At which point is balsa no longer a suitable material?

[RomCat]

Hello All,

Brand new to the forum and this is my first post. I started making model rockets with my son a few months ago and caught the bug. Now I'm scaling up and building my first D & E motor rockets. I'm starting to question the suitability of balsa wood on this larger LPR build, but I don't have the experience to tell me if my instincts are correct or if I'm just being too cautious. I'm going to sink a lot of hours into this build and apply an expert finish, and it would be a shame if it didn't make it past its first launch.

My build is a BT-60 body with 40mm fins. OpenRocket tells me that with an E-motor it will hit about 1,600 ft at apogee with a velocity of 125 m/s and a max acceleration of 130 m/s^2. Is that too much for a 4mm balsa fin to handle? For what it's worth, I'm thinking of sinking the fin root right down to the motor mount on this one for extra strength, but that will make repair of the rocket that much harder if I snap a fin.

Thanks in advance.

 

[rharshberger]

Balsa TTW (through the wall, aka fins attached to motor tube) will work ok for your application, and if they are papered the balsa will be quite a bit stronger. Papering is putting a layer of common printer paper over the fin as a skin, personally I use self adhesive label paper and seal the fin edges with CA glue.

 

[RomCat]

Thank you for the advice. I'll share the build with everyone when it's done.

 

[Nytrunner]

Balsa is surprisingly resistant to flight forces. The key points I've been picking from reading and watching flights is not to go nuts and exceed "the speed of balsa" by using too strong a composite motor. Like Rharsh said above, papering can really help to reinforce it and get some more speed out of them. Then for landing, make sure your parachute will slow it down enough to ensure no snapping of corners or swept back fins.

And if you're cutting your own fins, make sure you don't make the grain line up with the rocket. When you step up again, You should check out the Estes Star Orbiter for an example of Balsa find on a 29mm motor rocket.

 

[rstaff3]

This is a gray area. I've shredded an Estes rocket (not TTW) on a B14 and flown one (with TTW) on a G. I think a TTW rocket should hold up to any E.

 

[Cabernut]

I have a custom-rolled 44mm dia rocket with large 1/8" papered balsa fins surface mounted and it holds up to E motors just fine so far. Biggest danger for fins is damage in transport to or from the field, followed by rough landings. 4mm or even 3mm(1/8") papered balsa fins with the grain in line with the leading edge can handle quite a bit as long as they aren't elephant-ears.

For papering, I would make the distinction that if you're after ease of finishing then stick-on label paper is so easy it feels like cheating. However if you are only after strength and not afraid of a bit of work, then office paper with a thin layer of glue makes fins really stiff.

:2:

 

[K'Tesh]

I'll toss in a link to a tutorial for papering fins that I think can help...

https://www.rocketryforum.com/showthread.php?65628-Papering-balsa-ply-Fins-A-foolproof-method

 

[JumpJet]

The Estes Star Orbiter uses 1/8" balsa with through the wall construction. No papering of the fins is needed. I bet you can fly this model on a G80 without the fins ripping off if the entire model is built properly. There are many things that will effect how strong your models fins are such as shape and size of the fins, grain direction, density of the balsa and how they are attached to the model. Find yourself a nice firm piece of 1/8" balsa that has some B\C grain in it and it will be supper stiff.


John Boren

 

[paulm]

For me, anytime the fin extends below the motor it gets basswood instead! Impact forces cause far more damage than flight forces.

 

[retortec]

I have flown balsa fins to Mach 1.35. It took a lot of prep to get there. The fins were treated with epoxy and sanded multiple times. Rocket proxy fillets were added and the roots were set in a complete epoxy "fin can". I guess the point being you can take it as far as you like but you need to plan for it.

 

[Rex R]

I built a minimum diameter big bertha(surface mount balsa fins) and flew it on a G67...fins survived w/o damage
Rex

 

[aerostadt]

I have used papered 1/8" balsa on the Double Shuttle with large wings using a cluster of E12 motors never having a wing or fin failure. Now E12 motors failures that is another matter. I always use 20 pound stationery paper with white glue, papering both sides. I do the paper drying on a large drawing board with a layer of aluminum foil and heavy books on top. There will be warping during the process, but I alternate both sides using with weights and don't take the weights away until the material is completely dry. The resulting material is light and tough.

 

[terryg]

For me, anytime the fin extends below the motor it gets basswood instead! Impact forces cause far more damage than flight forces.

+1 on Basswood, it is much easier to finish and stronger then balsa.

 

[dr wogz]

Remember as well, that balsa will have different strengths / density depending on where in the tree the sheet was cut from. As you search out the balsa sheets, you'll see some are harder / stiffer than others. Don't just pick the first / top sheet at the store.. Look thru the pile at the store and choose the right one!

 

[dmgrime]

The Estes Star Orbiter uses 1/8" balsa with through the wall construction. No papering of the fins is needed. I bet you can fly this model on a G80 without the fins ripping off if the entire model is built properly. There are many things that will effect how strong your models fins are such as shape and size of the fins, grain direction, density of the balsa and how they are attached to the model. Find yourself a nice firm piece of 1/8" balsa that has some B\C grain in it and it will be supper stiff.


John Boren

I actually did shred the fins on my stock built star orbiter using one of the economax G76 motors. Awesome flight but the fins came back like this:






Awesome rocket though! I got another as well as repairing this one with TTW basswood 1/8" fins. Yet to fly again but planning another G76!

Dave

 

[rstaff3]

Some of this also depends on the model diameter and weight. A Big Daddy on a G isn't the same as a BT-60.

This is one of those things that people have done, but more have been unsuccessful.

 

[RomCat]

Well, I'm getting there. I think I'll end up papering the fins just in case. The whole thing seems over engineered, but then again, I'm naturally risk averse. I attached the fins directly onto the motor mount and cut out slots for the centering ring and then brought them up through the body tube, which was complicated by the fact that it also has a boat-tail. I also gave the fins a semi-airfoil and didn't bring them down to a knife's edge to make it less likely they'll snap on the landing. Here's what it looks like:


The bottom of the body-tube is in rough shape from the whole fitting process, but hopefully I can clean it up during the finishing. Lessons learned. And, yes, that's a retaining hook on there as silly as it seems. I didn't want to friction fit it or use tape.

 

[dr wogz]

Most will point out that the wood grain "should" run parallel to the fin's leading edge. You've got it (the grain) perpendicular to the tube. That's better than parallel to the tube! Adding paper will add strength.

I assume the motor mount (MMT) is held by both the top & lower centering ring (CR). Personally, I would have made the MMT tube longer, to allow for 1 caliber (tube dia) or more between the CRs..

 

[JumpJet]

Dave,


Sorry to see your fins tear apart the way they did. I've heard of others flying the model on G motors with no issue. Just curious do you remember if the balsa in your kit was on the lighter side. Much of our 1/8" is such a high density I find it hard to believe they would rip apart like they did on your model. I see you had nice glue fillets as well.

John Boren

 

[RomCat]

Most will point out that the wood grain "should" run parallel to the fin's leading edge. You've got it (the grain) perpendicular to the tube. That's better than parallel to the tube! Adding paper will add strength.

I assume the motor mount (MMT) is held by both the top & lower centering ring (CR). Personally, I would have made the MMT tube longer, to allow for 1 caliber (tube dia) or more between the CRs..

Thanks for the tip. I beefed up the front centering ring as you can see to give it a bigger bite inside the tube and the rear CR has the added benefit of being directly up tight against the shoulder of the boat tail, although on second thought, I should've bonded it there at that point as well. We'll see what happens. Just have to add some nose cone weight and the recovery system and she'll be ready for a paint job!

 

[Incongruent]

Most will point out that the wood grain "should" run parallel to the fin's leading edge. You've got it (the grain) perpendicular to the tube. That's better than parallel to the tube! Adding paper will add strength.

I assume the motor mount (MMT) is held by both the top & lower centering ring (CR). Personally, I would have made the MMT tube longer, to allow for 1 caliber (tube dia) or more between the CRs..

The perpendicular orientation offers more flex resistance. The only issue is that in swept back fins, such as those, the back of the fin can snap off more easily. Papering would fix that and you can also wick CA into the balsa. Papering has the added benefit of finishing the fin surface, CA will strengthen and allow the paper (now soaked in CA) to be sanded smooth and harden it.

If you wick CA into the bottom of the tube, it will strengthen the area and prevent the layers of paper from peeling. Just make sure to elevate the motor hook so it doesn't end up glued to the tube.

You don't need to add another centering ring lower on the mount; you can cut and glue some balsa spacers to support the tube lower down as well. This option should be lighter and as it is hidden, doesn't detract aesthetically.

 

[Cabernut]

...

You don't need to add another centering ring lower on the mount; you can cut and glue some balsa spacers to support the tube lower down as well. This option should be lighter and as it is hidden, doesn't detract aesthetically.

It would be much easier, lighter and more simple to add a small cardstock centering ring to the rear.

Also, if the tailcone tip can attach directly to the motor mount tube, that would work as well. That is how it's done in some kits. The last thing you want is for the motor mount to come loose and "gimbal" itself. A D12 has a peak thrust of 3kg for a split second, so as long as the motor mount can hold that steady, you're good.

 

[Incongruent]

@Cabernut
So the centering ring connects to the boat tail?

 

[RomCat]

It would be much easier, lighter and more simple to add a small cardstock centering ring to the rear.

Also, if the tailcone tip can attach directly to the motor mount tube, that would work as well. That is how it's done in some kits. The last thing you want is for the motor mount to come loose and "gimbal" itself. A D12 has a peak thrust of 3kg for a split second, so as long as the motor mount can hold that steady, you're good.

Not sure if I'm thinking about this right, but the TTW construction of this rocket anchors the motor mount to the body tube with fillets along the entire 70mm length of the fins. If this motor mount comes loose during flight, then one of two things has happened. Either 1) the flight forces caused the external fillets to fail in multiple locations causing the whole mount to jar such that it is no longer concentric with the BT or 2) the double glue joints where the fin roots meet the motor mount fail (unlikely).

I can see the problems this design might have if the fins were glued directly to the BT, but the TTW construction seems to me to give the motor mount the stability that it needs. Or am I missing something?

 

[Cabernut]

Not sure if I'm thinking about this right, but the TTW construction of this rocket anchors the motor mount to the body tube with fillets along the entire 70mm length of the fins. If this motor mount comes loose during flight, then one of two things has happened. Either 1) the flight forces caused the external fillets to fail in multiple locations causing the whole mount to jar such that it is no longer concentric with the BT or 2) the double glue joints where the fin roots meet the motor mount fail (unlikely).

I can see the problems this design might have if the fins were glued directly to the BT, but the TTW construction seems to me to give the motor mount the stability that it needs. Or am I missing something?

I'd be worried about buckling of the body tube as it's a narrow attachment point.

I'm sure you'll be fine though. Keep us updated, it looks good so far.