[roc-chat] Using Aluminum Hardware: Lighter but strong enough?
- From: Richard Dierking <richard.dierking@xxxxxxxxx>
- To: roc-chat@xxxxxxxxxxxxx
- Date: Sat, 18 Aug 2012 10:20:06 -0700
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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