[FPSPACE] Soyuz DM roll during ballistic entry profile

[Javier Casado]

Understood, Antonin, thank you. So we are not talking about rolling around
the simmetry axis, but around the velocity vector. In that way, I agree, the
lift is neutralized. However, it seems to be a quite difficult maneuver,
isn't it? I mean to roll the capsule around an axis that is not coincident
with any of the main inertia axes of the spacecraft... Are you sure of this,
or is it a guess? I mean the rolling around the velocity vector...

Unless you know positively that this is the way it is done (I don't really
know), it seems to me more probable the other possibility raised by me. Ok,
you say that 17 deg/sec is rather slow, and it is. But there are
spin-stabilized spacecrafts that roll at only 5 rpm (17 deg/sec = 3 rpm
aprox.), so it seems not to be so slow after all... Anyway, I confess I am
just guessing.

Regards,

Javier Casado


2008/5/5, Mgr. Antonín Vítek, CSc. <avitek at lib.cas.cz>:
>
>
> The "roll" is rather slow - 17 deg/sec and exyactly it is not simple roll,
> but precession o symmetry axis aroun velocity vector. Due to the fact,
> that CG has som offset from the symmetry axis of DM. ThereforeDuring the
> precession rotation the TF actually makes 360 deg turns with the above
> said (17 deg/sec) angular velocity. As a result, the trajectory is
> "ballistic" i.e. without any lateral force. As a resut, the descent it
> steeper, landing spot in nearer to the point orf EI and the landind time
> is aearlier than in normal (i.e. aerodynamically controlled) descent
>
> > 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>
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> >
> >
>
>
>
> Mgr. Antonin Vitek, CSc.
>
> Office: Main Library, Academy of Sciences of the Czech Republic
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> 611
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>
>
>