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Why would my Jolly Logic Altimeter 2 say I went Mach 29?

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SoCalChris

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At XPRS last weekend, I launched a Wildman Punisher 3" on an L1000 motor. With that motor, there wasn't room for the normal electronics, so I bought a Jolly Logic Altimeter 2 and strapped that in.

The altitude reported was pretty much dead on with what was simmed at 15,665' AGL, but considering my rocket came back to Earth I'm pretty sure I didn't do 22,102 mph as indicated on the altimeter.

What would cause this? This was a brand new unit, pulled out of the package a few minutes before the flight. Battery was around 80%, and there was a port in the air frame.

 

snrkl

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If the altimeter vent hole wasn't large enough, could the barometric pressure have changed in as exponential fashion?

If it was an A3, the graphs might have given you some additional data points...

Otherwise - maybe the accelerometers were just pushed beyond their limits?

I'm sure John Beans could probably give you a reasonable idea about what happened...
 

Buckeye

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Because hobby altimeters are very poor in calculating speed. Baro alts have to go in the wrong direction mathematically, and accel alts are flakey.

Trust your simulation. That is the best result you have for determining top speed.
 

markkoelsch

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Because hobby altimeters are very poor in calculating speed. Baro alts have to go in the wrong direction mathematically, and accel alts are flakey.

Trust your simulation. That is the best result you have for determining top speed.
This is the correct answer.


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Keisling

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Is your JLA2 sealed from ejection gasses? Maybe a sudden pressure increase/decrease at ejection caused the false reading.
 

SoCalChris

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If the altimeter vent hole wasn't large enough, could the barometric pressure have changed in as exponential fashion?

If it was an A3, the graphs might have given you some additional data points...

Otherwise - maybe the accelerometers were just pushed beyond their limits?

I'm sure John Beans could probably give you a reasonable idea about what happened...
The vent hole in the rocket was plenty big, probably considerably bigger than needed to be since I designed the electronics bay to have a camera facing out. The camera (Or other altimiter I have for the rocket) had to come out though, as everything wouldn't fit with that motor in it.

Aspirations of being a high achiever.
At XPRS they had trophies for highest altitude and longest loft time. I tried getting them to give me a trophy for fastest speed, but no dice :confused2:

Because hobby altimeters are very poor in calculating speed. Baro alts have to go in the wrong direction mathematically, and accel alts are flakey.

Trust your simulation. That is the best result you have for determining top speed.
I had assumed that the altimeter would use a different formula with the same data to calculate speed and altitude. The altitude was pretty much dead on with what I was expecting. OpenRocket simmed it to 15,340 and the altimeter reported 15,556.

Is your JLA2 sealed from ejection gasses? Maybe a sudden pressure increase/decrease at ejection caused the false reading.
It was rolled up in the chute protector, but certainly not airtight. I would have thought that enough pressure was getting to it for a more accurate reading. This seems the most likely culprit to me though. Hopefully John will chime in at some point on this one.

Thanks for the replies, everyone.
 

DavidMcCann

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I had assumed that the altimeter would use a different formula with the same data to calculate speed and altitude. The altitude was pretty much dead on with what I was expecting. OpenRocket simmed it to 15,340 and the altimeter reported 15,556.
Yes, but if it thought it went from zero, to 15,556 very quickly, it would still report the correct altitude, and incorrect speed.
 

snrkl

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Yes, but if it thought it went from zero, to 15,556 very quickly, it would still report the correct altitude, and incorrect speed.
That's where my thinking was at. It's things like this that led to me saving a little longer and buying the A3 - MOAR DATAZ!!!
 

Buckeye

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I don't know the details of JL altimeters, but the altitude measurement is probably baro, and that is reliable. The speed has to be derived either from altitude (not a good practice, numerically), or integrated from accel (correct numerical approach, but still flakey). Either way, I don't trust it, especially from a peak recording unit. You don't know if the peak is truly what you want, or if it is just a spurious spike in the data. A time history of the data is needed to know for sure and to select the correct peak of interest.
 

blackwing94

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You didn't launch that rocket..... during the eclipse by any chance.....:y::y::y:
 

Handeman

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What was the speed of the rocket? At or near Mach? The shock waves traveling over the vent hole might have thrown it off the way the old baro units couldn't handle the waves and had to have a Mach Delay set on them.
 

SoCalChris

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What was the speed of the rocket? At or near Mach? The shock waves traveling over the vent hole might have thrown it off the way the old baro units couldn't handle the waves and had to have a Mach Delay set on them.
OpenRocket simmed the speed at 1,272 mph, or around Mach 1.7
 

Tonimus

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The Alt 2 uses the accelerometer to calculate speed.
 

Handeman

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The Alt 2 uses the accelerometer to calculate speed.
Was it oriented correctly during flight? It didn't get move to a wrong position while going to the pad and be in that wrong position during powered flight?
 

Tonimus

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It's 3 axis, so it shouldn't matter what orientation it is in. I suspect that there was some sort of shock event (maybe in or out of mach) that made it think that was the launch start.
 

Larry Curcio

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It's 3 axis, so it shouldn't matter what orientation it is in. I suspect that there was some sort of shock event (maybe in or out of mach) that made it think that was the launch start.
Long story on the 3-axis and orientation. (It's a 6 DOF problem, not a 3-DOF problem)

My question is whether the altimeter was secured inside the rocket, or if it was dangling. The accelerometer records its own accelerations, and not necessarily those of the rocket.
If it were dangling, the instrument would integrate crazy accelerations in arbitrary directions as if they were all oriented vertically. That process would yield crazy velocities.

Sorry if this is science fiction. Trying to help, but wasn't there...

Regards,
LarryC
 

SoCalChris

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Long story on the 3-axis and orientation. (It's a 6 DOF problem, not a 3-DOF problem)

My question is whether the altimeter was secured inside the rocket, or if it was dangling. The accelerometer records its own accelerations, and not necessarily those of the rocket.
If it were dangling, the instrument would integrate crazy accelerations in arbitrary directions as if they were all oriented vertically. That process would yield crazy velocities.

Sorry if this is science fiction. Trying to help, but wasn't there...

Regards,
LarryC
It was bundled up with the chute. It may have shifted a bit during launch, but it wouldn't have been anything absurd, and it wasn't just shaking around in the tube.
 

jderimig

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Long story on the 3-axis and orientation. (It's a 6 DOF problem, not a 3-DOF problem)

My question is whether the altimeter was secured inside the rocket, or if it was dangling. The accelerometer records its own accelerations, and not necessarily those of the rocket.
If it were dangling, the instrument would integrate crazy accelerations in arbitrary directions as if they were all oriented vertically. That process would yield crazy velocities.

Sorry if this is science fiction. Trying to help, but wasn't there...

Regards,
LarryC
^THIS is the correct answer.
 

cerving

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Even baro altimeters are shock sensitive... it's a good idea to secure ANY electronics you have.
 

Buckeye

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Even baro altimeters are shock sensitive... it's a good idea to secure ANY electronics you have.
No kidding. How can a sensitive piece of electronics, especially an accelerometer, just dangle from a rope? I read through all the JLA2 information on the webpage, and scant mention is made of firmly securing the altimeter, other than to purchase the mounting clip. Otherwise, the tether loop is recommended. Not a good idea.

We only have the peak readout from the LCD screen, so it's impossible to diagnose the cause. It is like the doctor feels your forehead, declares you are sick, then walks out of the room!
 

OverTheTop

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I think this is a good theory, about the altimeter rattling around. I can imagine a scenario of whatever filtering was in the firmware could be overwhelmed with a serious shaky environment. The quality of signal acquisition depends on the hardware and how much processing power is available to handle the data. Choices are made, usually based on the hard limit of CPU horsepower. These are quite low power so they probably did the best they could with the processor and power budget available. If three axes are being filtered then processor loading is greater than single-axis.

I could also conceive a situation where vibration was of similar frequency to the filtering so all sorts of aliasing issues crop up.

Maybe try padding it into the airframe next time, see if that improves things. You could also deliberately let it rattle around and look at the effects.
 

jderimig

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It's 3 axis, so it shouldn't matter what orientation it is in. I suspect that there was some sort of shock event (maybe in or out of mach) that made it think that was the launch start.
Orientation doesn't matter but it has to stay in the same orientation throughout the throughout the flight.
 
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