Rocket gyro or accel

Apologies for being so newb on this one but I have read a few articles and just not been able to wrap my head around it

Let's say I have a rocket hurdling toward the sky. I want to detect any deviation from a perfect straight flight path. Do I want an accelerometer, a gyroscope, or do I need some combination? Which axiis do I need to track
Thanks!

I think it's gotta be a gyro... since if the rocket turns, the accelerometer will turn with it, and continue to report it rapidly accelerating straight forward (unless the rocket is firing sideways...), whereas the gyro will report that it's turning....

This is a very demanding application - you have very little time to react to a small problem with the direction before it slams into the ground or a bystander. It's a lot easier to build the rocket so it goes straight in the first place, without any guidance.

Why would you want to do that anyway

A model rocket will weathercock into the wind normally.
This is useful as it makes recovery easier, it does not drift too far down wind.

For power measurment just an accelerometer will do.

DrAzzy:
I think it's gotta be a gyro... since if the rocket turns, the accelerometer will turn with it, and continue to report it rapidly accelerating straight forward (unless the rocket is firing sideways...), whereas the gyro will report that it's turning....

This is a very demanding application - you have very little time to react to a small problem with the direction before it slams into the ground or a bystander. It's a lot easier to build the rocket so it goes straight in the first place, without any guidance.

Right, please do not assume I am trying to build an active guidance system or an inherently unstable airframe. This first go is all about detection and data logging.

So am i correct in understanding that the accelerometer is for detecting motion relative to earth, and the gyroscope is for detecting orientation of the object? That is what I thought at first, but IIRC the iphone uses accelerometer for changing orientation, not gyro. that makes me think i have it wrong.

Sounds like an interesting project, I used to fly some model rockets, so I am officially an amateur rocket scientist :sunglasses:

All the motion sensors have limitations, which you have to allow for. The flaws can be compensated by combining the different sensors (sensor fusion).

Accelerometer tells you acceleration, that includes gravity though, and you can't tell the difference between gravity and acceleration unless you know which way is "up". Gyros can sense change in direction. With 6 axes (accelerometer+gyro) you can do "inertial guidance", I guess you have to calibrate the initial state.

If you are not doing any guidance, logging raw data would be easiest, and crunch the data on a PC afterwards. I'm not sure what data rate you would need to get a decent track, depends on your rocket :slight_smile:

The gyro reports the rate of the rocket's spin. If your rocket nosed over sharply at its apex then it'd work but I don't think that's typical of any rocket.

Very complete discussion of the problem, as pertains to rockets, here: http://home.earthlink.net/~david.schultz/rnd/2004/

For an incomplete discussion, or if you flunked Algebra, then you'd get a barometric pressure sensor (e.g. BMP085) and wait for a steady altitude reading to denote your apogee.

Greensprings:
Rocket may pull too many G's for the sensor

When it's at its apex and starting free-fall?