L3 Post Mortem - Flight of the Terminator

Worsaer said:

More photos to come, perhaps tonight. In addition to understanding what went wrong, I will also detail several things that worked well. Sharing information in the spirit of helping others be successful.


For anyone with too much time on their hands, a copy of my L3 project document can be downloaded here:

https://s3-us-west-2.amazonaws.com/12294/WRiley_Terminator_L3-Ver_1.9.pdf

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Do another Terminator so that nice documentation won't have to be done over with a different rocket. Very comprehensive and after second thoughts do it again with perhaps a longer apogee well and more powder. Seems the sequence of events
worked fine except for the fact the rocket couldn't separate at apogee. So retract my KISS statement. You're almost there but dinged by that little bastard Murphy. Just blow him away next time. Kurt

One thing I've been doing is ground testing until I get a decent deployment then I usually throw in another 20% for good measure for my flight charges. You might have been just on the cusp at altitude.

rcktnut said:

...Don't tamp the dog barf down hard on top the stuff either, just enough to keep it in place. You want BP granules just hugging each other, not compressed to tightly together. Opposite when using smokeless powders...

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Hmm, not sure what you mean here. If you've ever loaded a BP firearm you know that you have to compress the bullet into the case against the powder. Doing the same with a smokeless powder is bad and can lead to a kaboom. Remind me not to stand next to you if you reload your own cartridges.

For any firearm propellent you get the best results if you use a long tube. This can be verified by looking at the velocities of bullets in various lengths of barrels. The velocity increases as barrel length increases because more of the powder is able to burn and produce pressure to propel the bullet. (Up to a point and then friction starts to slow the bullet down, typically about 16-18" or so) Even in pistols there is a noticeable difference in velocity between a 3" and 5" barrel with many cartridges.

After reading through all the posts I think that the real issue is an undersized charge. In an earlier post I note that the pressure generated is substantially lower than indicated due to the actual length of the AV bay when you take into account the coupler. Add in the 7 lb weight of the nosecone as reported by the OP and I think it's an Occam's razor solution. Looking at the photos and reading the OPs comments it's clear the charge holders were good, the 'unburnt' powder was actually just residue, and that everything worked as expected. But the dynamics of the flight were stronger than the pressure the BP charge generated hence the deployment failure. As the actual volume of the AV bay expanded past the calculated volume the pressure generated dropped below what was required to eject the drogue from the body tube.

Not being judgmental, just trying to get to a reasonable explanation of the root cause.


Tony

My vote is undersized charges as well. We've seen it on TRF over and over again. Yes 2g may work on the ground, without air pressure messing things up. I've seen Ultimate Wildmen flown into the ground by those who swear that 2g will do you. If you look at the surface area of a 6 inch cone - a right circular cone is 312 in2 and any other design would be more. That's a lot of surface are for air pressure to act against. Both the velocity of the rocket and the cross winds will act to hold the cone down.

In this day of fiberglass rockets, why not do the calculation then give it a good margin to work, and justify it by the above? After all, the calculators talk about minimums.....

Sympathies for your loss Bill. Better luck next time with the L3.

So, #4 shear pins? Approximately 0.2" diameter? Just ran some calcs using three pins of nylon 66 at 0.2" diameter, based on a shear strength of 69MPa. 69MPa is 69 newtons per mm2. Each pin is about 20mm2. Adds up to over 4000N. 422kg force is the number I got. Sounds quite excessive if I got everything correct in those calcs.

Your charges and canisters look remarkably similar to what I use. BP amount is what I would choose as well. I would put the eMatch in the well, with the BP near the exposed end. I then add wadding and tape with high-temperature Kapton tape. The tape is probably neither here nor there compared to what you used. I would have used two shear pins, M3, giving a 9mm2 area and force required of just over 60kg.

Hope the next flight goes well.

I too am sorry your flight was not 100% successful, but thank you for sharing the details so folks like me can learn. It was a really nice rocket and you can tell you put a lot of hard work, "BS&T"into.

I have no opinion to offer as I am far from an expert, I am specifically trying to learn and understand.

I have read several comments that the charge wells were undersized. I would have guessed from what I have read that they were oversized. I understand and agree with more length giving additional burn time but don't you have to consider total volume? Dog barf is highly compressible, it strikes me that a large charge well, especially one packed with dog barf slowly starts approaching uncontained BP.

I would have guessed a long, but no too long, skinny charge well would produce the best results. I would also assume especially with BP that you want to "pack" the dog barf in similar to loading your muzzle loader. I always tapped the end of my ram rod with a wooden block to seat the bullet and compress the powder.

I'm in the process of sizing charge wells now for my L2 so I m paying seriously close attention, I definitely want to get this as right as possible.

I keep thinking back to my success with fingertips, they have always worked, and are sized right to the charge, no room for dog barf. I don't like that the fingertips "float". I've been thinking about building the charges in fingertips then loosing securing them in a charge well. Which brings me back to this thread, given the compressibility of the dog barf is there not a point when the charge wells become too big?

This thread is tremendously helpful.

#4 screws .2" in diameter? I don't think so. Try .081", and recalculate. Minor diameter of a #4-40 is .0813".

Here's an excerpt from my project doc, referring to data from RocketMaterials.org, that I used to determine how many shear pins to use, and the pressure needed to shear them. From my simple minded perspective, three number four shear pins wouldn't be too much, as long as the change was sufficient to shear them. I thought my test bore that out, but I'm learning that there are other variables that should be factored into a 'safety margin'.

If 2 grams worked, I figured 3 would be a valid safety margin. I'm guessing 4 would have been ok, and 6 would have guaranteed deployment. But there's always some point where the line in crossed, and you could be doing more harm than good. Since I now have a chute that cannot fly again, I can also test to see if 6 grams crosses the line and toasts the laundry.

As K'Tesh noted, this is a 'learning rich' thread.

As a learner, I've got 2 questions:

Was it ever confirmed that the drogue shear pins were cut? I looked over the thread, but didn't see it. One of the early posts notes a lower backup charge spike as evidence of partial drogue bay extension.

Are 3 shear pins customarily used to help prevent cocking (and wedging) if one cuts more easily than another?

Charles_McG said:

As K'Tesh noted, this is a 'learning rich' thread.

As a learner, I've got 2 questions:

Was it ever confirmed that the drogue shear pins were cut? I looked over the thread, but didn't see it. One of the early posts notes a lower backup charge spike as evidence of partial drogue bay extension.

Are 3 shear pins customarily used to help prevent cocking (and wedging) if one cuts more easily than another?

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Unfortunately that end of the airframe was heavily damaged, so I cannot determine if the pins sheared.
It's been observed that three pins will shear and separate more reliably than two. I will add that reference later when I can locate it.

Not sure if you've answered these or not but if so I missed it:

* Did the ground test use the full 7 lb weight of the upper airframe section?

* was there significant horizontal velocity at apogee?

* what was indicated airspeed at deployment?

Just wondering if any these may have been additional contributing factors.


Tony

manixFan said:

* what was indicated airspeed at deployment?

Tony

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Altimeters are unable to determine horizontal velocity, they will indicate 0 speed at apogee no matter how fast the rocket is going.

I did my L3 cert on the T5. When I did the dry fit, I pondered over the orientation of the bottom fins. Specifically, whether or not I should install them with the bevelled edge up, or down. OP installed them down, I liked the look of having then up (like the forward fins).

Anyone think there is an aerodynamic advantage to either configuration?

manixFan said:

Not sure if you've answered these or not but if so I missed it:

* Did the ground test use the full 7 lb weight of the upper airframe section?

* was there significant horizontal velocity at apogee?

* what was indicated airspeed at deployment?

Just wondering if any these may have been additional contributing factors.


Tony

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To answer the first question, yes, the test was performed with all hardware, harnesses, and chutes in place. Just as for flight.

Apogee was not visible. GPS shows it was a fairly straight up (and down! ) flight. There was certainly wind resistance on the nose at apogee, but I cannot say how much.

Worsaer said:

There was certainly wind resistance on the nose at apogee, but I cannot say how much.

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If there was wind resistance on the nose there was also certainly wind resistance on the lower section and fins trying to separate the rocket. Usually the base drag > nose cone drag. Speed at deployment is rarely of significance in inhibiting ejection separation. If it was you wouldn't need shear pins in the first place.

#4 screws .2" in diameter? I don't think so. Try .081", and recalculate. Minor diameter of a #4-40 is .0813".

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Thanks cherokeej. I thought it was like a #4 drill diameter. 4-40 UNC is more sensible!

Recalculating, each pin is 3.35mm2 and will shear at 69MPa (69N/mm2). Total shear force per pin is about 231N or 23.6kg. That is each. About 70kg force needed to shear the pins.

Looking at pressure differential at 12k' we get something like 36900Pa drop as the rocket ascends, assuming no leakage from the airframe. That translates to about 262N or 27kg force on the NC base. Shear pins are looking to be a little too robust.

When I want to make the shear pins a bit thinner I use the larger pins but drill the center out with a small drill (say 1mm).

I normally run small breather ports (say one at 3mm) in each section of the airframe with any significant volume (typically the chute bays) to equalise the pressure on the way up. Hasn't seemed to effect the normal ejection processes so far.

OverTheTop said:

I normally run small breather ports (say one at 3mm) in each section of the airframe with any significant volume (typically the chute bays) to equalise the pressure on the way up. Hasn't seemed to effect the normal ejection processes so far.

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More than one person has pointed this out. I do this in other rockets, and don't have a good excuse for not doing it on this one. Had I done so, I probably could have used 2-56 shear pins instead, rather than trying to contain the pressure differential.

Worsaer said:

It's been observed that three pins will shear and separate more reliably than two. I will add that reference later when I can locate it.

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I guess you mean this:
https://web.archive.org/web/2015021...rials.org/datastore/cord/Shear_Pins/index.php

Reinhard

Reinhard said:

I guess you mean this:
https://web.archive.org/web/2015021...rials.org/datastore/cord/Shear_Pins/index.php

Reinhard

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Yes, exactly. This is very useful information, based on data. Thanks for linking it.

Worsaer said:

Then thanks to you also Eric. The sun wasn't at our back, so a lot of photos look like silhouettes. I'm please to get a few shots of the up portion of the flight.

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I take that back, it was his! Mine was extremely similar though the angle was ever so slightly different. Anyway, the important part was that I was able to remove some of the blowout -

More than one person has pointed this out. I do this in other rockets, and don't have a good excuse for not doing it on this one.

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It didn't contribute to your grief on this one of course.

jderimig said:

If there was wind resistance on the nose there was also certainly wind resistance on the lower section and fins trying to separate the rocket. Usually the base drag > nose cone drag. Speed at deployment is rarely of significance in inhibiting ejection separation. If it was you wouldn't need shear pins in the first place.

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I respectfully disagree.

I used to shoot a lot of video at launches and have seen several instances where a drogue was deployed when the rocket had a fairly significant horizontal speed component. if the ejection charge isn't sufficient what I've seen happen is the back is the rocket crash into the fore section before the drogue is out or the two sections never fully separate. Once the upper section is free of the fin can it slows down much faster than it due to its lower mass. The second charge then fires but to no effect because the volume is now open so little pressurization takes place. So speed can be a factor.

I don't want to derail the thread but after shooting many videos of dual deployment I have personally witnessed several failures due to a significant horizontal velocity at drogue separation. It does not sound like that may apply here but it is a possible failure mode if coupled with a weak ejection charge.


Tony

OverTheTop said:

..Looking at pressure differential at 12k' we get something like 36900Pa drop as the rocket ascends, assuming no leakage from the airframe. That translates to about 262N or 27kg force on the NC base....

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Deleted! I misread the post as force created by the difference in pressure between the constrained air in the payload as it expands at altitude. So my comment about the amount of air being displaced by the chute etc., are N/A.


Tony

manixFan said:

That assumes the parachute bay is an empty sealed compartment. From the OPs posts I get the impression that the volume was nearly fully occupied. So I think that calculation greatly overestimates the amount of force created.

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You seem to be arguing that putting a parachute in a tube decreases the pressure in the tube. Which is silly so that can't be what you are saying.

While it may appear that the space is full of parachute, there is still a lot of air. Decreasing the air enclosed would decrease the time constant due to air leaks. Intentional or otherwise. But then, the sentence stated "assuming no leakage".

Thanks to everyone for thought provoking questions and comments.

Here's my working theory, and it's the most obvious: the charges were simply not large enough to reliably shear the #4 nylon shear pins.

• The #4 nylon shear pins were probably unnecessary; could likely have used #2 shear pins.
• I put too much trust in a single data point; one ground test was insufficient. I should have tested with larger and smaller charges to obtain more data.
• Had I properly weighed the risks, I should have chosen the risk of charges too large (toasty chute) rather than too small (ballistic return, non-deployment, shredding the main).

Next steps:
Looking at the bright side, I have a test airframe to work with before building my next project. It will be awhile before I can setup the tests, but I will eventually follow-up this thread with some results. I will likely not purchase the replacement project, a.k.a "Judgement Day" until January.


In addition to "what to fix next time", there are also the lessons, "what went right". I'll add those notes in a bit.

manixFan said:

I used to shoot a lot of video at launches and have seen several instances where a drogue was deployed when the rocket had a fairly significant horizontal speed component. if the ejection charge isn't sufficient what I've seen happen is the back is the rocket crash into the fore section before the drogue is out or the two sections never fully separate.

Tony

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Tony,

certainly that collision can happen because the drag on the nosecone is much much higher after it separates from the rocket then before. But while its on the rocket the drag force on the nosecone is generally less than the drag force on the rest of the rocket below. You can model this in OpenRocket or Rocksim by looking at the drag component analysis. An example of one of my simple 4NC rockets is shown below.



Since the drag on the rocket below the nosecone is generally greater than one the nosecone any airspeed of the rocket actually HELPS the ejection separation, not work against it.

Worsaer said:

I built the AV bay with aluminum charge holders, and packed the BP firmly with dog barf, sealed with tape.

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What, if anything, was used to prevent the BP from mixing with the dog barf? If nothing, then vibration could cause them to mix making less BP available.

Worsaer said:

Thanks to everyone for thought provoking questions and comments.

Here's my working theory, and it's the most obvious: the charges were simply not large enough to reliably shear the #4 nylon shear pins.

• The #4 nylon shear pins were probably unnecessary; could likely have used #2 shear pins.
• I put too much trust in a single data point; one ground test was insufficient. I should have tested with larger and smaller charges to obtain more data.
• Had I properly weighed the risks, I should have chosen the risk of charges too large (toasty chute) rather than too small (ballistic return, non-deployment, shredding the main).

Next steps:
Looking at the bright side, I have a test airframe to work with before building my next project. It will be awhile before I can setup the tests, but I will eventually follow-up this thread with some results. I will likely not purchase the replacement project, a.k.a "Judgement Day" until January.


In addition to "what to fix next time", there are also the lessons, "what went right". I'll add those notes in a bit.

Click to expand...

All valid points that are of course easier to make retrospectively (retrospectroscopically) #2 pins maybe just two or three of them for apogee? Small vent hole over area in the tube where some dead space exists. You can get some cardboard, soak in borax/boric acid let dry. Cut a circle, set that on top of your "blow up the tube" apogee stratagem charge holders and put your dog barf on top of it and below the cardboard disk. When it blows, some of the flash can be reflected back down into the surrounding dog barf by the cardboard. Of course, some of the flame will go past the edges, moreso on the side that fires first tilting the cardboard end up and the barf on top can act as a flame quench. Fearful of burning the chute? Double wrap with a chute protector or at least do a double layer that is going to take the brunt of the flash (which may be blunted by some of the techniques above).

I find that "flameproofed" cardboard adsorbs a major part of the flash and the dog barf on top will buffer the rest of the flame. You take caution with the drogue with an extra layer of protection closest to the flame front and it should remain nice and pretty. You gotta remember, the drogue and harness isn't going to be as tight a pack generally so is relatively easy to blow out. (Unless it is tight in your situation. I can't quite tell from the diagram)

Don't wanna use cardboard? Use more dog barf and layer the chute protection. Who cares if there's a few burn holes in the drogue? As long as the main survives you're good.

The other thing Bill is now there is the impetus to do multiple tests. Heck you can use a beater drogue and perhaps a well used but not "holey" chute protector and similar "used" harness material on the tests. You get it dialed
in and then use the 1st string material for the final test or if the "used" stuff is very close to what you are going to fly with stop there.

Now if I understand it correctly, Your main deployed albeit under the high energy situation without issue aside from getting blown to bits by the forces? If so, you will have less trouble refining that, if any. Good luck on the next try.

I had a similar though much smaller scale failure when making a tracker test mule rocket. Apogee only motor deployment with a chute release. I thought I'd use two 2-56 nylon "screwpins" to secure the nosecone and be
very "precise" about the charge. I took an empty motor casing and threaded the ematch/canister wires through the forward bulkhead/charge well for a ground test. Calculated .9grams of 4f and did the test. Pushed the button on my wireless remote and everything was ejected and the rocket body jerked 2.5 feet (I measured it) forward. Hmmmmmm, that's perfect. No steek'in need to cut the charge or add the whole 1.3grams that comes with the motor.

So's I now add .9 grams 4f to the charge well the day of the flight and it goes in ballistic. Why? Charge blew? I made the fatal mistake of not accurately reproducing the conditions! Charge is directed with the canister and
powder in the motor well can dribble aft on top of the delay grain that blows "back" out the end and not available to pressurize the chute bay!! Mister smarty pants got bit bad there.

Your problem was way more subtle than what old stupidhead did here. I'm sure your TAPS and others would have pointed that out but it was not apparent. Especially with that nice video test. You got a whole lot of people out there thinking about their own situations after you shared your unfortunate experience. Well next time will go better for you and I bet a lot of "next times" will be better with mishaps prevented with other fliers too!! (Yeah may any reader not repeat the mistake I made above too.) Kurt Savegnago

Nice write, just read through the L3 doc.. very polished.

Looks like your sims were right on too..

We found some inconsistent eMatches recently, almost cost me a rocket and it did cost my TAP his Extreme Wildman...we had never given the eMatches a second thought besides continuity testing.

Will look forward to your next attempt and hopefully this will just be a footnote in your certification.

Kenny

Worsaer said:

...my home made T6061 containers are 0.5" ID, and roughly 1.75" tall. After adding BP and the ematch, I pack firmly with dog barf and seal with masking tape (criss-cross, then around the perimeter good and tight....

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UhClem said:

What, if anything, was used to prevent the BP from mixing with the dog barf? If nothing, then vibration could cause them to mix making less BP available.

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Unlike Dave I prefer empirical evidence and a bit of math over thought experiments. Many others (including myself) have mentioned they use the same method as Bill. If it was an issue I think we'd know about it by now.

But just for my own curiosity I grabbed one of my large aluminum ejection canisters - .75" ID and 2.75" long. Normally it's filled with 6 grams or more of BP so I figured they are a worst case scenario - lots of unnecessary volume for a small amount of BP. Using my balance beam gram scale I measured out 2 grams of BP and placed it in the holder and then filled it with dog barf and taped it shut with just blue painters tape. I resisted the temptation to really pack it down so it was just moderately packed. I then shook it very vigorously for about 30 seconds. Using long needle nose pliers I slowly removed the dog barf being careful not to shake off any dispersed BP. Once the dog barf was removed I weighed the remaining BP and there was a negligible loss due to dispersion.

Unless the charges were stored on a paint shaker I'm not sure how they would have been vibrated enough to make a difference.

Bill clearly states he firmly packed the DB. While an N of 1 isn't an exhaustive test, it at least shows Dave's failure mode is unlikely. Of course anyone can perform the same test and see if their results match mine.


Tony

Dog barf removal after shaking: (blue tape is covering the e-match and mounting holes on the bottom of the canister)


Weight of BP at bottom of canister after shaking and dog barf removal: