Eggtimer Rocketry Eggtimer User Manual page 31

can introduce an error, particularly if they're too big and you have two of them opposite each other
(you'll get a crossflow through the payload bay which makes the pressure readings very noisy).
Finally, differences in the processor's timing may introduce errors, although the readings are taking
at relatively precise intervals so it's going to be very small.
The good news is that the magnitude of these errors tend to be proportionate to velocity as the rocket
ascends, so they respond well to being filtered with mathematical noise filters. We use a variation of
a filter to smooth out this "noise", so transition to supersonic speeds and back can be detected with
relatively good accuracy.
As the samples are taken, maximum values for altitude and velocity are logged, at the rate
determined by the Ascent samples/sec setting.
Mach Transition...
As your motor continues to burn and the velocity increases, if the velocity exceeds 800 ft/sec
aerodynamic shock wave buildup can fool the pressure sensor into thinking that the rocket is
descending when in fact it is actually ascending at a rather rapid speed. If this were not taken into
account, the flight computer might deploy parachutes at near-mach speed, which would undoubtedly
break something and ruin your day, not to mention what an object falling from the sky at these
speeds could do.
To prevent this from happening, the Eggtimer uses a predictive mechanism to hold off deployments
until it's safely out of the mach "danger zone". Real-time altitude readings are run through a filter,
which "smoothes" the noise from the pressure readings. The smoothed readings produce a much
gentler velocity profile, which allows it to be used to obtain reasonably accurate velocity samples. If
the velocity reaches 500 ft/sec, it is assumed that the rocket may reach Mach speeds, and
deployments are inhibited so that the sudden pressure spike (and perceived loss of altitude) does not
result in a premature deployment. When the velocity drops below 100 ft/sec for at least 1 second
(presumably near apogee), deployments are re-enabled.
Note that if Channel B is configured in Airstart or Delay mode, it WILL fire the igniter during mach
transition. Once your rocket hits LDA, airstarts and delay triggering are under the control of the
timers and are not dependent on altitude readings. It is assumed that you have modeled the flight
and appropriately set the timers, so if you do NOT want the second stage to fire during mach
transition you must set the Airstart Delay to a higher value so that the rocket will coast for a longer
period of time before it ignites the sustainer's motor. Also note that if you use the Airstart Min
Velocity value and there is a pressure rise/altitude dip due to mach transition, your airstart may be
delayed until the pressure drops/altitude rises when the rocket comes out of the transition. If you set
the Airstart Min Velocity too high, it is possible that the airstart igniter may not fire at all (that's why
this value is limited to 700 ft/sec).
Apogee and Nose-Over
Assuming that your rocket is moving more or less straight up, it will continue to slow down during
the coast phase until it gets as high as it's going to go. If the rocket was going absolutely straight up,
the velocity at this point would be zero; it would simply start falling to the ground. In reality, this
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