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

[Lakestake Rocketry]

Good morning VehDev!

As these threads are getting long I have hacked apart the preceding post,
please refer to the previous letter for full context.

Colin, any parts of your post that I have removed I am in full agreement
with.

Matt Campbell wrote:

>
> >I am hoping to get at lot of additional input from this group on the
> layer
> >design for the fabric composites proposal. Roman, Colin, can you make
> >the first proposal on layers/orientations for a Carbon/Kevlar airframe
> >and motor? If not please respond with what steps need to be achived
> >before we can do this (do we need to gather loading data for available
> >weaves?).
>
> Colin reponds:
> You shouldn't have to gather any data on the weave. I have material
> properties for pre-pregs and wet lay-ups, so I could have a look at some
> rough calcs.
> One of the problems I can see with a hand lay-up is the porosity. Have you
> looked into RTM (resin transfer moulding)? RTM should reduce the porosity
> as the resin is sucked through the entire part usually filling all the
> voids.
> Are you looking at using a metal liner as the mandrel and leaving it in
> the
> lay-up? This would help with leaks and possible even temperature (if it's
> made of something like titanium).


Thank you for your comments Colin.

Thanks also for your offer to do some rough calculations. I am hoping we
will
have relatively few layers, for reasons of cost and ease of construction.

Great note about porosity, I hadn't given it due consideration.

I am worried about the weight cost of using a metal liner. As our propellant
has a relatively low efficiency we may need to milk our airframe design for
every ounce we can while ensuring mission success. This was the reason
behind the original proposal to do a composite airframe. I worry (without
any
data to back me up) that making the mandrel light enough to use in the final
rocket we may loose enough strength it won't work well as a mandrel. Your
(and the rest of the team's) experience and comments appreciated on this
topic.

Are you vacuum-bagging the lay-up?


Yes, vacuum bagging was the method proposed.

Have you done lay-ups like this before? From the nominal 0.188" (4.8mm)
> wall thickness you'd be looking at around 17 layers of 200gsm carbon. This
> will need to be de-bulked during the manufacturing process (basically laid
> up a couple of layers at a time). If you tried to lay up something this
> thick in one go you will end up with wrinkles in the composite. I
> shouldn't need to say that that's a bad thing.
> Also, I'm not sure if you know but there are different qualities in carbon
> fibre. Generally the local fiberglass shop will have the cheap stuff. You
> should be looking at an aerospace supplier otherwise the calcs will be
> wrong.


I have done both cloth and woven tube layups at various sizes up to 8"
with very consistent results using various fibers. The wall thickness is
not a requirement, exceeding the strength numbers provided by Richard
by a safety margin should be our goal.

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?

I have been using ends from a local special effects distributor in
Hollywood.
I get a wide range of materials from aerospace grade carbon fiber through
'boat grade' carbon and Kevlar ends. I usually can't get more than 10 yards
of any one weave, but it is at a substantial discount below list. It is all
fun
stuff to play with, it is very educational to see the differences that the
grade
makes in the final product.

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 just had a thought that may be just as good as filament winding but with
> a hand lay-up benefits. I was thinking of carbon fibre braided tube. This
> stuff comes in continuous lengths, basically like a sock that you slip
> over
> the mandrel, hence there are no joint in the part. If you were to get the
> diameter of the braid matched to the mandrel then you can achieve the
> optimum fibre angle. This being around 55deg for thin walled
> structures. You could also use a couple of layers of uni-directional to
> take up the hoop stress. I'll have a look into this and get back to you on
> an ideal lay-up. You can also get the braided tube in Kevlar or a mix of
> Carbon/Kevlar.


I have some unidirectional, great stuff! I was given it by one of my vendors
as a thank-you for past business. I assume that we want to use this
sparingly
due to price. 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.

>Once we, as a group, agree on a design I'll lay up a 1 foot square
> >section for initial testing.
>
> Flat section or cylindrical? They will be very different to
> manufacture. If I were going to test it then I would lay-up a section that
> is cylindrical this will give you a more realistic answer as to wether or
> not the lay-up is suitable.
> Another options is to just lay up the entire part and test it for
> strength. If it's not the required strength then you could always just add
> a few more layers to it.


I was proposing a flat piece for cost and ease of creation just to make sure
manufacturing techniques left us in the right ballpark. Once initial testing
didn't reveal problems I was going to build a short tube (48")? for both
pressure testing and for the short motor test. Your idea of chopping off
the end for testing is a great one!

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.

Richard, who offered to do our materials analysis?

Have a great week all,

Matt C
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