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Sources: NASA websites, the Space Shuttle Operator's Manual, launch footage and commentary
Second Stage Ascent
Second stage ascent runs from SRB separation through MECO. There are a few notable bits of information from NASA on what’s going on at this point.
Guidance
For this part of the flight, Shuttle guidance , nav and control software (GN&C) is calculating the actions required to bring the orbiter to a predefined set of conditions that describe MECO. These include specified distance from Earth in radii, flight path angle, attitude, and engine cutoff velocity as well as the nascent orbit’s ascending node, orbital inclination and longitude. There are conditionals in this loop as well; for example, there is an overriding-priority instruction to maintain an acceleration of 3g or less during the ascent. In order to reach this point, the GN&C software (which is run when the flight computers are set to ‘Major Mode 103’ on the commander/pilot’s keypads) runs in a cyclic mode.
This means that the guidance system continuously checks the vehicle’s parameters, and compares them to the desired MECO parameters. After each check, it runs a set of calculations that tell it whether any corrections need to be applied. If so, it applies those corrections, and then runs the check cycle again. This continues until the vehicle is on course and within parameters, at which point guidance is said to have converged (the desired state and measured state converge). Until this occurs, the estimate of time to MECO (TMECO) will fluctuate, steadying down to a regular countdown after convergence. The algorithm run during this loop is named (according to NASA) Powered Explicit Guidance 1 or PEG1.
Approx. T plus 7:40
At this point, the engines are throttled back to maintain 3g or less, as atmospheric drag and fuel load on the orbiter both lighten.
MECO minus 40 seconds
Around now, PEG-1 begins to ignore position and attitude constraints in favor of achieving the desired cutoff velocity . This prevents MECO from occurring during an attitude change, or attitude/position requirements from destabilizing the velocity window.
MECO minus 10 seconds
At this time, the MECO sequence begins onboard the orbiter, triggered by the Flight Computers.
MECO minus 3 seconds
The SSMEs are commanded to begin reducing thrust by 10%/second until they reach 65% rated thrust. They are held at this level for around seven seconds to ensure a clean shutdown, and then shut off.
MECO
MECO is a point of fairly furious activity as the vehicle begins to reconfigure itself from a booster to a spacecraft, as well as rids itself of the last bit of disposable structure. Several things occur almost simultaneously:
- Guidance is handed off to the Transition Digital Autopilot. During the coast phase, all vehicle attitude commands are locked out, and body rate damping (prevention of unwanted roll/pitch/yaw) is handled by the TDA using the Orbiter Reaction Control System.
- GN&C goes into a checkout sequence, which...
- Confirms SSME shutdown and sets MECO confirmed flag in the onboard computers
- Closes MPS prevalves for all main engines and sets another flag to trigger the External Tank(ET) separation sequence when that command executes
- The ET Separation software, triggered by this flag, does the following
- Closes 17-in orbiter/ET feed line LOx and liquid hydrogen disconnect valves
- Tests for good closure on the 17-inch lines
- Dead-faces the orbiter/ET interface (closes the lines down)
- Unlatches and retracts the 17-inch line disconnects into the orbiter’s aft fuselage
- Arms three orbiter/ET explosive bolt controllers, one forward and two aft
- Fires the ET Lox tank tumble vent valve
In an automatically handled ET separation (the norm) the onboard computers command ET separation 18 seconds after actual MECO. If any of the tests during this phase fail (valves don’t close, etc. etc) the separation will not occur unless the problem comes back into tolerance or the flight crew overrides the inhibit on the sequence. They would do this by setting the ET Separation Switch on orbiter control panel C3 to ‘manual’ and then pushing the ET Separation Push Button.
The doors in the orbiter’s hull that permit passage of the 17-inch LOx and liquid hydrogen feed lines are closed by the computers (or by the crew if manual override is necessary). The delay between the start of the separation sequence and the actual separation of the ET is around eighteen seconds. After separation, the slight thrust produced by excess LOx escaping from the tumble vent induces a rotation of the tank of between 10 and 45 degrees per second pitch. At this point, the orbiter and the tank are inverted relative to the Earth; therefore, the ET’s nose rotates ‘up’ and away from the orbiter. As soon as this begins, the GN&C re-enables the reaction control system and allows the TDA to command an RCS burn on the four forward and six aft negative-Z jets so as to achieve a four foot per second vertical separation from the tank (the orbiter translates ‘down’ from the tank, according to a ground observer). Once the orbiter has gained sufficient separation, the ET separation complete flag is set in the onboard computers.
During some missions, these parameters may be changed in order to allow the orbiter to photograph (or videotape) the external tank as the shuttle pulls away from it. There is an external tank camera mounted on the shuttle; in order to reach the proper viewing angle, the crew will usually use the translational hand controller to command a positive-X vector change for approximately 11 seconds.
The next phase is Orbital Insertion; the onboard computers (once the ET separation flag is set) change over to Major Mode 104 (OMS-1 insertion) automatically. Mission Control must issue approval (go/no-go) for the OMS-1 insertion via the data links before this can proceed. This is because they are still responsible for determining if an Abort to Orbit or Abort Once Around is required. During the ascent, Mission Control will update the crew on their abort possibilities with voice checks; “negative return” means that the shuttle has passed the point where it can make a Return to Launch Site abort.
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