Question about strength of BT-80

[BryanHedstrom]

I have a rocket design I've been kicking around for a few weeks and now I would like to start construction. I am planning on installing a 38mm motor mount but will probably fly with mostly 29mm motors but we'll see.

My question is about some Bt-80 tubes I have laying around from Balsa Machining services. At what rate of acceleration or velocity is a good point to switch over to Blue tubes or fiberglass the tubes I already have?

 

[pcotcher]

Standard BT80s or BT80Hs?

 

[Handeman]

Are they the standard BT-80 or the "heavy wall"? If they are the heavy wall, I'm not sure you need to switch to blue tube or fiberglass. A lot depends on how much weight they have to hold up under acceleration, i.e. nose cone, electronics bay, camera, etc.

The only motors I would consider glassing the tubes for are the Warp 9 loads like the H999N or I1299N because you have to use electronic deployment and the extra weight of that along with the 80g to 100g acceleration could cause some issues. If all you use is motor ejection and the tubes only have to hold up the nose cone, I wouldn't over build it for 29mm or 38mm motors that keep the altitude low enough to actually still see the rocket. If you use the 38/720 or 38/1080 cases and start pushing past 80% Mach, you also might want to use fiberglass for tip to tip coating on the fins and the lower tube that has to support the electronics, assuming you'll be using dual deployment.

 

[BryanHedstrom]

They are just the normal BT-80. The nose cone is custom and might weigh more than some plastic cones of similar diameter but I'm sure its lighter than solid balsa. I' m not finished with it yet so can't weigh it. Rocksim gives it an estimated mass of 0.7982 oz, I don't think it will be that light. I would like to add an electronics bay and give dual deployment a try. Now considering all that, would you glass the tubes?

 

[bobkrech]

We designed rocket before RS. Weigh the aireframe to get the weight per inch, and weigh the NC.

You shouldn't have to glass the airframe, but if you are using 38 mm motors, you probalby want heavy wall airframe. Fiberboard airframes are plenty strong, but thin ones can bend, fold over and buckle. A thicker airframe is stiffer and won't bend, fold or buckle.

Bob

 

[MarkII]

You can buy BT-80 coupler stock in 34" lengths from BMS, and use it to line the entire airframe. The coupler stock is stiff and reasonably thick-walled, but it is not particularly heavy in weight. I am doing this for an Estes Executioner that I am building with a 29mm motor mount. Doubling the Executioner's BT-80 wall with coupler stock will substantially increase it's strength and stiffness without adding too much weight, and it is dirt simple to implement. It is much simpler than fiberglassing the tube and it is cheaper than replacing the kit's tubing with blue tubes.

MarkII

 

[BAR0051]

I have a bashed Estes Executioner that I have flown on 3 F39's (94% G) using standard BT-80 with no problem. I does have a plywood baffle in the coupler so I think that gives it a bit more strength there. I would think that a 38mm G motor should not be a problem if properly constructed.

 

[MarkII]

Because of its diameter and the thinness of its wall, BT-80 initially appears to be a bit too fragile for anything larger than a slow-burning E9. But I don't doubt that you are right - once the internal structure is installed (centering rings, coupler, motor mount), it would probably be plenty sturdy for mid-power. In my project, I could forgo the coupler lining and just install extra centering rings along the MMT for extra support; I'll think about it. I know that this is counterintuitive, but I might still give the section above the upper centering ring, including the parachute compartment, a double wall (up to the inserted nose cone shoulder). With the plywood centering rings that I'll be using and the TTW fins, the fin can should be OK, but the forward area would otherwise be unsupported by any internal structure, and it could get rather stressed at deployment time.

This doesn't totally answer Bryan's question, though. Remember that he asked at what point would either reinforcing the BT-80, or else switching to the much stiffer blue tubing be advisable for his 38mm project. But to really answer the question, I think that we need a little bit more information, such as: how long of a BT-80 airframe are we talking about (how much BT-80 in total?), what is the size of the nose cone, and how much does it weigh? Oh, and will the fins be surface-mounted or TTW? Bryan, you don't have to reveal every detail of your design if you don't want to, but a little more information of a general nature could be helpful in addressing the issue. Otherwise, the answer might be, "It all depends." Also, although you said that you will mostly just fly it on 29mm motors, is there a chance that you would want to fly it on 38mm motors some day?

MarkII

 

[Phred]

I agree with Mark II: I have built several upscale model rockets (3-4 feet long) using BT-80 as the main tube, and have flown them on F21s with no ill effects.

I use a BT50H stuffer tube with 1/8" lite ply centering rings, and leave about 12-14" at the upper end for the parachute. If I am flying a really heavy plastic nosecone, I coat the inside of the parachute compartment section of the BT-50 tube with super thin CA for a little extra strength.

Phred

 

[BryanHedstrom]

Thanks for all the advice. I still don't know which way I will go yet but here is some more information.

The Nosecone isn't done yet so I can't weigh it, The bt-80 tubes combined would be 48" long. The seperation point would be 34" from the tail end of the body tube and the tube weighs 3.68g per inch. Fins and centering rings would be made of 1/8" aircraft ply with through the wall construction on the fins. Overall length with nosecone and tail cone would be 71.5 inches.

As you might be thinking, its a long pointed nosecone. In Rocksim I have it as hollow construction 0.090" thick fiberglass and the calculated weight is 0.7982 oz, I still think its gonna be heavier when I pry it out of the mold.

As far as the 38mm motors go, I'm sure if they will fit, I will stuff one in there eventually

 

[Handeman]

How big of 38mm are you going to eventually stuff in there? You have a range of G to J. The 38/1080 case is almost 19 inches long so you should be able to fit it in there.

Since your tubing is standard and not the heavy wall, I would glass the tube instead of putting a coupler in it. I think glassing the tube would be easier, faster, and make life easier then trying to glue a full length couple inside the tubing. I also don't think it will really be significantly heavier.

I glassed my first tubes on the last rocket I build. Some of my lessons learned is to use 60 minute thin/laminating epoxy to put the cloth on (I used US Composites). With 60 minute epoxy, there's no rush, but you'll probably be done in less then 10 minutes. In your case, I would recommend two layers of 3 - 4 oz cloth. I use 6 oz on mine with a 54mm MMT and expected Mach+ flights. Cut a piece an inch or so longer then the tube and wide enough to wrap two times around. Suspend the tube between two supports on a square piece of wood like a 2x2, it keeps the tube from rocking and rolling. Remove all glassine and draw a line on the tube as a starting point for the wrap. Use a disposable brush to paint the epoxy on the tube and let it soak in a little. Make the first wrap and paint on epoxy where ever the first wrap doesn't soak up enough. Use good rubber/latex gloves and use your hands to smooth out and distribute the epoxy so you don't paint on too much. It'll be obvious where the cloth is dry, it turns transparent when wet. Continue the second wrap and paint on enough epoxy to wet out that wrap.

After it drys, I used Bondo to fill in the weave of the cloth and get a smooth finish.

It's really much easier then it sounds and you'll be surprised at how quickly you can glass a tube. It takes longer setting up and mixing the epoxy then actually doing the glassing. Also, the little bit of extra weight doesn't really make any difference when you're flying G - J motors. Not only will the tubes hold up to the thrust of the motors, but also the shifting, falling and movement of "stuff" in the trunk on the way to the launch.

I would highly recommend glassing the tube before assembly. After the fins are in place and Tip to Tip glassed, if you want, fillets installed, then use the Bondo to fill in.

Good luck and hope to see the L1 and L2 pics sometime too.

 

[MarkII]

Handeman - do you ever vacuum-bag your tubes?

MarkII

 

[Handeman]

Handeman - do you ever vacuum-bag your tubes?

MarkII

Not yet. I had the fiberglass cloth and the US Composites thin epoxy, but I didn't have any of the release sheets or absorbent sheets so I haven't tried it yet. I do have my compressor in the garage and a copper line with quick disconnects plumbed from the garage to the basement, along with a venturi vacuum pump that should work. Maybe the next project will have to include some vacuum bagging.

 

[Mikus]

I have a bashed Estes Executioner that I have flown on 3 F39's (94% G) using standard BT-80 with no problem.

Boy howdy, Executioners love F39s don't they. :headbang:

Mine flew F39s with no problems built almost stock (modified shockcord mounting) and with no baffle.

 

[DAllen]

FWIW...I have a BT80 scratch built rocket which looks like a cross between a Big Daddy and a Tiny Pterodactyl. I did a very light glassing with a single coat of 3oz. glass. I have flown it on an F52, F40, G64, G78 (Pro38 1g smoky) and the G69 sparky. All this with oodles of nose-weight in it for stability and no sign of wear.

https://www.coolrocketry.com/images/NSL2009/images/fullsize/IMG_6049.JPG.JPG

-Dave