[AR] Re: Intertank coupling design
- From: Carl Tedesco <ctedesco@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 23 Oct 2013 12:17:54 -0700
Bob,
Just getting to this post...
Here are some comments on the couplers on our SDSU rocket
<http://eon.sdsu.edu/%7Erocket/>:
1. Our SDSU rocket was 8" diameter, 0.072" wall.
2. 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.
3. We riveted the coupling to the airframe/tank tube for sections that
we did not intend to seperate.
4. 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.
5. Our big problem was alignment of airframe sections, which many on
this list gave some good tips.
6. Picture of our inter tank adapter here
<http://4.bp.blogspot.com/-y9c5oqkLSw4/URW_PgeNrnI/AAAAAAAAAFc/Wpq8c8LMobA/s320/Rocket+project+1.jpg>
7. 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 <http://www.nakka-rocketry.net/fusestru.html>. 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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