1. Defence Equipment and Services, Ministry of Defence, Bristol BS34 8JH, UK e-mail:
2. Centre for Propulsion Engineering, Cranfield University, Bedfordshire MK43 0AL, UK e-mail:
3. Centre for Defence Engineering, Cranfield University, Shrivenham SN6 8 LA, UK e-mail:
4. Applied Mechanics Department, Chalmers University of Technology, Hörsalsvägen 7A, Göteborg 412-96, Sweden e-mail: