[FPSPACE] How tightly are the Soyuz modules bolted together?

[Geert Sassen]

Thanks a lot to all of you, this is a really interesting discussion,
>
> But, if orientation is lost (meaning roll command), in the worst case TF
> vector may be oriented "downwards" --> steeper descent --> very high
> deceleration (too high Gs) and extreme heat flux --> all is dangerous.
>   

That's indeed what I have always understood, if you attempt to make a 
ballastic reentry without a constant roll, and you have a bad day (TF 
vector orientated downwards) you will run into very high G's and very 
high heat flux. Note you are then no longer in a pure ballistic 
trajectory but diving much steeper down. To avoid this you either need 
to have a constant roll (zeroing the TF vector) or you need to be able 
to at least keep the DM stable in a attitude where you avoid a downward 
TF. Both scenario's require working thrusters, leading once again to the 
question how Soyuz 5 managed to survive if its thruster-fuel was depleted?

One other question which keeps nagging on my mind: the initial reports 
clearly stated an 'overshoot' to explain the fact that the craft was not 
spotted in the intended landing area, and there are reports that the 
recovery forces were sent to the East (also in line with the 
'overshoot'), however a ballistic landing results in a shorter 
trajectory, landing west of the intended area, which was exactly what 
happened. So what made them think there could be an overshoot?
> BTW, stable orientations due to CG offset are two: one with lower heat
> shield in the correct orientation and the second in the opposite sense.
> Terrible!
>
>   
 From what I understand this is not possible with Soyuz due to its 
'headlight' shape, the "upside down" orientation is not stable (as 
opposed to Apollo, which did indeed have two stable positions), once 
aerodynamic forces act on it, it will always turn "bottom down" no 
matter what it's original orientation. Only with the PM still attached, 
it will act as an 'arrow' and orientate itself hatch forward...

I was just reading a chapter on Gemini re-entry techniques ('Gemini' by 
David Shayler, ISBN 1-85233-405-3, page 317) were is stated that two 
reentry guidance techniques were tested, one is called the 'rolling 
reentry' were the guidance computer rolls the craft in order to steer 
the TF factor, using the offset CG, essentially the same what is the 
normal procedure during a Soyuz 'lifting' reentry, and a 'constant 
bank-angle' where, from what I understand, the craft was kept in a 
stable position with the TF pointing 'up' (offset CG 'forward') and the 
bank-angle is used to 'steer' the craft and control the sink-rate 
(basically this is what is done during a Shuttle re-entry where the 
pitch is kept constant). I do not have a good description of the Apollo 
reentry guidance at hand here, but it sounds like a computer-controlled 
guided reentry used the roll-control and a manually controlled reentry 
used the bank-angle control where you keep the CG 'forward' (TF pointing 
'up') and use the thrusters to change your bank-angle to 'steer' the craft.

As far as I know Soyuz does not have an option to use manual control of 
the DM during reentry (as was possible on most US craft) and guidance is 
always done by controlling the roll-angle, either you make a 
computer-guided lifting reentry or you set a constant roll and make a 
ballistic reentry.

Regards,

Geert.