Re: Dictionary please

From: Ed Davies (edavies@nildram.co.uk)
Date: Wed May 29 2002 - 12:01:25 EDT

  • Next message: Sue Wheatley: "Dictionary Education"

    Sue Wheatley wrote:
    > 
    >  What does OMS-2 mean? All the internet sites seem happy to use the term,
    > but I can't find its meaning.  Where can I get a good abbreviation
    > dictionary that covers satellite observing terms?
    > ---sue
    
    Here are some exerpts from the STS Plus program documentation which
    should make it clear.
    
    >      STSPLUS can generate launch simulation for Space Shuttle launches from 
    > Cape Canaveral, Florida. Prior to Version 9509, STSPLUS used only 2-Line 
    > Elements ("TLEs") and assumed that whatever satellite was being displayed 
    > "had always been in orbit" and that the current position could be 
    > accurately calculated from the TLEs, not quite in line with the typical 
    > situation for a Space Shuttle launch! Many STSPLUS viewers were confused by 
    > the fact that prior to launch, the orbiter appeared to be off the coast of 
    > Baja California rather than sitting quietly on Pad 39A or 39B at Cape 
    > Canaveral. The STSPLUS simulation did not catch up with the true orbit 
    > until approximately the OMS-2 burn (Orbital Maneuvering System, Burn #2) 
    > which typically occurs about 35 to 45 minutes after launch. The OMS-1 burn 
    > is not required for a nominal ascent trajectory but its place and number 
    > are reserved in the STS Timeline regardless of whether or not it takes 
    > place. I am always relieved by the CapCom call "OMS-1 NOT REQUIRED" because 
    > that means that the Solid Rocket Boosters (SRBs) and the Main Engines 
    > (SSMEs) did their job correctly.
    
    > ....
    
    >      Estimated pre-launch TLEs are usually prepared well in advance of a 
    > flight by Gil Carman or others at the NASA Johnson Space Center, Flight 
    > Dynamics, in Houston, Texas. These TLEs represent the planned orbit for the 
    > Space Shuttle for a nominal launch and are calculated for a time (the 
    > "Epoch" of the TLEs) following the OMS-2 burn. It is the OMS-2 burn which 
    > circularizes the orbit at the intended altitude; prior to OMS-2, the 
    > orbiter is in a highly elliptical orbit whose perigee (low point, 
    > technically the minimum orbital distance from the center of the Earth) is 
    > some tens of miles above the surface and whose apogee (high point) is at or 
    > near the planned altitude. The OMS-2 burn occurs at apogee and in effect 
    > raises the perigee to the planned altitude. With apogee and perigee both at 
    > approximately the same altitude, a nearly circular orbit is the result. 
    > Without the OMS-2 burn the orbit is not sustainable, dipping too far into 
    > the Earth's atmosphere, and the orbiter would have to make a landing 
    > immediately.
    
    In other words, the OMS-2 burn happens at the high point about half an
    orbit after launch - probably somewhere over the Indian ocean.
    
    I think that Shuttle operations have changed a bit since David Ransom
    wrote this in that now the OMS is, I understand, used during the ascent.  
    It's called OMS Assist and is used to help get heavy loads into the
    relatively high inclination orbit of the ISS.  I might be confused 
    here though as it is possible that OMS Assist refers to the use of
    the OMS-2 burn.
    
    
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