Back to the Terminal Count phase

The Space Shuttle mission sequence is divided into phases. The phase which is perhaps the most visually impressive is the First Stage Ascent. This progresses from SRB ignition at T minus 0.3 seconds to SRB separation, also known as SRB Staging. It involves the most son et lumiere of the whole exciting process. The event timing during the ascent may change from mission to mission depending on the profile and load, so I won’t be using T-minus notation unless NASA has provided it; instead, I’ll use an ordered list of events.

For all acronym expansion that isn't done inline, just follow the hardlinks.

Sources: NASA websites, the Space Shuttle Operator's Manual, launch footage and commentary

First Stage Ascent

SRB Ignition

As indicated, the SRBs ignite 0.3 seconds before liftoff. The SSMEs are at 100% rated power and gimbaled to launch position; all connections with the vehicle retract or are dropped (launch umbilical, crew access arm, etc.) and when the boosters go, POW. The Shuttle clears the tower in approximately seven seconds. During this time, the vehicle rises in attitude lock, making any necessary corrections to remain in vertical flight.

When the nozzles of the SRBs are approximately 41 feet above the lightning rod atop the launch pad (the highest point on the structure) the Shuttle begins its preplanned roll maneuver to bring the vehicle to its ascent attitude.

Roll Maneuver

The roll maneuver actually involves roll, pitch and yaw maneuvers. At the end of the maneuver the vehicle is positioned with wings level relative to the launch pad, upside down, ascending at a pitch of approximately 78 degrees. Put another way, the Shuttle has spun 180 degrees so that the tail and upper body of the orbiter are to the north, and then angled north (with the orbiter ‘down’) approximately twelve degrees off vertical. This maneuver is complete by approximately T plus 20 seconds.

T to T plus 90 seconds: Load Management

During the first minute and a half of flight, the flight computers are programmed to minimize loading on the vehicle surfaces at the expense of maintaining a precise flight profile. To do this, the computer has a table of elevon positions indexed to vehicle structural loads; it will assume the elevon positions indicated to relieve stresses that exceed flight parameters. There are load sensors on the external surfaces of the orbiter, external tank and SRBs.

Typically, the maximum load occurs on the wings; the computer will maneuver the elevons (thus changing the pitch attitude slightly) in order to protect the wings and speed brakes and rudder alignment.

In order to manage the dynamic loading, there is a preplanned throttle change at approximately T plus 26 seconds. The SSMEs are throttled down to approximately 75% power as the vehicle approaches Max Q, in order to avoid exceeding pressure limits; at approximately T plus 60 seconds the main engines are throttled back up to 104% power as the Shuttle rises into thinner atmosphere. One of the most recognizable control messages during launch is the phrase ”Go at throttle up;” this is unfortunately due to the fact that it was immediately following this message that mission STS-51L (The orbiter Challenger) was lost during ascent.

Max Q, approximately 580 pounds per square foot of vehicle surface, occurs shortly after throttle-up, and is announced over communications links. During this phase, the Shuttle achieves its maximum onboard acceleration of approximately 3 gee.

Early in this phase, the preferred intact abort mode is the Return To Launch Site abort.

During first stage ascent, thrust vectoring is used to control vehicle attitude and rate of attitude change. All through the first stage, control of the vehicle attitude is maintained by using the main propulsion system processor of the ascent digital autopilot. This unit uses body attitude error signals generated by deviations from the planned profile in IMU measurements to generate a set of required correction signals. During this phase, these corrections are fed into the ascent thrust vector controller system (four units) and used to produce pitch and yaw corrections for the SSME gimbal servoactuators, as well as for the SRB nozzle actuators. Corrections for the SRB processor of the ascent digital autopilot are referred to as rock and tilt as opposed to pitch and yaw in order to differentiate them from commands intended for the main engine thrust vectoring system.

The signals used to determine rate are drawn from gyros in the SRBs until the SRB guidance system is commanded to ‘null’ itself in preparation for SRB staging, at which point all guidance functions revert to orbiter onboard systems.

First Stage Ascent Guidance

Guidance and navigation during this phase is performed by onboard computers utilizing a preplanned (loaded before launch) set of tables that index relative velocity vectors with appropriate roll, pitch, yaw and throttle values for that phase of the ascent. These tables are consulted continuously. In the event of a main engine failure, there are three additional tables which are generated assuming the failure of each engine respectively – so if engine one fails, there is a table for flight settings after that point which throttles up engines two and three and corrects for the unbalanced thrust, as well as changes from normal ascent profile to one of the various abort profiles.

During this phase of the ascent, the preferred intact abort mode goes from RTLS to a Trans-Atlantic Abort Landing (TAL), and after that, as altitude and speed permit, to Abort Once Around (AOA). As soon as possible, however, the preferred mode is changed to Abort To Orbit (ATO). The Shuttle can reach orbit (and has) with one main engine shut down. Mission STS-51F was forced to an ATO mode but managed to complete its mission nonetheless.

During the first stage ascent, the crew onboard has little to do save monitor some basic indicators for vehicle health. They can check to ensure that the SRB chamber pressures are higher than 50 psi, which indicates operation; they can monitor throttle settings and dynamic load indicators. Ground controllers monitor engine and system status through telemetry links. Guidance is generated entirely by the ascent digital autopilot and its associated systems; however, if the crew selects Control Stick Steering, the orbiter rotational hand controllers are activated at the commander and pilot stations. If the stick is left centered (known as in detent) then the guidance systems continue to generate all requisite navigation signals. If the controller is moved outside detent, then its input is summed with guidance attitude input to produce a navigation signal; the severity of the maneuver is determined by the degree of offset on the controller. Once the controller is released, guidance reverts to the onboard systems and profiles. While the controller is outside detent, the load management software stops operating. All maneuvers continue to utilize thrust vectoring; no aerosurfaces are used.

SRB Staging

At approximately T plus 2 minutes the SRB staging occurs. The Orbiter General Purpose Computers (GPCs) watch for SRB chamber pressure to fall below 40 psi in both chambers within 4.3 seconds, indicating burnout. They also check that vehicle velocity and attitude are within appropriate ranges to ensure that the SRBs won’t hit the vehicle after separation; a three-axis attitude hold is commanded by the reaction control system for four seconds. The SRB nozzles are positioned to null, and the flight control system is switched to accept rate inputs from the gyros onboard the orbiter. Pyrotechnics are triggered to physically separate the SRBs from the external tank, and small thrusters fire to push the noses of the SRBs away from the vehicle. This causes the distinctive ‘arc’ outwards as they separate; low-power remnant thrust from the SRB motors pushes them away and out.

Four seconds after SRB separation, second-stage main engine guidance takes over. In addition, if a main engine shutdown is detected, the failed engine is positioned to null, and trim changes are commanded on the remaining engines.

…ad astra per aspera…

Back to Terminal Count phase | On to Second Stage Ascent

Log in or registerto write something here or to contact authors.