Republic XF-91 Thunderceptor     46-680

                                   The XF-91 was designed around the concept of a regular jet engine for cruise flight, augmented
                                   with a cluster of four rocket engines for added thrust during climb and interception.  By the time
                                   it was completed, however, regular jet engines had been introduced with enough power to obsolete
                                   the concept.  Only two prototypes were built, one of held claim to being the first US fighter to exceed
                                   Mach 1 in level flight. .  The aircraft illustrated above is the first prototype.