Eggtimer Rocketry Eggtimer User Manual page 5

A General Introduction to Flight Computers
A flight computer is used to record the flight path of a rocket, and to optionally deploy recovery
devices. Typically, you can use it to find out the maximum altitude (apogee) of your rocket, the
maximum velocity of the rocket, the maximum acceleration (G forces) on your rocket, and other
flight characteristics. Being able to see these after a flight helps to validate your own designs,
and a flight computer is essential for high-power rockets since the motors used in those rockets
often don't have built-in ejection charges. Typically, such rockets have two parachutes: a
smaller "drogue" parachute that is deployed at apogee, and a larger "main" parachute that is
deployed at a much lower altitude. The drogue parachute keeps the rocket from drifting too far
away... if the main parachute is deployed at 10,000', it is possible that your rocket could drift
over a mile from the launch site. Using two-parachute deployment scheme typically keeps the
drift down to 1000' or less.
They can also be used to "airstart" motors in flight, i.e. to ignite the second (or even third) stage
motors. Doing this is a more advanced technique than multiple recovery deployment, and it
requires accurate software flight simulation in order to select the proper time and/or altitude to
ignite the motor. Typically, the second stage is ignited either when burnout of the first stage
motor is detected or after a short delay afterwards.
Flight computers can use either a barometric pressure altimeter, an accelerometer "G-sensor", or
both. Each one has its own advantages and disadvantages. A barometric pressure sensor can be
used to directly determine altitude, and when read at known intervals can be used to calculate
velocity between intervals with fairly reasonable accuracy. You can in theory use one to
determine acceleration, but slight errors tend to be multiplied resulting in much larger errors.
Since they read barometric pressure which depends on temperature, and may be thrown off by
aerodynamic issues such as the transition to supersonic speeds, careful consideration needs to be
taken to ensure that the altitude readings are accurate. Barometric pressure sensors tend to be
relatively slow, so the sampling rate may be limited by the speed of the sensor. Also, errors in
barometric pressure readings are inevitable during high-speed flight, because the pressure is still
changing as the sensor is being read. Derived data such as velocity may be inaccurate at angles
that vary significantly from the vertical, since the recorded altitude will be less than the distance
that the rocket actually travels during that time interval.
An accelerometer uses the forces on the rocket to determine when the motor has fired and burned
out, and can be used to determine velocity over a known interval with better accuracy than an
altimeter. Since it reacts directly to the forces on the rocket, it is very easy to detect reaction-
based events such as burnout, and therefore is the best sensor to use for "air starting" a second
stage motor in flight. It can also be used to determine altitude by integrating (adding up) the
differences in calculated distance for each time interval, however since this is "distance-traveled"
reading it may not be accurate at angles much above vertical. Also, high-G accelerometers (> 50
G, which may be experienced by high-power rockets) are expensive chips, costing a lot more
than a barometer chip. In addition, they only work well when the rocket is going up, once it
slows down before apogee the acceleration is minimal and therefore readings will not be
accurate. On the plus side, they tend to be very fast, so higher sampling rates can be used
compared to barometric pressure sensors.
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