Sent: Aug 18, 2012 10:20 AM
To: 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
