My piece on the SpaceX abort is up at Popular Mechanics.

Here’s the bit that got left on the cutting-room floor:

But even as the incident validated the safety of the vehicle, it raises issues about its launch reliability (that is, the ability to launch on schedule). Every rocket design is a compromise of cost, safety and reliability. In the case of the Falcon, it has nine Merlin engines in the first stage because this allows it to use the same engine in both that stage and the upper stage, which only has one (with a larger nozzle for vacuum operation). This saved a great deal in development costs, and provides economies of scale in manufacturing, with a steady production of them 24/7 in the company’s factory in Hawthorne. It also provides a more forgiving design, allowing engines out on ascent, while also allowing the functional capability to “deep throttle” the stage by selectively shutting down engines to maintain gee limits for crew.

But nine engines also means nine times the things that can go wrong and prevent a launch. In fact it’s worse than that. It actually increases the unreliability exponentially. For instance, if the probability of an event like Saturday’s for one engine is one in a thousand, the probability of it not happening on any one of the engines is 0.999 to the ninth power, or .991, which means that there’s about a one in a hundred chance of an abort. If it’s only one in a hundred, that means that there will be an abort every tenth flight or so. And that’s just for Falcon 9. Falcon Heavy will have twenty-seven first-stage engines, which means a probability of abort of almost three in a hundred for a one in a thousand single-engine reliability, and a probability of one in four for one in a hundred.

The company doesn’t have enough experience with this vehicle to know what its true reliability is, but if they continue to have pad aborts, they may decide that they’d like to get bigger, and fewer engines.

But it also raises the issue of the value of a flight-readiness firing (FRF), as SpaceX performed a few days ago, with a hold-down test of the first-stage engines on the launch pad for a few seconds a few days ago, in preparation for this launch. The Space Shuttle also did this each time before the maiden launch of an orbiter, to ensure that all systems were ready to go before the first flight, but it was a reusable vehicle. Max Hunter, developer of the Thor that evolved into the Delta in the sixties, used to say that FRFs caused more problems than they solved, because a clean vehicle from the factory could be damaged or worn in the process, making it less reliable for actual flight. The valve seemed to have worked all right in SpaceX’s FRF, and it’s unclear (though SpaceX may know) whether or not the failure was a result of the FRF (I would bet they’re already reviewing the data to see if they saw any anomalies toward the end of the test, if they haven’t already).

But unlike the Delta, either ancient or modern, the company has a goal of full reusability for the vehicle, including the engines, so it may make sense to do FRF, at least once they start to refly, when they won’t have to do it for every flight. But the incident has no doubt given Mr. Musk and his team quite a bit to think about.

I will be curious to hear what they think the cause of the valve failure was.