Previous gossips about reason of explosion of Falcon-9 FT with Amos-6 communication satellite encapsulated under fairing proved to be true. According to engineers and specialists from SpaceX announced that main reason for explosion on 1 September 2016 at LC-40 at Cape Canaveral during routine tests was helium tank.
Investigation was finished after over four months and finally it is possible for SpaceX to resume flights of their Falcon-9 FT. Engineers evaluated that one of three helium tanks, which is used for pressurizing liquid oxygen tanks, ignited rapidly ten minutes before fire test. Now main question should be what exactly changed in the construction of the tank from Falcon-9 1.1 variant to present Falcon-9 FT version. Question is simple - nothing changed in design of helium tank, everything in fueling procedures. Present version of Falcon-9 is fueled with supercooled (-198°C) LOX and RP-1 with also lower than room temperature. Fueling is performed 35 mintes before launch. Liquid oxygen with decreasing of temperature is increasing its density, what helps in filling tanks with additional amount of oxidizer and engines can work during flight with full thrust (that is why this version is named as FT - Full Thrust). LOX is pushed into pipes with helium stored in aluminium tanks insulated with strong carbon based composite. Tanks are installed inside fuel tanks to keep both LOX and helium in similar temperature. Filling tanks with supercooled propellant caused probably, that trapped oxygen inside carbon insulation overwrapped fibers and caused ignition. Unfortunately due the short time between beginning of anomaly to explosion (93 milliseconds) it was extremely hard to evaluate true reasons. Experts and engineers from SpaceX managed to find out key factor thanks to analyzing all data collected before explosion and tests of helium pressurizing system performed at McGregor Test Center in Texas. According to report:
"The accident investigation team worked systematically through an extensive fault tree analysis and concluded that one of the three composite overwrapped pressure vessels (COPVs) inside the second stage liquid oxygen (LOX) tank failed. Specifically, the investigation team concluded the failure was likely due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV,”
Probably lower temperature of propellant caused more intensive reaction of insulation with oxygen accumulated under wrapping. SpaceX evaluates, that for the moment solution for the problem is changing fueling procedures. Company also declares redesigning helium tanks in future:
“In the short term, this entails changing the COPV configuration to allow warmer temperature helium to be loaded, as well as returning helium loading operations to a prior flight proven configuration based on operations used in over 700 successful COPV loads. In the long term, SpaceX will implement design changes to the COPVs to prevent buckles altogether, which will allow for faster loading operations. ”
It seems that Falcon-9 FT still will be loaded with superchilled propellant, but probably time of fueling and pressurizing will be extended to avoid rapid reactions inside tanks. Company also declares to increase helium temperature to last level when anomaly was not present. Changes in construction should solve this problem and after installing new helium tanks it will be possible to shorten fueling time again. It was not confirmed if that change will affect the parameters of Falcon-9 FT like thrust. It could be possible mainly due the lower amount of propellant fueled before start (LOX will have higher temperature due the prolonged time of fueling).
SpaceX plans to resume flights with first launch in 2017 on 8th January from Vandenberg Air Force Base with ten Iridium NEXT satellites.