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From: moe@nuchat.UUCP (Norman C. Kluksdahl)
Newsgroups: sci.space.shuttle
Subject: Re: Direct ascent trajectory
Message-ID: <18419@nuchat.UUCP>
Date: 14 Jan 90 04:31:13 GMT
References: <6124@uhccux.uhcc.hawaii.edu> <1990Jan12.213536.7453@utzoo.uucp>
Reply-To: moe@nuchat.UUCP (Norman C. Kluksdahl)
Organization: Houston Public Access
Lines: 40

In article <1990Jan12.213536.7453@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>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?
>
>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.)

I'm not absolutely certain of the details myself, but as near as I can
figure, the direct ascent trajectory results in a highly elliptical
orbit, which requires the OMS 2 burn to circularize.  Now, from my
reference on ballistic trajectories, the perigee and apogee can be
uniquely determined from the flight-path-angle at burnout and the velocity.
We know that the shuttle must do an OMS-2 to circularize, but the tank
doesn't.  Thus, from an approximation of the altitude of non-negligible
atmospheric drag, one can determine the 'range angle' of the ET's trajectory.
Simple ballistic missile calculation.  

What does all that mean?  My interpretation is that the flight-path-angle
at burnout is relativly near zero, giving the ET a very large range angle,
and thus putting the ET down in the Pacific.  Remember, just because
the burnout is over the Atlantic, the perigee might be elsewhere, especially
if the flight-path-angle is nonzero at burnout.

(Ref: Bate, Mueller, & White, "Fundamentals of Astrodynamics"  Dover Press,
ISBN 0-486-60061-0.  Copyright 1971.  From the USAF Academy, hence the
details on ballistic trajectories.  Good book.  theory and examples,
unsolved excercises, but unfortunately no answers.)

=====================================================================
Norman Kluksdahl  
  ...!nuchat!moe