1. Univ. of Surrey, Guildford, Surrey GU2 7XH, U.K.
2. Professor, School of Engineering, Univ. of Surrey, Guildford, Surrey GU2 7XH, U.K. (corresponding author). E-mail: a.crocombe@surrey.ac.uk
3. Surrey Space Technology Ltd., Guildford, Surrey GU2 7XH, U.K.