SpaceX CEO Elon Musk has noted that the preliminary assessment of the second Falcon I flight shows that the second stage shut down only a minute before schedule – and still managed to deploy its satellite mass simulator ring.
The shutdown appears to have been caused by the sloshing of propellant in the LOX tank, increasing observed oscillation, which would normally have been successfully dampened out by the second stage Thrust Vector Control (TVC) system. However, the impact on the second stage nozzle during separation caused a ‘hard slew’ correction, over-compensating previously simulated scenarios.
The Falcon I launch vehicle lifted off from its Kwajalein Atoll launch pad in the Pacific Ocean last week, putting behind the failure – which occurred just seconds into first stage flight ascent – of its debut launch a year ago.
Although the video of the launch webcast cut off just after 5 minutes of flight, SpaceX have now gained visuals from the entire flight, past the point of the premature second stage shutdown.
‘Except for a few blips here and there, we have now recovered video and telemetry for the entire mission, including well past 2nd stage shutdown, which only occurred about a minute before schedule (roughly T + 7.5 mins),’ noted Musk to NASASpaceflight.com. ‘Including all the video, we have somewhere close to a terabyte of information to review.
‘This was far too much to send over the T1 satellite link from Kwaj and had to be brought over in person after the flight. A number of our engineers have only just returned from Kwaj and we have not had a chance to caucus, so please consider this still a preliminary assessment.’
As observed on the webcast, an increasing level of oscillation could be seen on the second stage. While this is initially being blamed on the sloshing of propellant in the LOX tank, SpaceX had simulated – and planned for – such a scenario. However, the impact on the second stage nozzle, which was subsequently corrected by the TVC system, added an extra – unexpected – parameter for the TVC to counter.
‘In a nutshell, the data appears to show that the increasing oscillation of the second stage was due to the slosh frequency in the LOX tank coupling with the thrust vector control system,’ added Musk.
‘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 an impact on the stage during separation, followed by a hard slew to get back on track.’
While the impact observed during separation failed to damage the second stage engine’s nozzle, the cause is currently being blamed on the vehicle’s rotation being fives times higher than the expected maximum.
‘The nozzle impact during stage separation occurred due to a much higher than expected vehicle rotation rate of about 2.5 deg/sec vs. the maximum expected of 0.5 deg/sec. As the 2nd stage nozzle exited the interstage, the first stage was rotating so fast that it smacked the niobium nozzle,’ Musk noted. ‘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.’
Initial commentary in the media was based mainly on five minutes of webcasted video. However, now that the full video and data has now been acquired by SpaceX, a number of bonuses have been noted.
‘On the plus side, the data shows that this is the only thing that stopped the Falcon 1 test flight 2 from reaching full orbital velocity,’ said Musk. ‘The second stage was otherwise functioning well and even deployed the satellite mass simulator ring at end of flight!’
More evaluations will follow, as SpaceX engineers pore over the vast amount of data they now have available to them. The initial findings raise the hopes of medium level modifications being able to correct the issues noted.
‘We definitely want both the diagnosis and cure vetted by third party experts, however we believe that the slosh issue can be dealt with easily by adding more baffles, particularly in the LOX tank. The Merlin shutdown transient can be addressed by initiating shutdown at a much lower G level, albeit at some risk to engine reusability.
‘Provided we have a good set of slosh baffles, even another nozzle impact at stage separation would not pose a significant flight risk (although obviously we will work hard to avoid that).’