Post Mortem on failure

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BrAdam

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I have another thread going about this but thought those in the Mech Eng field might want to figure this one out.

I was doing an ejection ground test and used too much powder. The tube is2.6" diameter and only 10.5" long and has about 5.5" of it used by e-bay and nose cone. The chute and cord take up the rest. So overstuffed is an understatement. .5 grams of BP did not do the job so the hammer mechanic in me said use more. I used 1 gram. Yes I know that is too much and told my son we would blow it out or blow it up. I did both. Chute came out like a champ. Tube blew out as well.

I have the tube drilled for 2 sheer pins but only used one. I usually only use one but drill two for symmetry just in case I need two. During the test the tube failed. Paper tube that is. The nose cone contains a brass shim to help cut the sheer pin(s).

Looking at some crude video it appears the nose cone starts to leave the tube and then the tube fails. Looking at the tube, it appears it failed at the unused sheer pin hole. The tube then tore from there then failed along the wrap.

The top edge of the tube is completely saturated in CA. The holes for the pins are also saturated with CA.

Anyone want to take a guess why it failed at the hole? Or have a mathematically supported reason?

My theory, The pin on the opposite side sheered cleanly. In fact the head is still in the tube and cleanly cut on the inside. When the nose cone ejected past the open hole the pressure found the weak spot in the tube and the tube erupted and the cone and chute and cord continued on their original path.

IMG_3551.jpgIMG_3549.jpg
 
My theory, The pin on the opposite side sheered cleanly. In fact the head is still in the tube and cleanly cut on the inside. When the nose cone ejected past the open hole the pressure found the weak spot in the tube and the tube erupted and the cone and chute and cord continued on their original path.

I disagree.

You have the tube drilled for two shear pins, but only used one. The nosecone "pivoted" on the one pin, until it sheared. This means the nosecone was cockeyed for a short bit in the tube, causing the shoulder to put an outward force on the tube, opposite the one shear pin....right where the other hole was.

-Kevin
 
+1 on Kevin's comment.

You always use at least 2 shear pins to prevent cocking.

If you were using 2-56 nylon screws, it takes a 50 pound load to shear them, or 25 pounds per screw. The cross-sectional of the airframe is ~ 5 square inches, so a 50 pound load divided by 5 square inches is 10 psi. 10 psi would not fail the airframe. As Kevin said, the NC pivoted on the single screw and point-loaded the empty side of the airframe before the screw sheared.

Bob
 
I disagree. This section has been ejected many times before with less powder and with only one sheer pin. And it has worked each time. There is no travel between the brass shim and the pin. If that was the failure mode, it was due to stress of the tube over time.

I also disagree with the entire theory that a nose cone pivots in the tube. If the fit between the tube and the cone is one that it takes friction to overcome insertion and there is say 2 - 3 inches of inserted length (on a 2.6 body tube). There can be no pivot. There is no physical space for this to take place. You would see stretching of the pin as failure instead sheer. And you would see failure at different points in tubes including the area where there is likely the most failure points built into the tube, the top cut edge.

This again is my theory and want to see others.
 
Ill add this as well since the responses forced me to look at the video again. it was filmed at 120 frames / second. Far to few to really capture what is happening but filmed non the less.

In the frame at cone ejection it does not appear the tube is ruptured. Also, the chute is still in the tube. The next frame the chute is at the top of the tube and the rupture is happening. The frame following that the chute is out and the tube is open. Again, I think the hole is the weak spot for the over pressurized tube to burst.
 
Not saying this is what happened, but I have seen charges blow a hole in the wall of an airframe when they are orientated in a way that the blast is directed perpendicular to the length of the tube. For this reason, I am careful to orientate charges with the length of the airframe.

I don't believe 1g of black powder is excessive for a rocket that size.

-Eric-
 
This does seem consistent with charges directed at the tube or in too confined of a space. My father had his 6.5" Mega Nuke blow out the fiberglassed sonotube when the charge was located between the RATT Works K240 hybrid casing and the wall of the tube and directed at the tube.
 
Interesting. The charge well, which is far to large for what is needed is 1/2" diameter brass tube inside a copper end cap The tube is about 2 inches long. It is mounted on the end of the electronics bay facing the opening of the tube.

Also, interesting is that I looked at the inside of the body tube and it is completely black on the side that failed and clean on the side with the pin. Could just be nomex rubbing on one side but that would also concentrate the pressure against the tube.
 
I disagree. This section has been ejected many times before with less powder and with only one sheer pin. And it has worked each time. There is no travel between the brass shim and the pin. If that was the failure mode, it was due to stress of the tube over time.

I also disagree with the entire theory that a nose cone pivots in the tube. If the fit between the tube and the cone is one that it takes friction to overcome insertion and there is say 2 - 3 inches of inserted length (on a 2.6 body tube). There can be no pivot. There is no physical space for this to take place. You would see stretching of the pin as failure instead sheer. And you would see failure at different points in tubes including the area where there is likely the most failure points built into the tube, the top cut edge.

This again is my theory and want to see others.

Cardboard isn't inflexible, nor is plastic. Under force, there can indeed be a pivot. Less powder = less power, meaning not enough to pivot to rupture. More powder, more power, pushes harder and ruptures the tube.

The hole may have been the failure point, but it's because the nosecone was trying to go out of it sideways. If it was overpressure going out the hole that blew it, the pressure would have been enough to blow the sheer pin head out the other side, which you said was still in the tube.
 
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Have not had time to read the last one but I found it interesting that nobody, including myself noticed the autocorrect version of shear pin that I have been using. Sheer as opposed to shear. Stupid computers.
 
I realize that none of the materials in this problem are inflexible. I am just saying the forces needed to pivot the cone are way to great compared to what it takes to shear the pin. I don't think the other pin needs to come out. We are talking about a gas medium and once it finds a way, it will follow it. It takes quite a bit of force to remove a 2-56 thread in tension/compression. Also for the pivot to happen the force of the gas would have to be significantly concentrated on the opposite bottom corner of the shoulder from the pin. This is not happening.

Not sure if anyone has tried this but grab one of your rockets and try to pull your nosecone out sideways.
 
The unused hole is acting as a vent, which, it appears, failed due to overpressurization. Had it had another shear pin in it, I suspect you would have found the initial 0.5g charge was more than sufficient.


Later!

--Coop
 
No, the original .5 grams was enough to eject. The packing of the tube was the issue. To tight and on one side of the tube. Because of the way the eye bolts extend into the tube there is not enough room to center the laundry. The gas blew out one side of the top. This was easy to see when I tested. The laundry wedged in the tube and stayed in. The new tube will be about 2 inches longer and I don't foresee a problem. Just thought those that have active minds like me would be interested in the problem.

Contacting MSOE to see if a student may want to noodle on this as well. It's a fun problem to look at.


Sent from my iPhone using Rocketry Forum
 
Brad,

Don't be so quick to dismiss those that speak of issues with using a single pin. The clear standard for high power rocketry is two or more evenly spaced pins. While you may be able to get away with just one pin, there are reasons why 2+ is the standard.

Eric
 
I know. Sometimes I like to poke the bear.:neener: Also, I question lots of things. As I asked in my original post, nobody has given a scientific reason for more than one. I understand it is a standard but, as is often the case in many businesses, just because we have been doing it for what seems like forever doesn't mean it is THE way it has to be done. Or even the most efficient.

Again, if someone can provide me with the science I'd be happy to put the stick down. I'm not trying to make people mad. Frustrated maybe:wink:. I am just trying to get people to think about some of the things that seem simple.
 
Keep in mind, though, that by immediately dismissing people (and in a way that came across as a bit flippant, whether it was intentional or not), you discourage people from trying to help you.

You stated "There can be no pivot. There is no physical space for this to take place."

You're talking about cardboard, which flexes. It wouldn't cause the nylon to stretch noticably, and still allow it to pivot just enough to compromise the cardboard.

Have you ever had a "tight" nosecone in a rocket? Get it off alignment just a bit, and it jams. That's what I believe happened in this case. Once the shear pin sheared, it allowed the ejection pressure to force the nosecone off.

Getting a nosecone cockeyed in a tube is an amazingly easy thing to do, and it's easier on a tight fit than it is a sloppy fit.

-Kevin
 
Again, if someone can provide me with the science I'd be happy to put the stick down. I'm not trying to make people mad. Frustrated maybe:wink:. I am just trying to get people to think about some of the things that seem simple.

One thing to consider is that as the number of shear pins increases, the force at the base of the nose must increase to shear them. This sets a lower bound (forces will probably still be transient and rising until the nose clears the airframe) on the force which needs to impart sufficient momentum for the nose to overcome any remaining friction and also pull out the recovery harness and the laundry.
 
I know. Sometimes I like to poke the bear.:neener: Also, I question lots of things. As I asked in my original post, nobody has given a scientific reason for more than one. I understand it is a standard but, as is often the case in many businesses, just because we have been doing it for what seems like forever doesn't mean it is THE way it has to be done. Or even the most efficient.

Again, if someone can provide me with the science I'd be happy to put the stick down. I'm not trying to make people mad. Frustrated maybe:wink:. I am just trying to get people to think about some of the things that seem simple.

I see an awful lot of people in this hobby arrive with science and depart in failure.

One should not dismiss anecdotal evidence or practical experience.

That is all.
 
I will agree that the point where holes are in an airframe are now the weak spot. While CA may increase tube strength, I believe it does decrease it's plasticity (i.e., makes it more susceptible to fracture because it does not flex as much). In piping design, they use area replacement to beef up the weak spot with more material, and that's according to code. That is why you see pipe fittings at branch junctions called "olets" used in pipes under moderate to high pressure. IMHO, CA is not doing much for you to strengthen the hole. It's better to have a small piece of metal there with a hole it, or a wrap of fiberglass.

IIRC there was one study that mentioned that nylon pins tend to stretch before fracture, whereas plastic (as in styrene) rods more cleanly fractured. If you got a pin that stretched for a moment, and that moment the rotation was just long enough to further weaken the already weak area, then it is entirely possible that during the recovery device transit the internal pressure exceeded the hoop strength of the airframe. Resulting in airframe blowout.

Regardless, with a single pin and you are going to have an asymmetrical load within the airframe, and binding is possible.

My :2:

Greg
 
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Ok, I apologize for sounding flippant. I am not dismissing peoples thoughts. It is hard to type with inflection. This is probably not hard to believe but my wife often gets frustrated with me and the "language" I speak. Apparently my english is different from hers. At work I am often called a f ing a hole but not by the people I work with. Figure that out. Preston and Eric cannot provide that answer.

Now to tick more people off. If you are going to tell me that it just happens or because, I will not accept that as an answer. If there is evidence of this happening I would like to see it (not sarcastic). This is something that happens within a fraction of a second and I doubt many people have witnessed this event. If you have the 10000 fps camera that helps us see an event like this, I will offer the tube and cone for destruction. Test has to happen at a location of my choosing though. Because of this I asked for some numbers. Nobody provided any. So, I challenge the accepted way of thinking by poking the bear to get others to critically think about the problem. I have worked at to many places that say this happened once so never again. The understanding of the event that caused the change was never explored to understand it. If I had a 10 pound rocket and a motor with 5 pounds of thrust and asked why it didn't go up I think everyone could provide a mathematical answer. This problem is just a little more complex.
 
Not sure if anyone has tried this but grab one of your rockets and try to pull your nosecone out sideways.

I think this is exactly why the failure could have occurred. Use a sacrificial body tube and a nose cone to do the experiment. Pull sideways AND use the shear pin as a pivot point (basically grab the tube in one hand and keep that hand stationary, and use the other hand to pull (one vector) and also push the nose cone perpendicular to the shoulder (the other vector)). You're going to pull one side of an oval cross section of cone shoulder through a round tube, and while the pin side (pivot) stays stationary, that oval gets bigger and bigger until the round tube cannot take it and either splits, or the pressure becomes too great and it blows out due to pressure (whilst the pin is still acting as a pivot). Makes sense to me at least.

Also, consider that the blow out happened just now, but the damage was being done in prior ejections, weakening the tube and perhaps even fracturing it, but invisible to the naked eye (flexible paint, sooty tube).

Finally, someone ought to be able to decipher the fracture pattern on the tube and be able to hypothesize on a failure mode.

Cool stuff. Rocketry isn't all about build and fly, it's also about destruction and failure mode analysis (...and back to the drawing board)! Lovin' it!!!!
 
So what's the mathematics behind drilling two holes but only using one?
 
Ok, I apologize for sounding flippant. I am not dismissing peoples thoughts. It is hard to type with inflection. This is probably not hard to believe but my wife often gets frustrated with me and the "language" I speak. Apparently my english is different from hers. At work I am often called a f ing a hole but not by the people I work with. Figure that out. Preston and Eric cannot provide that answer.

Now to tick more people off. If you are going to tell me that it just happens or because, I will not accept that as an answer. If there is evidence of this happening I would like to see it (not sarcastic). This is something that happens within a fraction of a second and I doubt many people have witnessed this event. If you have the 10000 fps camera that helps us see an event like this, I will offer the tube and cone for destruction. Test has to happen at a location of my choosing though. Because of this I asked for some numbers. Nobody provided any. So, I challenge the accepted way of thinking by poking the bear to get others to critically think about the problem. I have worked at to many places that say this happened once so never again. The understanding of the event that caused the change was never explored to understand it. If I had a 10 pound rocket and a motor with 5 pounds of thrust and asked why it didn't go up I think everyone could provide a mathematical answer. This problem is just a little more complex.

I use a single shear pin when appropriate, that is in a small diameter fiberglass or carbon rocket. Makes sense there as the material is so strong/rigid the shear pin doesn't stand a chance and the hole isn't going to elongate and the wall of the airframe isn't going to give like cardboard. On cardboard I typically use anywhere from two to four 2-56 or 4-40 dependent on the airframe and application. In cardboard they are evenly spaced around the diameter why? Cardboard flexes and you want balanced forces across the aft face of the nosecone so you don't get tilting, a stuck cone and possibly blow out your airframe (as demonstrated).
 
I also disagree with the entire theory that a nose cone pivots in the tube. If the fit between the tube and the cone is one that it takes friction to overcome insertion and there is say 2 - 3 inches of inserted length (on a 2.6 body tube). There can be no pivot. There is no physical space for this to take place.

So your paper tube doesn't stretch under pressure?
 
One thing I was told long ago.

1.Always use more than 1 Sheer Pin
2.Always use an odd number of sheer pins

So I use three. Keeps the nose straight and true when it comes out.
 
Not to be blunt, but...you don't need math to prove this. you don't need a 1,000 fps camera. You created an asymmetrical load. the NC blew out the side. Use two or three pins. solved. next?

Just because everyone is doing it doesn't mean there isn't a reason for it.
 
2.6 tube, total of 10.5 inches in length, 5.5 of which is taken up by av-bay. This leaves us 5 inches of working room for laundry. Okay.


2.6 tube's ID is 2.49 inches. At 5 inches of length, 1.0g FFFFg in this will generate 79.6 PSI. 79.6PSI on base of NC= pi*(2.49/2)^2*79.6=387.61 lbs.


The 0.089" hole for your unused #2-56 screw is attempting to vent those 79.8 PSI. It's failing, by the way. It can't flow all that much, really. I mean... we use a 0.25" hole to vent 100 in^3 quickly for apogee deployment...

Looking at the nose cone. 387.61lb pushing against it. The screw will shear at ~28lb. That'll take 28lb off. 359.61 lb remaining, but now it's cocked to the side, as it took that fraction of a second to cut the screw. How much? Oh... probably not a whole bunch --a smidge. But a smidge higher on the side opposite the shear pin--which would direct the gases to the other side. Where that hole is. The cocking itself creates a small pressure wave, increasing the pressure at the hole even further. And here comes that 79.8PSI...

How many PSI do you suspect it takes to tear paper?


Your initial charge would have been fine to shear 2 #2-56 pins. If you're going to rebuild, I'd suggest you drill 3 at 120 degrees from each other, use all three, and start your ground testing at 0.5g. Probably best to increase by 0.1g/charge, rather than doubling it.



Later!

--Coop
 
Also, I don't believe the fracture started at your hole. Take a look at the lower half of the horizontal crack. I see some paint showing cracking. Thats perhaps where the highest stress occured and started the blow out, again, whether by ejection pressure or more likely by my hypothesis of the cockeyed cone shoulder's growing oval shape as it tried to pivot on the shear pin.

I think I'm wrong here, but it looks by your dimensions and picture that the cracked paint seems to be right around 2" below the top of the tube. How long is your nose cone shoulder? It's not 2" perhaps? Like I said, probably wrong, but that's my diagnosis.

So I believe the fracture went through your hole. I'm not advanced enough to give you any engineering behind it, other than to conjecture that perhaps your drilling of the hole created a stress "riser" (normally we drill holes to stop an existing crack from propagating...so that contrary thinking on my part perhaps).
 
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