[FPSPACE] Soyuz DM role during ballistic entry profile [was: How tightly are the Soyuz modules bolted together?]
Charles, John B. (JSC-SA2)
john.b.charles at nasa.gov
Fri Apr 25 13:05:01 EDT 2008
Antonin, thanks! This is very interesting. I have assumed that the initial report of an "overshoot" was a mistake (did the recovery forces really believe it and depart to the east?), and that, assuming an otherwise nominal de-orbit maneuver, every planned Soyuz re-entry is in fact an overshoot: the lift vector is "up" (more or less) to make the entry trajectory as shallow as possible and keep the g-loading as light as possible. A ballistic entry occurs when the lift vector is cancelled out by continuous rolling of the DM. And as Antonin notes, it is a bad day when the lift vector is "down". Does anyone know what the g-pulse looks like in that case, and also the thermal load?
Also, what is the DM roll rate in the ballistic case? I was describing it to some colleagues who were concerned by the possibility of an excessively fast roll rate, and I realized I didn't know what the actual rate was.
>From: fpspace-bounces at friends-partners.org
>[mailto:fpspace-bounces at friends-partners.org] On Behalf Of
>Mgr.Antonín Vítek, CSc.
>Sent: Friday, April 25, 2008 11:52 AM
>To: fpspace at friends-partners.org
>Cc: Geert Sassen
>Subject: Re: [FPSPACE] How tightly are the Soyuz modules
>> Thanks, Geert...
>> My understanding of the 'role of the roll' on the Soyuz DM
>is that it
>> nulls out the lift and helps focus the likely descent path into a
>> predictable area. The roll is not necessary for stabilization, which
>> is achieved through the center-of-mass distribution on the DM and
>> would occur even without a roll. In that case, however, the lift
>> vector could be in almost any direction, and linger there if
>> did not attain a significant roll rate. This could throw it over a
>> much wider area, making the ground search much more difficult. Does
>> this interpretation coincide or contradict the impressions others
>I agree, but this comment:
>Yes, the angle between symmetry axis of DM and the velocity
>vector (angle of attack) is mantained automaticaly as the
>rotation moment generated by aerodynamic force and inertia
>force is naturally zeroed, leading to the nonzero angle of
>attack. Nonzero angle of attack generates some "lifting force"
>(rather say "transverse force" - TF). Vector TF (its
>direction) is mantained by roll control. During the normal
>descent, the TF is at least in early stages of re-entry
>oriented "upwards" and this leads to slower descent rate -->
>smaller deceleration. Also heating rate is somewhat smaller
>(but total heat amount might be higher).
>If ballistic descent is chosen, TF vector makes 360 deg turns,
>effectively zeroing the TF.
>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.
>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.
More information about the FPSPACE