[roc-chat] Re: Using Aluminum Hardware: Lighter but strong enough?
- From: Richard Dierking <richard.dierking@xxxxxxxxx>
- To: roc-chat@xxxxxxxxxxxxx
- Date: Sat, 18 Aug 2012 16:40:28 -0700
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
> **
>
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