Certification Pass or Fail

[sr205347d]

The fins are secured with 4mm nylon bolts. There is no glue involved.



The damage was done on landing. The only damage was to the nylon bolts, a quick and easy field repair. The fins were not damaged, only muddied.

I was surprised at the amount of discussion about this being a pass or fail. What do you think?

 

[cwbullet]

The fins are secured with 4mm nylon bolts. There is no glue involved.

View attachment 613587View attachment 613588

The damage was done on landing. The only damage was to the nylon bolts, a quick and easy field repair. The fins were not damaged, only muddied.

I was surprised at the amount of discussion about this being a pass or fail. What do you think?

As a Prefect and Level 3 NAR, I have been involved in certification for 10 years. These is a significant amount of variability in defining "no major damage present" or a "minor zipper". I have always gone by the standard that it must be ready to fly same day or within an hour.

If someone glued on a fin and it popped off, it would fail. I can see why it would make them think you should also fail.

 

[NTP2]

Maybe nylon is not the best choice, metal would be better probably. Edit also a coarser thread might help.

 

[Worsaer]

Maybe nylon is not the best choice, metal would be better probably. Edit also a coarser thread might help.

… and a slower descent speed. If it sheared five nylon screws it may have landed too hard to be safe.

 

[Scott_650]

It isn’t necessarily the actual damage - what caused the damage could be a bigger factor than the damage itself. As already mentioned, the damage could be an indicator of other factors - building techniques, recovery methods, etc…that could lead to a down check on a cert flight.

 

[thomas]

I would not even discuss this. If this would be my rocket, I would never want to get a level for that flight.

 

[les]

I believe the issue is the landing forces are not evenly distributed. Looking at the picture, the fins are not even against the body tube and primarily rely on the 5 nylon screws. There maybe a little force from the friction effect of the bands. Depending on how much play the screws had in the holes (basically tolerance) it could have been the equivalent of a zipper effect. The landing probably put more force on the bottom most screw which failed, which then applied more force to the next screw and so on until all 5 failed

 

[cwbullet]

I would not even discuss this. If this would be my rocket, I would never want to get a level for that flight.

Same here.

I believe the issue is the landing forces are not evenly distributed. Looking at the picture, the fins are not even against the body tube and primarily rely on the 5 nylon screws. There maybe a little force from the friction effect of the bands. Depending on how much play the screws had in the holes (basically tolerance) it could have been the equivalent of a zipper effect. The landing probably put more force on the bottom most screw which failed, which then applied more force to the next screw and so on until all 5 failed

Good point!

… and a slower descent speed. If it sheared five nylon screws it may have landed too hard to be safe.

Good point. That would probably be obvious when watching the recovery.

 

[rfjustin]

The fins are secured with 4mm nylon bolts. There is no glue involved.

View attachment 613587View attachment 613588

The damage was done on landing. The only damage was to the nylon bolts, a quick and easy field repair. The fins were not damaged, only muddied.

I was surprised at the amount of discussion about this being a pass or fail. What do you think?

Flier of record needs to make some minor improvements and fly it again.

That be a fail IMO.

 

[ThirstyBarbarian]

I don’t think I would pass that. My tendency would be to say, if this is an easy field repair, and it can fly again the same day, then let’s do that — repair it and fly it again, and let’s see if it works. The only penalty for failing a certification flight is you get to try again. That’s not the worst thing in the world.

 

[sr205347d]

Maybe nylon is not the best choice, metal would be better probably. Edit also a coarser thread might help.

The intention is that the bolts fail before the fins do. It worked as intended.

The LOC IV fins are only 1/8 inch ply. It is surprising that this kit is highly recommended for Level 1 certs. The bolt-on arrangement was my modification to protect the fins.

 

[samb]

I agree that shearing 5 bolts might indicate a fast descent. Did you do a descent rate calc ? Not surprised at all that a lengthy discussion ensued. There was damage. Did you discuss this build technique with your cert team before the flight ? I’m pretty sure using metal screws would have brought up the “substantial metal” question. Anyway, what was the verdict @sr205347d ?

 

[ThirstyBarbarian]

The intention is that the bolts fail before the fins do. It worked as intended.

The LOC IV fins are only 1/8 inch ply. It is surprising that this kit is highly recommended for Level 1 certs.

That’s not exactly a standard technique, but it does make some sense. So knowing the rules are that damage that would prevent the rocket from flying again results in a certification flight fail, and knowing your rocket has parts designed to fail, you might want set that expectation in advance.

Before the flight, you could tell the person witnessing your cert, these bolts are sacrificial parts designed to fail before the fins fail. It’s an intentional part of the recovery system to prevent real damage to the rocket. I have the replacement bolts right here with me, so if any of the bolts break on landing, I’ll replace them immediately after recovery and restore the rocket to a flyable condition. And then ask if that’s an ok plan for the cert flight.

I think if I were the witness, I’d agree to that if it were explained in advance. But if it weren’t explained beforehand, and the rocket were recovered with broken bolts, I’d ask to see the repair and possibly another flight.

 

[GalantVR41062]

I would have a few questions also and could agree that the parts are ment to fail before the fin, and can be replaced between flights.
I would also ask how the pieces between the fins are attached to the booster, do you have those parts replaceable also? What was your recovery weight, decent rate down and horizontal velocity, how about booster swinging or rotating as it lands.

How quick was the decent for the flight and what was the ground it landed on? Do you have a calculation/measurement on Gforce of landing, what's the shear strength of your nylon hardware and is this grater then the possible thrust of the motors you intend to fly?

The screws are not only in shear load but under tensile load also, as the fin hits the ground if the booster is spinning it will torque the fin and apply tension. I would recommend a new set of screws for every flight and have a recording altimeter and video of the flight.

I have sacrificial parts (fin tabs held on my 1/4-20 nylon screws) on my 1/4 scale Viking 7 sounding rocket and discussed these with the RCO and LCO before flying it, they are only there to complete the scale look and the rocket flies just fine without them.

 

[sr205347d]

I would have a few questions also and could agree that the parts are ment to fail before the fin, and can be replaced between flights.
I would also ask how the pieces between the fins are attached to the booster, do you have those parts replaceable also?

The brackets are held on by the clamping force, and are prevented from sliding off the back by the fin tabs. They are very robust, and I do not have spares on hand.

The ground was fairly firm dried dirt.

I don’t know about the rest, except to say that the rocket flew earlier that day without any popped bolts.

 

[ThirstyBarbarian]

I think incorporating sacrificial parts into a rocket to prevent real damage to important parts can make a lot of sense. But I don’t think it’s necessarily a good idea for a certification flight, from the point of view of making things easy on the witness.

One of the things a witness is supposed to judge is whether the rocket was recovered without damage, and if it was damaged, then they have to make a judgement call about whether the damage is severe enough to be disqualifying. Why put them in the position of having to make that call? It’s not a good test-taking strategy.

If you are taking a test of pretty much any kind, it’s probably best not to put the judge in the position of having to consider too many unusual factors. Like you wouldn’t necessarily want to take your drivers license test in a 100-year-old car with a hand-cranked starter and unusual driving characteristics. Keep it simple.

 

[NTP2]

Keep it simple.

Your forgetting a S…

 

[tsmith1315]

the rocket flew earlier that day without any popped bolts.

Perhaps they were compromised in the earlier flight, but didn't fail until re-used.

If the perspective is those bolts are a consumable item, then I could see failing the flight if they had been flown before and failed this time.

 

[troj]

I think incorporating sacrificial parts into a rocket to prevent real damage to important parts can make a lot of sense. But I don’t think it’s necessarily a good idea for a certification flight, from the point of view of making things easy on the witness.

Sacrificial parts, to save the rocket in case of a recovery system failure, isn't bad. However, that also leads to bad recovery behavior.

The utmost goal should always be for the rocket to recover at a safe descent rate.

-Kevin

 

[watheyak]

The intention is that the bolts fail before the fins do. It worked as intended.

The LOC IV fins are only 1/8 inch ply. It is surprising that this kit is highly recommended for Level 1 certs. The bolt-on arrangement was my modification to protect the fins.

As a prefect myself, it would have been a no.

It would have survived just fine if you for your plan to "protect the fins". 1/8" fins are more than adequate. Until you perforate then with a bunch of bolt holes. You're lucky it was only the boldts that broke.

You're creating a solution for a nonexistent problem. There's a reason that rocket is highly recommended for level 1. No need to mess with something that works.

 

[troj]

The fins are secured with 4mm nylon bolts. There is no glue involved.

The damage was done on landing. The only damage was to the nylon bolts, a quick and easy field repair. The fins were not damaged, only muddied.

I was surprised at the amount of discussion about this being a pass or fail. What do you think?

If you sheared 5 nylon bolts, your descent rate was way too high. This is a failure.

As a flier, if this had been mine, I would've cut any discussion short and said it wasn't successful.

Back when I was a prefect, I would've told you No, this is a fail.

You either need a larger parachute or you need to resolve why your recovery system didn't fully deploy and result in a proper descent rate. Even with nylon, this had to have smacked the ground fairly solidly.

-Kevin

 

[les]

At a minimum, as a consumable part, I would have performed the certification flight with brand new ones. Not some that had already been stressed and potentially weakened by other landings. If they had not broken the issue would not have come up.....

The descent rate may have been OK or too high - you can use a descent rate calculator based on your rocket and chute selection to see what would have been expected.

But then you need to account for additional forces as other mentioned, such as the rocket swinging on the shock cord. That can add a lot of "speed" besides the actual vertical velocity under the chute.

If you sheared 5 nylon bolts, your descent rate was way too high.

If all 5 bolts went simultaneously, I fully agree. My belief is still that first one bolt let go (perhaps partially cut through from a previous landing), that then put more stress on the next, then when the second went even more stress on the third, etc. Watching "Engineering Disasters" on TV has shown for many bridges or even buildings and airplanes, one bolt/rivet fails that then causes more stress on the subsequent ones until it becomes a "chain reaction" causing the ultimate failure.

 

[les]

I tried to find out via a web search, but I was unsuccessful in the time I was able to spend. Just what is the rated shear strength of your 4mm bolts?

 

[sr205347d]

I tried to find out via a web search, but I was unsuccessful in the time I was able to spend. Just what is the rated shear strength of your 4mm bolts?

As you can see from the one remaining bolt, they don't shear. The heads pop off.

 

[gldknght]

I don’t think I would pass that. My tendency would be to say, if this is an easy field repair, and it can fly again the same day, then let’s do that — repair it and fly it again, and let’s see if it works. The only penalty for failing a certification flight is you get to try again. That’s not the worst thing in the world.

Plus one!
If it can be repaired and flown again SUCCESSFULLY, then, it's an absolute pass. But I think with everything else being the same, this would most likely fail a second time.

 

[troj]

If all 5 bolts went simultaneously, I fully agree. My belief is still that first one bolt let go (perhaps partially cut through from a previous landing), that then put more stress on the next, then when the second went even more stress on the third, etc. Watching "Engineering Disasters" on TV has shown for many bridges or even buildings and airplanes, one bolt/rivet fails that then causes more stress on the subsequent ones until it becomes a "chain reaction" causing the ultimate failure.

Strong disagreement.

For that to happen, the stress has to continue to be applied. This means that the landing was likely very hard, and thus the descent rate was too high.

 

[sr205347d]

The certification team talked about it, then looked at the checklist which asks them to "Verify that no major damage is present. Minor impact damage or 'zipper' is acceptable." In the end, it is their judgement. They deemed the damage to not be major, but rather minor impact damage, and awarded me my Level 2.

This makes sense to me, because the criterion is about damage, not about the descent rate under parachute. This rocket's descent rate was just fine on two previous flights. Another rocket with stout 1/4" thick plywood fins could likely have withstood the same impact on the hard dirt, but then could crack a fin landing in a gravel parking lot.

Whether damage is major or minor, to me, depends on the extent of repair effort required. Likelihood of damage is a function of the wind gusts and the landing surface as much as it is the size of the parachute. With a normally acceptable parachute, if you are unlucky and land on a paved road, or hit the side of a barn, or catch a gust, there will likely be some damage.

The builder can mitigate the level of damage by building hell for stout, or by making it damage tolerant. But a stout fin can still get busted up, if you are unlucky. Then some major surgery is required. If you can replace some nylon bolts, or worst case put in a spare fin, the extent of repair time and effort is much reduced and the damage is minor.

 

[samb]

Congrats on the L2 cert. NAR/Tripoli both leave a lot of wiggle room for cert team judgement. FWIW I think descent rate should be a factor to consider on all high power flights. Since I wasn’t there I can’t say for sure if a high descent rate contributed to the damage shown.

 

[JackC]

The certification team talked about it, then looked at the checklist which asks them to "Verify that no major damage is present. Minor impact damage or 'zipper' is acceptable." In the end, it is their judgement. They deemed the damage to not be major, but rather minor impact damage, and awarded me my Level 2.

This makes sense to me, because the criterion is about damage, not about the descent rate under parachute. This rocket's descent rate was just fine on two previous flights. Another rocket with stout 1/4" thick plywood fins could likely have withstood the same impact on the hard dirt, but then could crack a fin landing in a gravel parking lot.

Whether damage is major or minor, to me, depends on the extent of repair effort required. Likelihood of damage is a function of the wind gusts and the landing surface as much as it is the size of the parachute. With a normally acceptable parachute, if you are unlucky and land on a paved road, or hit the side of a barn, or catch a gust, there will likely be some damage.

The builder can mitigate the level of damage by building hell for stout, or by making it damage tolerant. But a stout fin can still get busted up, if you are unlucky. Then some major surgery is required. If you can replace some nylon bolts, or worst case put in a spare fin, the extent of repair time and effort is much reduced and the damage is minor.

You’re forgetting that all aspects of the flight must be safe to achieve certification. If you told me that you designed the fin attachment to fail to protect the fins, I would’ve failed you because you flew a rocket designed to fail which is not safe. Using metal screws would have met the criteria to design a safe rocket.

 

[JackC]

As a prefect myself, it would have been a no.

It would have survived just fine if you for your plan to "protect the fins". 1/8" fins are more than adequate. Until you perforate then with a bunch of bolt holes. You're lucky it was only the boldts that broke.

You're creating a solution for a nonexistent problem. There's a reason that rocket is highly recommended for level 1. No need to mess with something that works.

I agree. The original poster overthought the whole design. There are plenty of designs that have robust fins simply by laying up epoxy fillets. I always recommend the KISS method when certifying.