SAX: Launch Details

SAX has been successfully launched on April 30 04:31 GMT, at the very first attempt! It has been placed in orbit by an Atlas I launch vehicle, consisting of

Orbit requirements

The SAX mission uses the parking orbit ascent trajectory design to place the spacecraft into a 600 Km circular, near equatorial final orbit by the Atlas I launch vehicle. Atlas stage performance has been evaluated using the Inflight Retargeting (IFR) algorithm in the flight computer at the beginning of the Centaur first burn. The final orbit inclination target (which has been used in the Centaur second burn) has been adjusted based on this knowledge.

A high performing Atlas booster will result in a final orbit inclination lower than nominal and vice versa. The spacecraft's design lifetime is approximately two years, but is expected to function for at least four years. In the figure below we show the orbital planes and maneuvers used for the SAX launch.

Orbital Planes and Maneuvers Used for SAX Launch

Atlas Mission Profile

The AC-78 has been the 100th launch of an Atlas Centaur combination, signifying an historic milestone in the successful Atlas program since its first flight in 1957. The mission ascent profile is shown schematically in the figure below. Key flight events, called mark events, are reported from Building AE Telemetry Lab during flights.

Mission Ascent Profile

The SAX mission uses a standard Atlas I ascent profile. The flight begins with a short vertical rise, during which the vehicle rolls to the flight azimuth of The SAX mission uses a standard Atlas I ascent profile. The flight begins with a short vertical rise, during which the vehicle rolls to the flight azimuth of 105.1 degrees. At an altitude of about 300 m, Atlas starts its initial pitch-over phase. At about 2500 m, it enters into a nominal open-loop zero-pitch and zero-yaw angle-of-attack phase to minimize aerodynamical loads. The launch day zero angle-of-attack steering phase is implemented through the open-loop ADDJUST designed steering program, which enhances launch availability by reducing wind-induced flight loads. Booster engine cutoffs (BECO) occurs when the INU senses an axial load factor of 5.5 g. Booster phase flight ends with jettison of the booster engines and associated thrust structure.

The Atlas sustainer and Centaur phases of flight are controlled by the Centaur's closed-loop guidance. The Centaur insulation panels and payload fairing are jettisoned during Atlas sustainer solo phase. Atlas sustainer flight continues to propellant depletion.

After sustainer engine cutoff (SECO), the Centaur stage separates and ignites its main engines (MES1). The Centaur first burn, the longer of the two Centaur firings, injects the vehicle into a slightly eccentric parking orbit.

Following Centaur first main engine cutoff (MECO1), Centaur and spacecraft enter a coast period. At a guidance-calculated start time, just north of the equator, The Centaur main engines are re-ignited (MES2) and the vehicle is steered by guidance into the required final orbit.

The SAX spacecraft requires the Centaur to provide a separation attitude such that the spacecraft +X axis is pointing to the Sun, the spacecraft +Z axis (Centaur +XB) is pointing North, and the spacecraft +Y axis is normal to the North Pole vector. The figure below shows the marked events along the projected SAX orbit onto the Earth.

SAX Subvehicle Trace

This material has been taken by the brochure "SAX: AC-78 Mission Overview" prepared by Lockheed Martin, and distributed during the launch in Cape Canaveral.

This file was last modified on Friday, 28-Nov-1997 08:52:07 CET by Mauro Orlandini

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