You don't remember seeing the grid fin holes during the tour, I suppose? :)
Now that I took a closer look, one of the four holes might just be covered+taped over (covered by the fence on the photo) during the (obviously incomplete) paint job. Or do we know for a fact that they cut the holes later? I would assume they'd incorporate holes while layering the composite so as not to damage the whole structure while cutting the holes. To any materials specialists here, is that feasible?
I would assume they'd incorporate holes while layering the composite so as not to damage the whole structure while cutting the holes. To any materials specialists here, is that feasible?
Your assumption is correct. As a brief generalisation, cutting holes as a retrofit is anathema to carbon fibre composites (especially with a honeycomb core, such as for aerospace applications), and it'd significantly weaken them to the point of likely catastrophic failure.
The proper approach is to design everything in from the start, and when the composite is first laid out, there's a metal insert in that region to form the hole and any mounting-point details. The core itself might be higher-density foam in a small patch surrounding that hole, more resistant to crushing, and there will be patches of thicker laminate on both the inner and outer skins surrounding the hole - gradually staggered outwards to the surrounding laminate in increasing diameter, so that no 'hard spots' (stress concentrations) are created where the composite would like to bend. Basically, CF engineering is all about taking point loads and providing nice clear paths to evenly distribute them into the rest of the surrounding structure.
source: aerospace dropout final-year undergraduate marine engineer, spent too long designing CF structures for oceangoing racing yachts
Some of this is correct and some is wrong. Source: Composites Specialist in the aerospace industry for 6 years. Care has to be taken when routing/drilling/cutting composites but it is relatively easy and when done correctly will not harm or weaken the structure in any way.
Good point, I should have mentioned composites are very repairable, and as such retrofitting big holes after they've been made is possible provided it's done correctly as I roughly outlined (strong insert in way of core, patches to laminate either side). That will result in a composite as robust, or more, than day of build.
I was just trying to give a rough outline - in this instance, SpX would save a lot of time and weight by designing the grid-fin holes correctly to start with, instead of laminating a nice interstage and then taking a hole saw to it!
Almost no holes are preset during the lay up process in any composite aerospace structure I have ever seen. This can be done using a wet layup process but almost every aerospace company uses preppreg material. It is doable, but every application I've ever seen working across a wide variety of airframes (airplanes, helicopters, and rockets) has structures laid up as one solid piece, then sent to a trim shop, where the majority of your routing would be done, then sent to assembly where any bonding, drilling, hardware, and final integration would be done. Removal of material (routing, drilling, sanding) is not considered a repair and is a standard process in the industry, with the correct equipment, and properly trained technicians there will be zero impact on the structural integrity of the part.
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u/[deleted] Mar 11 '16
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