Abort Control Sequences

The abort control sequence is the software that manages, among other items, the OMS and aft RCS configuration and thrusting periods during ascent aborts to improve performance or to consume OMS and aft RCS propellants for orbiter center-of-gravity control.

Premission-determined parameters are provided for the OMS and aft RCS thrusting periods during aborts since the propellant loading and orbiter center of gravity vary with each mission.

The premission-determined parameters for the abort-to-orbit thrusting period are modified during flight, based on the vehicle velocity at abort initiation. The premission-determined parameters for abort once around are grouped with different values for early or late AOA. The return-to-launch-site parameters are contained in a single table.

The abort control sequence is available in OPS 1 and 6 and is initiated at SRB separation if selected before then or at the time of selection if after SRB separation.

ATO and AOA Aborts

The OMS and aft RCS begin thrusting as soon as an ATO or AOA is initiated with one main engine out.

For some aborts, an OMS-to-aft-RCS interconnect is not desired. A parallel aft RCS plus X thrusting period using aft RCS propellant and the four aft RCS plus X thrusters will be performed during the OMS-1 thrusting period to achieve the desired orbit. If a plus X aft RCS thrusting period is required before main engine cutoff, the abort control sequence will command the four aft plus X RCS jets on if vehicle acceleration is greater than 0.8 g and will monitor the RCS cutoff time to terminate the thrusting period. If an RCS propellant dump (burn) is required before MECO and vehicle acceleration is greater than 1.8 g, the abort control sequence will command an eight-aft-RCS-jet null thrust and monitor the RCS cutoff time to terminate the thrusting period.

In other abort cases, an OMS-to-aft-RCS interconnect is desired. This thrusting is performed with the OMS and four aft RCS plus X thrusters to consume OMS propellant for orbiter center-of-gravity control. More aft RCS jets can be commanded if needed to increase OMS propellant usage. For example, for an OMS propellant dump (burn), 14 aft RCS null jets can be commanded to thrust to improve orbiter center-of-gravity location.

If the amount of OMS propellant used before MECO leaves less than 28 percent of OMS propellants, a 15-second aft RCS ullage thrust is performed after MECO to provide a positive OMS propellant feed to start the OMS-1 thrusting period.

The OMS-to-aft-RCS interconnect sequence provides for an automatic interconnect of the OMS propellant to the aft RCS when required and reconfigures the propellant feed from the OMS and aft RCS tanks to their normal state after the thrusting periods have ended. The interconnect sequence is initiated by the abort control sequence.

In order to establish a known configuration of the valves, the interconnect sequence terminates the GPC commands to the following valves if they have not been terminated before honoring a request from the abort control sequence: left and right OMS crossfeed A and B valves, aft RCS crossfeed valves and aft RCS tank isolation valves.

A request from the abort control sequence for an OMS-to-aft-RCS interconnect will sequentially configure the OMS/RCS valves as follows: close the left and right aft RCS propellant tank isolation valves, open the left and right OMS crossfeed A and B valves, and open the left and right aft RCS crossfeed valves. The OMS-to-aft-RCS interconnect complete flag is then set to true.

When the abort control sequence requests a return to normal configuration, all affected OMS/RCS propellant valve commands are removed to establish a known condition; and the interconnect sequence will then sequentially configure the valves as follows: close aft RCS crossfeed valves, close left and right OMS crossfeed valves and open aft RCS propellant tank isolation valves. The OMS-to-aft-RCS reconfiguration complete flag is then set to false, and the sequence is terminated.

Return-to-Launch-Site Abort

An RTLS abort requires the dumping of OMS propellant by burning the OMS propellant through both OMS engines and through the 24 aft RCS thrusters to improve abort performance and to achieve an acceptable entry orbiter vehicle weight and center-of-gravity location. The thrusting period is premission-determined and depends on the OMS propellant load.

The OMS engines start the thrusting sequence; and after the OMS-to-aft-RCS interconnect is complete, the aft RCS thrusters are commanded on. The OMS engines and RCS thrusters then continue their burn for a predetermined period. The interconnect sequence is the same for ATO and AOA aborts. The OMS and aft RCS will begin thrusting at SRB staging if the abort is initiated during the first stage of flight or immediately upon abort initiation during second stage.

Contingency Abort

A contingency abort is selected automatically at the loss of a second main engine or manually by the flight crew using an item entry on the RTLS TRAJ or RTLS TRANS CRT displays. For the contingency aborts, the OMS-to-aft-RCS interconnect is performed in a modified manner to allow continuous flow of propellants to the aft RCS jets for vehicle control and to allow contingency rapid dump (burning) of OMS and RCS propellants. The abort control sequence tracks the total time the OMS and aft RCS are on to determine the amount of propellants used.

The request for an interconnect will cause the interconnect sequence to configure the valves sequentially as follows: open the aft RCS crossfeed valves, open the left OMS crossfeed valves A, open the right OMS crossfeed valves B, close the left and right aft RCS tank isolation valves, open the left OMS crossfeed valves B and open the right OMS crossfeed valves A. The OMS-to-aft-RCS interconnect complete flag will then be set to true.

If the rapid dump is selected before MECO, the OMS-to-aft-RCS interconnect occurs, and both OMS engines and the 24 aft RCS jets are commanded to thrust until the desired amount of propellant has been consumed. The rapid dump will be interrupted during external tank separation if the thrusting period is not completed before MECO; otherwise, the thrusting period terminates when thrusting time equals zero or if the normal acceleration exceeds a threshold value.

Upon completion of the thrusting period, the OMS-to-aft-RCS configuration flag will be set to false, and the sequence will be terminated. A return-to-normal-configuration request by the abort control sequence will cause the interconnect sequence to configure the valves sequentially as follows: open aft RCS propellant tank isolation valves, close the aft RCS crossfeed valves, and close the left and right OMS crossfeed A and B valves. The OMS-to-aft-RCS interconnect complete flag will be set to false, and the sequence will be terminated.