Whistle Rocket

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

Sodium Salicylate

Red Iron Oxide

Vaseline

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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