SpaceX Investigation Points to Composite Overwrapped Pressure Vessels
SpaceX AMOS-6 Anomaly Update 28 October 2016
“The investigation team has made significant progress on the fault tree. Previously, we announced the investigation was focusing on a breach in the cryogenic helium system of the second stage liquid oxygen tank. The root cause of the breach has not yet been confirmed, but attention has continued to narrow to one of the three composite overwrapped pressure vessels (COPVs) inside the LOX tank.”
“Pending the results of the investigation, we continue to work towards returning to flight before the end of the year.”
Previous statements:
– SpaceX AMOS-6 Anomaly Update 23 September 2016
– SpaceX Anomaly Statement September 2, 6:45pm EDT
The question is whther they will modify the COPV to provide better protection from the LOX outside it. Whatever method is used, testing to burst pressure should follow. If they decide to change the design, the current invenory coul be used for test purposes.
I don’t think I am correctly visualizing this tank and apparently not using sufficiently fine search keywords.
Is the LOX tank containined within the fuel tank? Or are the tanks discrete?
I always wondered about temperature and pressure management for these very dissimilar chemicals, both on the pad and when the tanks are being filled or depleted in flight It’s a dramatically changing environment.
The LOX and fuel tanks are separated by a blkhead as Brian points out. Inside the LOX tank are three ~10-liter COPVs containing gaseous helium at about 350 bar. That much inert gas under that pressure has as much potential energy as several kilograms of TNT and could easily account for the small initial fireball.
As I thought I knew; I don’t think I realized the helium was contained inside the LOX tanks.
The LOX tank and fuel tank have a common bulkhead between them.
The second example in this image, LOx is the top tank.
http://i.stack.imgur.com/m8…
Inside of which are four COPV He bottles, similar to this:
http://www.psi-pci.com/imag…
Which are wrapped in layers of carbon fibre. Visible in this failed version:
http://abilitycomposites.co…
The He bottles are inside the LOx tank, because they need to be cold to create the necessary density of Helium. Looking down into the LOx tank, via a camera at the top. The He bottles are arranged around the bottle of the tank.
http://spaceflight101.com/f…
(The blue liquid is the remaining LOx, the tank is nearly empty, near the end of the burn.)
Misleading title, the release actual points to propellant loading procedures tangentially connected to the COPVs, not anything to do with COPV construction or manufacturing flaws.
Apparently the feeling is that the helium was quite cold, below 54K, the freezing point of oxygen. Possibly this ocurred when the helium, having passed through the LOX in the tank, expanded from the supply line into the COPV. This may have caused crystals of solid oxygen to form between the composite layers.
However if the problem of LOX intrusion into the composite is not dealt with there are other failure modes that may be induced, I would guess any situation where LOX might boil after itrusion for example, such as a launch scrub, and particularly on the booster where the COPVs would be subject to repeated exposure.
The densified propellant process will be used for SpaceXs Load and Go with the Astronauts as LC39A. This means they will strap the Astronauts in the Dragon capsule, then start the LOX/RP-1 densified propellant loading into the launch vehicle. We saw what happened with this process at LC40 in September.
By far MUCH safer to have humans in a capsule-cum-launch escape system than to have them be forced to walk around and enter the vehicle once it has been loaded.
The biggest concern with this accident, explosion, and investigation is that, according to other Internet articles, this densified propellant process will be used at LC39A for SpaceXs Load and Go process with the Astronauts. So a rushed, let’s hurry up and launch again mentality could be extremely risky for the Astronauts.
The countdown is constrained by orbital mechanics; all flights to the ISS have to launch during the brief period when the orbit track is overhead. Safety is achieved by repeated tests of the launch system and modifications to eliminate potential failure modes.
It’d be an exciting test of the abort escape system!