High strength aluminum alloys for motor casings?

It is known that carbon fiber composites can save about half off the weight over that of standard aluminum propellant tanks and casings for both liquid and solid propellants. And carbon fiber has been used for both commercial and amateur rockets to save on vehicle dry mass.

So I always assumed carbon fiber was the best you could do with current materials. But in reading about the Super Loki suborbital rocket, I found it used a specialty high strength aluminum alloy to reduce the booster stage dry mass and this alloy's strength was close to that of carbon fiber.

In looking up other high strength aluminum alloys I found there are even aluminum alloys that even exceed the strength of carbon fiber composites!

See for example this comparison between standard 6061 aluminum alloy and high strength aluminum alloys:


https://precisionarmament.com/tennalum-7068-aluminum-alloy/


The 7075 is close to twice as strong as the 6061 alloy and the 7068 is close to 2.5 times as strong. The 7075 though is 2 to 3 times as expensive as 6061 and the 7068 is 3 to 4 times as expensive. But because of their higher strength, not as much material would be needed for the casing so the price would actually not be much greater than using the standard 6061.

I wanted to get an idea how much is the thickness common for standard aluminum casings of commercial motors. Say for 4" and 6" casings. Anyone know that? Then because of their higher strength the 7075 would be thinner by about 1/2 and the 7068 by a factor of 1/2.5, resulting in lighter casings.

I gather the 7068 is not as common but the 7075 appears in several thicknesses:

https://www.futuremetals.com/products/tubular-products/aluminum-tube/

Bob Clark

The stronger alloys may not be extrudable or even very machine-able, or the may not react well to motor temps. All that can play into design of the motor case and material selection.

I believe several of the alloys you mentioned do not take heat well, which could be a problem.

Some quick and dirty aluminum info. We use O and a T6 alloy on the B-17 we are building. O is easier to work with as it is very malleable. T6 is much harder and brittle. You have to bend aluminum against the grain or it can crack. T6 will definitely crack when bent with the grain. You can form parts in O and then heat treat to make it stronger, but it can warp when heat treated.

There are always trade offs in materials. What might be strong may not be able to be cold formed, machined or cast at all or economically. It may have a much lower fatigue limit, be too brittle or prohibitively expensive or impossible to source at the size you need...

For commercial casings, the raw material is likely a small fraction of the cost to produce it. Tooling, labour, etc have to be accounted for.

For one off projects where cost and time aren’t as important, you have more options.

RGClark said:

It is known that carbon fiber composites can save about half off the weight over that of standard aluminum propellant tanks and casings for both liquid and solid propellants. And carbon fiber has been used for both commercial and amateur rockets to save on vehicle dry mass.

So I always assumed carbon fiber was the best you could do with current materials. But in reading about the Super Loki suborbital rocket, I found it used a specialty high strength aluminum alloy to reduce the booster stage dry mass and this alloy's strength was close to that of carbon fiber.

In looking up other high strength aluminum alloys I found there are even aluminum alloys that even exceed the strength of carbon fiber composites!

See for example this comparison between standard 6061 aluminum alloy and high strength aluminum alloys:

https://precisionarmament.com/tennalum-7068-aluminum-alloy/


The 7075 is close to twice as strong as the 6061 alloy and the 7068 is close to 2.5 times as strong. The 7075 though is 2 to 3 times as expensive as 6061 and the 7068 is 3 to 4 times as expensive. But because of their higher strength, not as much material would be needed for the casing so the price would actually not be much greater than using the standard 6061.

I wanted to get an idea how much is the thickness common for standard aluminum casings of commercial motors. Say for 4" and 6" casings. Anyone know that? Then because of their higher strength the 7075 would be thinner by about 1/2 and the 7068 by a factor of 1/2.5, resulting in lighter casings.

I gather the 7068 is not as common but the 7075 appears in several thicknesses:

https://www.futuremetals.com/products/tubular-products/aluminum-tube/

Click to expand...

dhbarr said:

Tough, strong, dense, stiff. Please don't pretend like those are all the same thing. After you do young's vs. density, add a dimension of toughness vs. cost.

Aaaaand we're back on familiar territory because *shock* these aren't surprises. Al, CF, etc. are used precisely because of where they sit in that space.

Click to expand...

Someday I'm going to really -tag- one of these windmills.

https://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-temp/metals.jpg
https://www.burnsstainless.com/images/technology/YieldStrength-2.gif
https://www.nakka-rocketry.net/pix/ftycht2.gif
https://www.researchgate.net/profil...uminum-alloys-7075-T6-2618-T6-and-2219-T6.png
https://media.springernature.com/fu...07-5/MediaObjects/40038_2015_7_Fig43_HTML.gif

Just a quick not quite random selection of pics scraped from a google search. They might help get some of the concepts across.

Gerald

Stainless or Chrome/Molybdenum steel might also fit into your equation..

At least one of the hi-strength Al alloys has a problem with crack propagation. Don't recall which one.

Best -- Terry

Zeus-cat said:

Some quick and dirty aluminum info. We use O and a T6 alloy on the B-17 we are building. O is easier to work with as it is very malleable. T6 is much harder and brittle. You have to bend aluminum against the grain or it can crack. T6 will definitely crack when bent with the grain. You can form parts in O and then heat treat to make it stronger, but it can warp when heat treated.

Click to expand...

In regards to the 7075 alloy, it already comes in tubes at certain thicknesses:

https://www.futuremetals.com/products/tubular-products/aluminum-tube/

That’s why I wanted to know the thickness for common 4” and 6” motor casings. The 7075 could be about 1/2 the thickness thereby saving casing weight.

Bob Clark

Also high strength aluminum alloys such as the 7075 can now be 3D printed:

Engineers Have Found a Way to 3D Print Super Strong Aluminum.
B. Ferguson/HRL Laboratories
by Dom Galeon September 22, 2017 Hard Science
https://futurism.com/engineers-have-found-a-way-to-3d-print-super-strong-aluminum/

Bob Clark

A solid fuel rocket motor is a straight tube with fairly simple end caps, possibly the least suited object for 3D printing I could think of. Extruded tubes are popular in a vast range of pressure vessel applications for a reason. Hybrid and liquid fuel motors on the other hand can be (and in some cases already have been) greatly improved in performance and cost efficiency with 3D printing and SLS technology.

I'm not sure how you think you can improve on what already exists when you have little idea of both the current state of motor cases and the practical properties of the materials cases could be made from. You are doing the academic equivalent of a triathlon before figuring out walking.

From NFPA 1125, for commercial motor.
7.4.5 Reloadable metal motor casings shall be made of 6061*T6 aluminum alloy or other aluminum alloy with equivalent mechanical properties.
7075 and other materials listed would not be "equivalent".

Mark
Still not related to Bob.

MClark said:

From NFPA 1125, for commercial motor.
7.4.5 Reloadable metal motor casings shall be made of 6061*T6 aluminum alloy or other aluminum alloy with equivalent mechanical properties.
7075 and other materials listed would not be "equivalent".

Mark
Still not related to Bob.

Click to expand...

Is that only for reloadable cases? Also, these only would be for experimental launches or even commercial launches, i.e., for sale. They would not be for amateurs gathering at rocket meets.

Bob Clark

RGClark said:

Is that only for reloadable cases? Also, these only would be for experimental launches or even commercial launches, i.e., for sale. They would not be for amateurs gathering at rocket meets.

Bob Clark

Click to expand...

If you are selling the motor it has to be certified as far as I know. If it is experimental it is an amateur gathering at a rocket meet.

7075 does not handle heat well. It is more prone to brittle failure. It is more expensive. All excellent reasons not to use it.

2024-T3 is a better choice. And you pay for it.

Gerald

RGClark said:

In regards to the 7075 alloy, it already comes in tubes at certain thicknesses:

https://www.futuremetals.com/products/tubular-products/aluminum-tube/

That’s why I wanted to know the thickness for common 4” and 6” motor casings. The 7075 could be about 1/2 the thickness thereby saving casing weight.

Bob Clark

Click to expand...

I wasn't expecting you to roll your own tubes, I was just giving you some general info on the properties of the aluminum that I have used. I have to admit that when I started working on the B-17 I was very surprised to learn aluminum sheets have a grain and that you really need to be aware of that when planning a three dimensional part. For reference; the grain runs the same direction as the lettering printed on the aluminum sheets. You MUST avoid bending along the grain.

LithosphereRocketry said:

If you are selling the motor it has to be certified as far as I know. If it is experimental it is an amateur gathering at a rocket meet.

Click to expand...

What I mean is it would be something like the Princeton teams Spaceshot rocket that had its own reserved time at Spaceport America. Or like USC’s attempt at a suborbital space flight which uses carbon fiber composite casing.
There are some amateurs who on this forum who have made their own composite casings, either fiber glass or carbon fiber, so I imagine they would not be launching those at rocket meets.

Bob Clark

RainierWolfcastle said:

A solid fuel rocket motor is a straight tube with fairly simple end caps, possibly the least suited object for 3D printing I could think of. Extruded tubes are popular in a vast range of pressure vessel applications for a reason. Hybrid and liquid fuel motors on the other hand can be (and in some cases already have been) greatly improved in performance and cost efficiency with 3D printing and SLS technology.

I'm not sure how you think you can improve on what already exists when you have little idea of both the current state of motor cases and the practical properties of the materials cases could be made from. You are doing the academic equivalent of a triathlon before figuring out walking.

Click to expand...

One thing that's potentially interesting would be printing non-mandrel-able cores. For instance, a helical or honeycomb structure to dramatically increase the first 0.1s of surface area on a plateau propellant.

G_T said:

7075 does not handle heat well. It is more prone to brittle failure. It is more expensive. All excellent reasons not to use it.

2024-T3 is a better choice. And you pay for it.

Gerald

Click to expand...

The aluminum alloy used for the Super Loki booster was 2014-T6:

DESIGN, DEVELOPMENT AND FLIGHT TEST OF
THE SUPER LOKI STABLE BOOSTER ROCKET
SYSTEMS.
Bruce Bollermann, et al
Space Data Corporation Phoenix, Arizona
30 June 1973

https://www.dtic.mil/dtic/tr/fulltext/u2/766737.pdf

Here are the tensile strength ratings for this alloy and some others:

Aluminum 2014-T6; 2014-T651
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA2014T6


Aluminum 2024-T6
Tensile Strength, Ultimate Min 427 MPa Min 61900 psi
Tensile Strength, Yield Min 345 MPa Min 50000 psi
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA2024T6


Aluminum 6061-T6; 6061-T651
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061T6


Aluminum 7075-T6; 7075-T651
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6


The 6061 is the standard aluminum used for motor casings and many other common uses. The 2024 has only its minimum values listed but based on that, the 2014 would be preferred. The 2014 is not as high as the 7075 but is proven for the solid motor casing use so may be preferable to the 7075 for that use.

There is also the new 7068. It has been described as being used for "ordnance", which presumably means artillery shells. In that case it might also work for motor casings.

Bob Clark

dr wogz said:

Stainless or Chrome/Molybdenum steel might also fit into your equation..

Click to expand...

Or some titanium alloys. Remember it's strength to weight that's key here. On the graphic above titanium 6AL-4V is similar to 7075 on that measure, and is already used in large, commercial solid motors. It is quite expensive though.

Bob Clark

RainierWolfcastle said:

A solid fuel rocket motor is a straight tube with fairly simple end caps, possibly the least suited object for 3D printing I could think of. Extruded tubes are popular in a vast range of pressure vessel applications for a reason. Hybrid and liquid fuel motors on the other hand can be (and in some cases already have been) greatly improved in performance and cost efficiency with 3D printing and SLS technology.

Click to expand...

There is also the consideration of weldability and availability. According to that 3D printing article, the strongest aluminum alloys have problems with weldability. See the video here:

[video=youtube;8YwlenA4bdg]https://www.youtube.com/watch?v=8YwlenA4bdg[/video]

And because they are not in as common use as 6061 there may also be limitations of the availability of the thicknesses required.

Bob Clark

Bob,

I never care about room temperature numbers for rocket motor material. I'd want to see the numbers for elevated temperature. Assume you are going to have a bit of a thermal issue. What works best then? That is a safer approach.

I'm not saying 2014 is not a good choice. It may be. I don't have its elevated temperature data in front of me. But the Loki Dart is not a good model, It only had to survive a couple seconds as it was a boosted dart and the booster was expendable. Different requirements, different engineering solution.

Also, T6 or T651 is not always the answer.

Gerald

Regarding the triathlon vs. walking comment, I couldn't agree more. While I do know how to walk in this area, and I have a fair to middlin' ability to run, my swimming and biking are weak. So I'm not going to try to give any answers, just a few comments and questions.

First, are we right about your level of knowledge? From the OP, it seems that you've left out a bunch of factors that make it look like you're no expert; are you actually a PhD in materials science in disguise?

Next, remember that, all else being equal and when money is no object, a higher strength to weight ratio is still not always better. That's because if you take advantage of a really, really high ratio to obtain the required strength with super low weight, you sometimes end up with such a thin object that it has grossly inadequate stiffness. Any aluminum alloy has good strength:weight, but I still make my LP fins out of balsa, not aluminum foil.

Super low weight also means low thermal capacity. If the lighter casing takes the same heat load as the heavier one, its temperature will spike higher, so it needs not the same property vs. temperature relationships, but better ones.

All that said, if you want to become an expert runner, swimmer and biker and then look into exotic materials for motor casings, take a look at titanium aluminide (TiAl). I know it has good strength and good high temperature properties. I don't know about any of its other important properties, and it'll cost you a whole lot, but it'd be interesting to run the numbers if you're running such numbers on other alloys.

And finally, always remember, the most important thing to know in engineering (and most other things) is the limit of what you know. "A man's got to know his limitations." Because if he doesn't, then he doesn't know what to improve on. So go out, learn a bunch of important materials concepts, methods of calculations, sources of good data, etc. etc., and then invent a better mouse trap. We would all- OK, many of us would be happy to use it. (Some of us will stick with the old mouse trap as long as it's good enough and cheaper.)

I'm hardly an expert in rocketry or materials science. That's why I'm asking questions. My background is in pure mathematics.

About titanium aluminide, I believe the titanium 6Al-4V alloy that appears in the first graphic is of this type.

Bob Clark

Oh, no indeed. Ti-6Al-4V is, as one can read in the name, 6% aluminum, 4% vanadium, and the balance (90%) titanium. Those are by mass.

TiAl is 50:50 by mole fraction, which makes it 36% aluminum and 64% titanium by mass.

EDIT: Or, it would be 64:36 if my simple calculation were the whole story. But it's not. I just read up a little more, and found here to my surprise that more aluminum than that is used, for reasons unknown to me. And, it's no surprise that small amounts of other elements are typically included in such alloys as well. That article gives me more hope that a casing made from this stuff might be good, but I still don't know either how difficult it would be to make or how expensive (though I can guess that the latter is pretty nasty.)

And while on the subject of expensive, exotic alloys, take a look at this. Tony Stark would be proud. Of course, hardness isn't everything, any more than strength is. (In this application, probably less.)

VW has a laser beam welding process. It handled automotive grade aluminum magnesium alloys just fine. That’s why the roofs were so smooth. There are industrial ways to weld 7075T6 but I am not familiar with those processes due to my limited exposure in industry as a mech engineering student. The welding companies will advertise easy to weld metals for welder instructions on welding tools that are hand held MiG or TiG processes.

A limiting factor on very thin wall aluminum cases is machining tolerances for processes in manufacturing. So if the tube wall is too thin the machinist will complain of using a lathe because converting a solid round bar to a threaded hollow casing may not turn out completely round or uniform when you push the edge on thinness of materials. There will be a practical point where the vibrations of the machinery operations destroy the tube from chatter and harmonics. Some of the exotic aluminum alloys are not sold in tubes common to rocketry casings but as solid round bar. Then you have to bore out the center. I’ve heard a machinist complain the 3D printed structures weren’t as strong as milled or forged parts. The firearms industry for military M4 receivers uses forged 7075T6 for strength and durability. Milled parts are useful but considered rubbish in comparison of thousands of rounds fired. 3D printing allows complex curves or internal features. It is unnecessary to make a tube by 3D print save that for a curved nozzle a machinist won’t do for any price etc.

In theory land if you go about heat treating the metals, cold drawing, or cold hammer forging it you can increase properties further than published but the details of those processes are beyond undergrad level ie go find professionals in industry. Material science will open the doors to diffusion processes to embed a material with not a coating but a chemical layer infused to the metal structure molecular level with a penetration depth for time and temperature of exposure to improve hardness or corrosion resistance. NP3 is a common treatment. The tricky bit is all the details.

Pratt and Whittney used chemical milling on turboshsft components for jet aircraft with tolerances so tight that parts reportedly seized in several NTSB reports by dissimilar metals with different thermal expansion rates when pilots pushed service ceilings and angles of attack which blocked cooling the turbine engine on some rear engine jet airliner designs. They basically beat CNC tolerances. I don’t know much of chemical milling.

Porsche uses superplasticity processes to form fenders along with many other automotive firms of complex alloys. The process is slow and very expensive. There are many ways to form aluminum parts. Tactics will vary from industry to industry based on needs and costs.

G_T said:

7075 does not handle heat well. It is more prone to brittle failure. It is more expensive. All excellent reasons not to use it.

2024-T3 is a better choice. And you pay for it.

Gerald

Click to expand...

I’m looking for some ref’s on wall temperature for solid rocket motors if anyone knows any. Best I could find was a simulated motor test firing by Richard Nakka:


https://www.nakka-rocketry.net/therm.html

It shows with multiple paper layers as insulation, and with burn time of a few seconds, the temperature only goes up to 50C.

Motors for amateurs do commonly burn for only a few seconds. But I don’t know if the wall temperature only reaches 50C as a max with typical insulation used by amateurs.

I did see some more detailed specs on the 7075 alloy that says it weakens worse than the 6061 at a temperature of about 300C, as you and others suggested here. So we need to know what is the actual wall temperature range for small solid motors at only a few second burn.

Bob Clark

If nothing else, heat soak at the nozzle is going to be way over that. The temperature might not get down as low as 50C for 15 minutes after the burn for a 7600 with a full diameter graphite nozzle, and that still isn't a very big motor.

I consider long burn motors to be single-use (at least, with full diameter graphite nozzle, and uninsulated forward bulkhead). Your first burn might be ok, and if the fins are forward of the nozzle the rocket itself might be ok. But going for a second burn is asking for it. T6 may well be T1 or 2 on the second burn attempt (or T0 if you don't wait a week for the aluminum to age before the second burn). The duration of the heating matters. Longer heating improves the annealing of the metal (in a practical sense though over-annealing does exist where crystal grain size increases).

Now if you are running a low pressure motor where the case is way over spec for the job, that might still be fine with some mild overheating and mild loss of temper. My typical target pressure is 900psi. Sometimes, higher. I prefer higher pressure and a greater nozzle expansion ratio, to improve delivered ISP. I don't think it would be fine then. Most commercial motors are lower. 300psi is about the min you ever want to try at sea level BTW (to my understanding), or the flow through the nozzle might not behave correctly.

The combustion temperature of the propellant is going to come into play. With a phenolic liner in a 38mm motor, I can (read as been there, done that) make it single use with just 4 bates grains. Only the forward bulkhead is sort of reusable, though eroded. That's from high pressure high temperature and a more reactive gas mix. Even the LE phenolic liner is nearly entirely consumed, in a small fraction of a second. That's without cato. Barely.

Please note the nakka site data is more in line with sugar motors, than with some HTPB possibilities. The latter can be thousands of degrees higher in temp.

The nakka site does have a graph which shows why I suggested 2024-T3. You can actually get it in tubes, but it is easily several times the cost of 6061-T6 and options are limited. But sometimes you can start a project with acquiring a tube, and designing from there. You might pay about $600 for a 5' piece of 3" tubing 0.120 wall, for instance, delivered. It might make a nice single use motor case for BALLS.

Generally, one can improve performance in small increments by throwing money at a problem. But sometimes the answer is just to go cheap and go slightly larger. The economics are often better. If I recall my numbers correctly, a single use P motor designed for highest performance to dollars ratio and going a little outside conventional propellant choices, could potentially have about a half to a third the materials cost (all materials to get to a ready to burn motor) of a conventional P attempt for the same target payload and altitude.

Gerald

G_T said:

If nothing else, heat soak at the nozzle is going to be way over that. The temperature might not get down as low as 50C for 15 minutes after the burn for a 7600 with a full diameter graphite nozzle, and that still isn't a very big motor.

I consider long burn motors to be single-use (at least, with full diameter graphite nozzle, and uninsulated forward bulkhead). Your first burn might be ok, and if the fins are forward of the nozzle the rocket itself might be ok. But going for a second burn is asking for it. T6 may well be T1 or 2 on the second burn attempt (or T0 if you don't wait a week for the aluminum to age before the second burn). The duration of the heating matters. Longer heating improves the annealing of the metal (in a practical sense though over-annealing does exist where crystal grain size increases).
...
Gerald

Click to expand...

As I suspected, the 7075 tubes available at https://www.futuremetals.com/products/tubular-products/aluminum-tube/ do not have the T6 heat treatment required for high strength.
Do you think it is possible for amateurs to supply the needed heat treatment?

Also, on that Nakka site, he discusses making tubes from flat sheets. Because of their low thickness you can roll it by hand around a solid rod. The 7075 T6 has greater availability as flat sheets. So this may work for it. There is the question of welding. The 7075T6 has poor welding capacity. The method of friction-stir welding may work without degrading too much its strength. Another possibility with rolling a flat sheet is have sufficient overlap so that even welded, most of the strength of the alloy is maintained at the overlap.

I’ll also enquirer of the authors of the 3D-printing method to see if it maintains most of the T6 strength in the small thicknesses we need.

Bob Clark