@PeterH
My hunches for the main drivers:
Before unlocking:
- Assure sufficient Mach number (M 1.4 plausible number) to stay clear of transsonic regimeīs agressive and complex dynamics;
- Other factors assumed of lesser or no relevance.
Margins to respect after unlocking:
- Pressure distributions in the supersonic and hypersonic regimes (no complex transients) within prescribed limits, as determined by a.o.:
- - Trimmable elevons within prescribed trim angles
- - Other aerodynamic control inputs within prescribed limits
- - No reaction control inputs or within prescribed limits
- Sense and magnitude of actuation forces from pneumatic feathering system within prescribed limits
- Thrust vector misalignments within prescribed limits (determined from design criteria in combination with static motor tests and powered flight test data recordings), or else early motor cut-off.
Margin to latest permittable unlocking, determined mainly by:
- Aerothermodynamics of unfeathered re-entry, driven mainly by vehicle energy (altitude, speed, weight, i.e. by duration of motor burn)
Other than that I can say very little about the re-entry.
The aerothermodynamics of the re-entry is probably the least known of all factors involved, even for the vehicle designers.
The same can be said for Apollo lunar return re-entries at the time and for the first Space Shuttle Orbiter re-entries, the actual margins were only established after the first flights and there have been narrow escapes.
In comparison SS2 has no or limited thermal protection and the design concept is to not need it, hence the feathers and the unlocking as soon as safely possible, thatīs my best guess.