munck%Mitre-Bedford@sri-unix.UUCP (04/14/84)
This morning's Boston Globe had a large front-page picture of
Challenger "roaring flawlessly toward orbit yesterday atop its
booster rockets." The picture shows the Orbiter sitting on top
of the ET and SRBs and going up at an angle of about 40 degrees
from the horizontal.
I didn't see the launch, so I have two possibilities; either
Crip got tired of going up upside-down and decided to try a whole
new way, or the Globe printed the picture sideways. I've noticed
that broadcast commentators seem to be somewhat uncomfortable
with the standard launch position, which I assume is necessary
to keep the ET from ripping itself off.
-- Bob Munck (munck@mitre-bedford)al@ames-lm.UUCP (Al Globus) (04/17/84)
Launch attitide has not changed. Tidbit: the shuttle stack turns upside down shortly after lift off so the pilot can see the horizon and orient himself in case of an abort.
ks@astrovax.UUCP (Karl Stapelfeldt) (04/19/84)
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)ks@astrovax.UUCP (Karl Stapelfeldt) (04/21/84)
Phil Karn points out (quite correctly) that my earlier posting is essentially too restrictive. The orbiter can direct its thrust through the center of mass of the orbiter/ET combination and still accelerate upward and downrange. In an effort to salvage credibility, I note that the geometry of the combination (for a given flight path angle with respect to the orbiter wing plane, coincident with the thrust vector) dictates that the cross-sectional area of the vehicle to the direction of flight (and thus the aerodynamic drag force on the combination) is a minimum for the configuration that we are all used to seeing. Thus while the vehicle is still ascending through the atmosphere, it seems to me that there is a strong fuel economy incentive for the orbiter-below-ET attitude. However, once the vehicle has climbed above most of the atmosphere (essentially the last 3/4 of the ascent), there seems to be no restriction of this kind on the orientation of the orbiter with respect to the tank. Thanks for your correction, Phil.