what is lighter aluminum or fiberglass?

chipotle mg

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aluminum or fiberglass....for the purposes of performance and strength. what would be lighter yet still strong enough for an avionics coupler bulkhead? either end would be attached to kevlar and hold the rocket together under parachute.

thanks in advance
 

rocket_troy

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aluminum or fiberglass....for the purposes of performance and strength. what would be lighter yet still strong enough for an avionics coupler bulkhead? either end would be attached to kevlar and hold the rocket together under parachute.

thanks in advance
Difficult question to answer precisely because there are different grades and tempers of Aluminum alloy resulting in vastly different strengths. Then there are fibreglass composites consisting of vastly different glass:resin ratios with different types of fibreglass and different filament/fibre geometries resulting in completely different structural properties depending on the geometric support.
If we start with some assumptions: Al Alloy is comparable to grade 6061 or 6082 with a T6 temper and we assume the fibreglass composite is a bog standard 90 deg woven cloth layup (say 200g weave) with a 75:25 glass/resin ratio that has been sufficiently cured and post cured. Then you can roughly say the UTS (ultimate tensile strength) of the 2 materials will probably be comparable. The glass composite will be lighter (less dense), but the alloy will offer superior physical attributes in other areas such as shear, compression, temperature etc.

TP
 

Kelly

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A pound of aluminum weighs about the same as a pound of fiberglass.
Which is a smartass way of saying, you need to precisely define what it is you are trying to optimize. If it is simply tensile strength for a sample of fixed weight, fiberglass will be stronger. But there may be other considerations, some other complex function, which you are trying to optimize, and that will determine the best choice of materials.
 

cerving

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One of the big advantages of fiberglass (or composites in general) is that they are easy to mold into shapes that would be difficult to produce with the same weight of sheet metal. That's why the original Corvette had a fiberglass body... they couldn't stamp it out of steel. You can control the properties of the material so that you can concentrate the strength (and weight) where it's most needed, and in the direction of the stress, you can't do that with sheet metal unless you add reinforcements (and weight). Once you start laminating composites over cores like Nomex honeycomb it's game over for sheet metal... they don't come close in strength-to-weight.
 

Steve Shannon

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aluminum or fiberglass....for the purposes of performance and strength. what would be lighter yet still strong enough for an avionics coupler bulkhead? either end would be attached to kevlar and hold the rocket together under parachute.

thanks in advance
If you truly want a strong material and light weight you use neither aluminum nor fiberglass, but carbon fiber composite. However, in the text of your post you indicated that your application is for an avionics coupler bulkhead. Many times the bulkhead isn’t truly a strength member; it’s just a cap to seal the avionics bay. The load bearing strength member is frequently a threaded rod or two, usually steel, that run from end to end in the avionics bay. If this is the case, and you connect an eye or loop at each end, you can even use plywood for the bulkplate, which might be lighter yet.
 

jqavins

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Fiberglass has typically around 60% of the mass of the same amount of aluminium.
And by "amount" let's assume you mean volume.

A pound of aluminum weighs about the same as a pound of fiberglass.
Which is a smartass way of saying, you need to precisely define what it is you are trying to optimize. If it is simply tensile strength for a sample of fixed weight, fiberglass will be stronger. But there may be other considerations, some other complex function, which you are trying to optimize, and that will determine the best choice of materials.
One of those oft' overlooked considerations is stiffness. In the extreme, Kevlar cloth without any resin is very strong for its weight but would make a lousy bulkhead. For any given material, the stiffness of an object goes up with the cube of its thickness, and is proportional to a material property called Young's Modulus. If one material's strength to weight ratio is a little worse than another's but it's modulus is substantially better, the added thickness that the other needs to meet the stiffness requirement may make the lower strength material the lighter choice in the end. That's one reason that sometimes balsa is a better choice than harder and stronger woods; to get some required strength, the balsa is so thick that it winds up being stiffer.

Also, how much difference is this going to make in the end? How much will the bulkhead weigh, whichever material you use, and how different will they really be? What fraction of the complete rocket's weight does that difference constitute? And how critical is saving every gram for your design goals? I bet you'll find it really doesn't matter.
 
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