SpaceX Suffers Stage Separation Anxiety but Problem Solved
Sorry, but I couldn't help the title. In short, the third flight of the Falcon 1 failed because the time between first stage engine cutoff and second stage separation was a tad too short. The third flight incorporated fixes to the problems seen in the second flight as well as a change to the first stage engine (from the ablatively cooled Merlin 1 to the higher performance, regenatively cooled Merlin 1C).
The ablative engine uses an engine liner -- made of something akin to fiberglass and not so different from a capsule heat sheild in function -- to keep the engine from melting. The liner keeps the engine cool by slowly burning away as the engine runs. The burning bits leave the engine, carrying the heat with them, which prevents the unburnt part of the liner from heating up. This method is often used, such as with the Apollo Lunar Excursion Module's descent and ascent engines. It's great stuff and makes for a simple engine design. Fuel and oxydizer are mixed at the top, burnt, and thrust shoots out the bottom. But the thrust performance changes over the time of flight due to erosion/burning of the liner's surface.
Regenatively cooled engines run their fuel and/or oxydizer through a cooling jacket surrounding the inside surface of the engine. This method keeps the inside surface of the engine from melting, removing the necessity of an engine liner, and also heats up the fuel/oxydizer for more efficient energy transfer. This is a more complex engine design, but it has the advantage that the engine performance remains constant throughout the flight.
Now comes the problem. When an ablative lined engine shuts down, it shuts down, the fuel/oxydizer valves are shut and the engine shuts down at once. Not so with a regenatively cooled engine, when it shuts down, there's still a bit of fuel and oxydizer left in the cooling jacket that continues to burn. This creates a bit of excess thrust which takes a bit of time to fall off. This extra bit of time needs to be added to the delay between first stage cutoff and second stage separation. Time was added, but just not quite enough. In the video you can watch the stages separate then clunk back together. The statement from Elon Musk, head of SpaceX, talks about this issue. He also indicated that since the problem is well understood, the next flight of Falcon 1 can proceed within the next month.
This is the first rocket company I've seen that has been so open and honest with the public at large. The video footage from each launch is invaluable and allows not only SpaceX, but the public to witness and maybe even assist the development of SpaceX's technology. Great stuff.