BY MICHAEL BESSANT
In two of my previous Nuts & Volts articles (August and September 2007
issues), I presented the design of a compact and lightweight flight recorder
for use in model rocket development. An example application that was given
described taking recorded signals from a three-axis accelerometer and
exporting the post-flight data to a PC spreadsheet for display and analysis.
The design was based on a PICAXE-18X microcontroller and non-volatile
FRAM storage, powered from a single three volt supply. Connections to the
flight recorder's analog inputs and digital I/O lines were via a sensor port
that can be programmed in Basic to interface with signal conditioning
circuitry and a wide range of
sensors. Figure 1 shows a copy of
the original flight recorder schematic.
The previous articles also introduced
a number of compatible sensors, but
did not describe the associated
This article covers the addition of instrumentation to
measure a rocket’s rate of roll, relative altitude, and
detect booster-stage separation. Such information is very
useful when developing multi-stage model rockets that are
capable of reaching speeds in excess of 300 km/h.
Important design objectives can often include using
fin-induced roll to improve directional stability (but without
tangling the recovery chute) and minimizing the risk of
airframe damage by deploying the chute when the rocket
is near apogee and travelling at low speed. These and other
flight characteristics can be investigated using the sensors and
circuitry shown in Figure 2 and described in detail below.
■ Multi-stage model
rocket on launch rail.
The three-axis MMA7260 accelerometer breakout
board described in my previous articles is retained
(available from SparkFun Electronics; www.sparkfun.com),
34 January 2009
but only the vertical axis is recorded. Compared with the
eight-bit readadc command used previously to measure
acceleration, improving resolution is achieved by the use
of the readadc10 command, but this has made the revised
record subroutine slightly more complicated. This is
because the latter command generates word variables and
the writei2c command can only write bytes to memory. In
addition, using two memory locations to store each 10-bit
ADC sample is less efficient (and using an entire byte for
only two bits, is even less), but the storage capacity
remains adequate for recording 400 samples/second
throughout the duration of a typical model rocket flight.