[FPSPACE] Soyuz DM roll during ballistic entry profile

[Javier Casado]

Sorry for coming so late with comments about this matter (I had lots of
unread mails), but I disagree with the role of the roll that has been
proposed here for the Soyuz ballistic descent.


As Antonin said:


 "the angle between symmetry axis of DM and the velocity vector (angle of
attack) is maintained automatically 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)."


Ok, I agree with that. However, rolling the capsule doesn't mean you'll roll
the TF vector: since the DM is a revolution body, its roll does not affect
its aerodynamic properties (except for a very small Magnus effect that may
appear; this would create a small lateral velocity vector, in addition to
the lift called TF by Antonin). In other words: if the angle between the
capsule's velocity vector and its symmetry axis (that is, the angle of
attack) does not change, then the lift direction does not change, it doesn't
matter if the capsule rolls or not. So, rolling the capsule does not mean to
roll the lift vector, thus "neutralizing" it; the lift keeps pointing the
same direction, independently of the roll.

What is the purpose of the roll, then? In my opinion, it is simply for
stabilization. Jim said that "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", but this is not exactly true. I mean, yes,
the required attitude is naturally attained by means of the center-of-mass
distribution, but without some kind of added stabilization (that supplied by
the roll), this attitude would be much prone to perturbations, oscillations,
etc. The capsule is spin-stabilized during descent to maintain "stable" the
natural attitude attained by its center-of-mass position.


In a "normal" descent, however, the capsule is not spin-stabilized, but it
has a 3-axis stabilization achieved by its ACS thrusters. I suppose that
this is changed to spin-stabilization in the case of ballistic descent
simply for safety (no dependence on the ACS).


Regards,


Javier Casado
http://es.geocities.com/fjcasadop

<fpspace at friends-partners.org>