Balsa Body Tubes

Sure. It's larger around than a paper or vellum tube, but it's much lighter, and tremendously more durable than vellum.

1. Find a mandrel. I use a piece of K&S 1/2" tubing with one wrap of thin card stock glued to the outside with 3M77, then wax paper.
2. While you're at the hobby shop, get some .025 music wire.
3. And pick up some A-grain 1/32 contest balsa.
4. Cut some strips of balsa about 1.56-1.58 wide (pi*.510 minus a little), and a little longer than the desired tube.

5 Tape the wires to your flat workbench about 2" apart, and use them as jigs to sand down the balsa (set in between the wire - sand with a rigid block until you hit the wires - voila, .025 balsa) 6 Soak the balsa for a while in hot water 7 Wrap around mandrel, then wrap with Ace bandage 8 wait overnight 9 Remove bandage - balsa tube! While still on mandrel, carefully glue ends together. I use Titebond or Pica GLUIT, but Ambroid or Sigment will work. The ends won't quite meet. Put glue in the joint and then stretch it to meet with masking tape. You can use CA, but you had better be willing to destroy a few tubes when they stick to the mandrel. Let dry thorougly. 10 Still on mandrel, sand carefully, and clean up ends. If desired, boat-tail the tail end. 11 Still on mandrel, put on a few coats of butyrate dope. Don't use nitrate for obvious reasons, unless you just happen to enjoy flaming destruction. 12 Still on mandrel, apply Jap tissue or OO silkspan. I use jap tissue WET, which is only for those with a lot of experience. OO silkspan makes a very sturdy tube. Jap tissue is lighter. Don't be scared to use shrinking dope (Brodak or Supercoat Clear). And put on a few coats. Let dry and outgas until smell is mostly gone, on the mandrel. If you take it off too soon it will shrink and be crooked, and bubble up from the ejection charge. I cook mine off in the food dehydrator for a couple of days on about 150 degrees, but I'm generally impatient. It won't be perfectly round, but it's not critical. 13 Build rest of model as usual. If you did a good job, there's not particular need to cut away the tissue. Just glue the fins on with Ambroid or Sigment, and it will all melt together into one piece.

1. Of course, you will need a custom nose cone to fit the thicker wall, turn out of balsa.

Larger tube are of course easier, but it doesn't take long before you don't need them anymore, as you go up in impulse. These models can easily be built below optimum mass even for A's, so it's only necessary for performance reasons in the lower classes.

Another thing to watch for is stabiliy. With a 2.2 gram model and a 1/4A engine, they tend to come out a little bit tail-heavy. You can add mass, but I think it's clearly more efficient to make the fins larger - of course, just large enough.

I'll send a picture of one of mine, and try to post it to a.b.m.r.

Brett

Would lining the inside with Nomex paper help with the burn issue?

nOrM

Or shellac (ignore tech post)

Hi Norm,

I've made a set of mandrels for 13, 18, and 24mm motors out of aluminum on a miniature Sherline lathe. I've wrapped 1/32 balsa over the mandrel, glued it into place, put it back in the lathe, and turned it down, for a nice round tube. It helps to tape the tube securely in place.

The lathe is great for making matching nose cones that fit perfectly.

Adapting the rocket design for the lighter tube can be a problem. You can sweep the fins back for more stability, but this increases the chance of shredding them. I had the bright idea of using a bigger streamer--but the rocket wasn't heavy enough to unfold it fully--it looked like an accordian coming down... Thinner streamer material didn't quite work either--another shred.

Zack Lau W1VT NAR 80361

No, but I tried really hard to get a mirror-like turned finish--the cosmetic imperfections don't seem to matter in actual use.

I used a dead center--but next time I'll get a live center to reduce the heating and themal expansion problems I encountered.

Tissue strengthens fins--1/32 tissued TTW fins worked OK on my BT-50 B PD rocket, but you probably want to know what holds up when you use pistons on a minimum diameter bird--so would I.

I've been too busy learning the tougher events like C DELA, 1/2A CA, F DELD, ARG, B FW, PMC...and E HD and D FW next month to do much test flying of minimum diameter duration rockets.

Zack Lau W1VT NAR 80361

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nice post

Thinner balsa sheets may also be available from some FF supply sources, but I have only used the common 1/32" balsa.

Most contest models are easily built below optimum mass, so I prefer the ease and durability of conventional tubes for most models. Bob Kaplow has used balsa for transitions on egglofters to good advantage. I have used balsa on superoc where the strenght and stiffness at low weight is very important (fiberglass might even be better).

Alan

It's *a lot* cheaper. I can't imagine the degree of waste it takes to get .016 balsa sheet from a log - gotta be a factor of 3 or 4. I'm sure that's why it's so disproportionately expensive. It's easy enough to sand it down with the wire jigs, it's not worth it.

Sure - it's only really an advantage in the small motor classes. I use regular old Estes tubes for anything bigger than A because they can be built underweight in general, and the frontal area hit is more significant. For reference, the 1/4A model in the pictures on ABMR is also a legal superroc, too, although it's too short to be competitive under the current rules.

Bear in mind, of course, that duration models might still be better off light even if the the boost is sub-optimal, because the descent rate can be lower. I can tell you from experience that a 2.2 gram model with a burnt-out engine casing, and a 22" parachute, comes down awfully darn slow! One downside is that with the chute ejected, but undeployed, a real light model might be too slow to open the chute. I had a 5 minute OOS flight on one 1/4A PD flight even after it boosetd to ~350 feet, then fell to about 75 feet before the chute finally opened. Doesn't take much to thermal these little guys.

Brett

I don't think they cut the sheets that thinly from the log. They sand them down to final thickness. It's not worth it for ordinary models.

BTW, I read one story about a guy who bought a glider kit, but the balss in the kit was just one big block. He took it to his friend with the bandsaw. When he came back later his friend was so proud at the success he had cutting the balsa. However, he had cut it as end grain. :(

I still don't think it is worthwile for small motor events. However, I will note that most people think of optimum mass with the full coast to apogee, while the available motors often have shorter delays resulting in reduced optimum mass to ejection altitude. Balsa tubes are worthwile for superocs at high q (larger motors), and things like the egglofter transition where the high strength to weight ratio is needed and the structure is not confined to a constant diameter tube shape.

Alan

Of course - they can probably cut it to maybe around 1/16, but the saw blade is probably 3/32 or more - so that's 133% as much balsa going up the exhaust chute compared to the end product right there. Then sand it down to .016 (or whatever your favorite size is), and lose 75% of it. So you start out with 5/32" worth of wood, and end up with 1/64 balsa. Thats a 10% yield.

Brett

I've always wanted to try that to use laminated between fiberglass / kevlar sheets. End grain balsa is VERY strong...

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!