An intelligent system that schedules Space Shuttle activities
Donna Kautz · 9th Computing in Aerospace Conference · 1993
This paper will present an intelligent system called the Ground Processing Scheduling System (GPSS) that is being developed and utilized at the Kennedy Space Center (KSC). This paper will focus on the capabilities of the GPSS utilizing constraint-based programming methods combined with iterative repair and simulated annealing techniques, and user interface characteristics. The GPSS schedules and reschedules ground processing task activities of the U.S. Space Shuttle fleet through the Orbiter Processing Facility (OPF). The OPF is a highly dynamic processing environment in which the Orbiter work activities include postflight inspection and dese~ic ing, and maintenance and testing in preparation for the next scheduled mission. Frequent rescheduling often occurs because the OPF is dependent on the integration of work from twenty-four major orbiter systems. GPSS improves OPF scheduling by providing an integrated scheduling tool that predicts conflicts between scheduled tasks, and aids the user in resolving those conflicts. The tool is a constraint-based scheduler that repeatedly attempts to improve schedules by seeking repairs for violated constraints. GPSS contains an automated conflict resolution feature that allows the human to interact with the scheduler to resolve problems and to find ways to work around unresolved problems. This tool takes away the human scheduler from the role of manually examining and resolving schedules for conflicts. GPSS also contains a search mechanism allowing for a near-optimal schedule by utilizing its automated optimization module. The optimization module allows the scheduling tool to search for schedules that not only meet all required constraints, but also optimizes the schedule for selected, specific criteria. The GPSS scheduling tool is in an operational test pilot currently underway at KSC. The project is a cooperative effort that exchanges technologies between the NASA Ames Research Center and Lockheed Space Operations Company. The Kennedy Space Center is responsible for the ground turnaround and support operations, pre-launch checkout and launch of the Space Shuttle and its payload in preparation for the next space mission. Approximately 40,000 technician man-hours are spent preparing the Orbiter for its next mission. Orbiter and facility configuration, limited resources, and late part deliveries all constrain work in the OPF. Approximately 50% of the work for each orbiter flow is standard and predictable. The remainingg work is driven by factors such as the uniqueness of a particular payload, the testing requirements specific to each payload, new orbiter modifications, and in-flight anomalies. A Space Shuttle mission begins when the orbiter lands at KSC or Edwards A.F.B. in the California desert. The ground processing of the orbiter begins immediately upon landing. On the runway, initial ground inspections are performed, hazardous gases are purged, and the orbiter is secured for safe ground operations. If the orbiter lands in California, jt is then ferried to the Shuttle Landing Facility (SLF) at KSC atop a Shuttle Carrier Aircraft (SCA). The Orbiter is then removed from the SCA, towed to one of three OPF bays where the payload is removed from the cargo bay, maintenance, testing, modifications, and repairs are then completed. Shuttle vehicle integration begins with the stacking of the Solid Rocket Booster's (SRB's) on the Mobile Launcher Platform (MLP) in one of two VAB high bays. The External Tank (ET) is then transferred from its checkout cell and mated with the assembled boosters. When OPF processing is complete, the Orbiter is transported to the Vehicle Assembly Building (VAB). A processed orbiter is rolled over to the VAB after horizontally-integrated payloads have been installed. It is hoisted into position along side the ETISRB stack and mated. Following vehicle checkout of the entire system, the assembled shuttle is rolled out to a launch pad. On-the-pad test requirements are met, vertically integrated payloads are installed, and pre-launch propellant servicing and ordnance work is performed. Final preparations are completed and the launch countdown begins. KSC ground systems verify the vehicle's flight readiness and initiate the Shuttle launch. Minutes after the launch, recovery crews at sea begin the retrieval of the spent solid rocket boosters to start the process of vehicle turnaround. The booster casings are towed back to KSC facilities on Cape Canaveral Air Force Station and refurbishment begins. As the Shuttle mission is performed on orbit, KSC crews continue preparations for the subsequent flight. Vehicle elements being processed for the next launch may include either the orbiter currently in flight or another from the fleet. Recovery of the orbiter at KSC or other landing site begins its turnaround for another mission. Orbiter Processina Facilitv (OPF) Initial OPF operations start with a series of vehicle operations. They include post-flight safing, deservicing, trouble-shooting, orbiter modifications, and some payload installations. Copyright @ 1993 American Institute of Aeronautics and Astronautics, Inc. All rights reserved. First, the orbiter is jacked up off its landing gear and leveled, OPF workstands are moved into position, and preparations to gain access to various orbiter areas are begun. The orbiter is connected to ground power, facility ground coolant and purge air, and the Launch Processing System (LPS). Safing operations then occur. Some of these operations are hazardous which require the OPF to be cleared of all non-essential personnel. These operations must be completed before post-flight trouble shooting begins on anomalies which resulted from launch, spaceflight, or reentry. Orbiter components are removed and repaired, or replaced, as required based on anomaly reviews, and then retested in parallel with other processing activities. Modifications may be performed to meet future mission requirements, resolve an identified deficiency, or to replace existing hardware with new, improved designs to enhance vehicle performance. Space Shuttle cargo processing is performed in parallel with vehicle processing so that fully integrated and tested payloads are ready for installation in the orbiter at the appropriate time to support the launch schedule. Upon completion of all cargo installation activities or any work being performed in the payload bay, the payload doors are closed and latched. All ground support and access equipment is then removed and the orbiter is towed into the VAB. Hundreds of shifts of work are scheduled in each OPF flow. Each of these shifts of work must have all the constraints discussed in the following section taken into consideration during the dynamic scheduling process. OPF Schedulina Constraints To perform a task in the OPF a number of factors must be coordinated. These factors can be summarized as paper, parts, people, and configuration. l3RSx Every operation performed on the orbiter must have explicit engineering instructions. These instructions are a step-by-step guide for technicians and quality inspectors to follow in order for work to be accomplished. Engineering paper is released to. the shop floor after review by NASA's other contractors to ensure compliance with requirements and standards. The Engineering document is given a number and scheduled for work.