GRAHAM WARWICK / WASHINGTON DC
When the Columbia Accident Investigation Board (CAIB) releases its report later this month, its recommendations could include a boost to a programme under way to improve sustaining engineering of the Space Shuttle by creating digital models of the vehicles two decades after they were built. The Digital Shuttle project has already helped during reconstruction of Columbia to identify debris and locate its position on a three-dimensional computer model.
In addition to converting the original two-dimensional paper drawings into the three-dimensional solid models now used in vehicle design, the Digital Shuttle project aims to encourage the development of extensions to current computer-aided design (CAD) tools that will allow non-geometric information and product life-cycle knowledge to be associated with the 3D models.
"The Shuttles are unique vehicles," says Paul Keller, Digital Shuttle programme manager at NASA Ames. "They are reconfigured extensively for each mission, so the engineering portion of the life-cycle cost is much higher than for aircraft. Despite the fact the vehicles are relatively mature, there is a lot of engineering activity."
To save money, NASA has never updated the original drawings to reflect all of the design changes - called engineering orders (EO) - made to the Shuttle. There are over 1 million drawings and EOs per orbiter, and maintaining and reconfiguring the vehicle requires engineers to mentally process the drawings and EOs. "This is extremely time-consuming and error-prone," Keller says.
The first step in the Digital Shuttle project involves taking the original drawings, incorporating the EOs and converting them into 3D CAD models of the orbiter "as designed". At the same time, all non-geometric information on the drawings - including notes made by the design engineers and maintenance technicians - is being associated with the solid models in the form of XML files.
"The 3D CAD industry has concentrated on good geometric models, and has not taken a step back and taken a look at what's involved in sustaining the use of solid models through a product's life cycle," says Keller. "There is a lot of text on drawings," he says, noting that for maintenance engineering 2D drawings are usually generated from the 3D CAD model. This raises issues of configuration management across the drawings and model. "Geometry is handled well by CAD, but we have to be able to add the non-geometric information."
NASA is working with CAD vendors to develop extensions to existing design tools that will allow non-geometric information to be associated with solid models in an open XML format. Four companies were awarded contracts to convert selected drawings and EOs to solid models and XML files for "high traffic" areas of the orbiter where there is a lot of engineering activity. Although NASA opened the competition to different CAD formats, it then specified the industry-standard Catia v5. After proving its feasibility, the project is moving on covert of a broader range of drawings.
Vehicles are not always built exactly as designed, and the second part of the Digital Shuttle project is aimed at creating digital models of the orbiters as built and as maintained. This is being achieved by scanning the actual orbiters using a variety of different reverse-engineering techniques including laser scanners and photogrammetry. "The aim is to produce a parallel as-built model of the orbiter. We don't have the time or funds to resolve which is right or wrong, so we need both," says Keller.
The as-built model will be used as a digital mock-up for virtual assembly, location identification and process planning. Because the as-designed model will always be more accurate, due to errors inherent in the scanning, it will be used to validate any engineering changes required.
Both models contribute to what NASA calls the Shuttle "ontology" - or knowledge model. "Ontology moves beyond the design and considers the life cycle, generating relationships [between models and information] that span the life cycle," says Keller. CAD specialists, particularly in Europe, are working on extending their geometric models to include related information, he says.
Ontology is the science of modelling knowledge in ways that humans and computers can understand, so "smart links" from solid models would take the user to information relevant to the task at hand, whether designing, reconfiguring or maintaining the vehicle. Knowledge modelling would allow the user to know everything relevant about a part, even decades after its design.
NASA is determined to return the Space Shuttle to flight, and to keep it flying, but the CAIB is likely to recommend sweeping changes to the inspection, maintenance and operation of the vehicles. With the Shuttle likely to be upgraded and operated for another 20 years, capturing the knowledge of the designers and maintainers in a digital form is increasingly critical.
Source: Flight International
