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From: ks@astrovax.UUCP (Karl Stapelfeldt)
Newsgroups: net.space
Subject: Re: Challenger's new launching attitude
Message-ID: <297@astrovax.UUCP>
Date: Wed, 18-Apr-84 23:04:34 EST
Article-I.D.: astrovax.297
Posted: Wed Apr 18 23:04:34 1984
Date-Received: Thu, 19-Apr-84 03:50:00 EST
References: <12075@sri-arpa.UUCP> <205@ames-lm.UUCP>
Organization: Princeton Univ. Astrophysics
Lines: 24


     The many references to the erroneous Boston Globe photo of the Challenger's
recent liftoff seem to be missing an important point.  If the thrusting force
of a rocket is to impart translational acceleration (and not angular acceler-
ation) to the vehicle, then it must thrust through its center of mass.
     Close inspection of the aft region of the space shuttle orbiter will show
that the main engines' neutral position points their thrust vector *below* the
plane of the orbiter's wings, as well as in the orbiter's forward direction.
This is necessary because the combined orbiter/external tank assembly has a
center of mass more nearly inside the tank than the orbiter.  (ET take-off
weight is something like 1 million pounds, whereas the orbiter's is around
250,000 pounds).  The main engines must gimbel during the ascent to keep
the thrust vector constantly pointing through the orbiter/ET center of mass;
remember that the ET mass is constantly changing due to fuel expenditure during
the ascent.
     The bottom line is that the orbiter *must* be below the tank during liftoff
so that the combined system accelerates upward and downrange.  Any configuration
with the orbiter above the tank could never produce thrust in the direction
of the local vertical (which, after all, is where the shuttle is designed to
go.)  Including the effects of SRB thrust direction (and aerodynamic forces)
will complicate the discussion for the first two minutes of flight, but
essentially produce comparable reasoning.

					      Karl Stapelfeldt
					      Princeton U. (and NASA ROTC)