Certing Level 2 tomorrow with a CTI K675 Skidmark!! WOOOOOOOOO!!!!

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Mach_Seven

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So sorry for you, family. How old was your pup?
Only 7! Went crippled about 6 weeks ago. Then the cancer just ravished the poor little guy. But he was in good spirits the whole time. He just took life as it came. We knew time was short, so his diet the last month was fit for a king: steaks, chicken, hard boiled eggs, salmon, sweet potatoes, watermelon, and ice cream. He was deteriorating rapidly yesterday (unbeknownst to me because I was at the launch and nobody wanted to concern me with it), so when I got home, we invited all his friends over, grilled him a beautiful ribeye with salted caramel ice cream for dessert, and surrounded him with love. It was a beautiful evening so we did it outside.

Thanks for the condolences. Sincerely. It's incredible how these little creatures can steal our hearts without ever saying a word.
image_50379521.JPG
 

UhClem

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Go look up Euler buckling load.
What is important is the longest unsupported length. The motor mount and centering rings stiffen the fin section a lot so it isn't a problem. The altimeter bay tends to do the same. That long stretch of tube between the two on the other hand...

Also, the important material parameter isn't absolute strength but elasticity. Paper doesn't do very well there and of course carbon fiber does much better.

The formula is linear in modulus of elasticity but quadratic in length. So shortening the tube has a big effect.
 

bobbyg23

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Just curious if you have pictures of how the fin can was constructed?
 

SDramstad

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I took M7's sim and ran the plot with the K675, attached.

It sims well above mach 1, but seems less than mach 2 based on altitude, if I understand the velocity/altitude tables correctly.

Does anyone see anything interesting? Seems to me that it would have been at mach 1.5-ish at the 2.5 second-ish change of direction and I didn't think that would be a danger zone for fin flutter due to a mach transition.

Having said that and based on previous issues with rotating machinery and natural frequencies, does the fin shape, material, attachment etc., give a fin a natural frequency that could be excited by an air flow across it that is totally unrelated to the issues that happen during a mach transition? Seems likely based on 'Galloping Gertie'. . .

Sandy.
Can you run that again with the stability graphed?
 

Mach_Seven

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I've never done that before, so all I did was click the drop-down for stability vs time. If there's a better way, please let me know.

Sandy.
Just an FYI - regardless of what the sim says, the CG was 2.3 calibers ahead of the CP when fully loaded (including motor).
 

Sandy H.

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Just an FYI - regardless of what the sim says, the CG was 2.3 calibers ahead of the CP when fully loaded (including motor).
Understood. I've never looked at that plot and it seems weird to me by default, so I wanted to make sure to say I was just posting what somebody wanted to see. I'm not entirely sure how to interpret that graph or if launch-site conditions would matter.

Sandy.
 

Mach_Seven

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Sorry to hear about the disaster! That was a nice rocket. I’m looking forward to your next attempt.

You mentioned the high G acceleration of that motor in this rocket a couple of times. Do you think it was the acceleration that caused the failure, or the speed? I would tend to suspect the rocket folded due to aerodynamic forces due to speed, not due to the high G forces.

What do you think might have happened with different motor choices?

For example, if you picked a CTI J1520, that would have more thrust and greater acceleration, but lower total impulse and lower maximum speed. Do you think it would still fail due to the acceleration?

On the other hand, if it was a K300, that would have lower peak thrust and lower acceleration, but more total impulse, and would probably achieve a similar speed to your K635 and sustain it for longer. Do you think it would hold up due to lower acceleration? Or would it fold due to the speed and aerodynamic forces?

I’m not sure the answer, but it’s something to think about. Good luck on your next attempt.
Sometimes I conflate the two. Aerodynamic forces is the likely culprit. Not acceleration.
 

Tim51

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Except that is exactly how the certification process is structured.
Good discussion. Here in the UK, it would not be possible for a flier to cert L3 on an M then go straight to higher impulse without obtaining special large rocket scheme insurance, so that effectively adds another review point - not another cert, but a self regulatory check, involving the flier and UKRA. Obviously, the solid research motors you cite are not covered (as home made solid propellant isn't legal under UK law, although research hybrids are), but the point stands.
Personally my main concern re certifications is that (afaik) all rocketry organisations currently allow L2 certification with motor ejection, yet many L2 motors are plugged. So the L2 practical test flight does not test the full range of skills one would need to safely use all L2 motors.
 

KenECoyote

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My thinking was, "If I don't intend to fly J motors, why cert with one?" If I fail my Cert attempt, than I am not ready for these big motors yet.
Actually maybe a flight using a J motor to maiden and test the airframe first (while also certifying) would've been helpful... inspection of the rocket, data and flight video after might've shown some anomalies (ex. stress marks in the airframe, fin flutter, data or camera showing some anomaly, etc.). Sometimes going back to basics makes sense and even Estes recos first flights on smaller motors before bigger. Plus I really like J motors, so maybe I'm biased. 🙂
Btw It was a beautiful build...your skills are remarkable!
 
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Mach_Seven

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Actually maybe a flight using a J motor to maiden and test the airframe first (while also certifying) would've been helpful... inspection of the rocket, data and flight video after might've shown some anomalies (ex. stress marks in the airframe, fin flutter, data or camera showing some anomaly, etc.). Sometimes going back to basics makes sense and even Estes recos first flights on smaller motors before bigger. Plus I really like J motors, so maybe I'm biased. 🙂
Btw It was a beautiful build...your skills are remarkable!
Thanks for that last line. I needed it! My confidence is a bit rattled right now.
 

Mach_Seven

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One thing about cert flights is that there really isn’t a penalty for failing. You just try again later. But they do attract more attention than other flights, so it could be embarrassing. And you do have to involve others as your witnesses, so if there are too many failures, it starts wasting other people’s time. One failure or even two is ok, but you don’t want make a habit of it!

In general, I don’t think it’s a great idea to try something new on a cert flight. It should not be your first DD flight, for example. But as long as you’ve already practiced your techniques before on rockets and motors within your current certification class, I don’t see anything wrong with an “aggressive” certification flight if that’s what you like.
Good point. My previous Iris performed beautifully before she ended up in the power lines (another hard lesson learned). This was just a "suped-up" version with improvements I wanted to make (like an external "ARM" switch) with the same proven recovery system. I fiberglassed tip-to-tip for the first time and double-walled it. I really thought she was good to go.

If I had to list ten ways the rocket would fail in order of likelihood, the airframe folding over during boost would've been number ten.
 

Steve Shannon

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Sometimes I conflate the two. Aerodynamic forces is the likely culprit. Not acceleration.
I would suspect a combination of both. But one thing appears clear to me. The parachute came out while the rocket was under thrust. I don’t think there’s no other way to veer 90° so rapidly. That would have folded the rocket over if it weren’t already. What I don’t know is whether the nosecone came off and the chute came out because the airframe began to fail (that’s what I would suspect) or whether the nosecone came out for some other reason which then resulted in the sharp 90° turn and then breaking the airframe.
 

Mach_Seven

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I would suspect a combination of both. But one thing appears clear to me. The parachute came out while the rocket was under thrust. I don’t think there’s no other way to veer 90° so rapidly. That would have folded the rocket over if it weren’t already. What I don’t know is whether the nosecone came off and the chute came out because the airframe began to fail (that’s what I would suspect) or whether the nosecone came out for some other reason which then resulted in the sharp 90° turn and then breaking the airframe.
The nose cone, payload section, and avionics bay were all screwed together into one unit. The entire unit was still secured to a piece of the booster section with shear pins when I found it. The parachute came out of the airframe - not ejected like normal.
 
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Steve Shannon

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The nose cone, payload section, and avionics bay were all screwed together into one unit. The entire unit was still secured to a piece of the booster section with shear pins when I found it. The parachute came out of the airframe - not ejected from the like normal.
Pardon me, when I say the nosecone came out I mean whatever must dislodge in order for the parachute to come out, in this case the payload section, avionics bay, and nosecone assembly (which you told me before, but I didn’t remember). What I’m getting at is that something caused the rocket to take an almost instantaneous right angle turn. That doesn’t happen (in my experience) except if the parachute comes out under thrust. Fin shredding will do something similar but more slowly and with parts maintaining the original direction. Some motor problems could cause the same sudden right angle abrupt turn as well.

Since the shear pins were still retained to the lower section I would agree that the airframe failed first. Sorry for my confusion.
 

Banzai88

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54mm long CTI motor with a relatively long burn time..........entirely possible that front closure blow by pressurized the body tube and forced early separation, or at least forced enough separation to unbalance aerodynamic forces enough to cause the tube to fold over.

After all, CTI 54s, especially the longer ones, have known issues, which is why many folks put a second o-ring in (if there is a second groove) and/or also 5 min epoxy the fwd closure in place.
 

Mach_Seven

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54mm long CTI motor with a relatively long burn time..........entirely possible that front closure blow by pressurized the body tube and forced early separation, or at least forced enough separation to unbalance aerodynamic forces enough to cause the tube to fold over.

After all, CTI 54s, especially the longer ones, have known issues, which is why many folks put a second o-ring in (if there is a second groove) and/or also 5 min epoxy the fwd closure in place.
In this scenario, wouldn't the seperation be at the actual separation point? It seems unlikely to me it would blow out the airframe without separation. The upper section was still connected to the booster air frame piece with the shear pins. I'm still trying to locate the booster, so examining the casing isn't possible yet.
 

Banzai88

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In this scenario, wouldn't the seperation be at the actual separation point? It seems unlikely to me it would blow out the airframe without separation. The upper section was still connected to the booster air frame piece with the shear pins. I'm still trying to locate the booster, so examining the casing isn't possible yet.
Perhaps, depends on overall failure mode. That high speed gas from grain combustion cuts through the o-ring and around the forward closure and out the top of the motor casing right by the ejection charge well, and that super heated gas flows directly into the body tube. On two occasions over the last 5 or 6 years, I've seen the results burn through/blow out the body tube at the top of the motor. On the second one, the shear pins were still holding the avionics bay, similar to yours.

Fwd closure blow by is a known issue with CTI 54s in 5, 6, 6XL, and has become a sporatic issue with 38s also in 5, 6, 6XL.
 
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Mach_Seven

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Perhaps, depends on overall failure mode. That high speed gas from grain combustion cuts through the o-ring and around the forward closure and out the top of the motor casing right by the ejection charge well, and that super heated gas flows directly into the body tube. On two occasions over the last 5 or 6 years, I've seen the results burn through/blow out the body tube at the top of the motor. On the second one, the shear pins were still holding the avionics bay, similar to yours.
Wow. Interesting. A new theory in the mix. I've got to find that booster section. Nobody in my party saw it come down. I thought I did, but it was the forward section. When I got home and watched the video, you can clearly hear the LCO say "We've got a piece coming down over here still smoking." I never heard it onsite and was wandering aimlessly looking for it. Trying to contact him now to at least narrow down the search area. The grass and shrubs are tall enough you practically have to step on something to find it. Wish me luck.
 

Homer_S

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Or knowing your limitations. Which you shouldn't really be exploring with a certification flight. That should be something simple and low risk so you can clear that low bar. (it really is pretty low)

Then do something stupid. :)
Bummer of a flight :eek:,

I failed on my first two attempts at L1 with a "scratch built" scaled up rocket:

1) Forgot that Chute Release turns off after a ground test, it is even in the manual.
2) Calculated BP to put in ejection charge and then put in 20-50% more after forgetting to measure. This led to separation of the payload bay from the fin can as the overpowered charge blew out my baffle mount point.

Both halves were recovered, repaired and re-flown.

For my successful attempt, I went kit and did a test launch with the cert flight profile and a motor below L1 range.

All of this was valuable learning for my eventual L2 model, next year or two. I guess my point is that maybe a helper for certification is test flying the airframe before the cert flight. Might or might not have failed in a less catastrophic way. We tend to ground test most things but not always a lower power full flight.

Homer
 
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Steve Shannon

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Wow. Interesting. A new theory in the mix. I've got to find that booster section. Nobody in my party saw it come down. I thought I did, but it was the forward section. When I got home and watched the video, you can clearly hear the LCO say "We've got a piece coming down over here still smoking." I never heard it onsite and was wandering aimlessly looking for it. Trying to contact him now to at least narrow down the search area. The grass and shrubs are tall enough you practically have to step on something to find it. Wish me luck.
Good luck!
 

SDramstad

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I had something similar happen to my level 2 rocket 3 or 4 flights after I certed with it. I flew it several times before sticking an Aerotech L1090 in it. It turned sideways and ripped apart just before burn out. Best guess is the CP slid forward enough going supersonic to make it tumble under power. Shredded the fiberglass tube just as abruptly as yours did. Recovered all of the pieces, rolled a new tube and added a few ounces of shot to the nose cone. It has now flown several times on that motor without failure. Good luck next time!
 

RocketScientistAustralia

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I asked Chuck Rogers(RASAero) Rogers Aeroscience, to see if he could sim your camera shroud and see if the transonic shock wave was throwing out the CP. The Camera shroud is in line with one of the fins. Certainly with the position and size of this shroud there is a possibility of this shockwave preventing one of your fins being seen by the airflow. Your video shows a wobble before break up at about the point it would have been in that speed region. As you go transonic, your dynamic forces increase dramatically. Also your camera shroud would be giving you quite a kick sideways at the point your airframe was approaching max load and perhaps shielding one of your fins on a longer than normal airframe.
One the things with rockets, is that when they fail, it all happens really, really quickly. For anyone there, it's difficult to know what happened. For anyone not there, we're just doing our best to try and figure what happened with a best guess theory.
If Chuck gets some free time and can sim that shroud , it will be interesting to see what he says.
Good luck with the rebuild.
Norm
 
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Banzai88

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I asked Chuck Rogers(RASAero) Rogers Aerospace, to see if he could sim your camera shroud and see if the transonic shock wave was throwing out the CP. The Camera shroud is in line with one of the fins. Certainly with the position and size of this shroud there is a possibility of this shockwave preventing one of your fins being seen by the airflow. Your video shows a wobble before break up at about the point it would have been in that speed region. As you go transonic, your dynamic forces increase dramatically. Also your camera shroud would be giving you quite a kick sideways at the point your airframe was approaching max load and perhaps shielding one of your fins on a longer than normal airframe.
One the things with rockets, is that when they fail, it all happens really, really quickly. For anyone there, it's difficult to know what happened. For anyone not there, we're just doing our best to try and figure what happened with a best guess theory.
If Chuck gets some free time and can sim that shroud , it will be interesting to see what he says.
Good luck with the rebuild.
Norm
Interesting. I'm contemplating the location to put a RunCam shroud on a mach capable rocket. Thanks to this, I'll be sure to locate it between fins!
 

Mach_Seven

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I asked Chuck Rogers(RASAero) Rogers Aerospace, to see if he could sim your camera shroud and see if the transonic shock wave was throwing out the CP. The Camera shroud is in line with one of the fins. Certainly with the position and size of this shroud there is a possibility of this shockwave preventing one of your fins being seen by the airflow. Your video shows a wobble before break up at about the point it would have been in that speed region. As you go transonic, your dynamic forces increase dramatically. Also your camera shroud would be giving you quite a kick sideways at the point your airframe was approaching max load and perhaps shielding one of your fins on a longer than normal airframe.
One the things with rockets, is that when they fail, it all happens really, really quickly. For anyone there, it's difficult to know what happened. For anyone not there, we're just doing our best to try and figure what happened with a best guess theory.
If Chuck gets some free time and can sim that shroud , it will be interesting to see what he says.
Good luck with the rebuild.
Norm
The camera shroud was not in line with any of the fins. It rested directly between two of them in line with the external switch and the graphics. If that’s how it was showing in open rocket it was a mistake. The last time I saved it I may have had it in the wrong position.

I really appreciate you guys’ input. It’s been very educational.
 

UhClem

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I've never done that before, so all I did was click the drop-down for stability vs time. If there's a better way, please let me know.
Notice how it starts out unstable? The greater the length to diameter ratio, the more the CP moves with non-zero angle of attack. Also, I don't see the camera shroud or even a stand in and that couldn't help the CP at all.
 

RocketScientistAustralia

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Interesting. I'm contemplating the location to put a RunCam shroud on a mach capable rocket. Thanks to this, I'll be sure to locate it between fins!
if you're going through mach, make it as streamlined as possible. Or inside the tube. At the CG would be optimum for minimum twist. But you'd still get a sudden increase in side force as you go transonic if it's not balanced up with another lump on the opposite side. I think......... Its only one mans opinion...... :)
 

rklapp

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The camera shroud was not in line with any of the fins. It rested directly between two of them in line with the external switch and the graphics. If that’s how it was showing in open rocket it was a mistake. The last time I saved it I may have had it in the wrong position.

I really appreciate you guys’ input. It’s been very educational.
From your earlier photo, the shroud was attached to the ebay. Do you have a photo of it on the pad? It looks like a complicated rocket and admire your bravery and perseverance.
 
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