Keith Cowing
Wednesday, August 6, 2008
Editor's note: There was a media telecon with Elon Musk and Diane Murphy starting at 5:00 pm EDT today to discuss "specifics of the third flight of the Falcon 1 launch vehicle".
According to Elon Musk: "We have a definitive understanding of what went wrong on Flight 3. The problem was due to a design error not a production or quality assurance issue. The thrust transient was longer than it was for the prior flight."
The previous flight had an ablatively cooled engine. Flight 3 had a regeneratively cooled engine.
The gap between engine cut off and staging was 1.5 seconds - which was fine for the ablatively cooled engine on Flight 2. But on Flight 3, with the regeneratively cooled engine, there was some residual thrust after engine shut down and this caused the first stage to be pushed back toward the second stage after separation and there was a recontact between the stages.
Neither stage exploded but each stage got a little cooked. We did not recover the first stage because when the second stage fired it burnt the parachutes in the first stage. We did not recover the second stage either.
When we tested at sea level we had higher ambient pressure than what you have in a vaccuum, so this effect was effectively masked on our test stand during testing.
If we had increased the gap a little this would not have happened. If we had a rocket on the launch pad tomorrow we could make that timing change tomorrow.
We may have a vehicle on the launch pad as soon as next month. We will have assets on the island no later than next month and perhaps launching at the end of next month subject to range availability.
We have received a lot of customer support. Indeed, after every flight we have gained customers. We will be posting a detailed video showing everything that we know about this flight later today.
SpaceX Update: Timing is Everything
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. 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. 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
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.
Elon
