User`s manual

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Appendix D - Setting Up Your Eggtimer for Airstarts
Channel B can be used to fire airstart igniters, however with airstarts you have to take extra
safety precautions due to the consequences of an motor being accidentally fired on the pad. The
Eggtimer requires three separate events, in order, for the igniter to be fired in Airstart mode:
1) The Breakwire must have been tripped. This actually happens on the pad right after liftoff,
but it doesn’t get checked until the Airstart Timer expires. If it’s accidentally tripped prior to the
“Flight Started” warbling tone, the Eggtimer will fail the self-test and the flight program will not
start.
2) LDA must have been reached
3) The Burn Timer and Airstart Delay timers, which are started by reaching LDA, must have
expired.
To pick the right settings for LDA, the Burn Timer, and the Airstart Delay, you must use a flight
modeling program such as RocSim or OpenRocket, using the .rkt file for you model or as
accurate a build as you can make. Check the CG and the weight carefully against your
simulation. What you are looking for is the LDA and first stage burnout time and altitude. You
will use these to figure out the LDA, Burn Timer, and Airstart Timer as follows:
1) Find the time that your rocket will hit LDA. We recommend 200’ unless your rocket has a
thrust-weight ratio of under 10, in which case 100’ would be better.
2) Take the burn time of your motor and round it appropriately. This will be your value for the
Burn Timer.
3) Add the Burn Timer value to the LDA time to the Burn Timer value and note the elapsed
flight time from motor ignition. This will probably be slightly longer than your motor’s burn
time.
4) Determine where you want the second stage ignition to be. You may have to play around
with this time in your modeling program to get the altitude/velocity that you want. If you want it
to be at burnout, it’s going to be the same as the value in #3, and you can simply set the Airstart
Delay to zero.
5) Go backwards from the elapsed flight time of the second stage ignition to the expiration of
the Burn Timer that you noted in #3, the difference will be the value of the Airstart Delay.
Round up or down as appropriate to get the most accurate value. Note that if your igniter takes a
significant amount of time to fire the motor (over 1 second) 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.
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)