An Energia man's comment
An interesting opinion was expressed on a Russian space forum during duscussion of the TMA-11's ballistic reentry possible causes. Some poster said that, to his knowledge, a Soyuz capsule is very sensitive to pressure focus shifting during a reentry; not in just any direction, however, but only towards exceeding yaw channel disturbance.
By his words,
"The idea of the controlled descent is based upon an assumption that the COG is deliberately displaced along the vertical axis by a precisely calculated distance; at the same time, it can be displaced along the horizontal direction only in the adjustment limits (the limit switches of gyroscopes horz axis). The vertical gyros axis doesn't have those switches (or, more precisely speaking, their role there is different)... It all leans against an expectation that during a reentry the aero force exerted on the bottom would turn the capsule in the way such that the total force vector passes through COG, thus providing the necessary AOA.
Add to that that during the Soyuz design nobody put much effort to design the Descent Control System anew, it was taken as a ready part of a different hardware item, the one which had pretty stable COG location. This had been quite passable since Soyuz was intended for short missions with the same people comprising the crew, so it wasn't a big problem to predict the location of the COG. On the other hand, when the capsule diameter shrunk, the COG and the COP has neared each other pretty close, which made the entire thing very sensitive towards shifting of either. Now, with very small COG displacement, the capsule tends to turn to side and the AOA is sustained not only by the force acting on the bottom, but also on the bell part (top). Which is, IMO, very good, because thus a stable equilibrium is achieved much better than it would be according to the design specs. Meanwhile, it somehow turned out that the stable AOA in this position is exactly what it's ought to be and the landing goes nominal and nobody minds that the top part of the capsule is burnt so badly every time.
However, if we have a greater than usual disbalace on the horizontal axis, we are getting a yaw disturbance. The automatic control system is taught to assume only a bank disturbance and tries to straighten it up, then goes crazy and fails, sending the capsule into a BR.
So it might just happen that the little Korean was just too dolly in her side seat or the payload wasn't distributed accordingly.
Personally, I'd rather prefer it to be always a ballistic reentry, 'cause the g-load is greater, yes, but two times shorter in duration and the landing spot would be predictable better".
:hmm:
