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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