Impressed with the performance of the McDonnell F-4 Phantom II that had just been introduced into service with the US Navy, the Soviet Air Force asked the Mikoyan design bureau (OKB Mikoyan) for a successor to the MiG-21 "Fishbed" that could counter the F-4 on equal terms. The first attempt by the OKB was the experimental Ye-8 demonstrator that was based on the MiG-21. However, the Ye-8 suffered from numerous technical problems that led to the loss of the first prototype and before the second Ye-8 could be flown, priorities with the Soviet Air Force command staff had changed on the characteristics of the replacement design and now emphasized STOL performance as it was felt that the long runways needed by most Soviet fighters were vulnerable to attack by NATO's intermediate-range ballistic missiles. STOL performance would allow the fighter to be deployed to remote areas away from the main air bases in the Soviet Union and Eastern Europe. Mikoyan's studies of STOL configurations were subsequently narrowed down to two configurations- one using variable-geometry "swing wings" and one using lift engines. Although using lift engines would be simpler in theory, swing wings allowed for more space in the fuselage for fuel and weapons. Although Artyom Mikoyan himself favored swing wings, a 1961 report by the prestigious Russian aerodynamic institute, TsAGI, concluded that there were considerable aerodynamic issues as well as construction issues with variable geometry wings.
While there had not yet been any flight testing of swing wing designs in the 1960s in the Soviet Union, Mikoyan already had experience with STOL jet lift fighter technology, having flown in 1966 the Ye-7 (also referred to as izdeliye 23-01, izdeliye meaning "article", usually in reference to demonstrator or prototype) which was a MiG-21 modified with two lift jets in the center fuselage. Mikoyan divided his team into two groups- one group did further studies of the swing-wing design and the other group studied the same design, but with delta wings and lift jets instead of the swing wing. Mikoyan's nephew, Vano Mikoyan, was placed in charge of both teams. As jet lift had already been flight tested, it was agreed to fly that version first while the other team refined its work on the swing-wing version. The jet lift version was designated in-house as izdeliye 23-01 (a reuse of the Ye-7's designation) and had a single Khachaturov R-27F afterburning turbojet as the main cruise engine with 11,400 lbs of thrust and two Kolesov RD36 lift engines mounted in the mid-fuselage and angled 5 degrees forward from vertical to provide a slight forward thrust vector. A dorsal aft-hinged door opened up during flight along with auxillary inlet doors to provide air for the lift engines which exhausted through a belly-mounted grid of vectorable deflector vanes that could also direct lift engine thrust forward for braking on landing.
Because the new Sapfir radar system was larger than any radar system used previously, a nose inlet was impossible and the 23-01 had half circle lateral intakes with a central shock cone similar to that fitted to the Lockheed F-104 Starfighter and the Dassault Mirage series of fighters. To augment the lift from the delta wing of the 23-01, the flaps were blown with engine bleed air when lowered. The delta wing resembled that of the MiG-21, but was scaled up for the increased size and weight of the 23-01 design. While there was no radar system aboard the 23-01 nor were there any fire control avionics, a 23mm cannon was fitted under the fuselage just aft of the cockpit and dummy K-23 missiles were fitted under the wings (the K-23 had the NATO designation AA-7 Apex and in operational Russian service was designated R-23). The 23-01 prototype was even painted in the light gray color used by the air defense units of the Soviet Air Force and it made its first flight on 3 April 1967.
The 23-01's flight test program revealed that it flew just as poorly as the MiG-21 STOL demonstrator that proceded it, the Ye-7. Test pilots noted a considerable amount of instability bordering on loss of control on takeoff due to interactions between the lift jets and the wings. On landing, it was found that the exhaust of the lift jets was getting ingested into the main engine, robbing it of power. The only solution was to increase the landing speed, but then this negated any STOL benefit of having lift engines! Pilots expressed concern at what to do in case of the loss of one of the lift engines on takeoff or landing before the wing had generated sufficient lift. At altitude the lift engines were deadweight and it compromised the maneuverability of the 23-01. The short flight test program confirmed Artyom Mikoyan's favoring of the swing wing configuration, but the aircraft continued flying until the 23-01 could be demonstrated to the public at the Moscow Domodedovo air show in July 1967. With its operational color scheme and weapons loadout, the Soviet Air Force announced its designation as the MiG-23PD, giving Western analysts the impression it was a possible production type. In fact, NATO even went as far to assign the 23-01 the code name "Faithless" as a result, believing that jet lift would be a feature of the MiG-23! After the air show, the 23-01 was retired and donated to the Moscow Aviation Institute for maintenance training and subsequently got scrapped.
The first flight of the world's first production variable-geometry aircraft, the General Dynamics F-111, in December 1964 showed TsAGI's original 1961 report in error and work on the swing wing version of the 23-01, designated in-house by Mikoyan as the 23-11, was increased. In fact, the Mikoyan designers scrutinized every photograph and inflight footage of the F-111 to determine how the Americans had solved problems that TsAGI only 3 years earlier deemed too difficult to overcome with present technology. The 23-11 made its first flight on 10 June 1967 only one month before the STOL 23-01 was unveiled to the public at Domodedovo. The 23-11 became the first production version of the MiG-23, the MiG-23S (S referring to the Sapfir radar) which used most of the fuselage of the 23-01 and the empennage but added lateral box inlets and a new swing wing. NATO would assign the MiG-23 the code name "Flogger".
The flight test program of the MiG-23/23-11 showed that swing wings very closely approached the field performance of the jet lift STOL 23-01. While the 23-01 at a takeoff run of 1,500 feet, the MiG-23 had a takeoff run of 1,800 feet. Landing the 23-01 required 1,100 feet and the MiG-23 could do it in 1,400 feet. In addition, the MiG-23 proved to be a far more versatile design on account of the performance accorded by its variable geometry wings. Nevertheless, the jet lift 23-01 remains an unusual historical aviation curiosity that fooled NATO into thinking it was the production configuration of the MiG-23!
Source: OKB Mikoyan: A History of the Design Bureau and its Aircraft by Yefim Gordon and Dmitriy Komissarov. Midland Publishing, 2009, p255-257.