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From: BARRY@RICEVM1.BITNET
Newsgroups: sci.space.shuttle
Subject: Re: Direct ascent trajectory
Message-ID: <1641BARRY@RICEVM1>
Date: 14 Jan 90 21:17:56 GMT
References: <6124@uhccux.uhcc.hawaii.edu> <1990Jan12.213536.7453@utzoo.uucp>
Organization: Rice University - ICSA
Lines: 32

The OMS-1 maneuver is by no means obsolete.  The need for an OMS-1
is determined by the flight design -- orbital characteristics, ET-impact
footprint (hopefully it lands in the Indian Ocean), propellant loadings,
etc.  An OMS-1 is almost always required, regardless of the flight plan,
if a MECO underspeed occurs or an Abort to Orbit trajectory is selected.
OMS-1 is not performed with the ET attached (as I interpreted Henry's
posting).  In fact, for TAL cases, an OMS burn is performed in order to
avoid the probable violent rupture of the ET when it reenters the
atmosphere.  This, however, is not an "OMS-1" maneuver.

An OMS-2 maneuver is always required.  In some problematic cases, the
maneuver may be performed with the RCS, though it might be called an
OMS maneuver.  Consider the upcoming HST deploy mission, STS-31.  The
deploy altitude is 330nm, which requires protecting about 530fps
velocity change in order to deorbit to "steep" targets using the OMS
engines.  The OMS-2 maneuver is on the order of 480fps to raise
perigee to about 310nm.  These two maneuvers require a full OMS load!
Therefore, an OMS-1 cannot be performed, so the ascent must be a direct
insertion.  Post OMS-2, the orbit will be 330x310nm.  To circularize the
orbit, an RCS-1 maneuver will be performed a few hours after OMS-2 in
order to achieve 330x330nm.  Some folks refer to this maneuver as an
"OMS-3" maneuver, though it uses RCS thrusters and RCS propellant.

Hope this help clear things up.

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Matthew R. Barry
Department of Mechanical Engineering      Guidance and Propulsion Systems
Rice University                           NASA/Johnson Space Center DF63
Houston, TX  77252                        Houston, TX  77058
barry@ricevm1.rice.edu                    mbarry@nasamail.nasa.gov
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