Light-/Medium-weight Paper Airframe: How hard and fast?

[MountainRocketeer]

My basic question is, how much acceleration (g's) and velocity can a BT80 (2.6" paper, Estes-type) airframe withstand? I have gotten my L1 certification on this rocket and flown a half-dozen H motors, and now I'm trying to decide how far to push it.

(Some will chuckle or shake their heads that I am discussing paper airframes on a high-power forum, but I am pleased with my birds and enjoy what I do. My question is not how would you build a certification rocket, nor whether you would buy or build a new or different rocket, nor whether light paper, heavy paper, fiberglass, or carbon is the best material. My question is how far I can push this rocket.)

This rocket is a 42", 26 ounce, three-segment, dual-deployment rocket constructed of BT80 tubing. The nosecone a plastic 4:1 ogive, with epoxy filling the first few inches to anchor 4-20 threaded stock for a removable bulkplate, which contains a Big Red Bee transmitter. It carries a Raven AvBay housed in a 29mm coupler set in the airframe coupler's dual bulkheads. I used Balsa Machining Service's body tube and coupler stock because it is bit thicker and stiffer than Estes or Apogee tubing.* I already had a bunch of BT80 tubing, cones, etc. from my mid power projects, so that is what I used for my cert rocket (and it is light enough I can also use AT 29x40-120 motors for economical flying).

I epoxied coupler stock inside to sleeve the entire rocket, so it is a "double hulled ship" from the base up to the nose cone. I have never zippered any of the tubes, but the top two segments attach to the avbay with plastic rivers so I can easily replace one if I get a zipper, etc. Additionally, the fincan is a zipperless design with a centering ring flush with the base of the rocket and anothet flush with the front edge of the segment. A heavy-walled motor tube runs the full length, and a CTI 6xl motor case will exactly span the front and rear centering rings/length of the fincan.* The fins are 1/8" aircraft grade birch plywood, through the wall, and I used a finguide to set them true. All airframe junctions are carefully squared, with coupler overlaps of at least 2.5 inches, and with fore and aft coupler and airframe edges in contact at all joints (so weight/inertia is born by both the body tubes and the full-length coupler stock). It has always flown very straight with minimal rotation and no wobble or coning.

I have flown it on a half-dozen H motors, with impulses up to 298 N-s and 4000 ft altitude (CTI H159 Green); and max thrust of 254 newtons, acceleration of 23.5 g's, and velocity of 665 fps/.6 mach (AT H159 NBT).

I would like to fly it on the CTI I243 White, which is only a 25% increase in impulse to 381 N-s, but a 75% increase in peak thrust to 445 newtons, resulting in a 50% increase in acceleration (39 g's) and velocity (965 fps/.87 mach).

So, any thoughts about whether that acceleration and velocity is within the do-able range for this airframe?

How hard have any of you pushed a thin paper/Estes-type airframe with high-power motors?

 

[1tree]

In the name of making this real science, I say go for it perhaps getting some pics as it launches. And then just keep pushing the envelope.

Then again I on who believes that most over build their rockets by huge margins.


Kirk

 

[Coop]

With this rocket being double-hulled, it's not your typical Estes combo ANYWAY, and the full-length coupler would make the stock BT-80 stiffer--which you'd want with higher G flights.

I don't see an I-243 out of the realm of possibility. I think the greater challenge would be tracking the flight and recovering it!


Later!

--Coop

 

[Handeman]

I agree with Coop. This isn't a standard build and with the stiffer walls, you should have no problems with the Pro29 I243, I244, H340 or even the H399. I had a rocket that used 1/8" wall mailing tubes. These walls were thicker, but the mailing tubes aren't as well built as rocket BTs. It handled 80Gs from I1299N-P Warp9 motors without a hitch.

The Gs forces on the BT isn't that big of deal, unless you have a lot of weight in the nose cone. It's the section of tube between the av-bay and the fin can you have to worry about. As long as your joints are square and even so you get even loading all the way around the tube, it should handle any 29mm motor you want to throw at it.

 

[tsai]

Agree with everyone else. Because of the stuffer tube, you've increased the strength dramatically. I remember some equation I saw at one point which indicated that the compression strength increased as a square of the thickness.

For 29mm motors, I wouldn't be afraid to throw anything at it. I'm guessing you'll have to deal with fin flutter from speed before you need to worry about the tubes collapsing.

 

[g zilla]

When it shreds you know you went a little to far. Totally agree any 29mm motor shoud be fine.I would even try Cessaroni H-410 V max

 

[grouch]

The 410 is no big deal. The rocket he is describing can easily take a 410.


Sent from my iPhone using Rocketry Forum

 

[bill_s]

Interesting. You didn't say if this is BT80 or BT80H. The BMS T80H is .041" thick, double with 80H coupler tubing and that's huge. With the regular, double it and it's a little less thick than LOC tube, but with an epoxy layer and probably staggered spirals.

I've seen several cases lately of stress points. Problem with extra reinforced part is where it stops, that's where it's going to break, or get a little split at the spiral groove. Sounds like you've done good in that regard. Another variant is how much weight is carried by the tube, rather than just having to carry its own weight and the drag of a nose cone, which still increases with length.

I don't know the answer, could try testing a spare piece of tubing. "Test by destruction" is a sort of valid engineering technique, quick but has disadvantages, and the basis of our "experience" based methods of rocketry. Have heard of plain BT80 failing, and Aerotech tubing in long rockets such as the Mirage with extreme motors, so you could be approaching the limit.

 

[JRL303]

Only one way to find out...

 

[bobkrech]

Your rocket doesn't use model rocket construction techniques so it's not an "Estes type" rocket. (You certified with TRA, right?) It's a double-walled, fiberboard rocket with heavy wall fiberboard airframe and plywood fins similar to the high power LOC/Precision construction methods (good enough for M motors without glassing) but stronger because of the double-wall construction. Your airframe stiffness won't be very different than a FG rocket of the same weight because of the double-wall construction as density of fiberboard is half that of FG composite so the total cross-section thickness is about twice of the composite.

Stiffness is the critical parameter to prevent airframe buckling with is one of the two principles causes of shreds, the other being fin flutter due to lack of stiffness of the fins. Both failures are due to velocity, not acceleration.

Your rocket should withstand any 29 mm motor you can shove in it as long as the fins are not excessively large.

Bob

 

[MountainRocketeer]

Well, I guess I will push it to the limit of 29mm power. (Yes, I certified with NAR.)

I will see if I can post some photos of the rocket and the flight before too long.

Thanks, Bob and everyone else. I appreciate the informative feedback.

 

[timro]

I built a Madcow Striker 2.6" "MPR" and adapted it for dual deploy and used a BRB GPS in the nose. It is probably very comparable in materials (single fiberboard tube) and construction (I removed the aft centering ring to make internal fillets using aeropoxy structural adhesive) to what you describe. It came in around 860g dry weight without motor.

I flew it this January on a CTI I205 (6G Imax) and it was a very good flight. I'd not hesitate to fly it again on a 6GXL motor.