Following the most recent Falcon 9 Rapid Unscheduled Disassembly, Elon Musk tweeted that "There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause."
This article mentions "a potential pointer towards the helium pressurization system’s bottles in the Second Stage" or "Composite Overwrap Pressure Vessels" (COPV). Where are these on the upper stage? How likely is it that this could have played a role in the RUD?
On the Falcon 9 v1.0 that was used for the first 5 launches, they were down around the engine area, and required insulation and protection from the engines.
In the move to Falcon 9 v1.1 and the Merlin 1D engine they moved them into the LOX tanks on both stages. This de-cluttered the engine area a great deal.
COPV's are notoriously difficult to build, since they hold immense pressure (6000 psi in rocket use cases) and if they leak/burst/fail then they go off like bombs. With shrapnel.
SpaceX used to buy the COPV's from an external vendor but decided to bring manufacture in house. During development there was a failure of one of theirs, and it looked like minor bomb damage.
They have had flight delays due to issues with the Helium pressurization system on previous flights.
The current thinking floating around is not necessarily that one of the COPV failed (as in exploded very very violently as a 6000 psi vessel releases), but instead some component of them failed and leaked the Helium at a somewhat slower rate.
(I was cheering for the IDA breaking lose as the root cause but it looks like that has been ruled out. If it was the payload not the booster).
While @geoffc explained where they are, I'll explain why they are there. These tanks contain very high pressure helium. Helium is a very difficult gas to store, and as any gas, storing works best at low temperatures. Storing them inside a tank with already cold LOX makes it much easier to manage than storing it outside of the tank would manage.
Careful followers will note that there was an issue with the helium tanks in the preparation for the ORBCOMM launch the summer of 2014. Immediately before the static fire, there was a gas seen leaving the first stage, which caused a hold of the static fire test.
It should be noted that the failure in this case was quite a bit different than the ORBCOMM pre-launch failure, this seems to be related to the struts not working as designed, which the ORBCOMM failure seemed to be an issue with the tank itself.
The typical COPV have an inside metal shell generally aluminum. The carbon fiber reinforcing layers are wound up at ambient temperature on the shell OD using a resin to keep the layer sealed to the tank metal inner shell to prevent helium leaks. The holy grail of COPV manufacturing is to find a way to make the tank without the inner metal layup metal shell. That is hard to do since the layup of the carbon layer need a reasonably strong surface for the carbon outer layer to wind on. The tank will also need metal domed ends to contain the longitudinal inside pressure components. In general the tanks will require a fairly complex failure prone and QC intensive manufacturing process. The finished tank will have carbon and resin outer surface and storing it inside a LOX tank will be a challenge mainly because of the very corrosive environment created by the LOX will literary burn the OD carbon, leak the helium and result in a bad day for everybody on the pad.
