NEVER MIND THE DOG, BEWARE OF THE MISSILE!
NEVER MIND THE DOG, BEWARE OF THE MISSILE!
by Dave "Fritz" Katz #2349
All photos by the author except as noted.
Let me start by saying that I live in a pretty safe neighborhood, so when my next door neighbor drove home in a new HUMMER, I didn't think much of it. However, when my other neighbor pulled into her driveway in an Armored Personnel Carrier, I began to worry. It was then that I decided that I needed a home security system that would be effective against intruders and armored vehicles!
From my study of the available military hardware I determined that a Hellfire missile met my specifications for home defense. Unfortunately, I was unable to purchase one, so I decide to design and build my own improved version!
My missile, the Fritzfire II, would have the latest in target acquisition systems, including faux millimeter wave, IR, laser and video seekers. Finding a suitable transparent housing for the seekers was a major challenge, for without it the project could not go forward. Fortunately, I was able to obtain a surplus clear dome from an Army Copperhead Laser Guided Projectile. The configuration of this dome, with the addition of a custom-made shroud, would mate perfectly on a 5.5 inch diameter airframe tube.
I proceeded to order a 5 foot section of 5.5 inch tube, as well as a 54mm motor tube, centering rings and a bulkhead assembly from Cluster R. For a nominal fee, Cluster R cut the rear fin slots to my specifications.
The design of the Fritzfire II front shroud and rear boattail required experimentation with new construction techniques (well, new to me anyway!). The first thing I did was download a nice little program called shroud.ps from the rec.models.rockets archives at sunsite.unc.edu. This program allows you to enter the parameters for any size shroud, and then it prints out a perfect shroud template on a laser printer. For the boattail, I asked the computer to produce a shroud template that was 5.5 inches in diameter at one end, 4 inches in diameter at the other end, and 3 inches long. I transferred the template to poster board, cut it out and assembled it. I then took a 5.5 inch centering ring and drilled two 1/2 inch fill holes on either side of the motor tube, and epoxyed the ring 3 inches from the bottom of the tube.
The bottom of the shroud was formed from 1/4 inch thick plywood which was cut to a four inch diameter using a fly cutter and a drill press. Two "T" nuts were mounted on opposite side of the motor tube hole, which would be used in conjunction with a custom-made aluminum motor retention plate. The ring was them attached flush with the bottom of the motor tube.
Once the shroud was epoxyed to the two rings, the fun really started. I purchased a spray can of "Great Stuff", which is a brand of expanding foam insulation and sealant. Next, I injected foam into the shroud through one of the 1/2 inch fill holes. Although I only filled the shroud cavity about half full, the expanding foam continued to come out of both holes for quite some time. However, once the foam had cured and the excess foam had been removed, I was left with a very strong, solid, and lightweight boattail assembly. This process was repeated on the front shroud for the seeker assembly and again produced excellent results.
The Fritzfire II used conventional construction techniques for the remainder of its assembly. Fins were cut from 1/8 inch plywood, and the rear fins were mounted through the wall and epoxyed to the motor tube. The front fins were also mounted through the wall, but the short fin tabs were completely covered and filleted with epoxy, so as to minimize their protrusion into the inside of the body tube. This prevented the parachutes from becoming hung up on the front fin tabs. Finally, all shroud seams and airframe attachment joints were filled with Elmer's Fill and Finish.
With the missile almost completed it was time to confirm a suspicion I had held while assembling it. I felt that the combination of a boattail and the front fins might make the location of the Center of Pressure (CP) be too far forward and could cause the Fritzfire II to be unstable. To check my premise, I downloaded an evaluation copy of a program called "Center of Pressure Calculator" by Austin Aerospace. This program, based on the Barrowman Equations, allows you to input data about your rocket and then calculates the CP. It has a graphical interface and supports many rocket design parameters such as reducers, multiple fin sets, and boattails.
Since I had already determined the Center of Gravity (CG) for the missile (with motor and recovery system installed) I was able to enter that data into the program as well. The program then computed the CP, compared it to the CG, and reported that the missile was not just slightly unstable, but was completely unstable!
There weren't many things that I could do to move the CG forward of the CP because the fin size, fin locations, and length of the missile were already established. However, I could add weight to the nose, so I obtained some 1/8 inch thick lead sheet from my friend Tim. Tim told me that this type of lead sheet was used to line the bottom of shower stalls, before the tiles were attached.
I proceeded to cut a number of 5 inch diameter disks from the lead sheet and attached them to the bottom of the seeker assembly. It took 3 disks with a combined weight of about two and a half pounds to bring the CG safely in front of the CP. With the stability problem behind me, I was now ready to complete the missile.
After a few coats of white primer, I painted the entire missile a lovely shade of MIL-SPEC olive drab. Then I applied the yellow markings using trim monokote. Next, on a lunch break, I went to the shipping department where I work, and asked them if they had a stencil cutting machine. They said yes, and after I explained what I wanted to do, they even showed me how to use it. I made two stencils, one with the missile's name and the other with the missile's designation and serial number. Later that week I applied the stencils using white spray paint and then finished off the Fritzfire II with a clear top coat.
The last weekend of April '95 I discretely wrapped the missile in a blanket and drove it down to Culpeper, VA for the Spring '95 Battle Park launch. For the missile's test flight I decided to forgo the shaped charge warhead and instead replaced it with a recovery system! The recovery system consisted of three olive green parachutes, each four feet in diameter. These chutes were military surplus removed from mortar fired aerial flares. I also used an Adept timer as backup to the motor's ejection charge.
I chose a Vulcan I-500 with a ten second delay to power the test flight, and the loaded missile weighed in at just over eleven pounds. At ignition, the missile vaulted skyward, and after a nominal flight the motor's ejection charge expelled the parachutes (thanks Scott!). The Fritzfire II looked majestic floating down under the three canopies, and landed on the west side of the corn field without a scratch.
The success of the test flight has encouraged me to build a shipping container so that I can take the Fritzfire II missile on the plane with me to LDRS XIV. Meanwhile, the missile will take its place as part of my home security system, and you know... I feel safer already!