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One of the circumstances leading to the VSS Enterprise crash was the fear of the pilot to unlock the feather system too late, thus the ascent would have had to be aborted. The feather system must have been unlocked between a speed of Mach 1.4 and Mach 1.8. If not unlocked before Mach 1.8 the flight would be aborted, this was the rule in the VSS Enterprise flights (I dunno whether it still is in the modified VSS Unity), why was it like this? Note I'm talking of unlocking the feather system, not activating it (going into feathered mode). The unlock is to happen during ascent of the SpaceShipTwo.

The poor pilot didn't know that when unlocking the feather system too early the spaceplane would go into feather mode by air drag (they didn't make it clear to the pilots). He wanted to avoid abortion of the flight so he unlocked the system before Mach 1.4 and still in subsonic flight. So why was there the rule to abort the ascent if the feather system isn't unlocked before Mach 1.8 in the first place? What would have been the threat if the feather system is unlocked too late?

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If the feather system failed to unlock they would die on entry, so it had to be done before committing to the space portion of the flight.

Since the feather system is critical to a safe re-entry, it was designed to be both simple and mechanically redundant. It was essential that the feather locks remain locked during the transonic boost phase of flight while equally critical that they are able to be unlocked prior to extending the feather for re-entry. As a result of this design requirement, all aspects of SpaceShipTwo’s feather system (locks and booms) are capable of being both extended and retracted using only one of two independent activation systems. Human performance failure modes were not considered....

The requirement to unlock the feather locks during the boost phase was implemented as a safety mitigation against a high-altitude re-entry with the feather stuck in the down position. Aerodynamic analysis determined that the vehicle could safely return from space in the feather down configuration if the rocket motor was shut down not later than Mach 1.8. Mach 1.4 was selected as a speed which allowed sufficient margin above the critical Mach 1.2 speed where the feather locks were required to remain engaged while still providing sufficient time for the aircrew to shut down the rocket motor in the event the feather locks failed to unlock.

(Emphasis mine)

Source: https://dms.ntsb.gov/public/57500-57999/57529/577511.pdf

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