My son and I have been working on a flight computer for a few years and continue to refine it, sharing our experience with student groups and maybe open sourcing it at some point. We've flown it a few dozen times between 15K-30K feet, but long term we want to put it on a sustainer to >100K feet. We still use commercial altimeters (currently) for safety, but we have very reliable data to capture launch, max accel/thrust, motor burn-out, apogee, fore/aft separation, descent, landing, GPS tracking, and more. For Apogee, we are primarily relying on the barometer. We used to also use tilt detection, but we've had a few flights where the rocket went to 70+ degrees in high altitude winds, while continuing to climb 2K feet, so we don't trust tilt (passively monitoring our apogee logic). We have very reliable 9DoF data calibrated and going through Kalman Filters (using an EM7180 processor), but our attempts to use integrated accelerometer data to calculate apogee have proven unreliable (too early or too late), even when staying within the limits of the accelerometer (500Hz sampling). We have also tried combining the accelerometer readings after motor burn-out, subtracting for gravity, and watching for convergence to (near) zero, but that only works about 50% of the time, as spin or thrashing at apogee creates too much noise on one or more axis.
I've read a dozen previous threads on the topic and it doesn't look like there is a silver bullet, but I'd be interested if anyone has a reliable formula they are using (without giving away any commercial trade secrets). We know we can always use a timer, but we are looking for something more accurate. We can also make a timer better by sampling velocity/angle at lower altitudes and calculating apogee time/altitude, before the barometer cuts out (quadratic regression). Integrating GPS is another option, but regaining lock and getting quality readings is not very precise at apogee, although it is more reliable than we expected.
After a few dozen tests with different rockets, I've been surprised at the amount of instability "at the top" with a some flights. Is it plausible that a (stable) rocket could do a full 180 degree flip well before apogee? Not that tilt is a reliable measure of apogee (it could drop straight back down without tilting), but is it reliable to assume that a 180 degree flip has passed apogee?
Thanks for any ideas and feedback,
Mike
I've read a dozen previous threads on the topic and it doesn't look like there is a silver bullet, but I'd be interested if anyone has a reliable formula they are using (without giving away any commercial trade secrets). We know we can always use a timer, but we are looking for something more accurate. We can also make a timer better by sampling velocity/angle at lower altitudes and calculating apogee time/altitude, before the barometer cuts out (quadratic regression). Integrating GPS is another option, but regaining lock and getting quality readings is not very precise at apogee, although it is more reliable than we expected.
After a few dozen tests with different rockets, I've been surprised at the amount of instability "at the top" with a some flights. Is it plausible that a (stable) rocket could do a full 180 degree flip well before apogee? Not that tilt is a reliable measure of apogee (it could drop straight back down without tilting), but is it reliable to assume that a 180 degree flip has passed apogee?
Thanks for any ideas and feedback,
Mike