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Comment by nether | Hacker News Reader

Not to be pedantic, but it's not possible to inspect an entire rocket for all defects because so many of them are microscopic and internal (not on the surface of a solid part). These can be analyzed and predicted with fatigue models in aggregate with varying levels of certainty. This comprises the fascinating field of fatigue analysis.

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While you're correct, you're not pedantic enough. Using X-rays to validate the most critical engine parts has been done for literally years and is a relatively boring and old part of "rocket science". Don't expect SpaceX to spend some serious cash on this and techniques such as this (MRI and sonic / ultrasound tech comes to mind).

Source: a good friend of mine is a structural engineer for Aerospace Corporation (a rocket scientist if you will) and I'm an avid r/SpaceX reader.

X-ray takes care of macro internal defects. It doesn't reveal defects at the grain (nanometer) scale, outside of research labs. I think it'd be time prohibitive to scan at that resolution for every component of large systems.

Uh, you meant "Don't expect SpaceX not to ...", or "Expect SpaceX to ...", right? I had a negation parse error.

Well played sir, well played, and yes! :)

There are pretty sophisticated machines out there for non destructive machine inspections. Many based on X-ray or MRI technology depending on the materials.

Similar to the machines used to inspect shipping containers. I would not be surprised if SpaceX had at least considered if not obtained equipment for the ability to X-ray the entire assembled rocket for any internal defects.

Anyone curious can take a look at the equipment from Yxlon (http://www.yxlon.com) for examples of COTS X-ray based inspection that's available. And Yxlon and most others in this industry do custom work, making a robotic full sized rocket inspection setup, quite a realistic concept when the costs of failure are considered.

Yeah, we used X-ray inspections to get away with smaller safety margins at my last employer. Plastic deformations start at the grain level (nanometers) though, and I don't think there's any X-ray microscopy to do this en masse.

On the other hand, the same is true about inspecting an entire rocket for manufacturing defects.

You're right, of course. This is what I meant by "unanticipated source of entropy," though -- some form of breakage which is neither visible nor predictable with the available information.

Semantics - it is anticipated and quantified, but with error bounds ;)

It's true that there is fatigue but shouldn't most of the fatigue be mitigated by the fact that they slow the rocket down significantly upon reentry? I don't see why a falcon 9 should suffer much more fatigue than any jet that goes faster than 2x the speed of sound

Yes, mitigated compared to a full-speed descent. It's not really possible to compare it to an entirely different system though like a fighter jet. Fatigue is analyzed at a part level, like a single strut or a patch of aluminum reinforcing a small area, and is dependent on the type of loading, materials, magnitude/dir of stress, temps in some cases, frequency of cycles, and so on. Fatigue life in aerospace is a tradeoff between designing heavier, beefier, longer lasting parts and skimpier, lighter parts that lead to a lighter vehicle overall.

Any part can undergo fatigue damage for a given loading if it is designed to be weak enough.

It might not matter after you do the math, but on landing this rocket is plunging back into the atmosphere at supersonic velocity moving through its own rocket exhaust. This is a pretty visceral image that does sound like something causing fatigue.

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