[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.

John Charles
Houston, Texas

>-----Original Message-----
>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 
>bolted together?
>> 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 
>the cabin 
>> 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 
>> have?
>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.

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