SL-100 Velocity accuracy?

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JDcluster

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I haven't had a chance to put it up against other altimeters that have a built in accelerometers but, I would like some input?
On a recent flight that went 13k the SL-100 plot shows either; close to 2000 fps or 2000 mph
I can't find what it reports in??

I have nothing to compare it to as:
The secondary altimeter ( ARTS 2) died at 0.510 sec into the flight, due to power loss :( .
The flight was still a success.

I'd like to to plot the raw data on a spreadsheet but, I'm not that good with spreadsheets.

Thanks!

JD
 
I have flown a SL-100 with a RRC3 as back-up and I believe the PerfectFlite software translates velocity into fps since it reported very near the same velocity as the RRC3. That said I haven't ever flown it side by side with an accelerometer. Also, if I'm not mistaken the velocity is derived from barometric pressure change over the recorded flight time not actually recorded. The data can be input to a spreadsheet by opening a file, going to the "Data" tab, clicking "Inspect" the in the new window click "select all" then "copy" and then paste into a spreadsheet. As far as doing anything meaningful with the data after that...???

Hope this helps.
 
The velocity is only as accurate as the static pressure recording. The altimeter bay port area has to be sufficiently large to provide an accurate dP/dt record, and must be smoothed around the Mach transition to avoid obtaining an erroneously large velocity due to high dP/dt caused by the Mach transition shock wave passage over the static port.

If you provide the data file I'll put it in a spreadsheet for you.

Bob
 
Upon landing it beeps out velocity in MPH, but does the graph in FPS. I called and asked Pete many moons ago.
 
I haven't had a chance to put it up against other altimeters that have a built in accelerometers but, I would like some input?
On a recent flight that went 13k the SL-100 plot shows either; close to 2000 fps or 2000 mph
I can't find what it reports in??

I have nothing to compare it to as:
The secondary altimeter ( ARTS 2) died at 0.510 sec into the flight, due to power loss :( .
The flight was still a success.

I'd like to to plot the raw data on a spreadsheet but, I'm not that good with spreadsheets.

Thanks!

JD

Any chance you can post the raw altitude and velocity data? Sometimes (sometimes) sense can be made even of supersonic baro data.
 
It definitely went mach. I was playing with the data and there is a point in the timeline that shows negative altitude. I'm not home right now but will post it when I do.

JD
 
I replotted your data and added a simple velocity calculation by the dA/Dt method and then applying a 5 point smoothing function in an attempt to get some of the noise out.

JDsflight2data.jpg

I wouldn't say the velocity is accurate to 10% due to the mach transitions, most likely due to undersized sized pressure sampling ports.

You need to run a sim in concert with the time to apogee values to get a good estimate of the velocity profile, or use an additional high sampling rate accelerometer to get a better handle on the velocity and vehicle dynamics.

Bob
 
JD what was the burn time of the motor in this flight?
 
Here's the raw data exported to an Excel spread sheet.


JD

This graph is from a so-called Savitzky-Golay filter. Like a moving average, it uses a moving window of points. It fits a polynomial to the points in the window by least squares. Then it differentiates the fitted polynomial and uses the value at the middle point in the window as the velocity. The window is then shifted by a point and the process is repeated. This graph uses a 61-point window. The first points are fudged by a standard procedure of reflecting the first altitude points.

Alas, as Bob predicted, there is too much turbulence to make sense of the data. E.G.; Your rocket didn't suddenly start falling at 450 ft/sec during boost, but that [2.45 seconds] might be the mach transition moment.

The registered altitudes are somewhat low, BTW, because it was a hot day.

The AV bay temperature rises during flight, likely because of drag friction. (Adrian pointed out this effect on the forum some years back.)

As a wild guess, I'd say thrust probably lasted 5.4 - 6.2 seconds.

Regards,
-Larry (Nice flight, BTW) C.

V Graph.jpg
 
It was an old Kosdon M 2240 so, spec at 2.65 sec or so....

JD

Then I would have to say there is way too much lag in the pressure altitude response to have any hope of velocity estimation. There is no way the velocity of the rocket can be increasing after motor burnout unless the rocket is point down at earth at that point.
 
I always thought I oversized them.
I have 4 x 11/64" holes in a 4" x 11" compartment.


JD

Then I would have to say there is way too much lag in the pressure altitude response to have any hope of velocity estimation. There is no way the velocity of the rocket can be increasing after motor burnout unless the rocket is point down at earth at that point.
 
I always thought I oversized them.
I have 4 x 11/64" holes in a 4" x 11" compartment.


JD

The lag could be filtering in the SL-100.

I don't know about the altimeter filtering algorithm, but the acoustic time constant for the sampling hole area and the altimeter bay volume is ~0.4 seconds. That's sufficient to deploy at apogee, but insufficient to accurately track static pressure changes outside the vehicle.

After 1 second the expected velocity should be ~Mach 0.6+/- at an altitude of ~350' while the StratoLogger is reporting 100'. This is well below where transonic pressure effects are expected, so there is definitely a response lag somewhere. The peak velocity is somewhere around ~1400 fps at somewhere around ~2.25 seconds and an altitude around ~1800'. The altimeter is in the middle of reporting a mach transition at an altitude of ~300', nowhere near accurate.

Bob
 
Anyhow; I enlarged the port holes to 3/16".
All 4 holes were countersunk,.
I'll compare the data on the next flight.
That will probably be next year some time.

JD

I don't know about the altimeter filtering algorithm, but the acoustic time constant for the sampling hole area and the altimeter bay volume is ~0.4 seconds. That's sufficient to deploy at apogee, but insufficient to accurately track static pressure changes outside the vehicle.

After 1 second the expected velocity should be ~Mach 0.6+/- at an altitude of ~350' while the StratoLogger is reporting 100'. This is well below where transonic pressure effects are expected, so there is definitely a response lag somewhere. The peak velocity is somewhere around ~1400 fps at somewhere around ~2.25 seconds and an altitude around ~1800'. The altimeter is in the middle of reporting a mach transition at an altitude of ~300', nowhere near accurate.

Bob
 
A very interesting read,,,
Thank you all...
I never thought of this,,
the few seconds before apogee the rocket has slowed so much even severely undersized ports will work for detecting apogee and deploying....

I know for a fact my ports are undersized..........
I have to look into this...

Teddy
 
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