On August 2nd, SpaceX made the surprise announcement that the third flight of the Falcon 1 rocket system would launch at 8pm (PST) that day. The world rushed to watch the first commercial flight of this impressive private-sector rocket via the web from a live feed on board. The first launch attempt was aborted due to a minor parameter fluctuation of 1% out of “normal” operating conditions, but the launch crew very quickly re-fuelled and prepared Falcon 1 for a second launch attempt within the hour. The second launch attempt appeared to be flawless, Merlin 1c engine roaring to life, lifting the rocket into the atmosphere. All seemed good, SpaceX seemed on track and very confident. However, minutes into the flight, the live video feed was cut and it was being reported an anomaly had occurred. It wasn’t until later in the week that SpaceX CEO Elon Musk gave details about the “anomaly.” SpaceX recently released video footage of the entire launch, up to the point where the stage separation problem occurred, spinning the ill-fated vehicle out of control…
So what did happen on that frustrating Sunday evening? On August 6th, Elon Musk announced the findings of the investigation into the launch anomaly. According to the launch engineers, the SpaceX Merlin 1c engine in the first stage performed perfectly. Even after the false-start on the launchpad, the engine was ready to go within the hour. This fast turnaround from launch abort to re-launch is a huge advantage for the company, a great testament to the flexibility of the technology SpaceX has developed in-house. The problems started during stage separation at an altitude of 35 km.
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. – Elon Musk, Aug. 6th statement.
From this statement and from viewing the video, it would seem that during first stage separation a small amount of fuel left over creating a small thrust just after the stages were forced away from one another (a.k.a. “stage separation pusher impulse”). At separation, it would appear that just as the first stage was beginning to fall away from Falcon 1, it regained some forward thrust, making it crash into the second stage engine. This small thrust anomaly prevented the spent first stage from falling clear of the igniting second stage. This sequence of events is captured in the series of screenshots below:
As the first stage was not clear, the second stage engine fired into the spent first stage. This would have caused a loss in control in rocket trajectory. However, the SpaceX editors appear to cut the video from the instant the second stage fires to when the rocket is in full tumble, blacking the frames out in between with the text “Faring Separation.” It’s not obvious what this means and there is no mention of it in the accompanying text. Most probably it means the camera was blown away from the rocket after second stage ignition.
The anomaly was down to what has been called a “thrust transient” and Musk points the blame at the tiny thrust that couldn’t be measured on the ground during test firing as the force generated was simply too small to be detected. However, in the vacuum of zero-gravity space, any thrust, large or small, matters:
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 – Elon Musk, Aug. 6th statement.
Although this event is an obvious set back, and deeply saddening for SpaceX and the owners of the payloads Falcon 1 was supposed to put into orbit, lessons have been learnt and Musk is positive the next launch will be a total success. After all, no one said rocket science was easy…