[AR] Re: Intertank coupling design
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Fri, 11 Oct 2013 16:46:45 -0600
That seems like overdoing it. The tanks on a pressure fed rocket are
really strong, much stronger than they need to be for flight load
purposes. If the rocket crashes in a semi-survivable manner I'd much
rather destroy the coupler than the tank. By making them the same
strength you've lost control over what breaks.
Regarding the threaded ring: wouldn't it need to be two rings, or the
web between them to be really thin or slotted, for it to carry none of
load? You show it thinner but not completely strengthless.
Why so much effort to not put shear stress on the thread? Sure the
thread is weaker than a shoulder of the same major diameter but that's
only a concern if the total fastener count has to be minimized as the
very first design driver. I don't understand why it would be.
Ben
On Fri, Oct 11, 2013 at 3:23 PM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> 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
>
>
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