Engineers like the phrase “design spiral”, to refer to a developmentprocess characterised by a series of iterative improvements and additions,particularly where they may be needed to meet evolving requirements. The termcomes from software design, and is seen as a dramatic improvement on the lineardesign process which usually characterises more traditional manufacturedproducts.
What makes the spiral attractive is the potential to continuouslyrenew a system so that it always incorporates the latest technologies and meetsthe latest needs. In a linear design process, specified requirements would befollowed by design, prototyping, testing and manufacturing. Each step iscompleted before work flows over to the next stage, and hence the process issometimes referred as the “waterfall” design method.
Big aircraft programmes which begin with a request forproposals that lead to carefully detailed performance specifications are goodexamples of the waterfall approach. Given the enormous investment in toolingand materials required to make such a complex physical (as opposed to software)system, this linear approach seems unavoidable. However, because it’s easy tofall off a waterfall but very difficult to go back up, adding new technologiesor changing components is very expensive once an aircraft is in production.
To cite a specific and very current example, one of thefactors contributing to Boeing’s travails with its much-delayed 787 airlinerprogramme is arguably what amounted to an attempt at highly compressed spiral design, by carryingout the prototyping, testing and manufacturing phases simultaneously. Looked atfrom that perspective, the 787 would illustrate why in popular language theword “spiral” is so often associated with the phrase “out of control”.
But the language made a small spiral forward this week atthe DSEi defence equipment exhibition in London.Gianfranco Terrando, senior vice president for unmanned air systems at Finmeccanica’sSelex Galileo unit, referred to a flight test planned for later this year on aGeneral Atomics Predator B of a new open architecture concept in payloadintegration as “spiral zero”.
The concept is to create a family of options for packagingsensors and data management software so that customers can, ultimately, combineany sensors and data handling techniques with whatever airframe they like. Aspiral development process makes such a concept possible – if each new requirementdemanded that engineers go back to stage one and devise a system from scratch,the cost and time involved would, clearly be prohibitive.