Eggtimer Rocketry Eggtimer User Manual page 40

Two AAA batteries (or AAAA if you can find them) work very well, however the holders can be
a problem because they are usually spring loaded and it is possible for the batteries to lose
contact in flight. We do not recommend using AAA batteries unless the body tube is large
enough to mount them horizontally, so that the G forces of launch and deceleration cannot make
the batteries shift and lose contact. Even then, we recommend that after you install the batteries
in the holder you wedge a thin piece of wood (scrap from the sled is good) in between the spring
loops to take up any slack. You should ONLY use completely enclosed holders, such as Radio
Shack #270-414, and you should use at least TWO zip ties to mount it: one vertically and one
horizontally. Alternatively, you can also use a dab of epoxy to mount the battery holder, but you
still need to have at least one zip tie to keep the battery door from popping open during flight.
Multiple Deployment Hints
1) ALWAYS weigh your completed rocket with all batteries and the motor that you plan on
using, determine the CG, and run it through a program like RocSim or Open Rocket to check the
stability and predicted flight path of your rocket. Google to see if there is a RocSim file for your
rocket, and/or modify it accordingly if you have made changes or have a custom build.
Yes, this IS rocket science...
2) Similarly, use RocSim/Open Rocket to determine the best delay time for the drogue
deployment. Ideally, it should be within +/- one second of predicted apogee, when your rocket is
moving at a relatively slow speed (< 20 ft/sec is ideal). If you are going to use the motor's
ejection charge to fire the drogue, and the motor has a fixed delay time (i.e. one that you cannot
"drill out" to adjust), it might be a good idea to do this before you buy your motors, so you can
purchase them with the correct delay time. Of course, if you're using the Eggtimer for drogue
deployment you don't need to worry about this.
3) Prior to flight, perform a Ground Test procedure with a test load and your ignition battery.
We recommend using miniature Christmas Tree lights, they work very well for testing with
either 9v or 12v A23 batteries. You should perform at least one Ground Test with the
pyrotechnic charge that you are going to be using to be absolutely sure that it will fire with the
battery that you choose, and that it's strong enough to kick out your parachute. This obviously
should be done OUTDOORS in a well-ventilated area. There may be local regulations that
prohibit pyro testing where you may be living... check with your local fire marshal if you aren't
sure. If you can't test in your backyard, check with your local club to see where you can test;
they may have an area set aside during launches for such testing.
4) Batteries can be heavy, especially 9v "transistor" batteries. For best flight performance, use
the lightest batteries that you can get by with. 3.7v single-cell Li-Po batteries work great for
both the flight computer power and for deployment with low-current igniters like a Quest Q2G2,
but you need to charge them properly and solder 24 gauge tinned pigtails to the battery leads,
since most of them are very fine and can easily break under flight stress. After you solder the
pigtails, fold the pigtails over and wrap the battery with a couple of wraps of electrical tape to
strain-relieve the delicate battery leads.
- 40 -