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.

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 © 2015 Jeff Karpinski