posted November 29, 1999 05:47 AM         

Screaming Banshee rocket

Last November AFN published an article, 'Stinger Missiles', describing how one man made small, spin stabilized rockets that make an interesting sound as they lift off, much like a hummer or z-bomb. I used tooling of the same design and similar tubes to make an interesting, whistling rocket. These 'Screaming Banshee' rockets sound something like a gigantic bird of prey screaming its head off when fired, very different than an ordinary large caliber whistling rocket. The formula I used is a variation on the 'Screaming Rocket' formula that appeared in the PGI Bulletin last quarter. I had on hand some potassium benzoate, but none of the sodium salicylate called for in that article, nor did I have a set of whistling rocket tools for my 3/4" tubes. I did have, though, a set of tooling for making the 'Stinger Missiles', which has a spindle 1 1/4" long -- somewhat shorter than called for in the 'Screaming Rocket' article. On a whim I mixed up a small batch of propellant substituting the benzoate, milled to a fine powder for the salicylate, and pressed a few motors. The results astounded me. They were some of the loudest, highest performance whistling rockets I had ever seen.

For reference, the formula is as follows:
Potassium perchlorate (fine powder) 64%
Potassium benzoate 32
Red Iron oxide 1
Petroleum jelly 3
The benzoate I had was obtained in a prilled form, i.e., small round beads about the size of BB's. For these rockets the powder needs to be very fine. My wife found at a garage sale a small food processor for a pittance) that works well for reducing small amounts of the prilled benzoate to fine powder. I simply added the appropriate amount of red iron oxide catalst to the benzoate prior to milling to produce a reddish mixture composed of a ratio of 32:1 benzoate/red iron oxide. Next, I placed the required amount of petroleum jelly in a beaker over a hot plate and gently heated until it melted.

Donning my respirator, I removed the petroleum jelly from the heat, turned off the hot plate. Lacking pure toluene called for in Mr. Vreyens' article, I then added about 5 parts laquer thinner for each part of petroleum jelly by weight, continuously stirring to dissolve the two together. I next added the petroleum jelly solution to the fuel/catalyst mixture and stirred the mixture until homogenous. Then I added the oxidizer (the KCl04) slowly while stirring continuously with a wooden spoon for several minutes until homogenous. The mixture had at this point the consistency of a thick soup and beaker was warm to the touch. If the mixture seems too dry or thick, I can add extra laquer thinner at this stage. I spread the composition out in a layer about 1/2" thick on kraft paper over newspapers to dry overnight. I have found that it is important to be sure the mixture is thoroughly dry from the laquer thinner before pressing the motors. A slightly damp mix can cause some shrinkage of the propellant grain over a period of days or weeks, transforming the rocket into a large ground salute. I have also found the formula and method of preparation as given in Mr. Vreyen's article to be comparable in effect, and perhaps less subject to detonation than the benzoate version of the formula. The only significant difference seems to be that Vreyen's original formulation made with pyro grade sodium salicylate seems to produce a sound that is more of a clear tone, without most of the raspy effect characteristic of the benzoate mixture. It is certainly easier to use the pyro grade sodium salicylate than to hassle with milling the prilled benzoate to a useful powder.

When the composition had dried overnight, I then carefully ran the mixture twice though a 20 mesh sieve and then stored in a paper container so that trace amounts of solvent could evaporate. After several days, the mix was ready to press.

I used custom made tooling originally designed to make spin stabilized, Warhead Launcher type rockets from 3/4" i.d. convolute wound tubes. Interestingly enough, the 3.25" long tubes cut for the WHL rockets worked very well for these 'screaming banshee' rockets, too. The process for making the motors differs mainly in that the screamers use no clay choke (as with mosty, if not all whistling rockets), and in lacking a side vent. These rockets seem to require no significant length of open tubing below the propellant grain to achieve their characteristic wail. Mine contained a propellant grain that ends about 1/8" from the rear end of the motor tube, and several casual observers have assumed erroneously that they were looking at a red clay nozzle, when in fact they were actually seeing the end of the propellant grain. I found a one ton arbor press works satisfactorily for pressing the motors, using all care and caution due the sensitive composition. I pressed the composition in increments of about 1 i.d. per pressing, until reaching about 1 1/2 times the i.d. above the spindle. I finished with my favored garniture, and fasten to a stabilizing stick of appropriate length. I use a length of slow thermolite taped to the stick and just touching the outside of the propellant grain for ignition. Quick match inserted into the orifice, on the other hand, detonated the motor.

Of course, I exercise great care in both making and firing these high energy, 'screaming banshee' rockets. I think they have some potential as a display item, but are more prone to detonation than ordinary black powder type rockets. Certainly, the 'Screaming Banshee' is still subject to the same stabilization and fallout problems as are other stick stabilized rockets. I think you will agree, though, that the combined audible and visual effect of these rockets is unique and noteworthy.

Addendum: 12/92 Since writing this article, I have learned that Service Chemical Co. is now carrying a pyro grade potassium benzoate powder. This stuff, as far as I have been able to ascertain, is superior to the hand milled variety, and one heck of a lot easier to use. Additionally, there has been some suggestion that the tendency of the benzoate mixture to detonate in the larger sizes may be due, at least in part, to the wide variation in particle size of the hand milled benzoate used in the original experiments. This commercially prepared powder may alleviate that problem to some extent. I would appreciate hearing from anyone with experience supporting or detracting from this hypothesis.