With the additional insight from the Discovery News ref. (RetiredF4 post)
Once its rocket motor burns out and its fuel is spent, SpaceShipTwo’s center of gravity shifts forward, so that the tail generates downward force, like an airplane. In between boost and re-entry, aerodynamics forces dictate when structural loads shift for safe reconfiguration of the vehicle.
So not only must a sufficient Mach number have been reached (to be clear of the complex dynamics of the transsonic regime), also a sufficient CoG forward shift must have been made.
The first transition occurs around Mach 1.4, or about 1.4 times the speed of sound. It’s not an exact time, but dictated by a combination of several technical factors including vehicle speed, altitude and motor nozzle angle. Around Mach 1.4, the lock (which actually is a chunk of machined aluminum) is moved so that the feather is now mechanically free. It won’t move though because at Mach 1.4, aerodynamic forces keep it nailed back.
The Mach number 1.4 in the applicable flight regimes appears to imply both proper "clean" supersonic conditions and the required CoG for tail surfaces down-lift in that regime.
“In addition to the lock, the feather itself has a big actuator that drives it up and down. So just because it’s unlocked doesn’t mean it’s just flopping in the wind and it can go where it wants. In the (initial) boost phase, the aerodynamic forces can overcome that, which is why we lock it in place,” Moses said.
Which suggests that after "the (initial) boost phase" the aerodynamic forces cannot overcome the pneumatic downforces on the feather. Which would imply that positive lift on the tail surfaces up to a certain magnitude can be tolerated. Such positive lift occurances (assumed limited in time and magnitude) may result from the trim of the horizontal tail surfaces, elevon deflections and thrust vector variations (e.g. due to uneven burn and outflow).
Two main pneumatic 625-psi actuators with a 9.5-in bore and 31-in stroke, change the position of the feather ...
Some numbers related to force and stroke had been found by PeterH, with an estimate of tail boom dimensions (someone may have these?) these numbers may then be related to maximum adverse (positive) lift on the tails that can be tolerated.
Which may serve to answer part of the recent questions from PeterH.
And leaves the following questions:
- What keeps the booms in the feathered position, as there is no locking mechanism for this.
--- Pneumatic pressure?
--- Aerodynamic forces?
In line with the previous I would expect Yes and Yes. In addition the proper re-entry attitude relative to flight path - when not controllable aerodynamically - may be set up by the reaction control gas jets.
@thcrozier
Applying equal pressure to both sides of the piston would "lock" the piston in place,
A hard locking in any halfway position is not possible with pneumatics, and I would not expect the feathers to be in any other position than their extremes except when transitioning. Maintaining some pressure on the retreating side of the piston may serve though to smoothen the transition. In the feathered position I can imagine - rather than bi-lateral pressurisation - a uni-lateral pressurisation pressing against a hard mechanical stop.
@BOAC
- unless the 're-entry' is inverted. ,
I understand the the unfeathering motion to be driven by the pneumatics, possibly assisted with forward trimming of the horizontal stabilisers and up-deflection of the elevons to reduce required forces.
Upon inadvertant failure of the pneumatics during the unfeathering phase perhaps a roll to (partial) inverted attitude may be an emergency option (inside or outside of designed procedures). I recall this trick had been pulled by an aeroplane with structural wing failure (distant past, must search deep ...).
Other than that: bail out at safe altitude and speed.
It would be interesting to know if parachutes are forseeen for space flight passengers (?)