And Falcon I's Stage Separation Timing Was Off This Weekend

It was far from a perfect flight... but they now know what happened. That's the message Elon Musk, CEO and founder of Space Exploration Technologies (SpaceX,) had for employees Wednesday, regarding the company's unsuccessful Falcon I launch this weekend.

"On August 2nd, Falcon 1 executed a picture perfect first stage flight, ultimately reaching an altitude of 217 km, but encountered a problem just after stage separation that prevented the second stage from reaching orbit," Musk writes. "At this point, we are certain as to the origin of the problem. Four methods of analysis – vehicle inertial measurement, chamber pressure, onboard video and a simple physics free body calculation -- all give the same answer.

"The problem arose due to the longer thrust decay transient of our new Merlin 1C regeneratively cooled engine, as compared to the prior flight that used our old Merlin 1A ablatively cooled engine," Musk continues. "Unlike the ablative engine, the regen engine had unburned fuel in the cooling channels and manifold that combined with a small amount of residual oxygen to produce a small thrust that was just enough to overcome the stage separation pusher impulse."

Musk says the company was aware of the thrust transient, and had allowed for some variance... but admits SpaceX "did not expect it to last that long. As it turned out, a very small increase in the time between commanding main engine shutdown and stage separation would have been enough to save the mission.

"The question then is why didn't we catch this issue?" Musk continues "Unfortunately, the engine chamber pressure is so low for this transient thrust -- only about 10 psi -- that it barely registered on our ground test stand in Texas where ambient pressure is 14.5 psi. However, in vacuum that 10 psi chamber pressure produced enough thrust to cause the first stage to recontact the second stage."

Musk concluded by noting "Good Things About This Flight" -- including the excellent performance of the Merlin IC first-stage engine, and correct stage separation sequencing and fairing separation. While acknowledging problems encountered on each of the company's three unsuccessful flights, Musk notes "neither the near miss potential failures of flight two nor any new ones were present."

Perhaps most importantly, Musk notes wryly, "We discovered this transient problem on Falcon 1 rather than Falcon 9," which is substantially larger.

"It looks like we may have flight four on the launch pad as soon as next month," Musk (below) concludes. "The long gap between flight two and three was mainly due to the Merlin 1C regen engine development, but there are no technology upgrades between flight three and four. The only untested portion of flight is whether or not we have solved the main problem of flight two, where the control system coupled with the slosh modes of the liquid oxygen tank.

"Given the addition of slosh baffles and significant improvements to the control logic, I feel confident that this will not be an issue for the upcoming flight four."

FMI: www.spacex.com