Howdy Richard, Thought of a couple more ideas.If you're going for lightweight attach points, it's hard to beat glued in sewn shock cord loops. You could even sew a loop (or just glue it down) after passing it through a couple of slots in a bulkhead and glue it to the back side to make sort of a flexible U-bolt.
Another thing you could do is to make a bracket out of aluminum angle and capture it with a nut on one or more of your avbay's through rods.
Bombproof attach points only come into play when deployments are less than nominal, but might just save the goods for another try. I've had lots of failures but never on a glued in shock cord. You might be able to burn one, but they're really hard to tear out.
Pax, Ed Holyoke On 8/18/2012 4:40 PM, Richard Dierking wrote:
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.RichardOn 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 -- ROC-Chat mailing list roc-chat@xxxxxxxxxxxxx <mailto:roc-chat@xxxxxxxxxxxxx>//www.freelists.org/list/roc-chat