[FPSPACE] Soyuz DM roll during ballistic entry profile

[Geert Sassen]

The shape of the soyuz cabine is such that with aerodynamic forces 
acting on it, it will automatically seek a stable attitude in which the 
vector connecting centre of volume and centre of gravity is pointing 
exactly in the direction of movement. However, as the centre of gravity 
is offset in regards to the 'vertikal' axis of the DM, this attitude 
will generate an angle of attack of less then 90 degrees, which again 
will result in a 'lift' vector (TF) perpendicular on the vertikal axis.

Now, when you roll the DM a certain angle, the line connecting centre of 
volume and centre of gravity will no longer be pointing in the flight 
direction, and in order to seek a new aerodynamically stable position, 
the aerodynamic forces will cause a certan pitch and/or yaw of the DM. 
This in turn results in a change of the angle of attack, which then in 
turn results in a change of the direction of the lift (TF). So 
basically, by only using your roll thrusters, you will cause a change in 
pitch and yaw and this will cause a change of angle of attack and thus 
of generated lift.

Using this system you will not need to use the pitch and yaw thrusters 
during descent, the only thrusters used in this part of the descent are 
the roll thrusters, by controlling the roll angle you automatically 
control pitch and yaw due to the offset CG and the shape of the DM. To 
say the DM is 3-axis stabilised during a normal 'lifting' descent is a 
bit misleading, in fact you are only controlling the roll angle and the 
aerodynamic forces do the rest.

Using this system it is extremely important that you know the exact 
location of the CG (this is why the soyuz has moving counterweights near 
the chairs which have to be set in a certain position), any error in the 
location of your CG will result in an targeting error in your landing 
location.

During a ballistic descent you need to 'null' the lift force or else you 
might get into a situation where the lift-vector is accidently making 
your trajectory steeper instead of more shallow. The only method to 
'null' the lift is by introducing a constant, slow, roll of the DM. Once 
again, due to the offset CG and the shape of the cabine, this will 
result in a pitch and yaw movement (precession), in fact you introduce a 
'corkscrew' motion to the DM during descent, however as the TF ('lift') 
vector is now turning 360 degree circles, the average effect will be a 
zero-lift ballistic trajectory.

Mgr.Antonín Vítek, CSc. wrote:
> 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
>   Narodni 3, CZ-11522 Praha 1 - Phone: +420/221 403 255, fax +420/224 240 611
> Home: Kytin 127, CZ-25210 Mnisek p. B., Czech Republic
> Phone: +420/318 592 865, cell +420/603 148 201 - Coord.: 14.2178 deg E,
> 49.8485 deg N, 442 m ASL
> My satellite home page: http://www.lib.cas.cz/www/space.40/index.html
> Home e-mail: avitek at seznam.cz
>
>
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