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

[william colburn]

Excellent, all. 

A prime requirement and one that is not trivial is the
ability to attach successfully with the same safety
factor to the ends for closures. That is why I
suggested a 40 inch length for the test piece. It can
be hydrotested and then used, if we wish, for the
short stack burn. 

Generally a single simple bolt circle is not going to
do the job. Here are some solutions that have worked
for others:

Tapered joint- frequently seen on composite piping
systems- the outer surface of the end fo the pipe is
ground to a taper to fit the same taper on a fitting.
The length of the taper is roughly one diameter of the
pipe.

Insert and bolt circle- The closure is inset a bit,
perhaps 1/2 diameter and retained with an insert made
from the same tubing by removing a strip
longitudinally. The insert is held in place with epoxy
and a complex bolt pattern of several circles.

Pinned- seen on composite rocket motor cases- the
closure is about 1/2 diameter in length. hundreds of
small roll pins are used in holes which are drilled
and then reamed for optimum bearing strength.

Those techniques are used on unmodified composite
filament wound tubing. If the ends are reinforced, we
might get by with a lesser intense method. But we have
to prove that early on as it is crucial to the success
of the motor.

BC

--- Lakestake Rocketry <lakestake at gmail.com> wrote:

> 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.
> 
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