Stratologger/DT3U

[vahpr]

Having my 1st look at data from the PF Stratologger - I like this, very cool.

This plot is from the booster of a recent 2 stage flight. The unit appears to see a sudden change in pressure/velocity shortly after burnout, then gradually figures out it's still going up and correctly deploys at apogee. It also doesn't appear to have the altitude right - the RDAS in the upper stage had b/o at 1.5 sec and about 250'.

I'm thinking it would like more venting.........although both (baro) units functioned correctly.......

Thoughts?

Thanks,
Jerry

[bobkrech]

You need to learn how to interpret the pressure and accelerometer data correctly.

1. The pressure sensor measures pressure.
2. The measured pressure only corresponds to altitude if the pressure being measured is the static pressure.
3. Your pressure also measured the exhaust pressure for the igniton of the sustainer engine.
4. The higher pressure makes it look like the rocket was descenting but it wasn't.
5. This is clear when you look at the accelerometer trace.
6. The rocket accelerated during the booster engine burn.
7. Then when the sustainer motor ignited, the exhaust gas from it went into the booster and it slowed down quickly.
8. The altimeter trace shows the booster reached apogee at 10 seconds.
9. You had the altimeter set for apogee depolyment.
10. The accelerometer spike at 10 seconds confirms the charge went off.
11. The altimeter trace shows the rocket has a constant descent rate after the ejection charge showing the recovery system deployed.

You don't show the rocket design and location of the altimeters, and you don't show the data from the other altimeter, so I can't say much more, except that you need to prevent the motor plume from getting to the booster altimeter or you will eventually cook it.

Bob

[COrocket]

You need to learn how to interpret the pressure and accelerometer data correctly.

Bob

Quick note - the stratologger only has a pressure sensor. Velocity is derived from the baro data.

Bob was certainly correct that the 2nd stage motor ignition leaked and pressurized your avionics bay showing a decrease in altitude. Whenever there is a rapid change in pressure, the velocity data will also reflect the event. It also looks like something similar happened when your drogue went off. There is a similar pressure/velocity spike, which shows that the pressure of the ejection charge also reached the avionics bay. I'd check that all the connections running in and out of the altimeter bay are sealed, except for the static pressure ports.

Looks like a good flight nonetheless. I'd imagine that the stratologger has some logic built in to prevent the drogue from going off during the 2nd stage pressure spike.

[vahpr]

Hi Bob,

Good to hear from you Bob! Been a while since we chatted

So, the SL is a baro-only unit (no accel). It calculates velocity from pressure sensor data and with software is not just "mach immune" but also assures accurate apogee detection. So even though it saw an apparent press increase it deployed correctly. Yay.

The thing is, the booster was no where near the sustainer ignition event. The unit is fairly isolated from the outside, deep in the booster with only a static port to the exterior side. I've included some pics, although none show the vent tube it's there!

So why did the unit see a too-high decrease during boost and sudden increase (to what was closer to the actual outside pressure) at burnout when nothing else was going on? I'm thinking the static port was too small. Enough to get the job done but small enough to not allow the pressure to equalize quickly. So I've added another for the next flight at LDRS

Here's the video - must say it was the coolest failure I've ever had I WILL remember the snap ring next time!!!

https://vimeo.com/40664493

Jerry O

You need to learn how to interpret the pressure and accelerometer data correctly.

1. The pressure sensor measures pressure.
2. The measured pressure only corresponds to altitude if the pressure being measured is the static pressure.
3. Your pressure also measured the exhaust pressure for the igniton of the sustainer engine.
4. The higher pressure makes it look like the rocket was descenting but it wasn't.
5. This is clear when you look at the accelerometer trace.
6. The rocket accelerated during the booster engine burn.
7. Then when the sustainer motor ignited, the exhaust gas from it went into the booster and it slowed down quickly.
8. The altimeter trace shows the booster reached apogee at 10 seconds.
9. You had the altimeter set for apogee depolyment.
10. The accelerometer spike at 10 seconds confirms the charge went off.
11. The altimeter trace shows the rocket has a constant descent rate after the ejection charge showing the recovery system deployed.

You don't show the rocket design and location of the altimeters, and you don't show the data from the other altimeter, so I can't say much more, except that you need to prevent the motor plume from getting to the booster altimeter or you will eventually cook it.

Bob

[bobkrech]

Hi Bob,

Good to hear from you Bob! Been a while since we chatted

So, the SL is a baro-only unit (no accel). It calculates velocity from pressure sensor data and with software is not just "mach immune" but also assures accurate apogee detection. So even though it saw an apparent press increase it deployed correctly. Yay.

The thing is, the booster was no where near the sustainer ignition event. The unit is fairly isolated from the outside, deep in the booster with only a static port to the exterior side. I've included some pics, although none show the vent tube it's there!

So why did the unit see a too-high decrease during boost and sudden increase (to what was closer to the actual outside pressure) at burnout when nothing else was going on? I'm thinking the static port was too small. Enough to get the job done but small enough to not allow the pressure to equalize quickly. So I've added another for the next flight at LDRS

Here's the video - must say it was the coolest failure I've ever had I WILL remember the snap ring next time!!!

https://vimeo.com/40664493

Jerry O

Hi Jerry

I'm going to LDRS so we'll finally meet face to face.

I'm still not sure where the altimeter was located, however if the entire inside volume of the booster was the ballast chamber then as small tube vent near the top of the booster might give that behavior. The end of the sustainer is a flat plate and there is a cone on the top of the booster, That's going to give a strange flow and pressure fluctuations near the top of the booster. Depending on the exact location, the sampling tube could be located in an expansion zone or a compression zone. If the altitude is too high, it would be an expansion zone. Upon separation, you would jump uo to static pressure and depending on the orientation of the sampling tube to the flight direction, some ram air could enter the sampling port and give a higher than static pressure to indicate a lower altitude. Certainly the video shows a really clean trajectory for the booster with a slight cant after drag separation but with no tumbling.

The Stratologger is probably running a Kalman filter (Dave Schultz aka UhClem wrote several research papers on applying Kalman filters to MAWD altimeter data) and they do a great job of predicting apogee even with noisy data.

You build them well Jerry. In spite of the motor problem, it lives to fly another day. That's a fairly uncommon occurrence in the world of big birds.

[vahpr]

I am looking forward to it Bob!

The vent line is out the side a few inches down from the cone. Otherwise it's sealed pretty tight. I'll be interested to see if adding another vent changes anything. And I'll say too, that it's fantastic that new electronics are coming out regularly. My own flight was a beta for a new radio-control flight event controller designed for this project, (Triplefire - available at transolve.com) but I would never have imagined a baro-only unit could be so nicely improved. Not to mention the various new gps/telemetry units, etc. Very encouraging!

And sometimes it's better to be lucky!

Jerry O

Hi Jerry

I'm going to LDRS so we'll finally meet face to face.

[dixontj93060]

The Stratologger is probably running a Kalman filter (Dave Schultz, aka Quickburst, wrote several research papers on applying Kalman filters to MAWD altimeter data) and they do a great job of predicting apogee even with noisy data.

Although both from Texas, unless I'm mistaken, David Bachelder DBA QuickBurst, is not the same as my friend, Dave Schultz (Kalman filter research).

[bobkrech]

Although both from Texas, unless I'm mistaken, David Bachelder DBA QuickBurst, is not the same as my friend, Dave Schultz (Kalman filter research).

You are not mistaken, I screwed up.

I know them both thru TRF and many e-mails over a lot of years.

Doh. David Shultz is UhClem, King of the Kalman, and David Bachelder is QuickBurst, the fastest ignition in Texas. Don't know how I could confuse the two.

The world is full of Dave's. My office mates a Dave and I work with 10 other Daves.

Must be the heat in New England making my mind into mush......

Bob