SpaceX is going operational

After the flight termination a week or two ago, I promised our commentariat I would post information on the problems which caused the second test flight to not reach orbit. A few days ago Elon Musk released this statement:

Post flight review of telemetry has verified that oscillation of the second stage late in the mission is the only thing that stopped Falcon 1 from reaching full orbital velocity. The second stage was otherwise functioning well and even deployed the satellite mass simulator ring at the end of flight! Actual final velocity was 5.1 km/s or 11,000 mph, whereas 7.5 km/s or 17,000 mph is needed for orbit. Altitude was confirmed to be 289 km or 180 miles, which is certainly enough for orbit and is about where the Space Shuttle enters its initial parking orbit.

It turns out that as many of us suspected, there was a feedback between fuel slosh and the control equations:

In a nutshell, the data shows that the increasing oscillation of the second stage was likely due to the slosh frequency in the liquid oxygen (LOX) tank coupling with the thrust vector control (engine steering) system. This started out as a pitch-yaw movement and then transitioned into a corkscrewing motion. For those that aren’t engineers, imagine holding a bowl of soup and moving it from side to side with small movements, until the entire soup mass is shifting dramatically. Our simulations prior to flight had led us to believe that the control system would be able to damp out slosh, however we had not accounted for the perturbations of a contact on the stage during separation, followed by a hard slew to get back on track.

There was indeed a contact of the first stage with the bell of the stage two motor at stage separation and it was indeed not a big thing:

The nozzle impact during stage separation occurred due to a much higher than expected vehicle rotation rate of about 2.5 deg/sec vs. max expected of 0.5 deg/sec. As the 2nd stage nozzle exited the interstage, the first stage was rotating so fast that it contacted the niobium nozzle. There was no apparent damage to the nozzle, which is not a big surprise given that niobium is tough stuff.

The unexpectedly high rotation rate was due to not knowing the shutdown transient of the 1st stage engine (Merlin) under flight conditions. The actual shutdown transient had a very high pitch over force, causing five times the max expected rotation rate.

The vehicle will be launching a satellite on its next flight:

The reason that flight two can legitimately be called a near complete success as a test flight is that we have excellent data throughout the whole orbit insertion profile, including well past second stage shutdown, and met all of the primary objectives established beforehand by our customer (DARPA/AF). This allows us to wrap up the test phase of the Falcon 1 program and transition to the operational phase, beginning with the TacSat mission at the end of summer. Let me be clear here and now that anything less than orbit for that flight or any Falcon 1 mission with an operational satellite will unequivocally be considered a failure.

This is all very good news to the new space industry. There is also supposed to be some more good news this month: Bob Bigelow of Bigelow Aerospace, maker of fine inflatable space stations, is supposed to make an important announcement. I suspect it has to do with a next launch date and he may announce he is skipping more intermediate tests or perhaps even an early anchor tenant for an operational station… if we were to speculate even more wildly.

All of us in ‘the biz’ will be watching closing.