Model Rocket Flight Computer
Jeff Karpinski of Littleton
Colorado used ExpressPCB circuit boards to build a flight
computer for his
large model rocket. The rocket shown above is a 1/2 scale model
of the British ASRAAM air-to-air
missile. His rocket is 4' 6" in length and weighs 5 lbs when
loaded and ready for flight. The body of
the rocket is 4" in diameter and built of fiberglass. The
rocket is powered by a J450 engine with
125 lbs of lift.
The flight computer is
positioned in the rocket below the nose cone. Throughout a
flight, the rocket's
acceleration and the atmospheric pressure are logged to EEPROM
for post launch analysis. The
computer is also used to control the deployment of the
parachutes. When the computer detects that
the rocket has reached maximum attitude and begins its decent, it
fires an electric match which ignites a
black powder charge to eject a small drogue parachute. This chute
brings the rocket down from apogee
in a rapid, but controlled decent. The computer continues to
monitor the altitude as it falls to 1000 feet.
At this point, the main chute is deployed to reduce the decent
rate to under 15 feet-per-second.
See the
board layout - See the board schematic
The Basic programmable flight computer is based on a 20MHz
microcontroller from Protean Logic.
The barometric and acceleration sensors are interfaced to a 12
bit A to D. The flight data is collected
and stored in a 32K EEPROM. Two high current HEXFET are used to
drive the electric matches that
deploy the parachutes. Assembled, the flight computer is only
1.1" by 5" in size.
Once the rocket is retrieved
from flight, the complete flight profile of pressure and
acceleration data can
be downloaded to a laptop for analysis. This graph shows the data
captured from the flight in the picture
above. Altitude is shown in feet, while acceleration is shown in
raw 12 bit data. Because the output of the
accelerometer is inverted, the graph is opposite of what you
might expect. At T-0, you can see the
acceleration is "at rest" at 1G. After ignition,
acceleration quickly jumps to 6Gs at only T+0.25! The total
burn of the motor lasts 1.44 seconds, at which point the altitude
is increasing at 746 fps or 509 mph. The
negative acceleration spike at T+11.5 is the parachute being
deployed at apogee. The lunch site in
Colorado is about 4500 ft above sea level. The rocket reached its
peak altitude of 3345 ft above ground
at T+10.5. Using longer burn motors, this rocket has previously
exceeded 5000 ft.
The photographs are courtesy of Jeff Karpinski
Copyright © 2007 Jeff Karpinski
