Path: utzoo!attcan!uunet!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!elroy.jpl.nasa.gov!zardoz.cpd.com!dhw68k!ofa123!rick From: rick@ofa123.fidonet.org (Rick Ellis) Newsgroups: sci.space.shuttle Subject: Re: Why is the orbiter launched upside down? Message-ID: <1168.26F67790@ofa123.fidonet.org> Date: 18 Sep 90 17:57:43 GMT Organization: Universal Electronics Inc., Anaheim CA Lines: 42 On Gregory J Schaffer writes: GJS> Just curious...where on the vehicle is this stress lowered, and why? GJS> The orbiter's wings produce lift while en route to orbit...being GJS> upside down *ADDS* this force to the direction of gravity, creating what GJS> I would think would be *MORE* stress on the orbiter. Am I missing GJS> something? Maybe it is, if you'll pardon the expression, the "natural GJS> tendency" of the orbiter to "hang" on the rest of the assembly. From NSTS-7: The vehicle lifts off the pad 0.3 second after SRB ignition, rising vertically in attitude hold until the SRBs' nozzles clear the lightning rod tower by approximately 41 feet. The vehicle begins a combined roll, pitch and yaw maneuver that positions the orbiter head down, with wings level and aligned with the launch pad. The orbiter flies upside down during the ascent phase. This orientation, together with trajectory shaping, establishes a trim angle of attack that is favorable for aerodynamic loads during the region of high dynamic pressure, resulting in a net positive load factor, as well as providing the flight crew with use of the ground as a visual reference. By about 20 seconds after lift-off, the vehicle is at 180 degrees roll and 78 degrees pitch. During the first 90 seconds of flight, the flight control system provides load relief by making adjustments to reduce vehicle loads at the expense of maintaining a precise trajectory profile. A special schedule of elevon position with respect to velocity is followed to protect the wings from excessive loads and to hold the body flap and rudder/speed brake in place. The surface position indicator displays the position of the aerosurfaces. To keep the dynamic pressure on the vehicle below a specified level, on the order of 580 pounds per square foot (max q), the main engines are throttled down at approximately 26 seconds and throttled back up at approximately 60 seconds. This also reduces heating on the vehicle. Because of the throttling at this time, the term "thrust bucket" evolved. Maximum dynamic pressure occurs shortly after throttle up. -- Rick Ellis Internet: rick@ofa123.fidonet.org --------------------------------------------------------------------------