Build a 6#
Rocket

Article originally published at Passfire.com

Figure 1

Motor cross-section showing the nozzeless design.

Figure 2

Six pound ( 1.5” ID ) stinger tooling. Note short spindle length at
bottom and bulkhead/passfire rammer on left.

Figure 3

Homemade pressing sleeve made from a piece of slotted PVC and pipe
clamps. Note close spacing of hose clamps and intentional twist offset of
clamp screws.

Build a 6#
whistle rocket

This article deals with the construction of a quick, easy, reliable
rocket that can be scaled up or down to almost any size. The rocket shown
is a 1.5" ID rocket, but smaller rockets are constructed in the same way,
at lower pressing forces. These rockets are incredibly loud and quick
flying, and capable of lifting quite a bit of payload. The 1.5" ID version
shown will lift a 6" ball shell no problem – in fact, the shell may be too
high up for your taste when it displays!

Required Tools:

Since whistle mix must not be rammed, a press is necessary for loading
the rocket fuel into the casing. Due to the sensitive nature of whistle
mix, it is strongly advised that a lexan or thick wood blast shield be
placed between the press and the operator. For very small versions of this
rocket, 1/2” ID or smaller, a small arbor press should work just fine. A
1-ton arbor press such as the ones available from harbor freight for
around $40 is just right for the job. As the desired ID of the rockets
increases, so does the amount of pressure needed to consolidate the
propellant. A sturdy H-frame press, around 12-ton capacity, is perfect for
the job. While an entire article could be written on presses, suffice it
to say that basic presses of this type can be had or made for quite
cheaply (see Figure 6).

Due to the high energy output of whistle rocket propellant, short-cored
rocket tooling is required for this rocket. The tooling shown in Figure 2
is commonly sold as “stinger missile” tooling and is ideal for building
whistle rockets as well. This tooling is commonly available from hobby
suppliers, and the short spindle length makes it about as cheap as rocket
tooling gets (short spindles require fewer ramming drifts).

The only other specialty tool required for this rocket is a pressing
sleeve to fit around the outside of the tube. Hose clamps are necessary if
the sleeve is made of PVC. Pressing sleeves are available from Rich
Wolter, or can be made quite easily from a length of PVC pipe. A good
sleeve will fit snugly all around once it is clamped into place. Gaps will
create bulges, tears, or weak spots in the finished motor, so it is
important that the sleeve stays round. The sleeve shown accommodates 1.5”
ID, 2” OD tubes, and was made from a piece of 2” PVC which was clamped and
slotted with a dremel tool and simple jig. It’s important that the slot is
wide enough to remain open ever

Figure 4

The whistle mix has a strong pink tint due to the red iron oxide used.
The propellant is granulated into weak granuals that crush very easily
under low pressure.

so slightly, or
perhaps be just barely closed after is has been tightened down, since it’s
purpose is to help solidify the tube during pressing.

The Casing:

The tube required for this rocket measures 5" long with a 1.5" I.D. and
2" O.D. Finding high quality rocket tubes can often be a daunting task
among all the recycled tubes that are so prevalent these days. The tubes I
use are special ordered from New England Paper Tube Co. and are quite
strong. In fact, some destructive testing I did at the 2003 PGI convention
showed that they can take 8000 pounds of force without rupturing!

Paper tube companies will often make exactly what you want, down to
thousandths of an inch, in any grade paper, including virgin Kraft and even
Phenolic-impregnated, but there's always a minimum order size that is usually
too large for the average hobbyist to afford. Skylighter sells a "6# rocket
tube" that is the correct size for this project and should work just fine, but
they are nowhere near the quality of the New England tubes. Vendors at the PGI
conventions often have good quality tubes. Some people even roll their own
rocket tubes from poster board paper or manila file folder paper, which will
make surprisingly strong tubes if done correctly.

The Propellant:

Whistle mix makes an excellent propellant that creates less of a mess than
black powder and has a much higher burn rate. The formula for the whistle
propellant used here is shown below. The Vaseline serves to reduce friction and
impact sensitivity.

NOTE: Whistle mix should be treated with care, as it is nearly as explosive
as flash powder and can be ignited by impact and friction.

Potassium Perchlorate

76

Sodium Salicylate

23

Red Iron Oxide

1

Vaseline

3

There are a multitude of techniques for integrating these ingredients into a
uniform mixture. The fuel, oxidizer, and iron oxide catalyst must be integrated
thoroughly with one another, and the Vaseline must be integrated uniformly as
well. A very effective technique detailed by Dan McMurray can be found online at
passfire.com .
This technique creates a very soft, powdery mix which consolidates nicely under
relatively low pressure. Other techniques that are commonly used may create hard
granules of whistle mix as an end product. These granules are still completely
usable, but require somewhat more pressure to consolidate into a uniform
propellant grain.

Here’s a quick rundown on mixing 100 grams of whistle mix using the technique
mentioned previously. First of all, some preparations must be made. Spraying
yourself and your clean work area with anti-static spray is a Very Good Idea.
Clothing should be cotton or non-synthetic. Metal objects should be removed from
your pockets. Long sleeves and a face shield are also advised. Due to the
solvent involved, working in an open area is also important.

First, 23 grams of Sodium Salicylate and 1 gram of Red Iron oxide are
screened together through a coarse screen until the mixture is a uniform pink
color. 3 grams of Vaseline are taken outside or away from the shop and melted in
a heat and solvent-proof container, such as a tin can, over a hot-plate or
flame. Around 23 grams of naphtha are added to the Vaseline and the mixture is
stirred. The Vaseline should be fully dissolved in the naphtha by this time. The
vaseline/naptha mixture and the salicylate/iron oxide mixture is combined in a
stainless steel bowl and stirred until uniform. 76 grams of screened Potassium
Perchlorate are then added to this wet mix and integrated very thoroughly, with
rubber-gloved hands, until the mix appears uniform. This final mixing of the
propellant is a very important step which should not be hurried through. For
example, it generally takes a little over five minutes of active stirring,
squeezing and scraping to thoroughly mix a full kilogram of
propellant.

An advantage of propellant mixed in this way is that it dries relatively
quickly. For drying, the mix can be spread out on paper and placed in light
shade. Or, the stainless steel bowl of propellant can be placed into a container
of water that has been brought to a boil and then removed from the heat source.
Obviously, the heat source should be nowhere near the solvent-wetted whistle mix
– to do this would be to tempt fate. While the mix is less sensitive to friction
in this wet state, the solvent makes it all the more flammable – even from
remote sources of flame if the vapors are allowed to build up. Exercise caution
and work outside.

Stir the drying mix every ten minutes or so, scraping the mix from the edges
of the bowl, or spreading it out further on the paper. The heat from the
container of water serves to drive the solvent off, as well as moisture present
in the solvent. Keep breaking up any lumps that form. If all goes well, the
propellant will be nearly dry by the time the water has cooled. If further
drying in part-shade is an option, dry fuel should be ready to use within
another hour. If the sun isn’t out, simply heating an additional batch of water
and repeating the drying process should do the trick. The propellant will still
smell slightly like solvent for a period of time, but is usable. If the
propellant is going to be pressed at very high pressures, or if the climate is
very humid, it may help to dry the fuel for an additional day or two in a closed
container in the presence of desiccant. Usually the propellant is ready to work
with after the water-drying stage is complete.

Pressing the Rocket

Aside from the amazing sound and power of these rockets, perhaps their best
attribute is their straightforward, quick construction. At the most basic level,
a nice rocket can be pressed with whistle mix only. It serves nicely as both
propellant and a sturdy bulkhead. This rocket is also "nozzleless", as the burn
area is very large.

Figure 5

simple cardboard or brass scoop helps with keeping pressing consistent
from rocket to rocket.

Figure 6

Homemade rocket press using threaded and steel channel. The blast
shield is a piece of 1.25" acrylic sandwiched between two thinner pieces
of lexan

The first step is
to secure the tube in a pressing sleeve. The tubes should be about 5" in
length for the rocket shown, which is 1.5" on the inside diameter. If hose
clamps are used to secure the sleeve, they should be spaced about 1/4" or
so apart, and each clamp rotated slightly so that the flat spots below
each clamp screw do not line up. The reason for this twist is to prevent
any flaws or bulges that develop in the tube from lining up and causing a
failure.

After the tube is secure, the spindle is inserted into what will become
the nozzle end of the tube. If the tubes have been cut to length by hand,
be sure that they’re square, or use the factory end. Many a bent spindle
or stuck rammer could probably be traced back to an out-of-square tube!
Use a funnel and add an increment of fuel, which equals approximately one
tube ID in thickness. For a 1/2" ID tube, a finished, pressed increment
should be around 1/2" thick, and a 1" ID tube uses 1" thick increments. It
saves a lot of time in the end to make custom scoops for each type of
rocket tooling out of a piece of cardboard or brass tube.

This first increment is poured into the top of the tube and the setup
is then placed on a press. The hollow rammer is used press the propellant
into a solid grain. A good starting point for a 1.5" ID rocket, using the
"fluffy" whistle mix described previously, is a loading pressure of around
6000 PSI. Articles on loading pressure have appeared in past PGI bulletins
and on Passfire.com. Alternatively, a "P2F" converter from Hal Bentley
will do direct readouts directly in terms of force-pounds. The second
increment of propellant is added and consolidated with the hollow rammer,
and this process is repeated until the top of the spindle is covered by
about 1/2 of one tube ID worth of whistle mix.

The charge that is located between the end of the cavity and the clay
plug is known as the "delay" composition. A simple and nice way to finish
this rocket is to add spherical titanium to the whistle mix used for the
delay charge. About 1/2 teaspoon of spherical titanium is added to the
last increment of whistle mix, and the mixture is gently stirred together
before pressing it into the tube. This delay

Figure 7

Clay nozzle mix can be seen in a loose state – the rammer shown will
leave a passfire hole through the finished clay bulkhead to the delay
fuel.

Figure 8

Off-center passfire hole gives better timing on header effects.

increment is then
pressed using the solid rammer. Note that adding titanium to the entire
charge of whistle mix will result in damage to your spindle, as the hard
titanium will grind into the softer aluminum tooling and damage the
surface. However, adding titanium to only the delay charge will still
result in bright white sparks for the majority of the rockets travel,
since the delay charge will be reached almost immediately as the fire
propagates from the tip of the spindle cavity. Thus using titanium in only
the delay charge not only conserves use of this expensive metal powder, it
saves your tooling while still giving the effect of a rocket with a
titanium tail for the full duration of flight.

Note that this rocket is using whistle mix as a propellant, partial
bulkhead and delay. Whistle mix has pretty good structural strength
compared to other fuels, and in most cases will hold the internal pressure
of the rocket. Thus the end of the fuel grain also acts as a bulkhead to
keep the powder from blowing out the top of the tube. A small clay plug is
added on top of this to provide additional bulkhead strength. The plug
need only be about 1/2" thick..

If an effect heading is to be added above the engine, then a small
passfire hole will need to run through the clay plug. Most rocket tooling
kits come with a final drift that will make this hole for you. However,
these drifts will locate the hole at the center of the plug which is not
the ideal location.

When cavity rockets such as this one use a centered passfire hole to ignite
the heading, the heading will appear to burst too soon as the rocket is still
moving upwards. This is due to the conical shape of the internal flame front
propagation, which will reach the top of the tube before all the composition at
the sides has been consumed. The longer the internal cavity is, the more
noticeable this problem will be. For a short core rocket such as this one, you
may be able to get away with a centered passfire hole without any problems.

If a bulkhead rammer for forming passfire holes isn’t available, a drill bit
can be twisted by hand to create an opening through the clay near the wall of
the tube. NEVER USE POWER TOOLS TO DRILL INTO WHISTLE MIX, especially in the
presence of a hard metal such as titanium! This isn’t black powder – please
don’t tempt fate! When hand twisting drill bits, it is easier if the bit is
installed into a drill chuck so that you have something to hold onto while
turning it. The ideal way is to use a drill press (with the power off of course)
to hold the bit so that it can be pressed down into the clay using one hand
while manually rotating the chuck with the other hand. The whistle mix delay
should be clearly visible through

Figure 9

Taping the stabilizing stick onto the engine.

Figure 10

Fusing the engine-- note there is not contact with the fuse and nothing
obstructs the bottom of the engine.

Figure 11

Finished rocket with a multi-break canister heading.

the passfire
hole, but the hole should stop just as soon as it reaches past the clay
plug.

Finishing the Rocket

Depending on the size of the motor, this type of rocket can lift a
pretty good amount of weight. A 7/8” ID rocket should lift several ounces,
and I’ve seen 1.5” ID rockets lift 6” ball shells – a little too high for
some, but beautiful anyway. Small salutes, or pre-manufactured ball shells
are also good options.

Sticks are an often-debated item in rocket circles. Some make their
sticks short and fat, others use sticks that are long and thin. One school
of thought states that the rocket should balance on your finger at a point
just behind the motor casing. However, I believe this method isn't so
accurate. What is more important is to have lots of relatively lightweight
surface area behind the bulk of the rocket's weight. It's the drag, not
the weight, that ultimately stabilizes the rocket. Picture a 3/4"
lightweight birch stick versus a piece of 1/16" steel rod of the same
weight. The steel rod will be an ineffective stabilizer even though it can
be made to balance just behind the motor casing.

This rocket can get away with a shorter stick since it takes off so
quickly. A 1.5” ID rocket will fly nice and straight using only a 36” long
stick that is about 3/8” square. I rip sticks from poplar planks, and
inspect them for straightness before use. Any curve in the stick should
point toward underneath the rocket motor rather than away from it.

The stick is held firmly against the rocket motor with one hand, and a
piece of fiberglass tape is applied to the base of the motor and stretched
tightly (see Figure 9). At least two turns of tape should be used. The
stick is then further secured using another piece of tape near the top of
the motor. Some use hot glue in addition to the tape, but as long as the
tape is stretched tightly as it’s being applied then this method seems to
be bulletproof.

Figure 12

Finished rocket with ball shell heading.

Fuses
are attached by using a piece of masking tape to hold them firmly against
the stick near the base of the motor. Several layers of tape are necessary
to keep the tape from burning through before the motor ignites. The fuse
should be positioned so that it just barely touches the bottom edge of the
propellant – inserting the fuse into the core will likely cause the rocket
to pop before it has a chance to take off! It’s helpful to prime the end
of the fuse with meal slurry so that the propellant is guaranteed to take
fire. Applying tape across the end of the tube or using any kind of nosing
paper that obstructs the end of the rocket motor will likely result in a
"salute on a stick," as whistle rockets are pretty sensitive to
obstructions during firing.

A rocket of this size gives you a lot of options for header effects.
Figure 11 shows a two break 3" canister shell heading, while Figure 12
shows how a 6" ball shell would be attached. When making shells for rocket
headings, time fuse or spolettes are not required for the first break.
Instead, a hollow tube is used in place of the normal spolette or time
fuse when building the shell. When the shell is completed, three or four
sticks of black match are inserted into this tube to give an instant
passfire from the rocket motor to the shell burst without creating a
delay. Be sure that the passfire tube does not protrude too far from the
finished shell so that it will seat properly atop the rocket motor.

Because accidental ignition of the header shell from stray sparks would
result in instant detonation, it is important to make sure the shell if
firmly attached to the rocket motor and sealed around the connection joint
with hot glue. Figure 12 shows how strands of fiber tape are used to hold
a ball shell onto the motor. The shell is attached to the motor first
before securing the stick. This way the vertical bands of tape coming down
from the ball shell will be secured by the horizontal bands of tape used
to hold the top part of the stick to the motor.

That’s it! Make sure your launch tube is solidly mounted and long enough for
the stick, and pointing nearly straight up. Don’t ever try launching one of
these out of a bottle or something that could possibly tip over. Anything that
isn’t sturdy will be immediately knocked over when the propellant ignites.

Special thanks to Kyle Kepley of Passfire for additional help with this
article. Please visit Kyle’s website, Passfire.com, for more great articles,
tips, pyrotechnic formulas, forums, and calculators!

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