Some important events in the Shuttles launch sequence that relate to the questions about main engine shutdown and the release of the holddown bolts: T -10.0 sec. The "Go for main engine start" command is issued by the Ground Launch Sequencer (GLS). (This is the last command issued by the GLS, however it retains the capability to command main engine stop until just prior to SRB ignition.) Flares are now ignited under the main engines to burn away any residual hydrogen gas that may be in or around the main engine nozzles. This hydrogen burnoff system prevents damage to the orbiter and its engines. T -9.5 sec. The Shuttle's flight computers order the opening of valves allowing the flow of liquid oxygen & liquid hydrogen to the main engine's turbopumps. T -6.6 sec. The three main engines are ignited at 120 millisecond intervals. The engines throttle up to 90 percent power in less than three seconds. T -3.0 sec If the Shuttle's flight computers determine that all three engines are at the required 90 percent thrust, then the SRB ignition sequence starts. If not, then main engine shutdown is activated by the flight computers. T -0.0 sec. The holddownn explosive bolts and the T-0 umbilical explosive bolts are detonated by command of the flight computers. The SRBs are ignited by the flight computers and the shuttle is committed to launch. There are really no big problems related to shutting down the main engines prior to SRB ignition. There are two ways this can happen. The GLS computer, which monitors hundreds of parameters in and around the orbiter, will shutdown the main engines if something is out of normal tolerance, or the Shuttle's flight computer system will shutdown the engines if something in the Shuttle is out of tolerance or if the engines are not producing correct thrust. As far as the holddown bolts failing to let go, it is a remote problem as the system is highly redundant in several ways. There are eight holddown bolts with explosive nuts holding the Shuttle in place. (Four at the base of each SRB.) Each nut has separate sets of explosive charges controlled by three separate detonation systems. The entire system is tested hours before launch and is monitored throughout the launch sequence. The odds of a problem are so remote as to be virtually non-existent. In the event of a failure the joints are designed to shear causing minimum damage to the SRBs. As to the question about main engine failures and abort procedures. If the Shuttle looses one or more main engines or OMS (Orbital Maneuvering System) thrusters, or if a critical system failure occurs during accent, the mission may need to be aborted. There are four different intact abort modes that can be utilized depending on the problem. They are: o Return To Launch Site (RTLS) o Trans-Atlantic Abort Landing (TAL) o Abort Once Around (AOA) o Abort To Orbit (ATO) In an abort situation, the type and time of failure determines which abort mode will be used. There is a definite order of preference for an abort. In case of loss of performance (i.e. main engine failure, OMS failure, etc.) the order of preference (most to least) is: ATO, AOA, TAL, RTLS. The selection of abort mode is based on the most preferred mode that can still be achieved with remaining performance. In case of a critical system failure (i.e. cabin pressure, orbiter cooling system, etc.) ending the mission as soon as possible is the priority. In this case TAL or RTLS (in that order) are the preferred modes. Some actual instances of the problems discussed: o June 26, 1984: STS 41-D launch scrubbed following main engine shutdown at T -4.0 sec. o July 12, 1985: STS 51-F launch scrubbed at T -3.0 sec. due to main engine shutdown. o July 29, 1985: STS 51-F Abort To Orbit (ATO) executed after one main engine was shutdown by computer due to a faulty temperature sensor. -- Bob Day.