Orbit Insertion
Following
MECO, the orbiter's altitude and velocity will vary depending on
the mission requirements. For example, an 80-nautical-mile (92-statute-mile)
altitude with an inertial velocity of approximately 25,660 feet
per second would place the orbiter in a suborbital trajectory so
that the ET would enter following separation. In order to boost
the orbiter to a viable orbit that does not degrade appreciably
during the mission and satisfies mission objectives, two propulsive
thrusting periods are made with the OMS engines, except in the case
of a direct insertion, when only one OMS thrusting period is required
to circularize the orbit. The first thrusting period is referred
to as OMS-1 and boosts the orbiter to the desired apogee; the second
burn is called OMS-2 and typically circularizes the orbit.
The optimal
orbital altitude (the altitude that satisfies mission and payload
goals) is determined before launch. During flight, however, problems,
such as main engine and SRB performance loss and OMS propellant
leaks or certain electrical power system failures, may prevent the
vehicle from achieving the optimal orbit. In these cases, the OMS-1
and OMS-2 burns would be changed to compensate for the failure by
selecting a delayed OMS-1, AOA or ATO abort option.
The main events
that occur during the orbit insertion phase include execution of
the OMS-1 thrusting period, typically about two minutes after MECO;
an MPS propellant dump, which begins during OMS-1; positioning of
the main engine nozzles for entry; shutdown of the three auxiliary
power units; MPS power-down; and MPS vacuum inerting to ensure that
all traces of MPS propellants are vented to space.
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