I reached the legendary speed of balsa!

[graylensman]

First time this has happened to me!

At our section launch yesterday, I flew (first time) my upscale Midget (18mm motors, up from 13mm). I gave into peer pressure and popped in a C6-0 in the booster (and a B4-4 in the sustainer). Needless to say, she left the pad in a hurry. We *heard* 2nd stage ignition, then the sustainer opened a jump point and disappeared into hyperspace. The booster, meanwhile, came back as pieces. Two fins had shredded, leaving one attached.

Anyway, my question is this. Did the booster disassemble at 2nd stage ignition, or at some point prior to that? And what can I do to make the next one more sturdy, using standard LPR methods?

 

[rabidsheeep]

ive heard of using tissue paper in glue to reenforce things... why not use it to also reinforce it to the actual rocket with a strong apoxe?

 

[GL-P]

Is a midget thru the wall fins?

What did you use to attach the fins?

What thickness of balsa did you use?

Epoxy is kinda heavy for such a small rocket.

 

[rstaff3]

I haven't flown a ton of staged rockets, but was/is not uncommon for the lower stages on a Commanche-3 to disassemble at staging. I just collected what pieces I could and went from there.

 

[sandman]

I haven't reached the "speed of balsa" since Estes stopped making the B14 motor.

 

[rstaff3]

I've reached it on a D21

 

[brianc]

Originally posted by graylensman
The booster, meanwhile, came back as pieces. Two fins had shredded, leaving one attached.

Did the wood fail? Or did your adhesive fail?

got pictures? (we need a milk mustashe icon)

 

[cls]

on an E9 at max Q my Flis Drake shredded one wing and kept flying. the wing broke in the middle of the span, across the grain, pretty cleanly so I just glued it back together and it still flies. but no more E9s.

 

[Fishhead]

I reached it with an upscaled Mach 10 on an Estes D12. I didn't have enough launch lugs to do both sides of the rocket and at the time didn't realize what an important function they served. They not only allow the M 10 to be loaded onto the pad, they also act as extra stiffening for the big bottom wing. (And they also look a little like machine guns.) The rocket had barely cleared the pad when suddenly the air was full of balsa confetti. This was in the days before I owned a digital camera, so I missed out on catching the excitement for posterity.

 

[Gus]

I have rarely reached the speed of balsa.

But I have reached the "speed of elmer's" on a number of occasions.

 

[Karl]

I've 'reached the speed of balsa' with the estes Wizzard, At MaxQ it sounded like a monocopter
Karl

 

[edwardw]

At a local EX launch a guy reached both the speed of balsa and speed of glue on 13mm E motors

Edward

 

[DynaSoar]

Originally posted by graylensman
Anyway, my question is this. Did the booster disassemble at 2nd stage ignition, or at some point prior to that? And what can I do to make the next one more sturdy, using standard LPR methods?

Without seeing at least the hyperspace jump of the sustainer it's hard to say whether it was flying normally. If it was, the booster fins probably shredded after separation.

The speed of balsa depends on the angle of attack. When the booster kicks loose and starts tumbling, if it's going like the Road Runner, the fins will present sideways at high speed. Unhappy balsa.

I've done some paper-fin bonding. Works well. Fold a plain piece of paper lengthwise, trace each fin onto the paper with the leading edge at the fold. Do that on both sides. Put adhesive in the tracings. lay the fins in the tracings, fold the paper over, and press it between two flat things with a bunch of weight. When dry cut them out and sand the edges. Except the leading edge. Since that's the fold, this lets you use the paper to make your leading edge.

A lot of people use yellow wood glue. I was afraid it'd make the paper warp so I used 3M 77 spray adhesive.

Now, that's for shredding balsa. But you said "leaving one attached". If the others came off at the root, and didn't break off mid-fin, you reached the speed of your glue joint instead. If that was the problem, you can apply the same idea above to your fin/body joint. I just glue a piece of dowel or thin slice of balsa or basswood into the fillet area before filling it in. You could also try the epoxy putty that Apogee sells. It's apparently JB Weld mixed with putty. You can work it for quite a while after mixing and form a really nice fillet. And it really is epoxy, so it'll hold real tight. Doesn't take much either. Smash it down into a modroc fillet and one $5 package should do a few dozen rockets.

 

[graylensman]

Wow! What a great response! Thanks, guys!

Yeah, this morning i conjectured that the shredding took place at staging. As you can see in the photo, it wasn't so much the balsa as the BT paper - the glue joints and fillets are pretty much intact; the top layer of paper peeled off!

We have a TARC team practicing staging with a Comanche as well, and also experiencing fin shred on boosters.

Dyna, I'll try your suggestions for the next booster airframe... and stay away from type C booster motors, I think.

BTW, that's the sustainer in the pic, too. A young man found it downrange while retrieving his rocket. Made me very happy.

 

[surdumil]

I've only experience the dreaded speed-of-balsa shread once. I cloned a Centuri Swift boost glider. The wings were beautifully airfoiled, but were pretty thin. I didn't tissue 'em. The glider was very nicely trimmed for a long, long glide time.

On its virgin flight, I slipped an A8-3 into the booster pod. It shot off the pad, the wings ripped off and fluttered down, the rest of the glider separated from the pod at deployment and dived straight down... very stick-like.

 

[sandman]

It looks to me like you exceeded the strength of the glue joint NOT the balsa.

Try this when you glue on the fins;

1.) Mark the tube for the fins.

2.) Lightly sand a line on the surface where the fins will go to "rough up" the surface of the tube where the glue will go.
Remark the tube (cause you probably sanded off the marks) and draw the line for the fins.

3.) Along the new line mark off the top of the fin so you don't go past the top of the fins.

4.) With a push pin or straight pin punch a line of pinholes.

5.) Apply a line of glue to the fins and touch it to the line of pin holes.

6.) With a toothpick force some of the glue into the hole. The glue will make "glue rivets" through the tube.

7.) Let this glue dry (I use Elmer's white glue) for about 15 or 20 minutes. Don't let it dry too much!

8.) Apply a thin new line of glue to the edge of the fin to "reactivate" the glue and stick the fin on (It will hold pretty good).

9.) Stand the model on it's nose until the glue drys.

10.) After the glue dries lay the model down on it's side and apply a glue fillet to two fins (the fins that are "up"). Use your finger to smooth out the Elmer's on the fillet and IMMEDIATELY wipe your finger off on a paper towel!

11. When the fillet sets rotate the model to the next "set" of fins untill all the fillets are finished.

A very strong joint! The tube will fail first! If the joint fails after this...It was doomed anyway!

 

[prowlerguy]

Did you use the "glue rivet" method of attachment? If you make some small pinholes through the body tube along the fin root, you can create a much stronger glue joint. That, plus several thin fillet coats (say 4 or 5), and I think you might be OK. The Comanche didn't use any filets at all, so that was probably their problem.

Edit: Dang, sandman beat me to it.

 

[adrian]

Originally posted by surdumil
I've only experience the dreaded speed-of-balsa shread once. I cloned a Centuri Swift boost glider.

Yes, boost gliders bring a whole new level of shredding. I've had a few scratch-built gliders fall apart. In particular, one lost its wings and tail after being boosted by a C6-3. I found all the bits, glued it back together, reinforced the root joints, and launched it again. The wings ripped off again anyway. I found all the bits, glued it back together, reinforced the wings and part of the body with doped tissue, and launched it again. The glider held together.

The most impressive breaking of the speed of balsa I've ever seen was someone else's scratch-built, twin-boom, D12 powered rocket glider, several years ago. As it built up speed, the wings started to flutter. Then they started to flutter a lot. Then they fell apart. People still speak of the incident with awe at International Rocket Week.

 

[Stymye]

sounds like the booster got under the thrust line,,I understand it happens often with the comanchee 3.. so mabey it didn't actually reach the speed of balsa afterall,,,

 

[graylensman]

Thanks for the suggestion, Sandman (and Prowlerguy!). But I have a concern. The booster BT is essentially the motor mount. Would not the insertion of the motor -and the heat from the burning propellant - weaken those glue rivets?

 

[bsexton]

Definitely the glue rivet technique already mentioned when your engine mount is smaller than the BT (gotta have a little room for the "rivet"). Also, I have used the "cover the fin with paper" method and the "cover the fin with Japanese tissue and dope" method for strengthening balsa without adding too much weight.

I didn't realize the sustainer had been found...what good luck!

 

[DynaSoar]

Originally posted by graylensman
Thanks for the suggestion, Sandman (and Prowlerguy!). But I have a concern. The booster BT is essentially the motor mount. Would not the insertion of the motor -and the heat from the burning propellant - weaken those glue rivets?

Only after the burn. It'll take a couple seconds for the heat to get through the casing. By then the booster will be free and falling. The glue joints might get flexible for a bit after that. But as soon as it cools off they'll be fine. As far as I know, repeated heating and cooling doesn't cumulatively weaken wood glue joints like it does epoxy. You won't have much glue going through the tube to the inside, but even the extra surface area of the hole itself will give the glue more to grab to. Make sure to wipe off any glue that does come all the way through or it'll keep the motor from going in.

Some pin holes through the fin just next to the root edge might help too.

 

[sandman]

Would not the insertion of the motor -and the heat from the burning propellant - weaken those glue rivets?

Elmer's is pretty durable and the heat of the motor casing wouldn't bother it too much.

Just don't make the rivet too big or you will have a clearence/fit problem with the motor.

You may have to sand the motor tube inside just a bit when the glue drys.

Glue some 100 grit sandpaper to a wood dowel and keep it in your flight box just for this contingency.

The sanding dowel works very effectively for this. And for a whole bunch of other things too...a handy tool to make!

 

[rstaff3]

I haven't tried this for lpr/Elmers rivets, but for mid power I've wrapped a spent motor with wax paper during the riveting. This seems to work pretty well.

 

[astronboy]

Yup, your pic show the culprit to be the glue joint, not the balsa.

Yes, I have had motor heat melt wood glue and elmers in the past. As this is a minimum diameter rocket, you could use CA or even good old Ambroid glue. To strengthen the joint, I would suggest gluing a strip of gauze along the fin root/bt joint ala the old ESTES Scout. Or for a neater looking final product, re-inforce the joint with silkspan instead of gauze.

Phred

 

[r1dermon]

just use epoxy for fillets. its heavy, but on a rocket like that, the fin will break before the fillet. hehehe. then CA soak the balsa fins and make some really sharp airfoils. throw in a 2" streamer and let it rip on the biggest motor that will fit.

 

[wyldbill]

Originally posted by cls
on an E9 at max Q my Flis Drake shredded one wing and kept flying. the wing broke in the middle of the span, across the grain.

When I built my drake, I glued a dowel (1/8" I think...) in the fin/BT joint as well as the wing tip joint. No structural problems on an E9.

This may not help in your case, though it may prevent flex that may have lead to the split.

FWIW,
-bill

 

[Rocketjunkie]

I found the speed of balsa the first time I put an Enerjet F67 in a rocket that flew perfectly fine on FSI F100's...

 

[r1dermon]

i still havent had a shred with balsa. i put up a star dart on a D21, it just went really really fast, but it stayed together. never found it, but thats a whole different story. that was probably one of my fastest flights to date, maybe my AURA last week on a G80. but i never found that either. lol.

 

[rstaff3]

Little rockets fare much better the big ones on a D21...plus I'm sure I didn't build the older ones as well as I did after I discovered AP