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. 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.
We were aware of and had allowed for a thrust transient, but 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? 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.
It looks like we may have flight four on the launch pad as soon as next month. 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.
Good Things About This Flight
- Merlin 1C and overall first stage performance was excellent
- The stage separation system worked properly, in that all bolts fired and the pneumatic pushers delivered the correct impulse
- Second stage ignited and achieved nominal chamber pressure
- Fairing separated correctly
- We discovered this transient problem on Falcon 1 rather than Falcon 9
- Rocket stages were integrated, rolled out and launched in seven days
- Neither the near miss potential failures of flight two nor any new ones were present
