SpaceX had an engine failure on Sunday. Rand Simberg thinks that's a good thing.
From an engineering perspective, all of this is good news for two reasons.
First, prior to this flight, the idea that the Falcon 9 could still get to orbit even with an engine out was just a marketing claim. Proving it would have required a demonstration in a test flight. Now that claim has been demonstrated and validated in an operational flight, if accidentally. While SpaceX's competitors and opponents will point to the engine loss as a reason for concern, in reality it should increase confidence in the company’s product. Every rocket provider has problems (United Launch Alliance's Delta IV, one of Falcon’s competitors, had a second-stage engine issue itself just last week), but in this case the design was sufficiently robust to overcome them exactly as the designers intended.
Second, if the engine really had exploded, this would have potential safety implications for a crewed version of SpaceX's Dragon capsule. Consider that the upper stage of the Falcon 9 uses the same Merlin 1C engine as the first stage, except with a few changes such as a larger nozzle for vacuum operations and passive radiative cooling rather than the “regenerative” cooling in the first-stage engines (it pumps fuel through channels in the nozzles to carry away the heat). The biggest difference, though, is that there is only one Merlin engine for the upper stage. So if it fails, the mission fails.
There have now been four flights of the Falcon 9, with ten engines each (nine for the first stage, one for the second). Counting the one engine failure from last night’s launch, that means that the engine has a demonstrated operational reliability of 39 out of 40, or 97.5%. That means that there’s a 2.5% chance that the engine would fail in an upper stage (where it has no backup), and would imply that this is an upper boundary on the reliability of the rocket itself (because other things could go wrong). This is in the typical ballpark of mission reliability for expendable launch vehicles for the past half century.