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cerving

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And main at apogee is common enough. But probably due to a secondary issue if it wasn't planned.
I had it happen with my Double Shot at 14,000'... loose shear pins. It landed 5 1/2 miles away... yes, I found it.
 

heada

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t-0 launch
t+1.25 launch detect
t+1.5 to 2.6 pressure oddity
t+17.65 motor backup eject (huge pressure spike of 1,000' drop)
t+17.95 apogee detect
t+18.95 apogee charge fire
t+195.95 main charge fire
 

Banzai88

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Wouldn’t the dip / downward spike equate to high pressure inside the eBay?
Yes. He's asking if the MAIN was sitting on a piston in the forward section of the rocket. The aft shift of weight of the recovery bundle during boost might have been enough of a pressure spike in a leaky av bay to cause a spike in pressure.
 

jbrracer

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OK, that rules out CTI forward closure blow-by issues causing an ebay pressure spike for the early flight anomaly. I would think that in an AT motor, such a failure would otherwise be catastrophically kinetic based on the case design.

The motor case is fine no damage no signs of leakage.
 

jbrracer

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Yes. He's asking if the MAIN was sitting on a piston in the forward section of the rocket. The aft shift of weight of the recovery bundle during boost might have been enough of a pressure spike in a leaky av bay to cause a spike in pressure.
No pistons in this rocket.
 

Banzai88

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Was a motor delay used as backup to the apogee separation charge? It's entirely possible that variance in the delay time blew the motor ejection charge at the same time that the avionics charge went off, causing a massive pressure spike that blasted the forward section hard enough against the recovery harness that the nose shook off and you had an un-commanded deployment of the main-at-apogee.
 

Charles_McG

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Wouldn’t the dip / downward spike equate to high pressure inside the eBay?
Yes. Hence the search for a source of extra gas for pressure. A piston on top of the avbay, shifting down under thrust would compress the gas below and if it leaked into the avbay would make a low-altitude anomaly.

But I think you've found a likely culprit for the main-at-apogee event. Whatever happened at T+1.85sec, it didn't really matter to the rest of the flight.

But I'd track it down before trying staging. Swings like that really interfere with your staging ignition safety limits.
 

jbrracer

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Interesting theory, but I would think it strange that it would have enough blow-by pressure to spike the av bay, but not enough to separate the rocket prior to apogee and have a flight failure.

Was this a CTI motor? The 54s are known to blow-by the fwd o-ring, and it seems that more and more reports of it happening with the 38s are becoming common place, too (I've had 2 happen last year myself before learning about gluing the fwd closure).

@jbrracer, is there a shear pin/pins holding the av bay to the booster fin can? How long is the recovery harness from the fin can to the av bay? If there were a blow-by puff that resealed, you got very very lucky.

You say that you've flown this av bay before, but has it been in this same rocket? The whole bay or just the avionics sled?

Was a motor delay used as backup to the apogee separation charge? It's entirely possible that variance in the delay blew the motor ejection charge at the same time that the avionics charge went off, causing a massive pressure spike that blasted the forward section hard enough against the recovery harness that the nose shook off and you had an un-commanded deployment of the main-at-apogee. Regardless, the descent data looks like the main came out at the top instead of 1K feet like you planned.
The entire 4 inch eBay has been flown before visually successful.
 

jbrracer

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Yes. Hence the search for a source of extra gas for pressure. A piston on top of the avbay, shifting down under thrust would compress the gas below and if it leaked into the avbay would make a low-altitude anomaly.

But I think you've found a likely culprit for the main-at-apogee event. Whatever happened at T+1.85sec, it didn't really matter to the rest of the flight.

But I'd track it down before trying staging. Swings like that really interfere with your staging ignition safety limits.
We had high wings that day 12-16 mph ground level. Is that suspect of that big positive pressure? No pistons in this rocket.
 

jbrracer

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Yes. Hence the search for a source of extra gas for pressure. A piston on top of the avbay, shifting down under thrust would compress the gas below and if it leaked into the avbay would make a low-altitude anomaly.

But I think you've found a likely culprit for the main-at-apogee event. Whatever happened at T+1.85sec, it didn't really matter to the rest of the flight.

But I'd track it down before trying staging. Swings like that really interfere with your staging ignition safety limits.
I haven’t even considered air starts yet I’m still pretty green to HPR. I do appreciate the heads up although!
 

heada

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I've built many 4" LOC av-bays. How are you sealing around the eye-bolt and both threaded rods? How are you sealing around the wire for your ejection charges? They leak gases around all those if you're not doing something to seal them. For the eye-bolt and rods, I use neoprene washers and I hot-glue around the wire for my ejection charges.
 

Charles_McG

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We had high wings that day 12-16 mph ground level. Is that suspect of that big positive pressure? No pistons in this rocket.
To me, no. It climbed the first 300' or so as anticipated. Others have reported that winds aloft that day were much higher - thus an even longer recovery walk for you. But you said elsewhere that you had 3 1/4" vent holes - which seems like more than enough. At 300', you'd also have seen if it suddenly changed angle if wind shear shoved the angle of attack around drastically.
 

jbrracer

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To me, no. It climbed the first 300' or so as anticipated. Others have reported that winds aloft that day were much higher - thus an even longer recovery walk for you. But you said elsewhere that you had 3 1/4" vent holes - which seems like more than enough. At 300', you'd also have seen if it suddenly changed angle if wind shear shoved the angle of attack around drastically.
Can the av vent holes be too large, not necessarily resulting in these events but in general?
 

Banzai88

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Was the motor ejection charge used or did you leave out the black powder ejection charge?

What's the variance allowed on a delay grain and motor burn time? Isn't it like +/-10%?

Motor burn of an ATK540M is listed as 2.9 seconds.
Delay time is listed as 14 seconds.
2.9 burn + 14 delay + nominal burn time variance .29 seconds + nominal delay variance 1.4 seconds = 18.59 seconds.

That's amazingly close to the T @ N-O: 18.950 data reported.

If that's the case, AND your avionics triggered the drogue event, that's probably well more than enough energy to blast the forward section to the limits of the recovery harness and shake the nose off/main out.
 

heada

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I use 3x 1/4" vent holes on my 4" av-bays and never had any pressure oddities like that but I also don't launch when surface winds are that high.
 

Banzai88

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Can the av vent holes be too large, not necessarily resulting in these events but in general?
It's been discussed that there is a practical upper limit, but that's usually been calculated as some absurdly large number normally measured in hole saw size vs. drill bit size.

Most folks use these sizes without issue. One or two sizes up doesn't seem to cause an issue like too small does. 4 vs. 3 holes seems to cause issues only in the highest of high winds and bad luck in getting wind blown directly through the av bay.

 
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heada

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Was the motor ejection charge used or did you leave out the black powder ejection charge?

What's the variance allowed on a delay grain and motor burn time? Isn't it like +/-10%?

Motor burn of an ATK540M is listed as 2.9 seconds.
Delay time is listed as 14 seconds.
2.9 burn + 14 delay + nominal burn time variance .29 seconds + nominal delay variance 1.4 seconds = 18.59 seconds.

That's amazingly close to the T @ N-O: 18.950 data reported.
Motor eject happened at 17.65 seconds. (altitude went from 6466 to 5854 in 50ms)
The TRS reports firing the drogue at 18.95
 

Banzai88

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Motor eject happened at 17.65 seconds. (altitude went from 6466 to 5854 in 50ms)
The TRS reports firing the drogue at 18.95
Ahh....got my head wrapped around it now. I pulled the wrong data point.
 

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@jbrracer , do you have a sim of this flight? Could you export the T, Alt, V data and post the file? I use something a bit stronger than Excel and can easily merge and overlay the data.
 

cerving

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One question. How lol
How did they come loose, or how did I find it? #1, I overdrilled the holes. #2, Eggfinder, of course. BTW, after this recovery I did two things... first, I went out and bought a used Suzuki Quadsport for recoveries, second, I developed the GPS module for the LCD receiver. Together, they make these really long recoveries a lot easier.
 

jbrracer

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How did they come loose, or how did I find it? #1, I overdrilled the holes. #2, Eggfinder, of course. BTW, after this recovery I did two things... first, I went out and bought a used Suzuki Quadsport for recoveries, second, I developed the GPS module for the LCD receiver. Together, they make these really long recoveries a lot easier.
I didn’t see it was you Cris or wouldn’t have asked how you found it lol
 

Buckeye

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These altitude dips are typical of passing through Mach, but this rocket was nowhere near that speed (quick sim in Thrustcurve). It is unlikely any weird pressurizations took place, other than maybe strong winds blowing into the av-bay. I simply chalk it up to the altimeter flaking out briefly and then returning to normal.

mm.png
 

jbrracer

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@jbrracer , do you have a sim of this flight? Could you export the T, Alt, V data and post the file? I use something a bit stronger than Excel and can easily merge and overlay the data.
hopefully this is what you want.

Also I find it interesting that OR sim is max 4748 and G=20 when I got very different results (see above)
also just for poops and giggles I tossed the same data into thrust curve and get

6215ft 20.27g

Why and where is all this so far off.
 

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Banzai88

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hopefully this is what you want.

Also I find it interesting that OR sim is max 4748 and G=20 when I got very different results (see above)
also just for poops and giggles I tossed the same data into thrust curve and get

6215ft 20.27g

Why and where is all this so far off.
You would have to also post your open rocket file so that we can pick through it to be able to answer that question, but quite often it's because form factor and actual weight in the sim does not match the real world item.

If those are correct, then play with overall surface finish. Most of us get to within 5% easily, and many get within 1%
 
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