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

[3stoogesrocketry]

There were three 3/16” holes in the booster, one in the switch band, and one in the payload bay. There were also two 1/4” holes in both av bay bulkheads. The failure occurred at 1800 feet. Is that enough of a pressure change to burst the airframe do you think if it wasn’t vented?

Perfect it sounds well vented. If the airframe was not vented , it would not burst the airframe. What would happen though is the sudden pressure drop from launch to 1800 feet AGL would cause the pressure inside the airframe to separate the rocket under boost . In my case a 3 inch minimum diameter on a 6000 ns motor popped my cone off ( held on by 3 shear pins ) and threw the main at mach 1.7+.

 

[FredA]

Is there some skillset that we should require to be used before we certify someone?

Heck yes.....problem is we can't agree what the skill set should be.....
I personally think it must include a flight that goes mach or faster plus DD that follows the planned profile....but that's me....
Others seem to think everything/anything goes as long as nobody got hurt.
Must be a rational middle ground.

 

[Steve Shannon]

Heck yes.....problem is we can't agree what the skill set should be.....
I personally think it must include a flight that goes mach or faster plus DD that follows the planned profile....but that's me....
Others seem to think everything/anything goes as long as nobody got hurt.
Must be a rational middle ground.

That’s not a skill set, that’s a performance threshold and I agree that we are divided as a hobby about what should be required. Part of that has already been discussed above. What can be easily tested in the wider spaces of the west can’t be done at all at some launch sites in the east.
A skill set would be something like simulating flights or experience in laminating body tubes.

 

[ThirstyBarbarian]

I wouldn't post in these forums if I didn't have thick skin or couldn't handle some hard truths! lol I truly believed this rocket could handle the impulse. 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. My pride hates me for saying that, but the simple truth of the matter is this: ROCKETRY IS HARD!

I still think the Open Rocket sim of 19 G's was off somehow. Looked more like 30. It was doing 600 mph and still accelerating less than three seconds into flight.

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.

 

[UhClem]

A skill set would be something like simulating flights or experience in laminating body tubes.

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.

 

[Exactimator]

There were three 3/16” holes in the booster, one in the switch band, and one in the payload bay. There were also two 1/4” holes in both av bay bulkheads. The failure occurred at 1800 feet. Is that enough of a pressure change to burst the airframe do you think if it wasn’t vented?

What does the MW manual say for the qty and size of av bay vent holes? I can't tell from the video if the rocket shredded the vents happened or vice versa.

 

[Steve Shannon]

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.

Certification flights should reflect what the candidate knows and can repeatedly do. There’s no value to having a person successfully certify on a six inch fiberglass kit and M1297 just so that person is allowed to fly an R at a research launch as the next flight.

 

[Orien]

Certification flights should reflect what the candidate knows and can repeatedly do. There’s no value to having a person successfully certify on a six inch fiberglass kit and M1297 just so that person is allowed to fly an R at a research launch as the next flight.

Exactly my thinking. What if I certed on a J350, then slapped a full L in her on the next UNSUPERVISED, non-cert launch and she tears apart 100' off the ground and a hellacious L motor is doing loop-de-loops over the flight line? And what's the big deal about a re-cert? It doesn't cost anything.

 

[Banzai88]

Exactly my thinking. What if I certed on a J350, then slapped a full L in her on the next UNSUPERVISED, non-cert launch and she tears apart 100' off the ground and a hellacious L motor is doing loop-de-loops over the flight line? And what's the big deal about a re-cert? It doesn't cost anything.

That's where the RSO duties, coupled with experience and some safety charts, come in to play. Sadly, that is all too often just simply a 'shoot the breeze about me and my cool rocket' stop along the way to the pads/LCO.

And I'm not sure about you, but I don't get motors for free.

 

[ThirstyBarbarian]

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.

 

[Sandy H.]

Certification flights should reflect what the candidate knows and can repeatedly do. There’s no value to having a person successfully certify on a six inch fiberglass kit and M1297 just so that person is allowed to fly an R at a research launch as the next flight.

Your statement matches my personal belief very closely. I have been an L2 for almost 10 years and have only flown 1 K motor and a handful of J motors. I have been flying more simple rockets in recent years and would not build a brand new rocket intended to fly on an L and make that my next flight. I'd re-fly a few of my proven DD rockets to refresh my skills and then fly another K or two before putting up the new rocket. All that would just be for me to be more comfortable with that type of flying again vs. what I've been doing more recently.

I don't think it needs to be codified into the rules or something like that at all, though. I think we all need to follow the rules as written and if the club, RSO etc. have additional rules/limitations, we need to follow them as well. Finally, we add our own personal responsibility and things should generally go well.

The OP had a flight that seemed like it would be fine, but it just barely wasn't. It would be interesting to see flight data if it were available, just to know more about it so he and the rest of us could more clearly understand the exact failure and add that knowledge to our toolkits for future builds and flights.

Sandy.

 

[Steve Shannon]

Exactly my thinking. What if I certed on a J350, then slapped a full L in her on the next UNSUPERVISED, non-cert launch and she tears apart 100' off the ground and a hellacious L motor is doing loop-de-loops over the flight line? And what's the big deal about a re-cert? It doesn't cost anything.

That's it. It's worth pointing out also, that there are two different perspectives (at least. The approach that an organization must have with respect to certification is shaped by the need to award certification to those who are skilled enough to keep the hobby safe, yet deny those whose lack of knowledge, judgement, or skills might someday jeopardize the hobby, its participants, and the organizations that "regulate" it.

From a flyer's perspective, especially one whose priority is strictly certifying, UhClem is exactly right. The person who wants only to get certified does so easiest by keeping things simple and low risk.

 

[Orien]

Your statement matches my personal belief very closely. I have been an L2 for almost 10 years and have only flown 1 K motor and a handful of J motors. I have been flying more simple rockets in recent years and would not build a brand new rocket intended to fly on an L and make that my next flight. I'd re-fly a few of my proven DD rockets to refresh my skills and then fly another K or two before putting up the new rocket. All that would just be for me to be more comfortable with that type of flying again vs. what I've been doing more recently.

I don't think it needs to be codified into the rules or something like that at all, though. I think we all need to follow the rules as written and if the club, RSO etc. have additional rules/limitations, we need to follow them as well. Finally, we add our own personal responsibility and things should generally go well.

The OP had a flight that seemed like it would be fine, but it just barely wasn't. It would be interesting to see flight data if it were available, just to know more about it so he and the rest of us could more clearly understand the exact failure and add that knowledge to our toolkits for future builds and flights.

Sandy.

According to the RRC2:
Max alltitude: 1806'
Max velocity: 5230 fps

These are obviously skewed. She was going fast, but not a mile a second fast. That 1000+ G right turn might have scrambled it's eggs a little bit...

 

[Sandy H.]

According to the RRC2:
Max alltitude: 1806'
Max velocity: 5230 fps

These are obviously skewed. She was going fast, but not a mile a second fast. That 1000+ G right turn might have scrambled it's eggs a little bit...

I guess I was thinking about a logging altimeter. I wonder how it made it through the mach transition (i.e. did the shread happen right around there)? If so, I think that could point to fin flutter as the real root cause with the change in angle of attack folding the tube as the secondary effect. Just my thoughts.

If there wasn't a datalogging altimeter on board, I wonder if it would be realistic to sim the rocket (which you already did, obviously) and see what the speed would have been at the time the shread happened based on the video timing?

Sandy.

 

[UhClem]

There’s no value to having a person successfully certify on a six inch fiberglass kit and M1297 just so that person is allowed to fly an R at a research launch as the next flight.

Except that is exactly how the certification process is structured.

 

[Orien]

Perfect it sounds well vented. If the airframe was not vented , it would not burst the airframe. What would happen though is the sudden pressure drop from launch to 1800 feet AGL would cause the pressure inside the airframe to separate the rocket under boost . In my case a 3 inch minimum diameter on a 6000 ns motor popped my cone off ( held on by 3 shear pins ) and threw the main at mach 1.7+.

The av bay coupler was still in the booster section airframe and secured with shear pins when I found it.

 

[ThirstyBarbarian]

This is almost certainly the failure point. I found this in the field. Looks like she just folded right over. Goddammit so much…
View attachment 485976

It seems like if it folded there, it would have torn off right at the fold. I bet it failed where the tears are.

Or maybe the N sticker delaminated with such force it collapsed the entire airframe!

 

[OzHybrid]

200lbs at take off
175 for the majority of useful thrust. Transmitted through a cardboard tube. It's not a question of should I use a lower thrust motor and then what would happen if I accidentally put a much higher thrust motor in it. The thrust curves are all readily available. There would be nothing accidental or unknown about any new motor selection. All of your flights should be within the limitations of the materials you've used. You increased the length by 12". Go look up Euler buckling load. As you increase the length of an object it takes a lower load to create the same amount of buckling. Your max buckling load is in the middle of the "rod" Where the couplers tend to get put. So our favourite construction method inserts a strength reducer at one of the rocket points that gets maximum bending load.
Would I sit on that is a question all should ask before launching. The question is about right in general loading terms for most HPR flights. If the answer is no. Don't do it.
I wish you all the best in your next attempt. Just because you alone have to build it, does not prevent you from having your design reviewed by others before you commit.
Good luck
Norm

 

[Sandy H.]

View attachment 486069
200lbs at take off
175 for the majority of useful thrust. Transmitted through a cardboard tube. It's not a question of should I use a lower thrust motor and then what would happen if I accidentally put a much higher thrust motor in it. The thrust curves are all readily available. There would be nothing accidental or unknown about any new motor selection. All of your flights should be within the limitations of the materials you've used. You increased the length by 12". Go look up Euler buckling load. As you increase the length of an object it takes a lower load to create the same amount of buckling. Your max buckling load is in the middle of the "rod" Where the couplers tend to get put. So our favourite construction method inserts a strength reducer at one of the rocket points that gets maximum bending load.
Would I sit on that is a question all should ask before launching. The question is about right in general loading terms for most HPR flights. If the answer is no. Don't do it.
I wish you all the best in your next attempt. Just because you alone have to build it, does not prevent you from having your design reviewed by others before you commit.
Good luck
Norm

Per the video, it looked like it failed right around 2.5 seconds. Seems like it made it through the hardest hitting portion of the thrust curve you posted. Seems like if it were going to buckle due to force from the acceleration due to thrust from the motor, it would have happened earlier in the flight, right?

Sandy.

 

[Orien]

View attachment 486069
200lbs at take off
175 for the majority of useful thrust. Transmitted through a cardboard tube. It's not a question of should I use a lower thrust motor and then what would happen if I accidentally put a much higher thrust motor in it. The thrust curves are all readily available. There would be nothing accidental or unknown about any new motor selection. All of your flights should be within the limitations of the materials you've used. You increased the length by 12". Go look up Euler buckling load. As you increase the length of an object it takes a lower load to create the same amount of buckling. Your max buckling load is in the middle of the "rod" Where the couplers tend to get put. So our favourite construction method inserts a strength reducer at one of the rocket points that gets maximum bending load.
Would I sit on that is a question all should ask before launching. The question is about right in general loading terms for most HPR flights. If the answer is no. Don't do it.
I wish you all the best in your next attempt. Just because you alone have to build it, does not prevent you from having your design reviewed by others before you commit.
Good luck
Norm

Well, I did a build thread in the largest HPR forum in the world and openly invited input and feedback. I sent the OR file to the prefect along with build photos and the intended motor sims two weeks before the launch for him to review. I discussed at length with the manufacturer what kind of loads this design might be able to handle. The rocket was thoroughly inspected by the prefect before launch. Green lights across the board. And I kinda know what a thrust curve is. I hope the implication isn't that I just slapped some components together and slammed in the biggest motor that will fit.

 

[Orien]

I guess I was thinking about a logging altimeter. I wonder how it made it through the mach transition (i.e. did the shread happen right around there)? If so, I think that could point to fin flutter as the real root cause with the change in angle of attack folding the tube as the secondary effect. Just my thoughts.

If there wasn't a datalogging altimeter on board, I wonder if it would be realistic to sim the rocket (which you already did, obviously) and see what the speed would have been at the time the shread happened based on the video timing?

Sandy.

I'd love to provide that info, but after the latest Windows upodate none of my MS Office applications work.

 

[FredA]

Per the video, it looked like it failed right around 2.5 seconds.

Max velocity is usually right at burnout - depending on altitude it usually coincides with MaxQ.

 

[Steve Shannon]

Per the video, it looked like it failed right around 2.5 seconds. Seems like it made it through the hardest hitting portion of the thrust curve you posted. Seems like if it were going to buckle due to force from the acceleration due to thrust from the motor, it would have happened earlier in the flight, right?

Sandy.

I launched a 3 inch rocket on a K2047 motor at LDRS 27. It snapped at the coupler as it decelerated. My hypothesis is that it came out of mach more slowly than it went supersonic and the rocket was flexing which led to breaking the coupler, resulting in spilling the chute. Only the coupler and the parachute were damaged.

 

[Sandy H.]

I'd love to provide that info, but after the latest Windows upodate none of my MS Office applications work.

Can you post the OR file? I think it would be interesting to look at, especially based on Fred and Steve's comments above. I simmed a random 2 1/2" diameter rocket I already had built in OR with that motor's thrust curve and my random rocket showed that when the thrust started to go down (around 2.25 sec) the velocity almost went flat until burnout. I wonder what that velocity would be with your rocket. If it were at a critical velocity, that could be the smoking gun. From forum posts and conversations with fast fliers, I've heard that breaking Mach 1 isn't that big of a deal, but you need to go through 0.9-1.1 fairly fast and not dwell at that range.

This is just what I've heard/read and not from personal experience with rocketry. I know it does matter with rotating machinery that have natural frequency issues. Its fine to go above the critical frequency, but just blow through it and move on, don't hang out around there or things get 'wobbly.' I think that's a similar analog with regard to fin flutter around mach transitions.

Sandy.

 

[ThirstyBarbarian]

Can you post the OR file? I think it would be interesting to look at, especially based on Fred and Steve's comments above. I simmed a random 2 1/2" diameter rocket I already had built in OR with that motor's thrust curve and my random rocket showed that when the thrust started to go down (around 2.25 sec) the velocity almost went flat until burnout. I wonder what that velocity would be with your rocket. If it were at a critical velocity, that could be the smoking gun. From forum posts and conversations with fast fliers, I've heard that breaking Mach 1 isn't that big of a deal, but you need to go through 0.9-1.1 fairly fast and not dwell at that range.

This is just what I've heard/read and not from personal experience with rocketry. I know it does matter with rotating machinery that have natural frequency issues. Its fine to go above the critical frequency, but just blow through it and move on, don't hang out around there or things get 'wobbly.' I think that's a similar analog with regard to fin flutter around mach transitions.

Sandy.

That’s what I was thinking too. It failed because of aerodynamic forces, not thrust.. It didn’t fail during the time of maximum thrust, but after it started to trail off. And most likely that was near its maximum speed, and close to Mach. Maybe it dwelled near Max Q for awhile.

Someone else mentioned the camera shroud earlier. It is another factor that could have contributed to the aerodynamic loads. It could have caused some kind of turbulence, vibration, unusual pressure or some other force that triggered the buckling.

 

[Orien]

Can you post the OR file? I think it would be interesting to look at, especially based on Fred and Steve's comments above. I simmed a random 2 1/2" diameter rocket I already had built in OR with that motor's thrust curve and my random rocket showed that when the thrust started to go down (around 2.25 sec) the velocity almost went flat until burnout. I wonder what that velocity would be with your rocket. If it were at a critical velocity, that could be the smoking gun. From forum posts and conversations with fast fliers, I've heard that breaking Mach 1 isn't that big of a deal, but you need to go through 0.9-1.1 fairly fast and not dwell at that range.

This is just what I've heard/read and not from personal experience with rocketry. I know it does matter with rotating machinery that have natural frequency issues. Its fine to go above the critical frequency, but just blow through it and move on, don't hang out around there or things get 'wobbly.' I think that's a similar analog with regard to fin flutter around mach transitions.

Sandy.

Sure thing. Here it is. The airframe joint is about 42" from the nose tip.

I appreciate you looking into this. Let me know what you find. Normally I'd be all over it, but we had to put our beloved dog Remmy down last night. We're all a bunch of blubbering fools around here today. But he was a happy boy all the way to the end before the cancer won. Here he is just a week ago.

 

[Sandy H.]

Man, I'm really sorry to hear that. That sux. . .

Sandy.

 

[Sandy H.]

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.

 

[Orien]

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.

Hmmm. My analysis was a bit more simple in it's conclusion:

 

[rcktnut]

Sure thing. Here it is. The airframe joint is about 42" from the nose tip.

I appreciate you looking into this. Let me know what you find. Normally I'd be all over it, but we had to put our beloved dog Remmy down last night. We're all a bunch of blubbering fools around here today. But he was a happy boy all the way to the end before the cancer won. Here he is just a week ago.
View attachment 486093

So sorry for you, family. How old was your pup?