I discussed this idea with Mike Kramer this weekend. I'm resistant (and perhaps for not much reason) to passing a shock cord through the bulkhead. It seems to me it would be difficult to seal without gluing it in place. However, I'm going to create a couple different bulkheads that are fiberglassed on both sides and will test this with other methods. Richard On Mon, Aug 20, 2012 at 10:28 AM, Ed Holyoke <bicyclop@xxxxxxxxxxx> wrote: > ** > 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. > > Richard > On Sat, Aug 18, 2012 at 3:07 PM, Kurt Gugisberg <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 >> 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 >> //www.freelists.org/list/roc-chat > > >