FlightSketch questions

[shockie]

Since the FS mini had a 3 axis accelorometer that means it can measure acceleration in the xyz axes where the z axes would be vertical sorta like a 3d printers xyz axes.

Do these xyz axes change depending on whether or not the mini is laying flat horizontally on its side or upright and vertical ?

Since you can get acceleration data in 3 axes, does that imply that you could plot it's flight trajectory in 3 dimensions from the acceleration data? In conjunction with the velocity and altitude data?

Would the 3 axis data be able to show if the model was rotating at any point in it's trajectory or would a roll rate sensor of some kind have to be used?

From the time the rocket engine ignited how long in time or altitude does the data logging start?

Finally it is recommended that you use 3x 1/16" diameter holes. That is 0.0031 in squared x 3 = 0.0093 in sq total. Could I divide that 0.0093 by 4 holes ,creating 4 0.6mm holes. Would that work for the pressure differential required?

The purpose of this last question is I have drill bits that small and I want to minimize any additional drag effects from the holes.

Thanks in advance

 

[Handeman]

With the latimer flow along the rocket, I don't think there is much drag effect from the holes. I've left blue painters tape loosely stuck to a fin, both ends loose and flapping, and it was still stuck to the fin after the rocket landed.

As for size, that is based on some time constant. The size is calculated to allow equalization in a certain time for a certain difference in pressure. The larger the area of the holes, the faster it will equalize. I aways go a size or two larger just to make it equalize faster.

 

[BEC]

Since the FS mini had a 3 axis accelorometer that means it can measure acceleration in the xyz axes where the z axes would be vertical sorta like a 3d printers xyz axes.

Do these xyz axes change depending on whether or not the mini is laying flat horizontally on its side or upright and vertical ?

Since you can get acceleration data in 3 axes, does that imply that you could plot it's flight trajectory in 3 dimensions from the acceleration data? In conjunction with the velocity and altitude data?

Would the 3 axis data be able to show if the model was rotating at any point in it's trajectory or would a roll rate sensor of some kind have to be used?

From the time the rocket engine ignited how long in time or altitude does the data logging start?

Finally it is recommended that you use 3x 1/16" diameter holes. That is 0.0031 in squared x 3 = 0.0093 in sq total. Could I divide that 0.0093 by 4 holes ,creating 4 0.6mm holes. Would that work for the pressure differential required?

The purpose of this last question is I have drill bits that small and I want to minimize any additional drag effects from the holes.

Thanks in advance

Well, here's just a randomly picked FS Mini log for one of my flights. You can, of course, go to the online log (https://flightsketch.com/flights/) and download anyone's data to play with.

As you can see the data starts at a little over 4 seconds before liftoff. Like most such devices, the FS devices keep a few seconds' worth of data in a rolling buffer from the time they're turned on — or in this case, armed for launch — then use launch detect to determine what of that buffer to keep.

You can also see that all three axes' acceleration data are listed separately (in ft/s/s rather than Gs) so you might be able to plot as you suggest. I have not tried to do this. Velocity is based on the barometric data (I believe) so I don't know how useful it would be. The Z axis is up if the long side of the board is parallel to the centerline of the rocket and the Bluetooth module is on top.

I've nothing to add to @Handeman's comments about venting. In a small compartment (a couple of inches of BT-50) three pinholes are enough. The bigger the compartment, the bigger the required venting, just as he said.

 

[SolarYellow]

There are several formulas and/or calculators online to determine how big the vent holes should be based on the volume and expected performance of the rocket.

 

[waltr]

Rotation can NOT be determined from Accelerometer data. You need a Gyro to measure rotation Rate. Then apply offset corrections and integrate per time step to get angle.

 

[shockie]

I'm thinking of "centering" it laying flat within a 40mm diameter tube and only making the container at most .5 inch.rs in height. Perpendicular to the centerline of the rocket. So which axes would be vertical in that setup?

 

[shockie]

Well, here's just a randomly picked FS Mini log for one of my flights. You can, of course, go to the online log (https://flightsketch.com/flights/) and download anyone's data to play with.

As you can see the data starts at a little over 4 seconds before liftoff. Like most such devices, the FS devices keep a few seconds' worth of data in a rolling buffer from the time they're turned on — or in this case, armed for launch — then use launch detect to determine what of that buffer to keep.

You can also see that all three axes' acceleration data are listed separately (in ft/s/s rather than Gs) so you might be able to plot as you suggest. I have not tried to do this. Velocity is based on the barometric data (I believe) so I don't know how useful it would be. The Z axis is up if the long side of the board is parallel to the centerline of the rocket and the Bluetooth module is on top.

I've nothing to add to @Handeman's comments about venting. In a small compartment (a couple of inches of BT-50) three pinholes are enough. The bigger the compartment, the bigger the required venting, just as he said.

There are several formulas and/or calculators online to determine how big the vent holes should be based on the volume and expected performance of the rocket.

I used one and it provided me with this:
How do I interpret that?

https://rocketrycalculator.com/rocketry-calculator/test-vent-port/

 

[tsmith1315]

How do I interpret that?

Those are in drill bit size increments. Round the calculation to the nearest whole number.

You can't get a 0.91/64" drill bit, but you can get a 1/64". So, 1/64" per hole.

 

[cerving]

Rotation can NOT be determined from Accelerometer data. You need a Gyro to measure rotation Rate. Then apply offset corrections and integrate per time step to get angle.

Oh if it were only that easy... integration only works in one dimension, when you have three it gets VERY complicated. Look up "quaternions"...

 

[Dane Ronnow]

I'm thinking of "centering" it laying flat

While the FS Mini will work lying flat, it is recommended that the long axis of the board be parallel to the long axis of the rocket.

This is from the FS Mini User Guide: "The accelerometer will detect and correct for the orientation of the altimeter however maximum data range can be achieved by aligning the long axis of the altimeter board with the body axis of the rocket."

Either way, though, you'll get the same end-data—apogee, Vmax, flight duration, etc.

 

[shockie]

While the FS Mini will work lying flat, it is recommended that the long axis of the board be parallel to the long axis of the rocket.

This is from the FS Mini User Guide: "The accelerometer will detect and correct for the orientation of the altimeter however maximum data range can be achieved by aligning the long axis of the altimeter board with the body axis of the rocket."

Either way, though, you'll get the same end-data—apogee, Vmax, flight duration, etc.

Thanks for the info! I wonder what the range is? I'll only be using it with A3-6 motors with models that weigh between 21 to 28g and are 0.5m in length.

 

[cwbullet]

I really wish these products would return to productions. They easy to sue and have a small footprint. I have tied to email him but no answer.

 

[Dane Ronnow]

Thanks for the info! I wonder what the range is? I'll only be using it with A3-6 motors with models that weigh between 21 to 28g and are 0.5m in length.

The only range consideration is between the Mini and your phone app—they're connected during arming and launch. After launch, that connection is lost somewhere between 100' and 300' (the RF range), but the Mini is still recording flight data, and will continue to do so regardless of altitude or flight duration, then stop several seconds after landing when it no longer senses movement. When the phone app is back in range of the Mini during ground recovery, the two will automatically reconnect and you can view and download the flight data.

 

[Dane Ronnow]

I really wish these products would return to productions. They easy to sue and have a small footprint. I have tied to email him but no answer.

Agreed. They're a great little altimeter. I don't know exactly what the story is, but I gather there are supply issues with some of the components that make up the Mini. Not sure when or if production will resume. I hope it does. I'd like to have a backup for the one I have.