elastic tether
instrumentation bay
recovery chute
sun detector window
■ FIGURE 4. Configuration of rocket
used to test instrumentation.
chute ejection charge
delay charge
2 stage propellant
nd
to record the very start of engine ignition, the new technique
has proved to be very reliable in operation. The general
configuration of the instrumented rocket is shown in Figure 4.
Test Results
The Excel plot shown in Figure 5 was chosen as an
interesting example of a 'worst case' test flight. The spent
booster did not separate cleanly, causing the rocket to tilt
over before the second-stage engine ignited. As a result, the
remainder of the flight was nearly horizontal at an altitude of
not more than 20 meters! However, even this dangerously
shallow trajectory yielded a useful plot of the relative altitude.
It is possible to see that the recovery chute deployed
successfully after the rocket reached apogee but that the
time to impact was alarmingly brief. If we now turn our
attention to the 'sun' trace, it is possible to identify cyclic
variations in the light level caused by airframe roll, even
though the sky was completely overcast on the launch day.
separation switch
electric igniter
booster stage
The acceleration trace
provides a useful guide to the
timing of these events. For
example, it is possible to identify
booster stage separation,
second stage ignition, chute
deployment, and final impact.
Note that only the acceleration
trace is calibrated (the other
two traces have been expanded
in the interests of clarity) and
that the booster separation
detector switch failed.
Potential Enhancements
The low component count/power instrumentation
described here is capable of providing useful flight data
and has the potential for further development. Although
additional analog and digital channels can currently be
accommodated by connecting I2C interface devices to
the sensor port, a more direct solution would be to base
the flight recorder on a larger member of the PICAXE
microcontroller family. For example, the PICAXE-28X1
( 28 pin) would provide four analog and six digital inputs.
It can store 1,000 lines of code in onboard Flash memory
and has a maximum clock speed of 16 MHz. NV
To order back issues of the August/September 07
Nuts & Volts, go to our webstore at
http://store.nutsvolts.com or call800-783-4624.
■ FIGURE 5.
Example plot
from two-stage
rocket flight.
38
January 2009
