Falcon 9 Returns to Flight

SpaceX’s cutting-edge rocket launches once more

A Falcon 9 launch in 2015, which was followed by a successful 1st stage landing (source)

After September’s rapid unplanned disassembly, SpaceX is ready to return to flight. The RTF, as it is called, is the first time a launch vehicle returns to service after an accident.

The first step in the RTF process is understanding the cause of the accident which necessitated cessation of flight. In SpaceX’s case, this was challenging. The Falcon 9 FT is probably the most technologically advanced launch vehicle in operation today, and its boundary-pushing nature means that it is subject to failure modes which have never been seen before. September’s failure is an example of this.

The explosion of the vehicle (or ‘fast fire’ as Musk prefers) in September 2016 has been traced to a novel failure mode caused by interaction between the densified propellants and the composite-overwrapped pressure vessel, or COPV.

The COPV is a Pressure Vessel for holding helium (the PV in COPV), which is Overwrapped (the O in COPV) in a carbon fibre like Composite (the C). By wrapping the thin metal vessel in lightweight but strong composites, it is able to withstand greater pressure and weigh less than a comparable metal vessel. The function of the COPVs is to pressurise the rocket’s tanks with inert helium as propellant is drained from them. This pressurisation forces the propellant down towards the engines, but is primarily to ensure that the tank remains at optimal pressure for maintaining structural integrity. In a feature unique to the Falcon 9, the helium COPVs are contained within the liquid oxygen tank to allow for greater density.

The other player in September’s failure was the densified liquid oxygen. The Falcon 9 is powered by liquid oxygen and RP-1, which is refined liquid kerosine. SpaceX is experimenting with lowering the temperature of both propellants because they are denser at lower temperatures, and the denser they are the more fuel can be loaded into the vehicle. This extra margin provided by keeping the propellants near their freezing point rather than near their boiling point may allow for the Falcon to lift heavier payloads in the future. In the present, however, loading oxygen as cold as 206°C — a temperature at which the liquid oxygen is more like a slush than a liquid— is proving challenging. Competitor ULA’s VP George Sowers tweeted about densified propellents that they are “not worth the trouble, small gain for lots of headaches.” SpaceX disagrees and is continually making small improvements which cumulatively represent significant progress. For example, over its history the Falcon 9, its lift capacity to Low Earth Orbit has more than doubled.

Months of investigation into the interplay between these two components has revealed the cause the September disaster. The short answer is that a helium COPV submerged in densified liquid oxygen exploded because solid oxygen formed within the fibres of the COPV and compromised its structural integrity. The pressure released from the COPV then set off a cascade of failures resulting in the destruction of the vehicle.

This video by Scott Manley, below, provides a more in-depth explanation:

SpaceX has learned from the September disaster and has adapted their loading procedures accordingly. They have also safely loaded and fired numerous Falcon 9s at their testing facilities since the disaster. On paper, tomorrow’s launch is almost certainly safer than average, because every team has had extra time to prepare and all are aware of how damaging another failure could be. Despite this, the launch will be nervewracking. Another failure could set SpaceX back even further and reduce their capacity to further push the limits of innovation with projects such as the Falcon Heavy (scheduled for later this year), the first re-flight of a ‘flight proven’ Falcon 9 (also later this year), and the Red Dragon Mars mission (scheduled for 2018).

Relative to the conservative defence conglomerates which constitute most of their competition in commercial spaceflight, SpaceX is advancing with lightning speed. With progress, SpaceX also faces challenges which have never been encountered before. I hope SpaceX has a successful launch tomorrow and continuing successes into the future. Per aspera ad astra

You can watch tomorrow’s launch (and landing attempt) on SpaceX’s youtube channel.