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In June 2015, SpaceX's mission CRS-7 on Falcon 9 was lost when the rocket exploded on takeoff. SpaceX investigation concluded that the problem was a failure of a single defective strut. The strut's design load was 2,000 lb and it was rated for 10,000 lb, 5 times higher.

It sounds like a good news for SpaceX — just improve quality control to catch bad parts and you're done. But is it so easy? I wonder if there is a fundamental design flaw there. Why did a failure of a single strut bring down the whole system? Is every strut in Falcon a Single Point of Failure (SPOF) then? Or even just some of them? That would still be a dangerous design.

For space missions, redundancy is expected and engineers take precautions against failing parts, elements and subsystems. Every SPOF is an invitation of trouble. So assuming what SpaceX says is true, is it a sign of bad high-level structural design or what?

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    $\begingroup$ Typical spacecraft designs do not have redundant structure; this is weight prohibitive. Instead, the structures are designed to a factor of safety > 1 as explained in @TidalWave's answer. Imagine putting an extra vertical fin on the STS orbiter, or double walling the external tank. It would never fly. $\endgroup$ – Organic Marble Oct 25 '15 at 2:49
  • $\begingroup$ An example of redundant structure would be using 2 struts with design load of 5,000 lb instead of 1 10,000 lb strut. Latticework is a common feature of bridge and building engineering and it does exactly that. My house will not collapse if a single beam failed. I'd think rockets could be designed in a similar fashion. $\endgroup$ – Lesser Hedgehog Oct 25 '15 at 23:52
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    $\begingroup$ Your house doesn't have the same weight constraints. $\endgroup$ – Organic Marble Oct 25 '15 at 23:55
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No, it's indicative of insufficient Quality Assurance (QA), which is supposed to prevent defects, not of bad design.

I'm not sure what Factor of Safety (FoS) is used for Falcon 9 struts in question, but judging by numbers given and failure at 1/5th the design limit, it seems to be close to 2.5, assuming maximum load factor of 6 g (page 33 of Falcon 9 Launch Vehicle Payload User's Guide (PDF), kudos to Brian Lynch in the comments) and load of 3.2 g at the point of failure.

Even at FoS of 2.0, that's well within the industry norms, which, according to Wikipedia, are 1.2 to 3.0 for aircraft and spacecraft, depending on the application and materials. And Elon Musk has been quoted before that SpaceX works with industry leading safety margins, so it's not bad design.

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  • $\begingroup$ The Falcon 9 acceleration loading profile has a maximum of just over 6 g. So considering the 2000 lb load at 3.2 g, that would put the SF at roughly around 2.5. Things like pressure tanks have much higher design SF's since the failure modes are less robust -- a tiny crack can make a tank explode but wouldn't make a wing fall off. $\endgroup$ – Brian Lynch Oct 24 '15 at 22:16
  • $\begingroup$ @BrianLynch I intentionally didn't give a quote for pressure vessels that is included in the Wikipedia article I link to, not to confuse the matter. FoS of 3.5-4.0 would be used for, say, Composite Overwrap Pressure Vessels (COPV) helium tanks within the Falcon 9 LOX tanks, not for the stage cryogenic tanks or other stage structural elements themselves. That's why there's a separate quote for aircraft and spacecraft. COPV were suspected before as possible culprits for this Falcon 9 overpressure event failure, but it looks like they're now moving on to identify possible other causes. $\endgroup$ – TildalWave Oct 24 '15 at 22:22
  • $\begingroup$ @BrianLynch That point about SoF of ~ 2.5 is good tho. Do you have a source for the 6+ g max load and 3.2 g load at point of failure that I could use? $\endgroup$ – TildalWave Oct 24 '15 at 22:31
  • $\begingroup$ For sure, I was just addressing the 1.2 to 3.0 range. I believe most launch vehicle components will have a much higher SF compared to spacecraft payloads since the launch environment is far worse than space. $\endgroup$ – Brian Lynch Oct 24 '15 at 22:33
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    $\begingroup$ You probably can't afford a Falcon 9 but you can get the user manual here :) Check out figure 5-2 on page 33. $\endgroup$ – Brian Lynch Oct 24 '15 at 22:34
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Two and a half years later, NASA investigation concluded that yes, there was a design error, although of a different nature than what was proposed in the question:

NASA’s independent review team that investigated the destruction of a SpaceX Falcon 9 rocket and Dragon supply ship shortly after liftoff in June 2015 concluded a design error led to the loss of more than two tons of provisions and equipment heading for the International Space Station.

Lastly, the key technical finding by the IRT with regard to this failure was that it was due to a design error: SpaceX chose to use an industrial grade (as opposed to aerospace grade) 17-4 PH SS (precipitation-hardening stainless steel) cast part (the “rod end”) in a critical load path under cryogenic conditions and strenuous flight environments

Link If I unpack these words correctly, the strut may not have had as much safety margin as was designed in.

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  • $\begingroup$ Clearly at least one strut had negative safety margin! The review says that the design was bad because it specified an inadequate material. $\endgroup$ – Organic Marble Mar 15 '18 at 1:53
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    $\begingroup$ The lede is buried in that quote. 17-4 PH becomes martensitic (read: brittle) at cryogenic temperatures. They also made the rod end from a casting rather than rolled, heat-treated, austenitic bar stock (like literally every rod end I've seen used on spacecraft). This material choice means that the part would have voids and an irregular internal grain structure, which provides ample nucleation points for cracks to form under the high vibration flight environment. Add that to the brittleness experienced by that material, and you're just begging for failure. $\endgroup$ – Tristan Mar 15 '18 at 14:47

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