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Article by Paul Nortness

A few months ago, I began my journey into mid powered model rocketry. I had begun documenting the build process of the Estes Ventris, but as I worked out I found that the construction of a mid powered rocket is not that different from building a low powered rocket. With the exception of more robust materials and the use of epoxy in places to strengthen the bond, construction is pretty much the same. As we have covered model rocket building in previous articles I felt covering it again would be a waste of time. Instead I will discuss some of the new equipment needed to support the launch and recovery of a mid powered rocket.

First off, I need to upgrade my launch controller. While the Mighty D controller that I am using is a 12 volt launch controller, it only has 20 feet of cable. According to the National Association of Rocketry safety rules, I will need at least 30 feet of distance to safely launch an E through G powered rocket. Luckily, Balsa Machining Services created the Mighty D to be expandable. The cable uses a standard RCA plug. I was able to go to my local Radio Shack and purchase a 12 foot cable with RCA plugs for $3.99, and a female to female coupler that will allow me to plug in the launch button. A very handy and easy solution! Another thing I did was snipped off the smaller alligator clips and replaced them with more robust solder free clips. These clips have a screw that you wrap the wire around and tighten. This allows me to easily replace the clips if they get trashed from being directly under the motor when it fires. I purchased a handful of these clips at my local electronic supply store for $2.00.

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Next, I need a launch pad that can handle the extra weight and size of a large rocket. My Ventris rocket weighs 15 ounces without motor, whereas a typical low powered rocket will run anywhere from 1 to 5 ounces. There are numerous mid powered launch pads available commercially, but when have you known me to buy something commercially when I can make my own? So, I set out to find ideas to build my own. I already had a 1/4 inch launch rod, but I needed something to mount it on. In the end, the simplest solution turned out to be the best. I went to my local hardware store and purchased a 5 foot length of 1.5 foot PVC pipe and a three way coupler. This cost me a total of $4.87. On my way home, I stopped at a shop that sells flower pots. There I found a ceramic plate used to place large flower pots on so the water doesn't leak out. The one I settled on is 8 inches in diameter and cost me $1.50. This will serve as my blast plate. Once home, I cut the PVC pipe into three 1.5 foot sections. These sections will be the legs. Then, I get a 1/4 inch drill bit and slowly drill a hole through the center of the three way coupler. I need to make sure the hole goes through both sides so I can insert the rod. Satisfied with the fit, I then fasten the rod into place using epoxy. I use a specially formulated epoxy that bonds to metal. Finally, I find the center of the ceramic plate and very slowly drill a hole through it. This will allow me to slide the blast plate down the rod to the base. The 1.5 foot sections of pipe are then inserted into the coupler to create a tripod. The end result is a compact, but sturdy tripod launch pad that breaks down easily and quickly and can be transported in a backpack.

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In order to safely recover the Ventris, I chose to add Kevlar rope to the rocket. This is the only portion of the building process that I changed. Because mid powered rockets are much larger than their low power cousins, the ejection charges are much more "energetic". The extra force and hot gas can be murder on the standard elastic shock cord. Kevlar has a much higher heat resistance. The downside to Kevlar is it is much thinner and can greatly increase your chances of getting a "zipper". Zippers occur when a nose cone pops while the rocket is still travelling at a high rate of speed. The nose cone then gets dragged by the rocket and the shock cord rips through the top of the body tube. Elastic cords tend not to do this because they are springy and wide.

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So I decide to meet half way. I tie one end of the Kevlar around the motor mount just behind the last centering ring. This will anchor the cord. Then, I measure out just enough Kevlar to reach the end of the main body tube and tie the Kevlar and elastic shock cord together. To ensure the knot stays, I drop some CA glue onto the knot. Now, I have the heat resistance of the Kevlar while providing the spring of a regular shock cord.

In my next article, I will document my first mid power launch! Until then, have fun and happy flying!

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