PerfectFlite SL100 vs SLCF

Last Saturday I flew my Wildman Darkstar 4" DD using a PerfectFlite Stratologger 100 as Primary Altimeter and PerfectFlite Stratologger CF as Backup Altimeter. I'm trying to determine why the Main chute came out at Apogee. I'm thinking two 2-56 shear pins was not enough to hold pressure difference at over 6100 feet, but not sure. I did have pressure relieve hole in Payload section. Also thinking maybe need more that 1 sec apogee delay on Backup. Anyways, I've attached the Altitude/Velocity vs. Time graphs from each altimeter below. Graphs are pretty close except the larger velocity spike at 20 seconds on the SLCF. Anyone have any ideas why this difference between the two altimeters?

View attachment Flight2A-Darkstar4DD.pdf
View attachment Flight2B-Darkstar4DD.pdf

The velocity spikes are artifacts. They are not an indication of the velocity, but rather an indication of pressure which is how they are obtained.

The velocity is calculated as a delta altitude divided by delta time. This is obtained from a delta pressure divided by delta time. You get a pressure spike when the ejection charge goes off. You don't change your altitude, but you see a pressure spike that appears to be a big delta velocity.

As the PF altimeters sample every 0.050 seconds and are not synchronized timewise, you are randomly time sampling with respect to the pressure burst and since it is far faster than the sampling time, the recorded pressure increase is not indicative of the real value.

Bob

bobkrech said:

You get a pressure spike when the ejection charge goes off. You don't change your altitude, but you see a pressure spike that appears to be a big delta velocity.

Click to expand...

Why pressure spike? Altimeters are in separate section from BP charges.

I just had the same thing happen to me running the same setup, SL100 main, SLCF backup and my main popped right @ apogee, 3300, resulting in a long walk. However in my case it was not a mystery...I inserted the altimeter tray in upside down . After that launch the first thing I did when I got home was to key the sled so it will only fit in into the bay one way. You could literally assemble the rocket with your eyes closed now.

Now interestingly enough I was expecting to see more of a delta between the numbers reported from each altimeter. Although there were some differences, the altitude reported on the two launches was literally a 2' delta on the first launch and I think a 3' on the second.

Back to the nosecone. Is it filament wound? I think three or four is better, or possibly 4-40 screws.

The main charge didn't fire at apogee, so this was probably due to the shear pins being insufficient to hold against the shock of the apogee deployment. Use a longer or more springy shock cord or do something else to lessen the shock (masking tape on the shock cord, for example) and perhaps reduce the size of your apogee charge. I've never felt the need to use 4-40 screws on a rocket this size.

jeff2space said:

Why pressure spike? Altimeters are in separate section from BP charges.

Click to expand...

I see it all the time on both dual deploy and motor ejection only flights where the altimeter is in its own protected bay. I think you just get a high pressure area around the rocket for a fraction of a second immediately after ejection/separation and the altimeter records it. I have also seen an inverted spike at separation (down) and also one spike down and then immediately one spike up.

jeff2space said:

Why pressure spike? Altimeters are in separate section from BP charges.

Click to expand...

Because they are very sensitive. With a 24-bit digital A/D built into the pressure sensor, the altitude resolution is less than 1', that's really sensitive.

Bob

Another thought - how long is your drogue shock cord? Had that happen to me twice. All I did was lengthen my drogue shock cord.

bobkrech said:

The velocity spikes are artifacts. They are not an indication of the velocity, but rather an indication of pressure which is how they are obtained.

The velocity is calculated as a delta altitude divided by delta time. This is obtained from a delta pressure divided by delta time. You get a pressure spike when the ejection charge goes off. You don't change your altitude, but you see a pressure spike that appears to be a big delta velocity.

As the PF altimeters sample every 0.050 seconds and are not synchronized timewise, you are randomly time sampling with respect to the pressure burst and since it is far faster than the sampling time, the recorded pressure increase is not indicative of the real value.

Bob

Click to expand...

Right. Also, differentiating noisy altitude data gives even noisier velocity. If you really want to study velocity and acceleration, it is best to use an accelerometer altimeter and integrate, which is numerically more precise.

Zeus-cat said:

I see it all the time on both dual deploy and motor ejection only flights where the altimeter is in its own protected bay. I think you just get a high pressure area around the rocket for a fraction of a second immediately after ejection/separation and the altimeter records it. I have also seen an inverted spike at separation (down) and also one spike down and then immediately one spike up.

Click to expand...

Right. These spikes are what your altimeter beeps out as maximum altitude, which can lead you astray, especially if trying to match a simulation. The proper altitude measured needs to be interpreted from the time trace.

timbucktoo said:

Another thought - how long is your drogue shock cord? Had that happen to me twice. All I did was lengthen my drogue shock cord.

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Drogue is 14 feet of 3/8" tubular Kevlar shock cord attached to 1" Kevlar strap. Drogue Primary charge was 2.5 grams BP and Drogue Backup charge was 3.0 grams.

jeff2space said:

Drogue is 14 feet of 3/8" tubular Kevlar shock cord attached to 1" Kevlar strap. Drogue Primary charge was 2.5 grams BP and Drogue Backup charge was 3.0 grams.

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Just my opinion but thats a fairly short drogue shock cord for that rocket. Plus kevlar has very little stretch. I'm using 24' on my 3" Darkstar. As a general rule of thumb, I use 4-5 times the length of my rocket.

timbucktoo said:

Just my opinion but thats a fairly short drogue shock cord for that rocket. Plus kevlar has very little stretch. I'm using 24' on my 3" Darkstar. As a general rule of thumb, I use 4-5 times the length of my rocket.

Click to expand...

+1

Tubular nylon has some spring to it. I typically use 25' of 1" tubular nylon based on other rule-of-thumb posts here. On a handful of DD flights with (2) 2-56 shear pins, I haven't had a premature main yet.

mpitfield said:

I just had the same thing happen to me running the same setup, SL100 main, SLCF backup and my main popped right @ apogee, 3300, resulting in a long walk. However in my case it was not a mystery...I inserted the altimeter tray in upside down .

Click to expand...

What does this mean? What difference does it make which way a barometric altimeter is oriented?

Or do you mean the main was wired as the drogue and vice versa?

jeff2space said:

Drogue is 14 feet of 3/8" tubular Kevlar shock cord attached to 1" Kevlar strap. Drogue Primary charge was 2.5 grams BP and Drogue Backup charge was 3.0 grams.

Click to expand...

Your charges sound way too aggressive. Did you ground test? My 4" Frenzy uses 1.5g BP for both drogue and main. Main has three #2-56 shear pins, as well.

Buckeye said:

Your charges sound way too aggressive. Did you ground test? My 4" Frenzy uses 1.5g BP for both drogue and main. Main has three #2-56 shear pins, as well.

Click to expand...

I did ground test. Think I need to look over everything again, shear pins, shock cord length, and BP charge. Thanks.

Buckeye said:

Your charges sound way too aggressive. Did you ground test? My 4" Frenzy uses 1.5g BP for both drogue and main. Main has three #2-56 shear pins, as well.

Click to expand...

jeff2space said:

I did ground test. Think I need to look over everything again, shear pins, shock cord length, and BP charge. Thanks.

Click to expand...

I agree, I think the setup should be looked at again. My 4" rocket has 18" of empty space in the lower tube. I use 1.5g for the drogue and about 18' of 9/16 tubular nylon shock cord. The thing about drogue ejection is that you only need to get the drogue chute into the air stream. As long as that happens, the drogue will hold back the payload and the fin can will fall faster pulling any remaining shock cord out of the rocket. I tried z-folding my drogue cord, but it ended up landing without all of the taped z-folds opening. I've had recovery issues over the years, but never with the drogue chute.

My recommendation would be to ground test the drogue charge so it stretches the shock cord about 1/2 to 3/4 it's full length, especially if it's sliding on the ground. YMMV but that's worked for me.

Buckeye said:

What does this mean? What difference does it make which way a barometric altimeter is oriented?

Or do you mean the main was wired as the drogue and vice versa?

Click to expand...

The sled is hard wired to terminal blocks on each end, and the sled could, but not now, slide into the bay either way. So yes when the drogue fired the main was deployed. This is what happens when you are too busy talking and not paying attention while you should be focused prepping the rocket. I have seen others do this and always thought, how could they do such a thing, turns out it is much easier than I thought

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