My L3 Project...A 7.5" LOC Goblin...AMA

The Rocketry Forum

Help Support The Rocketry Forum:

firemanup

Well-Known Member
Joined
May 2, 2002
Messages
1,282
Reaction score
49
Now the fins AND road are curved. Went so fast he fluttered the road? Not so sure the video is showing flutter or just poor video quality showing artifact.

I would be suspect of any bond on a L3 build that was made with wood glue. You say the whole thing except the fillets was done with wood glue, no mention of that was made when you explained your build to one of the taps, dont know if you mentioned it to the other?

I’d go full on epoxy on the rebuild.
 

Attachments

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
This is going to be a fun post.

Did you TAPs (plural) talk to you about flutter??? If not, why not?
"That sucks" is rather late and poor feedback....
Ahh yes the good ol' 20/20 hindsight post mortem feedback. Always so helpful. Fred, I've had two TAPs involved from the very start. I have roughly 50 or so build pictures I shared with them as I was building this project that started back in January. So you could say I got a lot more very helpful feedback than "that sucks" but because of people like you who insist on making snarky posts in many threads here on TRF like the one above I am not going to share the details of those conversations for you to nit-pick apart. Plus I'm not even going to share with you who they were because again, of posts like the above. And don't give me this, "I just call it as I see it" nonsense like you've said before - that crap does nothing to help the situation.

So here's something else for you to consider. Do you know the details of the other shredded Goblin? No? Maybe you shouldn't try to compare that to this flight then like you were obviously trying to do by quoting chicagodave. Here's some details that I know from talking IN PERSON to the rockets owner. The other goblin was a 5.5" Wildman kit and it flew on a L - and that's all I know. I don't know the weight, the total thrust, simmed altitude, construction methods or anything like that therefore other than fin shape that was probably a different enough flight than mine unlike you I realize I don't know enough to make fair comparisons.

Yes I am having an attitude here in this post. If you don't like it learn some tact.


Now the fins AND road are curved. Went so fast he fluttered the road? Not so sure the video is showing flutter or just poor video quality showing artifact.

I would be suspect of any bond on a L3 build that was made with wood glue. You say the whole thing except the fillets was done with wood glue, no mention of that was made when you explained your build to one of the taps, dont know if you mentioned it to the other?

I’d go full on epoxy on the rebuild.
Lol fluttered the road...I think that's a video aberration from the fins vibrating the entire airframe so I'm sure all sorts of wonky things were happening here.

I do NOT want to turn this into a glue debate thread but the reality is, if the fins are going to flutter I really doubt just using epoxy instead of wood glue would have made a difference. In my rather limited understanding of the subject, to mitigate the risk of flutter you need to change the harmonic resonance of the fin itself. So I'll be glassing the fins themselves prior to attachment, then a layer of glass over the fillets that go maybe 1/3 up the fin and then another layer that goes tip to tip...or something like that. And yes, I did consult extensively with my TAPs about the wood glue thing and they have A LOT of experience building rockets this way and I of course will be consulting with both of them prior to commencing rebuilding...
 

Charles_McG

Ciderwright
Joined
Sep 12, 2013
Messages
2,575
Reaction score
632
Location
SE Wisconsin
I believe the wavy fin appearance is an artifact of how the digital camera scans.

I wonder if you can back out the frequency from the image. And if that would even be useful info.
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
My 7.5" has 3/8" plywood fins and I've never had problems except 1 flight where the fin was already cracked from landing on pavement. Flown on K through long burn N motors.
And right there is why I didn't even consider flutter as being an issue prior to this flight. The only difference in mine was that the leading and trailing edges were beveled but I have a really hard time believing that would cause flutter.
 
Last edited:

firemanup

Well-Known Member
Joined
May 2, 2002
Messages
1,282
Reaction score
49
If video aberration can flutter the road it can give the same appearance to the fins, which was my point and why I stated i wasn't sure you were seeing flutter but possibly artifact/aberration, didn’t state it as fact, either way your fins separated for sure.

It’s not “just” the glue, looking at your external fillets they seemed way too small to me, and i run six giant fillets to every fin, premium epoxy with milled fiberglass. The weakest part of your build is what gets tested during a L3 flight, any structural part starts to fail and it will unravel the entire thing.

Understand you’re not happy, don’t blame you, kind of hard to put a post flight update on a fatal flight in front of a bunch of rocket nerds and not get opinions though.

Takes multiple attempts for some, part of the game i guess, if it was easy evrybody would be doing it right? Or is it if it was affordable, hell i dont know, lol.

Was a hell of a flight to watch though, think thats the first L3 failure ive witnessed in that manner.
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
If video aberration can flutter the road it can give the same appearance to the fins, which was my point and why I stated i wasn't sure you were seeing flutter but possibly artifact/aberration, didn’t state it as fact, either way your fins separated for sure.

It’s not “just” the glue, looking at your external fillets they seemed way too small to me, and i run six giant fillets to every fin, premium epoxy with milled fiberglass. The weakest part of your build is what gets tested during a L3 flight, any structural part starts to fail and it will unravel the entire thing.

Understand you’re not happy, don’t blame you, kind of hard to put a post flight update on a fatal flight in front of a bunch of rocket nerds and not get opinions though.

Takes multiple attempts for some, part of the game i guess, if it was easy evrybody would be doing it right? Or is it if it was affordable, hell i dont know, lol.

Was a hell of a flight to watch though, think thats the first L3 failure ive witnessed in that manner.
Ohhh don't take it the wrong way. I'm certainly not unhappy with you. ;) I don't mind your input at all.

Took two tries to get my L1 and two tries for my L2...aw hell, why not 2 tries for my L3?
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
Check this out...only one fin had extensive damage on it. Does that mean only one fin fluttered? The 2 white ones in the middle could easily be reused. The black one only has that small patch missing....hmmmm
071F7219-FF68-4D12-A12E-FDC513180D84.jpeg
 

Not Quite Nominal

Well-Known Member
Joined
Jun 21, 2018
Messages
71
Reaction score
74
Check this out...only one fin had extensive damage on it. Does that mean only one fin fluttered? The 2 white ones in the middle could easily be reused. The black one only has that small patch missing....hmmmmView attachment 418162
From looking at the video, the glue joint appears to have held.

The only glue joint in a TTW fin structure that is heavily loaded in shear is the fin tab to MMT joint, and the longer the fin tab, the less the force on that glue joint. You have a nearly 2" tab, so I'm guessing no glue joints failed.

I may be in the minority here, but I think that the cantilevered wood fin will flutter long before the fully-boxed inner structure will fail.
 

crossfire

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jan 16, 2011
Messages
3,925
Reaction score
305
I seen 2 Goblins loose there fins this weekend. One was on an L motor.
 

boatgeek

Well-Known Member
Joined
Dec 27, 2014
Messages
2,567
Reaction score
1,003
Thank you for sharing you bad flight--gives us all a lot to talk about! It does suck, but good that you have a plan to rebuild the rocket. I'd be tempted to name the new fin can Steve Austin, since it will be rebuilt better, stronger, and faster. 😀

And right there is why I didn't even consider flutter as being an issue prior to this flight. The only difference in mine was that the leading and trailing edges were beveled but I have a really hard time believing that would cause flutter.
My experience with vibration in general is that it's unpredictable. You wouldn't think that beveling fins would make vibration worse since the exciting forces would be lower, but maybe that pushed the frequency of those forces into the "right" resonant range for your fins.
 

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
6,213
Reaction score
2,723
Location
Butte, Montana
Check this out...only one fin had extensive damage on it. Does that mean only one fin fluttered? The 2 white ones in the middle could easily be reused. The black one only has that small patch missing....hmmmmView attachment 418162
There’s a high probability that all of the fins suffered internal damage. That damage was bad enough in two of them that pieces shed. I wouldn’t use any of them again. In fact, I would use them as patterns and make the next set of fins out of at least one size thicker birch aircraft plywood.
 

Nytrunner

Pop lugs, not drugs
Joined
Oct 15, 2016
Messages
6,552
Reaction score
1,978
Location
Huntsville AL
You flew on the M1297 right Dave? I think Loc has a video of this kit on that exact motor


Where Is an NY resident flying a 6XL L3 project...
Never stepped foot in NY state, I'm from Texas and work in Rocket city (H-ville Alabama). How to fly the big stuff? Same as everyone else, take a trip out west.
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
There’s a high probability that all of the fins suffered internal damage. That damage was bad enough in two of them that pieces shed. I wouldn’t use any of them again. In fact, I would use them as patterns and make the next set of fins out of at least one size thicker birch aircraft plywood.
Good point...I wasn't planning on reusing them - just more or less mulling over the possibilities. I'm getting another set from LOC and am going to add a layer of glass so I would assume that will be the rough approximation of what you are suggesting.

You flew on the M1297 right Dave? I think Loc has a video of this kit on that exact motor
They sure do. In fact, it was that video that inspired me to get this kit for my L3 and fly that same motor.
 
Joined
May 29, 2009
Messages
535
Reaction score
171
You’ve got the right attitude, my friend. Rebuild, and go at it again! I would definitely do a tip to tip glassing on those fins, but that’s just me. As long as your TAPs are happy, it doesn’t matter what the arm chair quarterbacks say.
 

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
9,266
Reaction score
1,368
Location
Pasco, WA
There’s a high probability that all of the fins suffered internal damage. That damage was bad enough in two of them that pieces shed. I wouldn’t use any of them again. In fact, I would use them as patterns and make the next set of fins out of at least one size thicker birch aircraft plywood.
With that fin span my technique would be to skeletonize, fill the holes with endgrain balsa or composite foam and two layers of glass or carbon, and make sure the fin tabs meet the motor tube as well as locked in with fin pockets. I hate seeing that much work put into a rocket for the type of failure that occured.

How much did the rocket weigh RTF?
L3build40.jpg L3build43.jpg L3build81.jpg
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
How much did the rocket weigh RTF?
31 pounds on the pad.

Here's another data point if anyone is interested...Finally got around to looking at the RRC3 data and according to that little gizmo the top speed was 542 mph - if those fins could have held on for juuuuuuust one more second and additional 40 mph. Oh well.
 

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
9,266
Reaction score
1,368
Location
Pasco, WA
31 pounds on the pad.

Here's another data point if anyone is interested...Finally got around to looking at the RRC3 data and according to that little gizmo the top speed was 542 mph - if those fins could have held on for juuuuuuust one more second and additional 40 mph. Oh well.
Things happen fast.
 

Maxwelljets

Well-Known Member
Joined
Sep 19, 2015
Messages
223
Reaction score
104
Check this out...only one fin had extensive damage on it. Does that mean only one fin fluttered? The 2 white ones in the middle could easily be reused. The black one only has that small patch missing....hmmmmView attachment 418162
From these pics, it looks like the outer wood grain is parallel to the body axis of the rocket. Can you confirm that? You get quite a bit more flutter resistance when the outer plies are parallel to the leading edge. Seems like something LOC should change.
 

Glasspack

Lifetime Supporter
TRF Lifetime Supporter
Joined
Oct 2, 2011
Messages
620
Reaction score
132
Dallen,

I for one would like to thank you for sharing your flight data and experience. I certified LVL 1 years ago, and then the very next day with my DAD watching...…...Lit the whole dam thing on fire right on the pad. Seems in my second day prep, I allowed myself to get distracted and forgot the motor casing on a
CTI H143 Smokey Sam.
Ready for flight. SA-14 Archer 2nd Launch .jpg Seconds later.....SA-14 Archer Fire.jpg
"Wildman" Tim Lehr was kind enough to put out the fire...……….


I am currently working on my LVL2 project and find people like you inspirational !!
The honest, heartfelt, constructive feedback and MOST of the members; is what makes this forum so cool !!!
I have so much to learn, and I appreciate all of this forum.
Best of luck on your rebuild and next attempt !!
 
Last edited:

John Cummings

Well-Known Member
TRF Supporter
Joined
Jan 14, 2019
Messages
77
Reaction score
56
Location
El Paso, IL
Looking at the video I have of the flight everything looks good until about a second before destruction when the smoke trail from the motor sorta ripples or spirals maybe (rocket doesn’t appear to be spinning that much). Those with more experience maybe can tell if this is evidence of flutter causing the rippled smoke trail?
Here’s the first pic from my video showing everything normal:


6DAF590A-5C6A-4C09-8A18-19FE0185E938.png

Milliseconds later some rippling of the smoke trail:
7B176643-B429-4BE6-B971-6C17F9A3B3C7.png
And then right before destruction:
BB0D2AA5-7B58-4290-9E6C-F7511BC3EFB8.png
Zoomed in picture right before:
B655D038-5476-480F-82FB-FAAFE6FA3F6B.png
and Bad things happening:
64D72AC0-FD73-481D-9537-DD6658D94650.png
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
From these pics, it looks like the outer wood grain is parallel to the body axis of the rocket. Can you confirm that? You get quite a bit more flutter resistance when the outer plies are parallel to the leading edge. Seems like something LOC should change.
Three of the fins...Can't tell without sanding off the paint. The messed up one - yes the outer grain is parallel to the body tube. But, does that really make a difference considering how plywood is made? All the layers have the grain placed perpendicular to each other.
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,744
Reaction score
686
And my video for those who wish to examine it yourselves:
LOL...I'm surprised you couldn't hear me screaming, "WWWWWHHHHHHYYYYYYYY????" right after the shred haha. Yeah that funky smoke trail has to be from the flutter.


I am currently working on my LVL2 project and find people like you inspirational !!
The honest, heartfelt, constructive feedback and MOST of the members; is what makes this forum so cool !!!
I have so much to learn, and I appreciate all of this forum.
Best of luck on your rebuild and next attempt !!
WOW! Thanks for saying that! Glad I can be an inspiration! I learned a long time ago the best thing to do after a failure is just to get back on the horse so to speak. Just got word last night that my replacement parts are being shipped today. ;)
 

Maxwelljets

Well-Known Member
Joined
Sep 19, 2015
Messages
223
Reaction score
104
Three of the fins...Can't tell without sanding off the paint. The messed up one - yes the outer grain is parallel to the body tube. But, does that really make a difference considering how plywood is made? All the layers have the grain placed perpendicular to each other.
It actually does. If you have a sheet of 1/8" plywood, try bending it perpendicular to the outer plies vs parallel to the outer plies. You can feel the difference in stiffness. Essentially, wood doesn't really have a ton of stiffness perpendicular to the wood grain, so when plywood is bent that way it depends more on the inner cores. Like an I-beam or sandwich-core composite, to get maximum stiffness you want the stiffest material on the outer layers. When you bend plywood perpendicular to those outer layers, it essentially acts like those outer plies aren't there. This is why higher grades of plywood have more but thinner layers.
 
2
Top