Four reasons.

I should note, though, that it’s incorrect to say that the vehicle would have been destroyed if it had launched. They now know from simulations that it would actually have been fine, but better safe than sorry. I’ll have a piece up on this later today at Popular Mechanics.

Well, as I expected, that didn’t take long:

Engine pressure anomaly traced to turbopump valve. Replacing on engine 5 and verifying no common mode. #DragonLaunch

So it’s a good thing it shut down — it could have resulted in a catastrophic engine failure in flight, something that SpaceX has theoretically designed for (they claim to have engine-out capability from liftoff, and sufficient inter-engine blast shielding to prevent fratricide), but probably doesn’t want to test, at least this early in the program. Also, they had no performance margin on this mission, so it probably would have meant an abort to a low orbit, and perhaps an inability to get to the ISS.

[Update a while later]

Here’s the story over at Popular Mechanics.

As others have pointed out, the Falcon did exactly what it was supposed to do under the circumstances — it was a successful abort. It’s too bad that they have the three-day constraint for the next attempt. Hopefully there’s nothing wrong with the engine itself. They didn’t see this problem in the test firing a few days ago. But I wonder if there’s something about the geometry that causes the center engine to be a little higher pressure than the outer ones. If I were them, I’d be going back and looking at the pressures from previous flights and tests. If that’s the case, then the solution to this problem might be to just allow a slightly higher pressure on it.

[Update a couple minutes later]

Here’s the story from Spaceflight Now. Looks like if they have another scrub on Tuesday, they won’t have to wait another three days — they can try on Wednesday. If they launch Tuesday, that means a docking attempt on Friday, right in the middle of the ISDC.

Here’s an article describing the pre-rendezvous testing that it will go through. This is misleading, though:

“This is pretty tricky. And also, for the public out there, they may not realize that the space station is zooming around Earth every 90 minutes, and it’s going 17,000 mph,” said SpaceX CEO Elon Musk. “This is something that is going 12 times faster than a bullet from an assault rifle. So it’s hard.”

The velocity relative to earth isn’t really relevant, and doesn’t make it harder. All that matters is the relative velocity between the spacecraft and the ISS. And that, of course, is what they have to demonstrate their ability to control.

[Update a while later]

More thoughts from Tom Jones.

As of this morning, I’ve raised over $1800 of the $7000 that I need for my space-safety Kickstarter project. Keep it coming, and thanks to all who have donated so far.

From Andy Chaiken.