An Easy Launch Light for the EZI-65

by Roger Deran, LUNAR #569

Building a night-launch lighting system for my H.P. rocket turned out to be more complex than I at first thought, but I came up with some ideas and tricks to make it easier.

First, a light has to be very bright in order to be visible from the higher altitudes commonly reached by H.P. rockets. An EZI-65 on an H128-4W tops out at 1750ft. It is quite easy to see in daylight because of its 98mm diameter, but in the dark I expect it basically to vanish. Of course 1750ft is only a starting point for many of us, so you might want your lights to be extra bright too.

Light Emitting Diodes were just not bright enough for me, so I went with the 12V all-in-one incandescent units from Radio Shack. They come in red, green, or blue and have a hemispherical head that shines light in a wide pattern. I put four around the payload section wired in parallel. You can see at least two at all times, sometimes three.

These lights require a 1/2" hole. I just used a 1/2" reamer to punch and widen the holes. This of course created a huge burr inside, but by removing the handle from a 1/4" rat-tail file, you can get the handle end of the file inside the payload section and it will cut as you pull it outwards. This removes the burr quite quickly and results in a clean hole. The lights just pop in and hold themselves in place without washers or nuts. I think a 1/2" threaded light would tend to flatten the tube pretty drastically.

Another consideration is the battery. I wanted to provide plenty of current to the bulbs to keep them bright, so I started out with an eight-cell AAA pack. This was much too heavy, but a 9V transistor-radio type battery seemed to provide about 2/3 the overall brightness with a small fraction of the weight. Just be sure to use an alkaline battery rather than the old-fashioned carbon-zinc kind (as if you can even find them anymore or know what I am talking about.) A Ni-Cd battery has a low internal resistance also and will work great. Don't forget to get a fresh, charged one. The loaded output voltage of an alkaline battery decreases aproximately linearly as it discharges, so freshness is very important. A Ni-Cd tends to lose its voltage all at once right at the end, but a full charge is still a good idea. The light might have to run for a while both while sitting on the pad and during the possibly long search for the rocket once it is back on the ground.

The mounting of the battery was a head scratcher for a while, but a simple trick solved the problem.

At first, I wanted to wrap the battery in a bunch of packaging bubble wrap so that it would fit tightly into the payload section, but it wouldn't fit well with the lights in the way. Also, I would have to fill the entire payload section with bubble wrap, and even then the battery would be somewhat free to slide fore and aft under heavy accelerations, possibly damaging the lights.

A better solution was to attach the battery to a fixed support. A section of 1/4" threaded rod was the answer. The battery can simply be taped to the rod with duct tape, and the rod mounted to the payload bulkhead using wing or hex nuts with lock washers. You can use two nuts inside the payload section locked to each other, and one wing nut outside. I wouldn't rely on the threads holding in the wood alone. The 1/4" hole for the rod has to be drilled off-center to avoid the shock cord screw eye, but the battery still comes out near the centerline of the rocket because it is offset from the centerline of the rod.

The pigtails on the lights were a bit short, so I extended them with 22 gauge hookup wire, soldering all joints and insulating with heat-shrink tubing. This tubing is really great. You just cut off a bit, slide over the joint, and heat with a cigarette lighter. Don't use a match - it will work but will blacken the tube. Keep the flame some distance away and let it shrink slowly or the tubing can melt. The resulting joint is strong, reliable, and safe. Don't forget that an unintentional short circuit can result in a very hot, dead battery. By the way, black 'electricians tape' is never used by electricians. It unwraps by itself gradually, leaving a sticky, dangerous mess, and is actually harder to use than heat-shrink.

Information date: May 26, 1999 lk