evansb has it. Apologies for delay, lost my net collection for most of the morning.
The Republic XF 103 failed because the highly complex J67 turbojet/ramjet combination, a derivative of the Olympus, couldn't deliver adequate thrust. If you look at the pic again on the top right hand side you can see the type of view provided by the wide angle periscope which was the only forward vision available to the pilot. He was to have been provided with a 'supersonic escape pod' the top half of which was normally slid down. Only the basic flying controls were provided within the enclosure, which was deemed essential given the projected maximum speed of Mach 5. In best Gerry Anderson fashion the escape capsule also provided the means of entry to the aircraft. Due to the sheer size of the machine (a habit of Republic) the design was severely criticised for lack of access for maintenance and rearming.
