Mikoyan-Gurevich Ye-230 / Ye-231
Trials in the direct-lift approach started with the MiG-21DPD to validate the basic concept, but were then taken a stage further with the Ye-230 prototype. This was built in parallel with the Ye-231 variable-geometry prototype to ensure maximum commonality for any production type resulting from the twin programmes. The Ye-230 was of tailed delta configuration, was powered by a Lyulka AL-1F-1 afterburning turboj et, and had the same type of lift engine arrangement as the MiG-21DPD, namely two turbojets (probably Koliesov units) located vertically on the centre of gravity with air drawn from above past a rear-hinged louvred dorsal door and exhausted downward through a grid of ventral transverse louvres which could be angled by the pilot to provide a forward thrust component during transition to forward flight.
The Ye-231 variable-geometry prototype was almost identical to the Ye-230 apart from its lack of lift jets and the use of variable-geometry swept wings similar to those of the General Dynamics F-111, the first operational variable-geometry warplane. Comparative trials revealed the superiority of the variable-geometry arrangement. and the Ye-231 thus became the precursor of the MiG-23 fighter, later adapted with a modified nose and simpler engine arrangements as the MiG-27 attack aeroplane.