Eggtimer Rocketry Eggtimer User Manual page 50

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, if this option was selected. 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 must have expired.
4) If Airstart Min Velocity is used, the rocket must have reached it.
5) Low-speed detect ("Mach Unlock") must not have been reached, i.e. the rocket must be going
faster than 100 ft/sec.
6) Nose-over must not have occurred.
To pick the right settings for LDA, the Burn Timer, the Airstart Delay, and Airstart Min
Velocity, 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) Take the burn time of your motor and round it down to the nearest 100 ms. This will be your
value for the Burn Timer. For example, if your motor burns for 2.66 seconds, round it to 2600
milliseconds.
2) Find the estimated altitude at burnout, and use that to check the LDA value. The idea is to
have the rocket hit LDA before the motor burns out. For example, if your motor burns for 2.66
seconds and you see that the rocket will reach 400' at burnout, then 200' is probably a good
value for LDA. Don't set it the LDA to any value that cannot be reached in 2 seconds, or the
pre-LDA memory may wrap around!
3) 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 #1, and you can simply set the Airstart
Delay to zero.
4) Go backwards from the elapsed flight time of the second stage ignition from #3 to the
expiration of the Burn Timer that you noted in #1, 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
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