add a FAQ about the pyro on time

[web/altusmetrum] / FAQs / ontime.mdwn

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+# Why is the pyro "on time" of your products only 50 milliseconds?
+
+Altus Metrum flight computers are designed for use with low-current 
+e-matches.  The e-matches available for use in the rocketry hobby are
+mostly designed for use in the fireworks industry, where it's an
+advantage to retain continuity after firing to allow series strings
+of matches to be fired on a high-voltage DC ignition system.
+
+The problem this poses for us is that it means battery drain continues
+for the entire on-interval, even after the pyro device fires.  Since
+many of our products include radio transmitters necessary for helping
+to find the rocket after flight, "using up" the battery during pyro
+events limits the time available to locate and recover the rocket 
+after flight.  So, our goal is to balance "enough time to be sure we
+fire the pyro" against "so long we're just running the battery down".
+
+In bench testing some years ago, we measured an average firing time 
+of about 13 microseconds for commercial e-matches using a fully 
+charged single-cell LiPo battery.  That led us to choose 50 
+milliseconds on-time.  In practice, this has worked out brilliantly
+for more than a decade.
+
+# Can I change the pyro channel "on time" to something longer?
+
+On all Altus Metrum products, the dedicated apogee and main channels
+use a fixed 50 millisecond firing interval that can only be changed
+by modifying the firmware.
+
+On products with programmable channels like TeleMega, EasyMega,
+and EasyTimer, the programmable channels can have the "on time" set
+to whatever you want.
+