7' TOG and High Speed

HOLY COW !!! I launched my new SSTOG yesterday !!!! Right about max speed it shredded the fins. I guess I didin't realize how gast this thing would be moving on a J350. It was right around 8lbs. After loosing all three fins it continued on up to just under 1800' where it seperated and free fell to 400 where the 6' main deployed.
NONE, of my fillets were damaged in any way, it simply shredded the plywood. Lesson learned !!!! My DD went off perfectly though. I am going to rebuild the fins using thicker ply and I will glass them this time. I hope to one day fly it on an I600 !!!

With those large fins I am suprised they weren't glassed with at least 5oz. I would do a layer of 6oz and a 1 oz veil if you wanted but 6oz minumum.

Ben

Shredding and all it was still a magnificent flight. Even after the loosing the fins only about a third of the way to apogee the rocket continued straight up and recovery was perfect.

Originally posted by ben
With those large fins I am suprised they weren't glassed with at least 5oz. I would do a layer of 6oz and a 1 oz veil if you wanted but 6oz minumum.

Ben

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The 1oz veil would do absolutely squat.

For that size of fin, I would go with a single layer of 5-8oz over the thickened ply. That should be enough for what you plan to fly it on.

Originally posted by cjl
The 1oz veil would do absolutely squat.

For that size of fin, I would go with a single layer of 5-8oz over the thickened ply. That should be enough for what you plan to fly it on.

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that is why I said Veil. just to make filling the 6 or 8oz weave easier to fill.

Ben

I would glass or CF (not necessary) the fins with 8 oz. after bevelling them. Then I would do tip to tip with 5 oz. glass or CF. For asthetic purpose, cover it all with 1-2 oz glass.

Sorry to hear about the shred paul. I agree that a layer of 6 oz glass would do it. Given the price of carbon, I'd steer way clear of it unless absolutley neccessary unless you want and are willing to pay for the look of carbon

Joe

Wood veneers of any make are less-quality woods bonded with some sort of glue.

I suggest you use G10 next time.

Originally posted by kaycee
Wood veneers of any make are less-quality woods bonded with some sort of glue.

I suggest you use G10 next time.

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If it was aircraft ply, it was not lower quality wood. Honestly, I wouldn't use G10 for fins this size. I would go with either honeycomb or wood because it is lighter than G10.

Also, G10 fins (that size) might actually flutter more than the plywood...the fiberglassed plywood will dampen the effect...honeycomb is also a very good suggestion.

The rocket design was outstanding and really looked great on the pad and under boost...very straight boost.

Originally posted by cjl
If it was aircraft ply, it was not lower quality wood. Honestly, I wouldn't use G10 for fins this size. I would go with either honeycomb or wood because it is lighter than G10.

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If the fins were A/C Ply (probably were), they cannot be stronger than G10 at a given thickness. Yes, G10 is heavy compared to ply at the same thickness, but G10 of lesser thicknesses can provide the same or greater stiffness as ply thus off-setting the weight.
I've never has a G10 fin-equipped rocket strip the fins from the airframe, but I've seen many ply fins give up under lesser velocities.

Now you guys tell me !!!! LOL LOL, Where was this advice when I was building that beast !!!

I tend to overbuild so this time I thought "Self, you over build so let's take it easy this time" More recent conversation..."Self, go back to overbuilding, overbuilding means it comes back in one piece !!!"

Thanks LG, that vid you shot is pretty cool !!! Feel free to post it on here if you want.

Originally posted by kaycee
If the fins were A/C Ply (probably were), they cannot be stronger than G10 at a given thickness. Yes, G10 is heavy compared to ply at the same thickness, but G10 of lesser thicknesses can provide the same or greater stiffness as ply thus off-setting the weight.
I've never has a G10 fin-equipped rocket strip the fins from the airframe, but I've seen many ply fins give up under lesser velocities.

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I'm not saying they are stronger per thickness than G10. I would not be remotely surprised though if they were stronger per weight as far as resistance to flutter though. G10 is a wonderful material for many things. Massive fins is not one of them. Glass honeycomb would be a better choice if you were determined not to use wood, but properly made wood fins can take a heck of a lot of punishment.

Paul,

Was this a L2 cert flight?

Doug

Ick, shreds are brutal! But they sure do look cool

Keep us posted on the rebuild!

Paul

Is that the same light blue rocket with the tubes at the ends of the fins that you displayed in photos in a previous thread?

So the tubes ripped off and the straight body of the fins stayed intact?

All this and not a single picture Jeeeze!

Sorry about the shread Paul, its a cool looking model

I have a couple of observations.

I'm thinking the high drag of the large (relatively) tubes on the ends of the fins was a contributing factor.

I'd sure like a more specific description of the damage. Did the fins rip away from the airframe? Did the nacelle tubes rip off? Did the fins snap somewhere in between?

Further, suggesting that paul should have gone with a different building material for "fins of that size" - simply because of their size - simply isn't fair IMO. I don't think this was a failure because of massive fin flutter and wood alone. Something tells me that if you would remove those nacelle tubes from the ends of the fins and have a do-over, the wood fins would hold up just fine.

Paul's fins did not rip away from the airframe but shredded in the middle.
I have a video but it was done from a distance on my little Canon camera not meant for taking videos.
Watch the smoke trail and you can see the moment things shred.

https://www.youtube.com/watch?v=5aJvjba7e8U

Originally posted by Fore Check
I have a couple of observations.

I'm thinking the high drag of the large (relatively) tubes on the ends of the fins was a contributing factor.

I'd sure like a more specific description of the damage. Did the fins rip away from the airframe? Did the nacelle tubes rip off? Did the fins snap somewhere in between?

Further, suggesting that paul should have gone with a different building material for "fins of that size" - simply because of their size - simply isn't fair IMO. I don't think this was a failure because of massive fin flutter and wood alone. Something tells me that if you would remove those nacelle tubes from the ends of the fins and have a do-over, the wood fins would hold up just fine.

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I was there and you are on track. Two of the fins shattered in the middle in diagonal rip and one shattered closer to the body but still in the main fin portion. When I picked up one of the large fins that shattered closest to the body it was easy to see how much weight was out at the tip. Add to that the fact that these were open tubes with some obstructions ( the fin went partially through the tube to give more glue joint surface ) and you can see how this might have increased the stress exponentially on the fins.

I'm certain the fins - even this large - would have been fine had they not had the tubes on the end. Paul has flown many large finned rockets without problems. He has even flow larger fins than this with lightweight fiberglass on another J350 L2 attempt without fin related issues.

Originally posted by cjl
The 1oz veil would do absolutely squat.

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That just doesn't make any sense.

Drag from the wingtip tubes, maybe some flutter...
That "tube-in-a-tube" there attached to the central airframe...does that vent anywhere?...that may have created a turbulence that started things "moving" in the fin area.

Cool design...just didn't much like going real fast it seems. Bet a wind tunnel would be nice to have, eh?

Dave

Good Idea anyone got a 12' or so wind runnel I can barrow.

So many thoughts and ideas on what happened and so many are right on the $$$. It would have easily survived to give me my L2 if the Nacelles were not there. If I did that, It wouldn't be a TOG. To be honest though I should look for some lighter tubing for my Nacelles or leave them off. I need to get my hands on a 2" thin walled tube. I am even thinking that 1/4" ply with glass is too thin with the Nacelles.
The tube into a tube has been up three times now and this is the first time I have had a problem during boost. I know without a doubt the Nacelles did me in. I will rebuild and get it ready for Freedom Launch or maybe our club launch in June.

I have to admitt though, it was one KEWL flight !!!!!

I have already cut the remains of the fins off the BT and now I can proceed to start the rebuild between my MR Mom duties and my school work.

Originally posted by Ryan S.
That just doesn't make any sense.

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Why?

On fins this size made of decently thick ply, the relative strength of 1oz fiberglass is insignificant. It would make basically no difference to the survivability of the fins.

Paul that was a good flight. Honestly, you would be suprised at waht 6-8oz of FG can hold up to. I would leave the tubes on the end and take the glass and have it reach onto the tube. 8oz on each side of a fin will strengthen it enough to (in theory) reduce fluter without adding as much weight as 3/8" or 1/2" would. if you t2t it it will make it "one with the body tube" and should be alot stronger.

Ben