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  1. #1
    Join Date
    9th December 2015
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    73

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

    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.


    TRA 16109
    L1 - 07/09/2016 - Wildman Demon Sport on an Aerotech H195
    L2 - 07/08/2017 - Wildman 3" Punisher on an Aerotech J450DM

    Personal records
    Altitude - 15,556' AGL
    Speed - 1,272 MPH

  2. #2
    Join Date
    11th April 2017
    Location
    Brisbane Australia
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    536
    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...

    Reasonably new to rocketry and hailing from the land down under.. I speak metric... I know not of these feet and inches you speak of...

    QRS: #193
    AMRS: #148

  3. #3
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
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    Why would my Jolly Logic Altimeter 2 say I went Mach 29?
    Aspirations of being a high achiever.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  4. #4
    Join Date
    6th September 2009
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    1,574
    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.

  5. #5
    Join Date
    18th March 2009
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    Wisconsin
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    3,469
    Quote Originally Posted by Buckeye View Post
    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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    Mark Koelsch
    Tripoli 6155 L3
    Owner of http://www.rocketryfiles.com/
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    Keeper of the motor files

  6. #6
    Join Date
    22nd January 2017
    Location
    North Florida
    Posts
    13
    Is your JLA2 sealed from ejection gasses? Maybe a sudden pressure increase/decrease at ejection caused the false reading.
    -Dave

    -----------------------
    NAR# 103244
    Level 1 - PML AMRAAM 2 - July 8, 2017

  7. #7
    Join Date
    9th December 2015
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    Fullerton, CA
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    73
    Quote Originally Posted by snrkl View Post
    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.

    Quote Originally Posted by OverTheTop View Post
    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

    Quote Originally Posted by Buckeye View Post
    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.

    Quote Originally Posted by Keisling View Post
    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.
    TRA 16109
    L1 - 07/09/2016 - Wildman Demon Sport on an Aerotech H195
    L2 - 07/08/2017 - Wildman 3" Punisher on an Aerotech J450DM

    Personal records
    Altitude - 15,556' AGL
    Speed - 1,272 MPH

  8. #8
    Join Date
    12th March 2011
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    Delta Colorado
    Posts
    493
    Maybe the speed was correct?

  9. #9
    Join Date
    9th December 2015
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    Fullerton, CA
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    Quote Originally Posted by lawndartman View Post
    Maybe the speed was correct?
    This seems about as likely

    TRA 16109
    L1 - 07/09/2016 - Wildman Demon Sport on an Aerotech H195
    L2 - 07/08/2017 - Wildman 3" Punisher on an Aerotech J450DM

    Personal records
    Altitude - 15,556' AGL
    Speed - 1,272 MPH

  10. #10
    Join Date
    15th May 2016
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    1,989
    Quote Originally Posted by SoCalChris View Post
    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.
    David McCann
    Dave's Rockets | My Flights
    URRG |URRF 4| Level 2 | TRA# 14210

  11. #11
    Join Date
    11th April 2017
    Location
    Brisbane Australia
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    536

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

    Quote Originally Posted by DavidMcCann View Post
    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!!!
    Reasonably new to rocketry and hailing from the land down under.. I speak metric... I know not of these feet and inches you speak of...

    QRS: #193
    AMRS: #148

  12. #12
    Join Date
    6th September 2009
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    1,574
    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.

  13. #13
    Join Date
    6th January 2016
    Location
    Austin Texas
    Posts
    103
    You didn't launch that rocket..... during the eclipse by any chance.....
    NAR 101529
    Tripoli 17153
    L1 LOC Cyclotron 7/2/16
    L2 Madcow X-Celerator 1/7/17

  14. #14
    Join Date
    19th January 2009
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    Stafford VA
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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.
    Handeman

    TRA #09903 L3 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  15. #15
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    9th December 2015
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    Quote Originally Posted by Handeman View Post
    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
    TRA 16109
    L1 - 07/09/2016 - Wildman Demon Sport on an Aerotech H195
    L2 - 07/08/2017 - Wildman 3" Punisher on an Aerotech J450DM

    Personal records
    Altitude - 15,556' AGL
    Speed - 1,272 MPH

  16. #16
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    9th July 2014
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    Phoenix, AZ
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    The Alt 2 uses the accelerometer to calculate speed.

  17. #17
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    Quote Originally Posted by Tonimus View Post
    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?
    Handeman

    TRA #09903 L3 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  18. #18
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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.

  19. #19
    Join Date
    5th March 2009
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    514
    Quote Originally Posted by Tonimus View Post
    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

  20. #20
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    9th December 2015
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    Quote Originally Posted by Larry Curcio View Post
    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.
    TRA 16109
    L1 - 07/09/2016 - Wildman Demon Sport on an Aerotech H195
    L2 - 07/08/2017 - Wildman 3" Punisher on an Aerotech J450DM

    Personal records
    Altitude - 15,556' AGL
    Speed - 1,272 MPH

  21. #21
    Join Date
    23rd January 2009
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    Quote Originally Posted by Larry Curcio View Post
    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.
    John Derimiggio NAR/TRA L3
    MarsaSystems

  22. #22
    Join Date
    3rd February 2012
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    So Cal (ROC, TRASD, SCRA)
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    Even baro altimeters are shock sensitive... it's a good idea to secure ANY electronics you have.

  23. #23
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    Quote Originally Posted by cerving View Post
    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!

  24. #24
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    Quote Originally Posted by SoCalChris View Post
    OpenRocket simmed the speed at 1,272 mph, or around Mach 1.7
    THIS^ is the correct answer!

  25. #25
    Join Date
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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.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  26. #26
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    24th April 2010
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    Quote Originally Posted by Tonimus View Post
    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.
    For the best performance from the 3-axis accelerometer ... check out this ... https://www.apogeerockets.com/educat...sletter293.pdf
    -Scott Sager
    NAR 91621 L2
    TRA 15982 L2
    Woosh #558
    KC9WQK

  27. #27
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    Quote Originally Posted by Tonimus View Post
    The Alt 2 uses the accelerometer to calculate speed.

    I could be wrong, but last I saw they used a 24G accelerometer. If you spiked it.... all bets are off.
    David McCann
    Dave's Rockets | My Flights
    URRG |URRF 4| Level 2 | TRA# 14210

  28. #28
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    23rd January 2009
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    Quote Originally Posted by Tonimus View Post
    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.
    John Derimiggio NAR/TRA L3
    MarsaSystems

  29. #29
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    Quote Originally Posted by jderimig View Post
    Orientation doesn't matter but it has to stay in the same orientation throughout the throughout the flight.
    Are you saying altimeters can't choose their own orientation?
    David McCann
    Dave's Rockets | My Flights
    URRG |URRF 4| Level 2 | TRA# 14210

  30. #30
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    Quote Originally Posted by DavidMcCann View Post
    Are you saying altimeters can't choose their own orientation?
    They can choose their own orientation, but they may not be able to buy wedding cakes from certain bakeries.

    Zeus-cat
    NAR# 92125 L1
    Total Impulse for 2017: 1,493.8 N/s Flights: 56
    2017: 1/2A:0, A:6, B:11, C:2, D:12, E:4, F:1, G: I have NEVER launched a G motor, H:1, I:1

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