Eggtimer Rocketry Eggtimer User Manual page 51

takes a significant amount of time to fire the motor then you will need to SUBTRACT that time
from the Airstart Delay, in order to have the second stage motor fire when you want it to.
5) If you want to use the Airstart Min Velocity check, look at the estimated velocity that should
be attained at second stage ignition in #3, and use a figure of about 60%-80% of that value. This
will more or less ensure that the rocket isn't going horizontal at burnout. Note that the Eggtimer
has no way of knowing what direction your rocket is actually pointing, but this will at least
ensure that the rocket hasn't turned significantly horizontal before the second stage is ignited.
You also need to ground test whatever igniter and battery combination you're going to use to
make sure they will work together, you can use the CHB igniter test for this. (See Appendix B)
It is STRONGLY recommended that you use a good quality switch for the deployment battery,
and that you shunt the igniter until immediately prior to launch, releasing it right before you start
the Eggtimer's flight sequence by releasing the Reset switch. The sequence at the pad should be:
1) Connect your booster igniter
2) Turn on the Eggtimer, with the Reset switch enabled
3) Turn on the Deployment Battery
4) Release the Igniter Shunt (if used)
5) Shortly before the flight, release the Reset switch
6) Assuming that self-test passes and you get the Flight Ready tone, you're ready to launch
If you get an error and you cannot start the flight, the power-down sequence should be:
1) Turn on the Reset Switch to put the Eggtimer on hold
2) Turn off the deployment battery
3) Reconnect the Igniter Shunt (if used)
4) Turn off the Eggtimer battery
5) Disconnect the booster igniter
The igniter is now shunted and the batteries are turned off, so it's impossible for the igniter to be
fired. You may now correct the failed condition, and start over again.
Breakwire Tips
There are several ways that you can implement a breakwire, depending on how big your rocket
is. For a low to medium power rocket, you can use a piece of aluminum foil across two small
screws, and have a string tied to the pad that loops around the middle of the foil. When the
rocket lifts off, the foil will tear off, opening the circuit. For a higher power rocket, you can use
a piece of fine wire, similarly wrapped between two screws and broken by a heavy wire or
Kevlar® line tied to the pad. The important thing is that whatever you are using for a breakwire
can't break or come loose so easily that it might accidentally be broken while the rocket is
simply sitting on the pad, or at the prep table.
You can also use some kind of microswitch with a lever that gets pushed back by the rail or rod,
so that it releases when the rocket leaves the launcher. This has the advantage of being easy to
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