 The NKAP approved the preliminary specifications of the I @-@ 250 on 19 September 1944 which included a maximum speed of 825 km / h ( 513 mph ) a take @-@ off weight of 3 @,@ 500 kg ( 7 @,@ 700 lb ) and a time to 5 @,@ 000 metres ( 16 @,@ 404 ft ) of 3 @.@ 9 minutes . A mock @-@ up was inspected on 26 October and rejected because of the poor cockpit layout , although this decision was reversed because the fuselage of the first prototype was too far along for major changes that would have significantly delayed the program . That same month the combustion chamber was sent to TsIAM for testing which revealed it to be too weak . The complete powerplant was tested in December , but the drive shaft connecting them failed several times . 
