An experimental supersonic technology demonstrator, intended to evluate the performance of the cropped delta wing found in captured German documents, the tandem undercarriage and the delta wing caused unstability and an incedent where a crosswind crushed one of outrigger struts and with no control authority it rolled off the runway and the takeoff was aborted, the problems were fixed and it was ready for another test flight but Yakovlev wasn't interested in the project leading to it's cancellation but if it did fly it would have broken Mach speed at Mach 1.7 which would've broken the speed record, though the delta wing was tinkered with and did become succesful in the Mikoyan-Gurevich MiG-21 and the Sukhoi T-3 (VTOL for airbrakes)