Use of Al for recent rocket structures vs use of carbon for older ones?

[RPstress]

On TV I've recently seen aluminum isogrid for the SLS fuel tank (Horizon 2014). In the mid-eighties I worked on the Ariane V H10 interstage (the H10 interstage was canceled after the Challenger disaster in 1986 caused cancelation of the Hermes spaceplane (this is according to my memory and may be wrong)). This interstage was carbon-epoxy/Al honeycomb. Does anyone know the reasoning behind the selection of these materials? (The interstage held no fuel but had significant thermal loading, mainly radiation from the tops of the solids, as well as the main design driver of varying compression, also peaking at the position of the solids.) Why are these recent structures ally?

 

[SWComposites]

Maybe the cost of high speed machined Al has come way down.

Maybe the the efficiency of the rocket has gone up enough to make the interstage weight less critical.

Maybe the material was picked by some MBA Supply Chain weenie ......

 

[RPstress]

I don't have hard numbers on costs. While Al machining has dropped a lot (computerised automated 5-axis with minimal supervision), I'd be surprised if the cost of circularly curved aluminium plate has plummeted. Both carbon material and manufacture have reduced a lot, particularly for a stiffened shell. I'd also be surprised if Al structure cost has reduced more than for carbon structure. As for rocket engine efficiency, of course it's improved a bit (no numbers), but every ounce of structure saved is still almost an ounce of payload increased no matter how efficient the engine. Rockets sren't trucks (yet).

The implication is that these modern aluminum structures must somehow be weight competitive with carbon. Did the NASP structural fuel tank test failure require a change to much more conservatism in newer carbon designs?

Modern commercial aircraft (and NASP) are examples of how carbon is a preferred material for weight-critical structures. How is this different for modern rockets?

 

[Compositepro]

There is, and has been, a lot of research done into developing composite fuel tanks. Permeability to gas and micro-cracking due to thermo-cycling are two issues with composites. The weight added to address these issues probably makes aluminum competitive, at least at the current state of technology.

 

[RPstress]

Thanks Compositepro. I think NASP and some experimental composite fuel tank structures I've seen have all been monolithic carbon/epoxy. Fuel (or oxidiser) and sandwich seems like asking for trouble (single-use space structures are sometimes kept hanging around a long time and have weird rules for longevity - or they used to; I last worked on a space sttucture (a satellite) in the early nineties, which was carbon sandwich). Something like the interstage was a pretty simple structure made of three 120 deg cylindrical shells that spaced out the engine with room for its nozzle inside the length of the interstage, which was left sticking out after separation. Robust Al was used for the bolted rings top and bottom and also the separation ring about a fifth of the way down that had explosive bolts. The engine was attached to a carbon cone bolted to the upper ring. Annoyingly I've forgotten how the longitudinal joints between panels were made (they weren't very highly loaded).

It's quite possible that the modern Al fuel tank has issues to do with hydrogen storage that would make carbon/polymer heavier. I've never worked on a fuel tank. We did work on some pressurized spherical helium storage bottles that were Al. At the time I just thought that composite spheres were pretty hard to make (all our carbon then was woven).

 

[Compositepro]

Spherical carbon tanks would be filament wound. There was a large development program to make a tank for the Delta rocket, which ended badly, for predictable reasons. It was carbon fiber with non-perforated aluminum honeycomb. Structurally, this is the ideal way to make a large, light-weight tank. However, gas permeated into the core and blew the skins off of the core. That ended that program. There should be lots of information to be found about that.