The Boost Engine was a RB162 which was originally developed as a lift engine for one of the VTOL aircraft which never got built. The engine had a total loss oil system and needed to to be "oiled" on every flight even if not used. The RB162 contained a lot of composite (plastic) parts. Initially it was only to be used on take off and up to Fl100 with a maximum speed of (if I remember correctly 235kts)and not in icing conditions.It subsequently was permitted to be started below FL100 for approach and possible G/A if a main engine had failed. I tried that once with Number 3 engine shut down, the boost started ok (suprise) but failed with a loud bang some 2 minutes later. On inspection after landing the complete rotating assembly was missing and only a casing remaining!