[FPSPACE] Soyuz DM roll during ballistic entry profile

[palladium at aol.com]

Avete, amici!

Following the Soyuz near-disaster (which was, by the way, almost 
exactly duplicated the Soyuz reentry snafu recounted in my novel, Red 
Moon, which itself was based on the Soyuz 5 anomaly), a couple of 
questions came to mind:

(a) How much time did the crew spend pulling high Gs in "eyeballs out" 
mode? Did any of them show physical signs of such stress-- i.e. 
redenned eyes or skin?

(b) Is the orbital module jettisoned before or after retrofire?

(c) Could the steeper descent and higher re-entry speed translate into 
a higher landing (or "impact") speed as well, accounting for the big 
"dent" observed in the tundra? Or would this be effectively nulled out 
once the parachute opens?

D.S. Michaels

-----Original Message-----
From: Javier Casado <fjcasadop at yahoo.es>
To: Geert Sassen <geert at navtools.nl>
Cc: fpspace at friends-partners.org; "Mgr.Antonín Vítek,    CSc." 
<avitek at lib.cas.cz>
Sent: Mon, 5 May 2008 9:35 am
Subject: Re: [FPSPACE] Soyuz DM roll during ballistic entry profile

Absolutely clear now, thanks to both, Geert and Antonin.

In fact, as I see, the complication is not in the maneuver to get this
precession movement, but in the capsule design. What they indirectly 
achieve
with that CG offset is (in addition to the proper "natural" attitude for
reentry) to get a main inertia axis for the vehicle that is not 
coincident
with its simmetry axis. This way, a simple firing of the "roll" 
thrusters
produces a precession movement instead of pure roll. Ingenious.

It is amazing to me how those old designers did so much with so little.
Today, virtually no designer could calculate an inertia axis without 
CAD!

Regards,

Javier

2008/5/5 Geert Sassen <geert at navtools.nl>:

> 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 Casadohttp://es.geocities.com/fjcasadop
> <fpspace at friends-partners.org> <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
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>
>
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