[AR] Re: Intertank coupling design
- From: Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
- To: Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
- Date: Fri, 25 Oct 2013 13:04:36 -0600
Sent from my iPhone
On Oct 24, 2013, at 8:47 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx> wrote:
>
> Carl,
> Thanks for the info and link to Richard's page. I haven't decided on the
> exact coupler configuration yet so there's still room to improve the design.
> I assume one "caliber" means one diameter?
Yes, one caliber = one diameter.
> I saw that on many other HPR websites where the construction materials were
> composites or fiberglass. Some of them even suggested two or three
> diameters. I can't believe that you would need a 5-10 inch long coupler to
> hold two 5 inch diameter tube sections together.
In hindsight, they may be talking about the sections that separate for
parachute deployment, since these sections usually have 2 to 4 nylon shear
screws. The one caliber rule insures the section stays put during high speed
flight.
> An airplane fuselage is many feet in diameter and they usually only have a
> single frame section with a few inches on either side. Of course there
> stringer section running the length of the airplane.
Ours was about 2-3 inch long coupler for our 8-inch diameter airframe. If I do
it this way again I would probably use two rows of fasteners offset; still the
same number of fasteners.
> I can envision alignment issues though, especially if the ends aren't
> completely round and the fastener holes aren't drilled perfectly square
> to the ends. I was hoping to mitigate some of that by making the coupler a
> bit thicker than needed so it wouldn't deform as everything is bolted
> together.
>
> Was your fuselage skin also the pressure tank?
Yes, the pressurized tanks were also the airframe.
---Carl
> I had to go with 16 #10-32 screws on the 5 inch diameter to keep from
> exceeding the bearing allowable stress on the skin. With a conservative 2.0
> factor of safety, the tank end caps have to withstand around 18000 lbf each
> (which works out to about 1100 lbf per #10 fastener in shear).
>
> -Bob
>
> On 10/23/2013 02:17 PM, Carl Tedesco wrote:
>> Bob,
>> Just getting to this post...
>>
>> Here are some comments on the couplers on our SDSU rocket:
>>
>> Our SDSU rocket was 8" diameter, 0.072" wall.
>> The coupler was made from the same tube; we slit the tube longitudinally to
>> remove enough so that it could be fit into the original 8" tubing.
>> We riveted the coupling to the airframe/tank tube for sections that we did
>> not intend to seperate.
>> For sections we wanted to separate we used twenty (20) 8-32 screws. We had a
>> few screws strip in the thin 0.072" wall, so your method with inserts is
>> appealing.
>> Our big problem was alignment of airframe sections, which many on this list
>> gave some good tips.
>> Picture of our inter tank adapter here
>> An HPR guy once told me to make the coupler insertion length at least one
>> caliber. We did not follow that advice and hence could have contributed to
>> our alignment issues given the tolerances of our "rolled" couplers.
>> Why could you not use your "inner sleeve" as the coupling tube? Seems like
>> one extra part.
>> Richard Nakka has a nice little webpage on calculating loads on your
>> airframe here . It doesn't address loads at if your rocket has any
>> angle-of-attack, but it's still a nice guide.
>> --- Carl
>>
>> On 10/11/2013 2:23 PM, Robert Watzlavick wrote:
>>> I'm designing the couplings that will connect each of the propellant tanks
>>> to the rest of the structure for my rocket. The primary structure for the
>>> tanks is 5 inch OD, 0.125 wall 6061-T6 tubing. The question is: what wall
>>> thickness and length does the coupling tube need to be to connect two of
>>> the 5 inch OD tubes together? To simplify the discussion, ignore the tank
>>> bulkheads or any other structure for now. I don't have an estimate yet of
>>> the expected flight bending loads but for a first pass, one philosophy
>>> would be to make the joint at least as strong as the rest of the structure
>>> in bending so it won't fail at the joint. There would actually be 3
>>> concentric elements: 1) the two primary structure elements butted up
>>> end-to-end, 2) the coupling sleeve inside, and 3) another sleeve inside the
>>> first one. NAS 623 fasteners, installed radially from the outside in are
>>> used to hold it all together, with the primary structure and coupling
>>> sleeve drilled to match the "grip" diameter of the fastener. The only
>>> purpose of the inner-most sleeve is to have something for the
>>> fastener to thread into. By using a second sleeve, there are no shear
>>> loads on the fastener threads, only tension. Even though the inner sleeve
>>> (with fastener threads in it) will provide additional stiffness, I wasn't
>>> going to count its contribution since it will be cut down as thin as
>>> possible. The material for the two sleeves would also be 6061-T6.
>>>
>>> If the goal is to match the bending capability of the primary structure,
>>> then I would think the coupling sleeve only needs to be thick enough to
>>> match the moment of inertia of the primary structure. Then, the length of
>>> the coupling should only be a function of the shear tearout allowable for
>>> the joint. Am I on the right track here? Of all the allowables for the
>>> joint (fastener shear stress, bearing stress, shear tearout, net area
>>> tension), shear tearout and bearing stress appear to be the most critical
>>> for this design. One thing I'm not sure about is how to convert an
>>> arbitrary bending moment into the shear load per fastener.
>>>
>>> See attached sketch for details. Any advice would be appreciated.
>>>
>>> -Bob
>>>
>>>
>>
>> --
>> Carl Tedesco
>> Flometrics, Inc.
>> 5900 Sea Lion Place, Suite 150
>> Carlsbad, CA 92010
>> tel: 760-476-2770 ext. 515
>> fax: 760-476-2763
>> ctedesco@xxxxxxxxxxxxxx
>> www.flometrics.com
>
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