
projectile from the barrel.

Insert a length of fuze into the fuze hole, pushing out any remains of
burned fuze from a previous launch.  Make sure some significant length of
the fuze reaches into the combustion chamber to ensure positive ignition. 
For fire safety, put a small piece of tape over the fuze right where it
enters the side of the can.

Add a small amount of Pyrodex RS powder into the launch tube.  When I say
"start small," a good amount is a teaspoon or less.  You can always work
up to more if the mortar is holding together well.  The powder should pile
up at the bottom of the barrel, and be touching the fuze wire enough to
ensure positive ignition.

WITHOUT placing the projectile into the barrel yet, position your mortar
at the launching site.  I recommend keeping it nearly vertical, or at
least tipped no more than about 20 degrees from vertical.  If the mortar
falls to one side after launch, it can damage itself.  If the mortar falls
over during fuze burn, there is NO TELLING where that projectile is going
to go sideways!

When you are ready to launch, slip the projectile into the barrel, again
testing to make sure it doesn't bind or scrape too badly on the way in. 
Light the fuze and go and run really, really far away and hide behind a
LARGE SOLID OBJECT!  Boulders are great places to hide behind, but make
sure the projectile doesn't come down on your head afterwards.  :)

To reuse this mortar, remove any unburnt or partially burnt fuze and/or
gunpowder.  Make sure no embers remain.  It is usually not necessary to
clean the mortar between launches, unless enough grease builds up that it
becomes difficult to insert the fuze or load the powder.

BEFORE re-use, carefully inspect the mortar, looking particularly at the
concrete for signs of structural failures, cracks, chips, bulges, etc. 
If the mortar becomes cracked, THROW IT AWAY!  DO NOT CONTINUE USING A
CRACKED MORTAR.  THE NEXT TIME IT WILL MOST LIKELY EXPLODE!

Theory of Operation:
~~~~~~~~~~~~~~~~~~~~
The ignition of the gunpowder will generate gas pressure inside the
combustion chamber, and hopefully the weak link will be the soft drink
can, which should go skyrocketing out of the mortar.  If you load too much
powder or weight into the can, you may rupture the mortar.  This brings me
to Load Notes.

Load Notes:
~~~~~~~~~~~
The more powder you put in the mortar, the more total expanded gas might
be generated during ignition.  I say "might" because if you choose a very
light projectile (an empty soda can), then it will leave the barrel long
before the powder has burned fully, thereby relieving pressure on the
mortar.

Conversely, if you choose too heavy a projectile, it will not have left by
the time the powder fully burns, and depending on the amount of powder and
gasses generated, it could exceed the structural strength of the mortar.

Remember that every time you make the projectile heavier, that means when
the powder burns it has to press a bit harder against the projectile to
get it to move.  It also means that the projectile will tend to stay in
the barrel longer, allowing more of the powder to burn under pressure,
generating higher and higher chamber pressures.  As all handloaders and
black powder enthusiasts know, a TINY increase in projectile weight or
powder load can result in a DRASTIC increase in chamber pressure, and
rapidly lead to structural failure.

IF YOU MAKE TOO MUCH PRESSURE INSIDE THIS MORTAR, IT WILL STRUCTURALLY
FAIL AND THEN EXPLODE LIKE A GRENADE!!!!

Lighter loads will extend the useful life of your mortar.  Heavy loads
will stress the concrete more and cause it to crack earlier.  But they
sure are fun!

Safety Notes:
~~~~~~~~~~~~~
This device can KILL!  Do not attempt any of this without careful
supervision from someone who has experience with constructing explosive
devices.  Do not attempt this without proper eye, hearing and hand
protection, in case of accidental detonation or mortar rupture.  Do NOT
smoke while operating this device, as accidental detonation may occur.

When the projectile leaves the barrel mouth, there will be a fairly
impressive blast wave emanating from the device, so be sure to wear
hearing protection.

The coffee can around the concrete MAY aid in the capture and retention of
concrete shrapnel should the mortar fail, but don't count on it.  Big
pieces of high-speed concrete can quickly ruin your day.  :)

As always, have fun, and please report to me your successes, your
failures, your improvements, etc.  My address is <kwd@netcom.com>, and you
may find updates to this project via anonymous FTP:

      ftp.netcom.com
      /pub/kw/kwd/pyro
