Aluminum alloy airframes

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muddymooose

Hoopy Frood
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Since I got back into rocketry as an adult the most appealing aspect to me has been continuously "pushing the envelope." My L1 and prospective L2 builds have been comprised of polymers, fiberglass, and plywood and I've enjoyed learning about their physical and chemical properties and how to best bond them for maximum strength.

While researching L3 and beyond, it would appear that complete fiberglass or carbon fiber airframes are the way to go. I'm talking high-acceleration, high-velocity, high-altitude stuff. At the same time, the cost and complexity of working with these materials seems to increase exponentially with performance goals. As a career machinist and metal-worker, this has given me pause. Personally it seems to me like doing L3+ work with aluminum alloys would be far less expensive and easier.

Yes I understand that the model rocket safety code says "no metal" and the high power code says "minimal ductile metal only when required." I would never build an aluminum rocket and recklessly launch it for ***** and giggles at a casual local site. However I would like to note that 20 pounds of carbon fiber will do as much damage at 600 mph as 20 pounds of metal at 600 mph.

That said, I'm intrigued with building a high-performance alloy rocket. Is BALLS the only launch where this sort of thing can fly? I'm not out to driving from Michigan to Nevada for something this cool, just curious if there's anything else around. Anyone have experience building alloy rockets?
 
https://www.kloudbusters.org/rules.html - Argonia has metal restrictions. Personally I think some of this is silly/nuts but it isn't my range and I don't make the call. I think it safer to attach metal fins to a motor case than some of what is done, if one knows good ways to do it. YMMV. You can even buy commercial metal fincans if you want. Metal tips on nosecones is quite standard too.

600mph is still big dumb rocket speed range. High performace rockets boost much faster than that. Big dumb rockets can break mach on boost (some of them at least; a 6" with an O for instance). High performance rockets will generally be over M2 and potentially quite a ways over. The challange can be keeping them from going too fast for the materials, and having structural or thermal issues.

Somewhat Assembled resized.png
Just some encouragement...

Gerald
 
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It is proven that carbon is twice as strong as aluminum. Therefore I do not see why metal should be discouraged.


Composite Addict
 
It is proven that carbon is twice as strong as aluminum. Therefore I do not see why metal should be discouraged.


Composite Addict

im all for metal. However the differences in density and frangabikity are likely why CF and Alare not judged the same
 
And Siri just murdered the word "frangibility". Ifnits even a word.
 
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Maybe carbon should be banned too.

M

One could argue that the strength of carbon fiber makes it a safer material. Our upscale Dragonfly is 12.75" in diameter by ten feet tall. He carbon fiber airframe is less than a sixteenth of an inch thick and weighs about six pounds. The entire rocket weighs less than fifty pounds loaded. If we had built it from cardboard sonotube and plywood, it would have easily weighed over a hundred pounds.


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Well the casings are usually 6061T6 aluminum alloy for a reason. Metal retains the advantage of having a known ultimate tensile strength on a graph for engineers to plunk math at in a "KNOWN" elastic region versus strain. It predictably deforms and will not suddenly shatter as more brittle materials do. You can calculate the life cycles easily with integrals for pressure vessels before rupture. It's a lot easier to do mechanics materials problems with. When you want to optimize structure thickness. Per pound its relatively cheap, easy to weld, excellent thermal ratings, known expansion on thermal loadings, and even easier to machine. I don't see you guys threading CF or making CF casings, lol. You can math structures with composites, but you need to break everything likely in lab to get your own data points of when it fails, and it will not be consistent failures same as polymers.

Composites may be stronger, but humans, especially engineers trust metals for more applications, because we simply KNOW more about predicting behaviors. Especially when you need to start throwing impact or fluctuating loads into the problem at hand. Take a course on advanced mechanics materials, there's a bunch of crap in composite structure design and there's matrices of stuff splitting nine different directions, I was all F--- this at that point, but hey maybe you're into that kind stuff...

I respect composites, the danger is there seems a lot of unknowns on actual engineering design. Entire new fields... A carbon fiber infused plastic, still behaves as a polymer does in failure. Now if they allowed steel alloy casings/nozzles you could really ramp up chamber pressures and use way hotter fuels.
 
Composite engineering is pretty well understood at this point. You'll find it in everything from race cars to military aircraft to civilian aircraft. Composites require proper processing and good practices and knowledge but you can do things with them you cannot do with any metals. I've made a carbon tube that tapers from about 3/4" to about 5/16" about a foot and a half long, that weighed 12g. It would support 7# cantilevered at the end without breaking. That was through use of a number of composite materials in defined layers. It was not a simple structure though it appears so from the outside. People throw composites at problems (non engineers) and think they know what they are doing, but mostly don't.

Could I make a composite motor case? Sure. Will I? Not likely. There are multiple measures of performance and some can be considered related to cost. Aluminum is cheap, anyone with a little machining background can work with it, and it is easier to design with than composites. It does have lousy thermal issues though.

Steel would help with the thermal issues, but other issues come into play. The walls of the case could be much thinner and still take the pressure, and they would have to be to keep the weight down. Steel is much denser than aluminum. So what happens when you take a tube of the same diameter and substantially thin the walls? You lose stability to ovalization. Young's modulus for steel is about three times that of aluminum, and the density ratio is comparable. So perhaps a steel casing could be done, but then at a third the thickness, bulkhead and nozzle get more interesting. I think steel makes sense for larger motors where the thermal soak is a greater issue but not for smaller motors. Besides, steel isn't allowed by the rules we play by.

What is safer between aluminum, steel, composites? Aluminum makes peeled low density fragments of considerable size so they slow down rather quickly. The fragments for steel of a properly designed motor (IMHO) are going to have about the same total mass and I don't think they will go much farther. Composites will shred and the fragments should not go very far as they will have less total mass.

Ballistic return? It isn't going to matter what the construction method. It will go through vehicles, houses, whatever, just fine (strictly speaking about high performance rockets of some size).

Now if you were inside a composite structure of carbon fiber, a safety rule of thumb is the layer closest to the fragile people should be composed of something that resists being ripped to shreds. The reason is when a carbon structure fails, the carbon shards are still possibly very strong and very sharp. The structure will literally shred the occupant. BTW, you HAVE to remove carbon splinters. The body will not encapsulate carbon fiber so instead the fibers can migrate inside the body. It is easy for them to do this as they are sharp and strong. This is NOT good!

High performance composites are pricy. 6061 T-6 (not the best choice but the common one) is cheap. Steel varies all over the place in price.

Steel nozzles are not going to cut it. Steel does not like 5000 degrees.

You can already ramp up chamber pressures. Use a thicker case. But honestly, why would you need to? I routinely run (or did until this year when I've had no time) 900psi in standard cases up to 114mm, and have run a few thousand psi in 38mm. Yes, that was single use but it held. Disassembly was a challange. All parts were damaged. But the propellant wasn't a standard propellant so you're not likely to hit that sort of thing other than by being foolish. The only time it might make sense to run higher pressures (for our uses, and IMHO) is with a plateau propellant. Our issue is not usually how to make a motor burn faster, if one wants max performance (measured as altitude vs diameter) but how to make it burn slower. Slower will generally lose a little ISP but will also usually gain altitude. That's due to the reduced base drag while the motor is burning. In extreme cases it is deciding between hoping the outside doesn't degrade too fast from the high external heating (punch a hole in the atmosphere method) vs reduced external thermal soak but increased internal heat soak. Aluminum starts fading fast at only a few hundred degrees. Other than cheap, low density, readily available, easy to work, it doesn't have much going for it for a rocket motor casing, or body going multiple mach numbers.

Oh, once you get off low solids effects propellant the temperature is up there anyway. The high aluminum content propellant options are not appropriate for our size motors as there isn't time for that much aluminum to burn, while avoiding nano metals. And I highly recommend avoiding nano aluminum. You won't survive trying to make the propellant.

Sorry for rambling.

Gerald
 
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Composite engineering is pretty well understood at this point. You'll find it in everything from race cars to military aircraft to civilian aircraft. Composites require proper processing and good practices and knowledge but you can do things with them you cannot do with any metals. I've made a carbon tube that tapers from about 3/4" to about 5/16" about a foot and a half long, that weighed 12g. It would support 7# cantilevered at the end without breaking. That was through use of a number of composite materials in defined layers. It was not a simple structure though it appears so from the outside. People throw composites at problems (non engineers) and think they know what they are doing, but mostly don't.

Could I make a composite motor case? Sure. Will I? Not likely. There are multiple measures of performance and some can be considered related to cost. Aluminum is cheap, anyone with a little machining background can work with it, and it is easier to design with than composites. It does have lousy thermal issues though.

Steel would help with the thermal issues, but other issues come into play. The walls of the case could be much thinner and still take the pressure, and they would have to be to keep the weight down. Steel is much denser than aluminum. So what happens when you take a tube of the same diameter and substantially thin the walls? You lose stability to ovalization. Young's modulus for steel is about three times that of aluminum, and the density ratio is comparable. So perhaps a steel casing could be done, but then at a third the thickness, bulkhead and nozzle get more interesting. I think steel makes sense for larger motors where the thermal soak is a greater issue but not for smaller motors. Besides, steel isn't allowed by the rules we play by.

What is safer between aluminum, steel, composites? Aluminum makes peeled low density fragments of considerable size so they slow down rather quickly. The fragments for steel of a properly designed motor (IMHO) are going to have about the same total mass and I don't think they will go much farther. Composites will shread and the fragments should not go very far as they will have less total mass.

Ballistic return? It isn't going to matter what the construction method. It will go through vehicles, houses, whatever, just fine (strictly speaking about high performance rockets of some size).

Now if you were inside a composite structure of carbon fiber, a safety rule of thumb is the layer closest to the fragile people should be composed of something that resists being ripped to shreds. The reason is when a carbon structure fails, the carbon shards are still possibly very strong and very sharp. The structure will literally shred the occupant. BTW, you HAVE to remove carbon splinters. The body will not encapsulate carbon fiber so instead the fibers can migrate inside the body. It is easy for them to do this as they are sharp and strong. This is NOT good!

High performance composites are pricy. 6061 T-6 (not the best choice but the common one) is cheap. Steel varies all over the place in price.

Steel nozzles are not going to cut it. Steel does not like 5000 degrees.

You can already ramp up chamber pressures. Use a thicker case. But honestly, why would you need to? I routinely run (or did until this year when I've had no time) 900psi in standard cases up to 114mm, and have run a few thousand psi in 38mm. Yes, that was single use but it held. Disassembly was a challange. All parts were damaged. But the propellant wasn't a standard propellant so you're not likely to hit that sort of thing other than by being foolish. The only time it might make sense to run higher pressures (for our uses, and IMHO) is with a plateau propellant. Our issue is not usually how to make a motor burn faster, if one wants max performance (measured as altitude vs diameter) but how to make it burn slower. Slower will generally lose a little ISP but will also usually gain altitude. That's due to the reduced base drag while the motor is burning. In extreme cases it is deciding between hoping the outside doesn't degrade too fast from the high external heating (punch a hole in the atmosphere method) vs reduced external thermal soak but increased internal heat soak. Aluminum starts fading fast at only a few hundred degrees. Other than cheap, low density, readily available, easy to work, it doesn't have much going for it for a rocket motor casing, or body going multiple mach numbers.

Oh, once you get off low solids effects propellant the temperature is up there anyway. The high aluminum content propellant options are not appropriate for our size motors as there isn't time for that much aluminum to burn, while avoiding nano metals. And I highly recommend avoiding nano aluminum. You won't survive trying to make the propellant.

Sorry for rambling.

Gerald

Gerald, not rambling at all. A high quality post covering several issues.


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Aside from Gerald's very informative quote unquote rambling, it appears as though you have successfully done a level I flight to 1500'. This is not meant to discourage you, but I would suggest that you take a little time and go through the cert process gradually and gain experience in the level 2 arena and then push the envelope a little further in the level 3 arena before trying an all aluminum Mach 3 high altitude screamer. Dreaming and planning is always a great part of the rocketry hobby, but there is long road of knowledge to be acquired to achieve a successful flight profile of this magnitude.
 
Dreaming and planning is always a great part of the rocketry hobby, but there is long road of knowledge to be acquired to achieve a successful flight profile of this magnitude.

There's hardcore engineering knowledge in calculating bolt spacing and shear stresses on each bolt. Let's assume you want to bolt a 7075T6 fin aluminum alloy fin to a L-bracket fastened to the airframe on each side of fin plate with cross bolts. Each bolt experiences double shear. Each bolt has a spacing value and known diameter to take the loading with a safety factor involved incase the material doesn't match its assumed "textbook' allowable stress value or forces exceed what was planned. This isn't a game anymore. It's just not a dream anymore. Math could kill someone if its wrong. It's not a whoopee do proposition here bud, we're talking about a fin ripping off a rocket when the math calculations are off. That's the kind of engineering problem the OP wants to mess with. I won't discourage him, but he might want either study up with a mechanics material textbook at-least or find a licensed engineer to check his work for his own personal safety. I'm just a mechanical engineering student. At some point you realize you don't know enough and you aren't dumb you just need an experienced person's help.

There's more to it than just rockets. 7075T6 doesn't take to arc welding. A machinist had told me it micro-cracks. Micro cracks invalidate an engineer's math, because the materials are assumed flawless. Normally you can calculate a propagation crack from a machine design book, but it doesn't work for microcracks which might rank in thousands to billons on the surface when you can't see the direction or length of crack or depth. The material properties changes states in metallurgy books and the temper changes weakening the material at welded joint below the published ultimate tensile strength. How much? I don't know...Basically most don't even try to open that can of worms.

/RANT By Student...
 
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And to not sound so negative, Bare Necessities was a successful L-3 HPR using 7075T6 aluminum plate fins, since the material had a higher tensile strength than 6061T6, and there are AeroFinSim files out there as reference along with technical reports. They used a thermal interference friction fit which was difficult to machine, by accounting for the thermal expansion of the materials themselves, and in addition to friction fit someone calculated the stresses, diameter, and spacing of cross pins not a let's cobble something together with a beer or three type deal.
 
Andrew_ASC: Relax. There are enough TAPs who are either P.E.s or have other relevant experience that if and when MuddyMoose builds a metal finned rocket it will have adequate oversight. Our safe distances and TAP reviews of such projects reduce the need to over-engineer our construction techniques and prevent math induced deaths.
MuddyMoose: Yes, there are other venues where you can launch a rocket that has a reasonable amount of metal construction. Ours is in Montana, but I doubt you have to go that far. If you attend a few large regional launches you’ll gain some idea of what’s allowed. If you haven’t been to Midwest Power, I recommend you do so. It’s this coming weekend.
There has been a lot of interesting work done by various people in mixing composites and aluminum materials. I had the pleasure of being one of the TAPs for one young man’s L3 flight at BALLS last month. He had machined alloy fins and researched how to bond them to a composite body. His build thread is on TRF and it’s very interesting. The work he did was beautiful.
https://www.rocketryforum.com/showthread.php?141232-Another-Level-3-Cert-Rocket
A few years ago Mike Fisher did some work bonding aluminum alloy fins directly to motor cases using a special solder (maybe alumaweld was the name) that alloys with the surface of the aluminum metal components to produce a very strong bond. There have been many others throughout the years who have built aluminum rockets for extreme flights. Most have flown either at BALLS or some other western launch site where high waivers and large spaces can take advantage of such construction. Look for project threads by Jim Jarvis, Gene Novaczyk, Mike Fisher. Their work is exciting and they have done a great job of sharing what they’ve learned. You may also find that you are more comfortable with all composite construction.


Steve Shannon
 
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Well they probably drank a bunch of beer after the cross pins didn't shear. Don't drink and derive math. Those dorks could have bought bolts from a hardware store rather than going through all the pain of thermal interference fit and custom machined cross pins. See? Engineers don't know any better anyhow. They can just size a hole, but they make everything harder than needed. Takes a machinist to explain to an engineer on how to build something faster or easier. Always.
 
Andrew, I’ve been a P.E. for 20 years. The practice of engineering requires that we be open to learn from people who actually know how to weld, machine, and build. Arrogance doesn’t get things done.


Steve Shannon
 
My experience with aluminum airframes....Works fine for smaller diameters but aluminum coupled to aluminum has a bad tendency to stick in the larger diameters. Counterintuitively, polished surfaces are the worst. A fairly rough crosshatch on both mating parts, coated with teflon spray and a dusting of graphite gives the best results. But if coupling to a motor casing, all bets are off once the casing heats up. The best plan may be to sleeve mating parts with a phenolic composite if your wall thickness allows for it.
 
There was a mention of bolt shear for bolt circles... Wrong approach, which achieves the wrong answer. One side is thin walled aluminum, and the bolt doesn't go very far in the other side typically. The failure mode won't be bolt shear. The aluminum will ovalize the bolt will tilt and then tear out a chunk of aluminum (how far it goes depends on how far down the failure road your case has progressed). Solve for the aluminum, then verify the bolt can take it. If the bolt is a decent grade of steel (as in certified to some standard and not just pot metal crap found at most hardware stores) you'll likely find the bolt is the least of your concerns. Just my suggestion.

I see this over and over, so I had to comment.

Gerald
 
Hey MuddyMoose and others, I apologize for what I typed that came out as arrogant and condescending. We all have a bunch of different talents and are trying to learn things well outside our comfort zone sometimes. And its not fair to disrespect others whom know more about other topics. One of the most amazing things a machinist explained was the EDM wire cutter process to cut tomahawk airfoil fins, the months machining of jigs to support the part, and the joking of it costing millions to make missiles because the scrap buckets were always full along with the stupidity of using metric fasteners that no one had gages for when a contract bounced back from Turkey with 200 out 28,000 parts complete. That same machinist did all the FEA for the Baja students so they never had a student injured in the program.
 
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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There's hardcore engineering knowledge in calculating bolt spacing and shear stresses on each bolt. Let's assume you want to bolt a 7075T6 fin aluminum alloy fin to a L-bracket fastened to the airframe on each side of fin plate with cross bolts. Each bolt experiences double shear. Each bolt has a spacing value and known diameter to take the loading with a safety factor involved incase the material doesn't match its assumed "textbook' allowable stress value or forces exceed what was planned. This isn't a game anymore. It's just not a dream anymore. Math could kill someone if its wrong. It's not a whoopee do proposition here bud, we're talking about a fin ripping off a rocket when the math calculations are off. That's the kind of engineering problem the OP wants to mess with. I won't discourage him, but he might want either study up with a mechanics material textbook at-least or find a licensed engineer to check his work for his own personal safety. I'm just a mechanical engineering student. At some point you realize you don't know enough and you aren't dumb you just need an experienced person's help.


I'm not sure how any of that is significantly worse than a build with composites which very likely involves a lot less math beforehand and the use of materials with a much higher variation in strength due to variations in the manufacturing process? Most of what we build is over-engineered, we aren't trimming grams to save tens of thousands of dollars on most flights. Even many light weight carbon fibre mach-busting rockets are built many times stronger than required to survive the flight profile.

If someone has vastly more experience working with metal than composites there really is no reason a rocket built by them from composites would be up being a safer or stronger solution. Either material formed into a rocket shaped object at L3 sizes has the capacity to do serious damage in a 'less than optimal' flight.
 
Sticking of the surfaces - yep, that's why machine tools such as the Bridgeport vertical mill have flaked ways and then use a special lube (way oil) that fills in the flaked out areas. I've wondered about doing the same thing with a rocket.

[video=youtube;D1eOQa1gYiU]https://www.youtube.com/watch?v=D1eOQa1gYiU[/video]

You know, once one is beyond the tube size that fits on one's lathe, the tubes aren't even going to be perfectly round. If it is the typical cheaply cut commercial tubing, a hydraulic clamp is used to immobilize the tube for cutting (assuming you didn't buy a 20' piece(s) straight from a mill run). It comes out slightly oval just from that clamping if the wall thickness isn't up to the stress. Sleeving such as Binder suggested is one approach that can solve this issue at the same time.

Working around these sorts of issues is where the fun is. High performance rockets have lots of these sorts of issues, and a bad choice in any one of them results in a failure or an increased likelyhood of failure.

Gerald
 
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