Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Direct ascent trajectory
Message-ID: <1990Jan12.213536.7453@utzoo.uucp>
Organization: U of Toronto Zoology
References: <6124@uhccux.uhcc.hawaii.edu>
Date: Fri, 12 Jan 90 21:35:36 GMT

In article <6124@uhccux.uhcc.hawaii.edu> goldader@uhccux.uhcc.hawaii.edu (Jeff Goldader) writes:
>Does anyone know what advantages the direct ascent launch trajectory 
>gives? I've heard of it being used several times, but never really 
>heard *why* it's used.  I do know it makes the OMS-1 burn unnecessary, 
>which leads me to believe it gives a higher apogee on the initial 
>orbit, but that's about it.

I was hoping somebody who knew for sure would answer this, but since nobody
has...  The old OMS-1 burn was a result of cutting off the main engines
on reaching a very low, in fact too low, orbit.  This gave better control
of where the external tank ended up, but required an immediate OMS burn
to prevent the orbiter from reentering with the tank.  Essentially this
amount to flying a "dip" maneuver, which cost something in itself, and
doing the last little bit of the ascent with the less-efficient OMS
engines, which ran up the cost further.  Going straight into orbit costs
less in fuel and hence gives either greater payload or a higher orbit.

I'd rather not be quizzed on the details, since I don't understand it
very well myself.  (In particular, at first glance I don't understand
why the tank ends up in the Pacific instead of doing one orbit and going
down into the Atlantic at perigee.)
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu