[SS-VehDev] Re: composite short stack/instrumentation section

[Lakestake Rocketry]

Thanks Colin,

Great information.

The whole airframe would not have to match the specs for the motor casing,
at least the top half can be a lot lighter construction.

I was told (by my vendor) that uni was harder to get and thus more
expensive.
That was several months ago. It sounds like either that was *? or the
landscape has changed in the carbon supply world. Good to hear.

How do we figure a liner into your calculations? If it has some mechanical
properties as well as providing insulation during the second stage burn we
may be able to build a lighter motor casing.

Richard, can your group recommend a liner for a composite airframe
that we can use to refine our model?

Colin, if you will be able to provide a wound tube how would it compare
with the current fabric design in terms of weight at the same strength?
Would you recommend that we use it as a motor casing only with a light
fabric airframe over the top, or would it be both the casing and aft
airframe which we attach the forward fabric airframe and fins to?

Matt

On 10/13/05, Colin Hutchison <colin at carboncopies.com.au> wrote:
>
> G'day all,
>
> Sorry for the delay. I've run some number on a range of lay-ups. I
> guess it comes down to Margin Of Safety and how close you would like to
> get
> to the limit. I've attached a screen grab of the Excel spreadsheet I've
> been using. The ply count runs from 15 to 32 plies, however, the braided
> tube was modelled using two separate uni directional plies. Therefore, for
> example, lay-up for ply count 29 is actually 5 layers of uni-directional
> and 12 layers of braid, totalling 17 layers. The braid angle was also
> modelled at +-60deg instead of the optimum of +-55deg. This makes the
> calcs a whole lot easier. I'll attach the complete lay-ups at a later
> stage. Each ply (as seen in the calcs) is a 200gsm layer of uni. So the
> braid would be equivalent to 400gsm.
>
> There are a few factors that affect all the calcs. One of them being
> the breaking strain of the Carbon. I've done the calcs for 4000us and
> 6000us. Typically Boeing use a value of around 4000us. This allows for
> there own safety factors, one of them being holes drilled in the
> surface. Another is the Young's Modulus of Carbon. I've used a slightly
> reduced value from typical pre-preg so this should be ok, but it comes
> back
> to my point that the supplier and quality of the carbon does matter. Also
> the quality of the epoxy will affect the final product.
>
> If you want to go to bare minimum the you could go as low as ply count
> 17 (1 layers of uni and 8 layers of braid). Allowing for 6000us then the
> minimum MOS is 0.1219. (Btw, the MOS is calculated for each ply in the
> laminate and then the minimum value is the MOS stated). The casing would
> come out at around 12.1lbs. If this were to be the lay-up then I would
> probably recommend that the ends be beefed up a little by adding more
> plies. Basically because you are going to be drilling holes in it.
>
> I'll also just not here that the holes should be at least 3 times the
> diameter of the hole to the edge of any composite.
>
> If you would like a little more safety, then ply count 22 (2 layers of
> uni and 10 layers of braid) gives a MOS of 0.1637 at 4000us and weights
> 15.7lbs.
>
> If you can afford the few extra pounds (and I know your pushing it)
> then I'd probably be going for something like the 29 ply count. This gives
> a good MOS (nearly 1) and at the end of the day will only weight 20.7lbs
> (41.4lbs for both stacks). If you were to swap from metal to composite
> then you could probably afford the weight and therefore little more
> safety.
>
> Each additional ply is approx 0.7lbs (so 1.4lbs for the braid).
>
> So I guess what I need to know now is, how close are you willing to cut
> it? I've also done the calcs for 1500psi and not 1100psi, so the minimum
> MOS will probably be enough. If you like I can run the numbers for 1100psi
> with a required MOS.
>
> Probably not the right spot to ask here, but what is the final grain
> structure? Will it have a cylindrical core or star shape etc?
>
> I still haven't found out about the filament winding, but I will
> hopefully soon.
>
> >I have added up to four layers at a time to a composite tube without
> bubbling
> >or wrinkling. Under what circumstances do you find this a problem?
>
> Laying up more layers than that and vac-bagging. Basically the plies have
> excess length and can't go anywhere and so they end up buckling and
> causing
> wrinkles.
>
>
> >For the amount of fiber we'll need for this project we should consider a
> >different source. I think that Roman may have good suggestions along
> >these lines. If you (Colin) or any other team member has a thought please
> >pipe up!
> >
> >Once we have a weave proposal everyone can start keeping their eyes out
> >for availability of those grades/weaves at their local haunts.
>
> I can get the stuff but unfortunately I have to pay for it.
>
>
> >I assume that we want to use this sparingly due to price.
>
> Not really. Carbon is basically sold by weight so it shouldn't matter what
> form it's in (unless you get down to fine weaves or really heavy
> weaves). Uni-directional should be the same price if not a little lower
> that woven cloth.
>
>
> >The braided tube works well, I worry about being able to find
> >the 'right size'. I have had to stretch the tube I get quite a bit to
> >match the
> >size of project I am working on. It still works well, you just don't get
> >'optimum'
> >performance as you noted.
>
> I know you can get it made up to specific sizes but then you are stuck
> buying a fairly large quantity. I'll figure out what size the braid needs
> to be to fit a 10in tube and sit at 55-60deg.
>
>
> >Just adding a few more layers makes sense, but since this is an
> international
> >project we may have to ship the part cross-country for testing.
>
> You could probably do the tests yourself. Basically if you have the ring
> that has been cut off the end of the casing, all you have to do is sit it
> upright and put a load on it. When you squeeze the ring the sides will
> deflect outwards a given amount. If you can measure the load and the
> displacement of the sides then I can work out the actual material
> properties. So there should be no need to send it cross country. I'd also
> need to know the dimensions of the ring.
>
>
> Colin.
>
>
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