User`s manual

- 14 -
2) Balance your model with the batteries in place.
AAA and NiMH batteries are fairly heavy, as are some of the larger Lithium batteries such as a
CR123. Alkaline 9V batteries are VERY heavy, almost 50 grams (nearly 2 oz!) You should
always weigh your rocket and determine the CG with the intended flight battery in place. We
strongly recommend using a program such as RocSim or Open Rocket to determine the flight
path of your rocket with the intended payload configuration (batteries, sled, etc.) and motor.
If the rocket was designed for egg lofting or multiple deployment, chances are pretty good that
just about any configuration with an Eggtimer isn’t going to cause any stability problems, since
those payload configurations are generally at least as heavy as a fully-loaded Eggtimer sled using
a 3v battery and LiPo battery for deployment. However, if you are building a rocket from
scratch or adding a payload bay onto a kit, particularly with a BT-60 or smaller body tube
diameter, you may end up with an over-stable rocket that wants to weathercock into the wind or
dance around on its tail after clearing the launch rod, due to excessive nose weight. Using a
larger motor or one with a higher average thrust (i.e. E20 instead of E9) to get more speed off the
rod will help that situation somewhat, as will using a longer launch rod. If you do the latter, you
might want to increase its diameter, to keep it from “whipping” as the rocket travels along its
path, or better yet use a rail launcher. If you can, you can also shorten the length of the booster
to bring the CG towards the rear; you should check this carefully with a flight simulator program
first before you start cutting, however!
Programming Your Eggtimer
Before you attempt to mount the Eggtimer in a rocket or fly it, you will need to know how the
programming screens work. The Eggtimer can be programmed from just about any USB-
compatible computer using the included USB-serial data cable. This cable has the interface
circuitry imbedded in the USB connector, so it doesn’t use up any board space or power on the
Eggtimer. It uses a Prolific PL-2303 serial interface chip, which is recognized as a native serial
port on Windows XP SP3, Windows Vista, and Windows 7, Windows 8, as well as by Mac OS
X and many Linux distributions. The interface is a simple text terminal, VT100/ANSI
compatible. No fancy GUI graphics, but it’s fast and easy to use, and free terminal emulator
software is readily available for every OS known to man. We like TeraTerm and PuTTY, but
any emulator will do; however, to download flight data you need to be able to “capture” the data
stream to a file, so make sure your terminal program supports this. If it doesn’t, you can get
around it by doing a Copy of the text that gets output to your screen, and doing a Paste into a text
editor such as Notepad.
If you do not already have a suitable terminal program, or you need the USB-Serial driver for the
cable, go to www.eggtimerrocketry.com, click on the Links tab, and you will be directed to a
suitable download location. Note that we try to support non-Windows operating systems since
the Eggtimer is OS-agnostic, however we’re Windows geeks so we can’t guarantee that we’ve
had a chance to try every possible configuration (especially with Linux!)