Aluminum alloy airframes

Andrew, I owe you an apology! I mis-read your post and somehow bolt circle what what I remembered later. That's what comes out I guess when I'm posting while home sick. The fin captured between a pair of brackets is a different problem. I think one can safely assume that bolt tilting won't be part of the failure modes.

BTW, if someone is doing that sort of mounting, and actually for anything near the motor or on the externals of a high performance rocket, I'd highly suggest significantly derating the Aluminum. The textbook values for the alloy won't be available for long once it starts to get warm. I wish I had really good data on how fast temper is lost in sustained intermediate temperature heating but I do not. I do have these pictures to share which might give a person pause who is first considering a high performance aluminum rocket:

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I threw in an unrelated pic showing loss of strength contributed by a fiber if the stress is not applied linear to the fiber. If the fibers don't take the load, then the matrix does... or fails trying to. I hope this also makes it clear why non-oriented fibers in an injected plastic part don't really accomplish all that one would want them to, unless the fibers are long enough compared to the features in the mold such that they self align to some extent to the expected stresses. Don't believe me??? Mix up some epoxy and throw in a bunch of milled fiberglass. Paint a thick ribbon on some wax paper. Let it cure. Bend it. Compare to un-reinforced epoxy. Since you poured it as a ribbon there will be at least some orientation from that, but not enough...

Gerald

PS - Notice how 7075 T-6 which looks so good in specs is actually one of the bad choices if some heating is going to be involved? I find it funny when people choose it for certain rocket parts. 2024 T-3 should be far superior as shown in the picture. It's just nowhere near as good at room temperature. But a high performance rocket doesn't live at room temperature. Unfortunately a 6' piece of 3 1/2" tubing (big enough that breaking 100k is possible) would run about 500 smacks.

Gerald

Andrew_ASC said:

So would one want to analyze the cross sectional area of the aluminum plate surrounding the bolt? The plate thickness*width-bolt diameter*plate thickness?? It's been a good while since I've covered the topic mechanics of deformable solids.

(This is for another scenario and possibly checking the outer bracket plate strength so the outer plate doesn't fail when a bolt isn't failing after the hole size is already calculated. I'm guessing side loads and moments on a bolt acting on a thin outer plate from fin aero forces, especially while busting mach, when turbulence is maximized, but I'm just the dumb student here.)

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The first stop is probably bearing strength of the aluminum, basically hole diameter times material thickness. That covers ovalizing the holes somewhat. There's also bolt prying and a raft of other failure mechanisms (fin bending, twist, fatigue for all of the above, etc.). I agree that the point of failure for bolted fins probably isn't the bolt. I don't do much with fasteners since we weld most everything in the marine world, except for bolting equipment to foundations.

If you don't have a repeatable filament winder/layer, composite construction is difficult to make repeatable. As one of my co-workers said, hand laid fiberglass is one of the only places where the final strength and weight depend on the skill of the lowest paid guy in the operation.

Well my only experience with composite construction was an L-1 multistage for SEDS at university level and as engineering students we didn't have all the data for a highly detailed thermal analysis (the casing temperatures during burn). Our fins were attached to outer airframe tubing wall near casing naturally. We dumped aluminum at the phase we decided to try to manufacture airfoils on a small scale, one thousandth tolerance that no mill could do, and that was nothing short of a slight miracle with printing tech out there with new materials. We knew very little of the theory to calculate structural analysis of composite structures as mechanicals, a nuclear, and a environmental engineer. Told one prof we didn't know FEA from a hole in ground on composites. Dude laughed. If I told you we weren't scared that's a lie. We truly didn't know what was going to happen. It wasn't simple to calculate like a metal structure. We went with Cotronics epoxy at a $100 a pint for it's 600*F and 11,100 psi tensile strength, because the Aeropoxy sales specialist and their engineers were telling us over the phone the Tg of a highly recommended epoxy were too low for a Mach 1.7-2.2 application. We were expecting 300F from aero friction and the casing temperatures just horrified us as the CTI claimed proprietary. Didn't have the budget to burn a motor and measure temps.

To simply get the drag coefficient before we even built the rocket, I had to ask a graduate level professor Dr. Sreenvias at UTC SimCenter, because he had 20 years hypersonic flow experience in research, he ironically knew people that had designed some of Iran missiles, his professor friends were also ex-NASA very old smarter than Sheldon type research dudes who heavily knew theoretical assumptions beyond undergrads/professors, and their Linux programs took our CAD models to simulate at environments the university didn't have a wind tunnel for, and it took 3 weeks to get a useful drag coefficient number for one drag force equation of this unique shape. They complained a curved airfoil in a CAD program was rough surface at the quality mesh they needed, and there was hours into fixing the model surface. I had asked how to set up 60 boundaries in ANSYS a CFD program for undergrads that we weren't fully taught on. They said setting up boundaries is hardest part of the simulations when you need answers beyond open rocket accuracy. The ex-NASA dorks were mocking OR was off by .4 mach, they computed with hand calculators and older texts. They wanted fuel specs from Aerotech (which hats off to aerotech for being generous enough to provide some required data), and could model gas particle flow exhaust interference along with the normal and oblique shock waves with the computers and code they had. They gave us a printout of the drag coefficient at expected flight angles too. They offered spin stabilized code and we declined because another university team had fins metal alloy type rip off a liquid rocket one year at 8000ft. Judge had warned to not try spin stabilized. We are students not a sounding rocket company. Lol.

Our first rocket imploded by the way... Half a second after launch with interstage coupler failure after sonic boom. We launched a different design rocket months later and placed top three.
I guess the team is okay with me yapping about that experience. Felt like gambling with $2.5K of R&D... What we learned from failure helped us later.

boatgeek said:

If you don't have a repeatable filament winder/layer, composite construction is difficult to make repeatable.

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To mere students with zero HPR experience, we didn't even try to form our own tubes, because we figured spiral wound tubes were simply better quality with less human errors than trying to build our own from scratch as those guys like Jim Jarvis and other experienced rocketeers do. We didn't even want to form our nosecone out of fiberglass for that reason with such low experience.

There was a lot of time consuming surface prep along with mixing epoxies to different ratios with careful attention to curing temperatures and time. It felt like a step up from just chunk a round bar on a lathe and turn at a certain rpm to desired diameter/etc. You could form any shape you couldn't with metal with right jigs or a vacuum. But it took a lot of focus to obtain the surface finish consistent along with good bonds. A lot of hand labor and sterile environments for mixing. We used a residential house with automotive respirators on to cure aerospace grade epoxy which is a dumb thing, but we did it, because the university ovens weren't large enough.

The only other aluminum alloy I could see that has good thermal values is Ti-6AL-4V. I've seen it used in hip implants at Smith and Nephew in Memphis, TN, and they had ability to 3D print it for bone fractures or matching medical implants to a patient's bone shape for a bunch of money. It's probably cost prohibitive to use in HPR. About $500 for a 1.5" by 12" round bar. Only 6 percent aluminum, but it's still an aluminum alloy barely. Hah.

Normally something is called an alloy based on its highest component fraction... 6Al4V is a Titanium alloy. The other common ones are 3Al/2.5V, and CP (commercially pure).

If you are not already familiar with the methods and risks of working Ti, it is better not to do so. For instance, the chips can self-ignite, sort of like Mg. Water acts as an oxidizer. Even CO2 can act as an oxidizer. The solition is to bury it in sand and wait it out. Ti also work hardens very fast. So you don't take light cuts. You take heavy ones to go below the previously work-hardened surface. That means bigger machines and power, preferrably flood cooling... I've avoided it. But it would be nice for a combination motor and body tube for a large enough rocket to be worth it. Or, perhaps for the tube of a boosted dart (though stainless should also be fine for that).

Gerald

Could the titanium alloy with aluminum work for a nosecone tip possibly in small quantities it shouldn't be outrageously expensive? Can the material practically be annealed then further heat treated to a higher strength later after machining? If it was annealed, it would work harden with each pass of a lathe turn, but it would be softer than if left alone right? It sounds like an absolute bear to work with. Never used it at all. I should take your advice and avoid it. Doesn't sound fun.

Andrew_ASC said:

Could the titanium alloy with aluminum work for a nosecone tip possibly in small quantities it shouldn't be outrageously expensive? Can the material practically be annealed then further heat treated to a higher strength later after machining? If it was annealed, it would work harden with each pass of a lathe turn, but it would be softer than if left alone right? It sounds like an absolute bear to work with. Never used it at all. I should take your advice and avoid it. Doesn't sound fun.

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Bar stock is normally provided in a normalized or annealed state, although mill treatment is generally not as precise as the procedures that would be carried out by heat treaters and may leave areas of slightly varied hardness. Given most material is priced mainly by the weight, and by lesser extent the processing used to get it to the shape as sold, Ti alloys are probably best suited for use in tubes for cases, couplers or bodies.

Normally a nose cone tip is the ideal place to use a heavier material to help get the CG as far forward as possible. Several grades of stainless steel are rated for exposure around 900-1000C, and if you really want to use an exotic material there are Inconel alloys that maintain full strength to over 600C.

Remember........
If you are launching at a a Tripoli event it is aluminum or copper alloys without prior approval of the BoD.

M

Alright where do we get a BoD application form? This sounds so cool.

MClark said:

Remember........
If you are launching at a a Tripoli event it is aluminum or copper alloys without prior approval of the BoD.

M

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I have always wanted to do a hand rolled/hammered copper skin rocket hand riveted to join the plates . Real steam punk / old school look . I was thinking a 5.5 inner fiberglass airframe with a 6 inch od copper skin . It would be heavy , but a big ol' honking skid would do the job .

Eric

Andrew_ASC said:

Alright where do we get a BoD application form? This sounds so cool.

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There’s no form; you just contact us via the contact links on the leadership page of the Tripoli website. You would have to convince us that it’s necessary. Being “so cool” won’t do it.


Steve Shannon

So if someone puts out a really detailed thermal analysis and structural stress report with calculations you guys could approve say a tungsten boosted dart? And if tungsten is approved for its strength or thermal properties by BoD then its okay compared to a long list of other possible materials? Since "cool" isn't scientific enough.

Andrew_ASC said:

So if someone puts out a really detailed thermal analysis and structural stress report with calculations you guys could approve say a tungsten boosted dart? And if tungsten is approved for its strength or thermal properties by BoD then its okay compared to a long list of other possible materials? Since "cool" isn't scientific enough.

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Existing safety codes, including NFPA 1127 and any landowner requirements would have to be considered. First, you would have to prove that it’s truly necessary. Do you have some problem with a planned flight that only this method or material can solve? If standard construction methods can and have been used to solve a similar problem, then you may have a challenge.
Second, you would have to convince us that you have the knowledge, experience, and judgement to construct and fly such a rocket safely. What’s your skill level? From other writings of yours it would appear that you are currently Level 1 and you’ve flown a very limited number of HPR flights, dealing more with group collegiate projects. If you haven’t individually mastered typical composite construction techniques there’s probably no need for you to build a tungsten dart. We don’t approve these types of projects simply as an alternative to learning traditional techniques.
Third, if we did approve a specific project, that approval is limited to that project. Our approval is not a blanket approval for you or anybody else to use a material or construction technique in the future.
Finally, if we sense that you are trying to baffle us with BS, we’ll reject it immediately.



Steve Shannon

Quote Originally Posted by Andrew_ASC View Post
So if someone puts out a really detailed thermal analysis and structural stress report with calculations you guys could approve say a tungsten boosted dart? And if tungsten is approved for its strength or thermal properties by BoD then its okay compared to a long list of other possible materials? Since "cool" isn't scientific enough.

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Steve Shannon said:

If you haven’t individually mastered typical composite construction techniques there’s probably no need for you to build a tungsten dart.
Steve Shannon

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No need to worry about that, see post # 9: https://www.rocketryforum.com/showthread.php?143153-Why-Dual-fins

rcktnut said:

No need to worry about that, see post # 9: https://www.rocketryforum.com/showthread.php?143153-Why-Dual-fins

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I luv the black glue fingerprints all over.....so artsy..eevn the fillets look like welded joints...so craftsy. :wink: