A failure of one part of the recovery system is my major concern when
it comes to using lighter weight hardware. For example, a robust
hardware attachment (and long Kevlar shock cord) saved one of my
rockets when a side deployment hatch didn't blow completely at
apogee. The main deployed at high velocity, and probably held because
of the fender washers, etc. However, if I ground test everything and
reassure that the deployment system is going to fire, I shouldn't have
to worry (much) about having very strong attachment points. It's
a combination of testing, risk, and benefit.
I don't seem to learn much from my mistakes. I've only learned not to
make mistakes.
The adjustable tap is interesting and I only found it by shopping on
McMaster-Carr. I will receive it on Monday. It has 3 small
adjustment screws to make the tap diameter smaller/larger. The first
10-24 tap I used was not adjustable and the nylon nuts used where a
little sloppy when installed on the threaded aluminum rod. If I could
have 'opened up' the threading diameter a bit, the nuts would have
been tighter. Do you see what I mean?
I recall you showing me the eye bolt that had straightened and yes,
that was amazing. However, consider that the eye bolt didn't get
pulled through the bulkhead. This too is remarkable. So, it makes me
think, could an aluminum U-bolt hold under this stress? I tested
bulkheads by pulling U-bolts through 1/4" plywood that didn't have
fender washers. Just hook your truck up (using Kevlar line) to the
U-bolt with the bulkhead attached to the workbench and drive away.
It's really fun. Anyway, this is when I started fiberglassing both
sides of the bulkheads. It's _much_ stronger.
So, in practice, we'll see if the lighter hardware holds for nominal
flights.
Richard
On Sat, Aug 18, 2012 at 3:07 PM, Kurt Gugisberg <kurtgug@xxxxxxxxxxxxx
<mailto:kurtgug@xxxxxxxxxxxxx>> wrote:
Hey Richard. Great work on the testing. It confirms my thoughts
on fin attachment. I am interested in seeing how nylon nuts hold
up under high shock loads. After pulling steel eye bolts straight
and seeing fender washers pulled through 1/2 plywood bulkheads, I
have my doubts about aluminum and nylon holding up.
Also, what is an /adjustable/ 10/24 tap?
Kurt
-----Original Message-----
From: Richard Dierking
Sent: Aug 18, 2012 10:20 AM
To: roc-chat@xxxxxxxxxxxxx <mailto:roc-chat@xxxxxxxxxxxxx>
Subject: [roc-chat] Using Aluminum Hardware: Lighter but
strong enough?
Since I've had the opportunity to go through some of my old
projects lately (cleaning out the garage), I've been taking
some rocket airframes apart including some destructive
testing. It's interesting to see what holds and where
weakness occurs. For example, for through the wall fins, the
surface fillet doesn't seem to do much. The most important
thing is that the fin slot is tight. So the lesson here is to
create tight slots and don't expect for epoxy to fill the gap
and hold strong.
For attaching the fin, creating many small slots (1/8"
deep) in the root edge for attachment to the motor mount works
well and doesn't require much epoxy. Bryan showed me how to
do this. If the surface is roughed-up with 80 grit sand paper
a fillet doesn't seem to be required. The failure I'm seeing
is the motor mount tube material. The surface of paper tubes
peels and phenolic fractures and peels. And, as expected,
fiberglassing the root to the motor mount tube works best, and
it doesn't require much resin. In fact, the lay-up can be
pretty dry and it seems to do fine. The extra amount of resin
I commonly use for a good finish is not necessary. I guess
drilling small holes in the motor mount tube where the fins
are attached might help hold the epoxy bond between the fin
and the tube. I haven't tried this yet.
*Another thing I've been experimenting with is the altimeter
bay. This is the important part of this message, and sorry it
took so long to get around to it.* Recently, I've done a few
deployment tests using 3/16" aluminum rods that are tapped to
10-24 tread and nylon hardware (nuts and wing nuts) to hold
the bay together. Also, I reduced the size of the U bolts
significantly. I replaced the 1/4"-20 U bolts (1" wide) with
U bolts from Home Depot that are in packs for cable clamps
(5/8" wide). Even though the cable clamps say they are 1/4"
size, the nuts are actually 6 mm metric. Just to give an idea
of the weight savings; Replacing the steel hardware and
reducing the size of the U-bolts on a 4" diameter altimeter
bay reduced the weight from 495 grams to 325 grams (34%
less). Also, instead of using metal fender washers, I've been
fiberglassing both sides of the bulkheads, and switched from
1/4" thick plywood to 1/8" thick. So, no more
fender washers. I'm wondering if I could make aluminum
U-bolts without causing too much stress on the aluminum rod
during the bend?
The deployment tests have gone so well, that I just ordered
some 3/16" high-strength aluminum (alloy 7075) rod, aluminum
nuts, and other light-weight hardware from McMaster-Carr. I
also ordered an adjustable 10-24 tap. I would appreciate any
advice on threading this new aluminum rod.
So, do you think this will hold? Are there problems with
using aluminum hardware I need to consider.
If you're interested in using light-weight materials and
techniques for your project, I would like to discuss at the
September launch. I'm planning on launching a two-stage
rocket with the 'new' hardware.
Richard
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