[FPSPACE] How tightly are the Soyuz modules bolted together?
geert at navtools.nl
Fri Apr 25 22:07:19 EDT 2008
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.
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
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