[roc-chat] Re: Using Aluminum Hardware: Lighter but strong enough?
- From: Richard Dierking <richard.dierking@xxxxxxxxx>
- To: roc-chat@xxxxxxxxxxxxx
- Date: Mon, 20 Aug 2012 10:49:10 -0700
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
>>
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>
>
>
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