(around 41.5 sec. after liftoff)
The M-V-4 has cameras to monitor the events of the vehicle during ascent. One of them, which was installed at the 1st/2nd interstage structure, caught the gush of many small foreign bodies in the ejected gas at about 25 and 38.5 seconds, respectively. And, at 41.5 seconds, an exteremely big foreign body was ejected to be broken into small pieces(Fig.4).
Synchronizing the ejection of foreign bodies in Fig. 4, the inner pressure in Fig. 5 decreased rapidly, and continued to decrease gradually without recovery till the 1st/2nd stage separation (75 sec.). The axial acceleration in Fig. 5 also shows the sudden decrease at the same time. The situation is that the inner pressure and the thrust became lower than planned in connected fashion.
You can find that the deviation of the measured inner pressure from the planned pressure occurred just after liftoff, started at about 38.5 seconds again, and escalated by two steps at 38.5 and 41.5 seconds. Taking the witness of ejection of foreign bodies by one of onboard cameras into consideration, the expansion of throat diameter proceeded in escalating fashion.
Summing up these data together with the temperature measurement, after X+41.5 seconds when the throat graphite was broken on a large scale, the hot gas which penetrated through a chink or a hole of nearby CFRP may have broken the metallic nozzle holder. And the hot gas may have leaked out of the nozzle at 51.6 seconds to damage the attitude control systems(See Fig.6).
2 The abnormal attitude of the 1st stage was everything.
3 What caused the abnormal attitude?
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