This paper describes the conceptual phase of a modification proposal for Saab Gripen, which aims to create the best possible basis for future aircraft development. One of the main tasks in this respect is to investigate different means to extend range. This paper describes the technical outcome from the concept generation, concept selection, and concept refinement at system and airframe level. The chosen concept involves relocation and redesign of the main landing gear. The original main landing gear bay is converted into fuel tanks. The relocation of the main gear enables a new ventral twin store carriage with supersonic jettison capability to be introduced. The concept proposal yields the following improvements:

· Increased range due to increased fuel capability

· Increased maximum allowed takeoff weight due to beefe d-up main gear

· Increased weapon flexibility and capability due to introduction of two new ventral store

· Introducing supersonic jettison capability

The paper also includes a description of how the work is related to the overall design process in general and the conceptual phase in particular, as described in design methodology literature.

There is an ongoing trend in the European Military a/c industry towards cooperation between nations when purchasing and between manufacturers when developing and producing a/c. Different manufacturers at different locations develop different parts or sub-systems. When using this approach a vital part of a fast and precise system evaluation is the use of simulation models. In order to stay competitive it is not only sufficient to be able to build large simulation models but also to do it fast.

This paper describes the conclusions regarding a modelling strategy of large fluid systems drawn from the building of a simulation model of the JAS 39 Gripen fuel system. An overall process is suggested into which the activities of building a model are fitted. This is however not the main objective; it is more important to identify the different issues and activities at the engineering level. If these are properly dealt with, the model development time will be reduced, if not, the wrong model may be designed. "Wrong" here means a model that does not do the job, or solves a problem other than the one intended by the stakeholder.

This paper describes how the framework of thc house of quality and design structure matrices are used to visualizee dependencies between top level requirements and engineering design properties. It is also discussed how quantification of the matrix elements may increase the understanding of how the top-level requirements impacts the low-level design parameters. lndeed, history has shown that overlooking combinatory effects between subsystems and night conditions may become expensive. Not only in terms of not goning getting the sizing right but more so if an entirely wrong concept is chosen.

This paper shows a matrix technique that has successfully been used at Saab and how this technique may facilitate the cconcept evaluation process of early fuel system design.

The matrix method aids the designer to take alk the relevant aspects into account when evaluating a design. Use of the method will also increase the understanding of what top-level requirement or combination thereof, which drives the choice of one particular concept rather than the other. The understanding of how the top-level requiremEnts impacts low level design parameters such as pump size or pipe diameter will also increse.

This paper describes early considerations that have to be made when designing an aircraft fuel system. Emphasis is placed on illustrating the impact of top-level aircraft requirements on low-level practicalities such as fuel system design. Choosing between concepts is one of the most critical parts of any design process. Different concepts have different advantages, and the concept that is the best choice is often dependent on the top-level requirements. This paper shows how optimization has been used successfully at Saab Aerospace as a tool that supports concept selection. The example studied is the design of a fuel transfer system for a ventral drop tank and the optimization results in different conceptual designs depending on the top-level requirements.

As the time between different development projects of new aircraft (a/c) extends, experienced personnel in the field of basic a/c system design are difficult to employ when being on the onset of a new design. Further on basic a/c system design is a field neglected in literature and in the educational system.

A text is under development that summarizes the Saab experience of the complete fuel system design with respect to the fighter a/c Viggen and Gripen, the commuter a/c 340 and 2000, the trainer a/c SK60 and also the conceptual a/c B3LA.

This paper is an extract of this text and describe early considerations that have to be made when designing a fuel transfer system. Emphasis is put on the top requirements on a/c level.

Choosing between concepts is often the most critical part of the design process. Different concepts have different advantages and disadvantages. The concept that is the best choice is most often dependent on the top level requirements. Sometimes there may also be a trade off between concept choice and the top requirements. In aircraft (a/c) fuel system design it has often proved difficult to find the switching point where the superior concept is changed. This sometimes makes the designer conservative and leads to the selection of a concept with too high a penalty. There is also a risk for the opposite and perhaps worse scenario: That the designer strives to reduce weight and cost and therefore, accidentally, chooses an under achieving concept and thus induces large downstream cost if late redesign or retro modifications are necessary.

This paper shows how optimization has been successfully used at Saab Aerospace as a tool that supports concept selection. The example shown is the choice of fuel transfer method for a ventral drop tank. The example also illustrates the impact of top-level requirements on low-level practicalities such as fuel system design.