Re: Dictionary please

From: Vitek, Antonin (avitek@lib.cas.cz)
Date: Thu May 30 2002 - 09:19:43 EDT

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    Jonathan T Wojack wrote:
    <snip> 
    
    > Is the Reaction Control Sytem (RCS) used only for minor changes in the
    > Orbiter's flight path (such as on final approach to the ISS) ?
    
    Correct. But the maneuver done during course of rendezvous with the ISS
    might be don o varioul levels of "force": either with two OMS engines,
    or with the single OMS engine only, with various selections of RCS
    engines. Terminal approach might be done also using veriner engines with
    extreme low thrust (see bellow).
    Special type of mauvering in last meters of approach (for you in
    Anglo-Saxon worl "in last feet...") or immediately after separation form
    ISS called "low-Z mode" is made with combination of forward and aft
    "horizontally" firing RCS combination. Their axes are not exactly 
    paralla to the log axis of robeter; therefoe their thrust (force) is not
    fully compensated and very small fraction of resulting force is left in
    the direction perpendiculary to the wings of the orbiter.
    The information about actual combination of engines used for a
    particular maneuver of shuttle is hidden eg. at orbatl info page
    
    http://www.spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/SHUTTLE/SVPOST.html
    
    The maneuvers (planned) are on the top of this page under the heading
    
    IMPULSIVE TIG (GMT)   M50 DVx(FPS)      LVLH DVx(FPS)      DVmag(FPS) 
    IMPULSIVE TIG (MET)   M50 DVy(FPS)      LVLH DVy(FPS)      Invar Sph HA
    DT                    M50 DVz(FPS)      LVLH DVz(FPS)      Invar Sph HP 
    ------------------------------------------------------------------------
    099/21:21:06.172           2.5               5.4              6.2    
    001/00:36:47.225           5.6               3.2              122.8  
    000/00:00:10.276          -1.2              -0.0              85.2  
    
    as blocks of three lines. The rightmost value in the first line of each
    row triplet (here 6.2) is total maneuver magnitude in fps, the leftmost
    value in the third line (here 000/00:00:10.276) is the burn duration (in
    hours:minutes:seconds). Dividing the first value by the second, ie. 
    6.2/10.276 = 0.603
    you obtain mean acceleration in feet per second sguared. As the orbiter
    mass doring the flight varies, the values are slightly  changing, but: 
    if this value is approx. 0.6 to 0.8 ft/s2 - one OMS is used
    around 1.5 two OMS are used
    less than 0.3 some combination of RCS is used.
    
    Engine type         thrust
    OMS           26.7 kN (kilonewton) = 6000 lbf (pounds of force)
    RCS           3.37 kN = 758 lbf
    Vernier       111 N = 25 lbf
    
    Theoretical acxcelerations are (witr orbiter mass of 250000 lbs = 113400
    kg, ie. approx. orbit insertion mass)
    one OMS: 0.24 m/s2 = 0.77 ft/s2
    two OMS: 0.49 m/s2 = 1.54 ft/s2
    one RCS: 0.03 m/s2 = 0.10 ft/s2
    
    The accelerations are about 10 % higher in final parts of flight, when
    cargo (eg. new modules) are left on station.
    Excuse of OT message, but some education...?
    
    Tony
    
    
    Mgr. Antonin Vitek, CSc.
    Office: Main Library, Academy of Sciences of the Czech Republic
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