Hi All,

I am working with Steve Eves who last year launched a 36' tall scale Saturn V rocket and in so doing, set a record for the largest model rocket ever launched and safely recovered. Having accomplished that he has now set out to build a 1/10 scale Saturn 1b with two stages.  What he needs, and what I am researching is a method to determine whether the second stage is at or near a vertical attitude before the second stage engines are ignited. I have been palying with a Parallax Javelin board and a Sparkfun two axis rate gyro, but as you have probably already guessed, a rate gyro in this application is useless.  So, I am currently looking at the 9dof Razor IMU and would welcome input on the best way to go about accomplishing my task. We basically need a go or no go signal if the vehical is off of local vertical by more than 10 degrees or so. Any input would be appreciated. Remember that at first stage burn out, the vehical will be experiencing zero G flight. Thanks!!

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The Sparkfun 9Dof is actually overkill, since you don't need the magnetometer. ArduIMU+ will do what you need out of the box (you won't need to use the GPS, because you don't care about yaw). The only thing is that I don't know how many Gs those sensors can handle (they're the same ones as the Sparkfun board).
The max G measured on the Saturn V was @6.72 g's at 1.5538 seconds into the flight. Cheaper is good because I may be donating this to the cause. I'm assuming the ArduIMU+ sends out a serial bitstream that the javalin could read and interpet. I'll check out the specs. Thanks!
I wouldn't necessarily assume that the rocket would reach the same g-force as the real deal. It might get more, it might get less. You should look at the specific thrust and amount of the fuel you are using and estimate it from there.
oops, no that G load was from the 1/10th scale model we launched last year. however you are correct, the new Saturn 1b should experience slightly higher G loads due to a higher thrust to weight ratio. I'm estimating a max G of about 9 to 10 on the new model.
Have you any info on the vibrations experienced in the previous flight? Is it possible to steer the second stage after first stage burnout. I'd have a look at trying to use a camera based system (maybe in conjunction with the solid state IMU). That said, the rocket requirements are very different from an IMU for a UAV.

For a rocket, flight time is very short, accuracy needed is likely to be substantially higher, accelerations are much higher, weigh constraints are negligible in comparison, you can't rely on flight mechanics (dihedral, etc) to stabilise flight.

Since flight time would be in seconds, would you maybe use a conventional gyro. http://gyroscope.com/d.asp?product=GIMBALS

It would be very interesting to compare an SS IMU and the conventional gyro

On the 36' Saturn launch, vibration after leaving the launch rail was surprisingly low amplitude.
Due to NAR rules it is not possible or legal to steer the vehicle after launch. Something about it then being a guided missle.. Accuracy would need to be within 5 degrees or so. We could tolerate up to a 10 degree offset from vertical. Actually the rocket should takeoff near vertical and after staging we are only interested in whether it is within the limits specified. Real gyros were considered but dropped due to difficulty in measuring offset. The zero G situation also adds to the complexity.
You must have pretty amazing aerodynamics to be sure that a 36' rocket flies straight. Is this done by imparting a rotation via the aft vanes. That way I suppose leaning a particular direction would even out.

5 degrees should be easily attained by the SS IMU. Zero g should not be an issue as it is the accelerations before this that will affect the orientation.

Would a manually initiated firing of the 2nd stage be possible based on the view from a bottom mounted camera?
when you have zero G, any of mems can be used, they get no comparision to the G they need. I would recomend you clasical mechanical gyro that can guide you through the whole flight...but no one sells it I think...
This is an application where thermopiles should shine -- VFR conditions, insensitive to vibration and acceleration, and lightweight.
Big enough for a trained monkey...or do the NAR rules say no to that too?
Think through the IMU approach. We use accelerometers to correct the gyro drift, Gravity will be much smaller than linear acceleration in your case making measurement of the gravity vector difficult. Accelerometers are also prone to vibration issues, and the gyros can be as well. I think it may be very difficult to have any gyro drift correction during the burn. You should look for gyros which will not have vibration issues and which will be stable enough to rely on for the time span of the burn without drift correction. You may need to characterize the thermal characteristics of the gyros and compensate for the temperature dropping too.
Mike, good point, but I'd be slightly worried that they'd be thrown off by thermal stuff from the rocket, including stage separation, which could give a false tilt signal.

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