of 1953. In April 1954, a Yak-120 prototype with the RP-6 passed its state acceptance trials, and on 13 May the Council of Ministers approved the production of the modified version with the designation Yak-25M. In addition to the radar, this version also incorporated several changes – the AM-5A Srs 1 turbojets were replaced with RD-5A (AM-5A) Srs 2 engines with the same rating, the wheelbase was increased by moving the nose gear unit 33 cm forward to improve directional stability during takeoff and landing, and the cannons were fitted with muzzle brakes. 406 Yak-25Ms were built at the Saratov factory, with deliveries beginning in January 1955. RAF RPF ELINT flights from October 1956 indicated that the Yak-25M had entered service, but that height-finding by ground-based radars was poor and so interception was ineffective above 35,000 feet. Further Yak-120 development In September 1953, after the first stage of the Yak-120's state acceptance trials concluded, the second Yak-120 prototype was re-engined with the new Mikulin AM-9A turbojet, an improved version of the AM-5. Rocket pods for ARS-57 Skvorets folding-fin aircraft rockets were added, intended for use against enemy bomber formations. A modified RP-6 known as the Sokol-M was also to be added, and what was designated the Yak-120M was to begin its state acceptance trials in September 1954. Due to delays occasioned by the late delivery of the engines and lack of radar, the aircraft was not completed until the end of 1954, and was instead equipped with a standard RP-6. In November, the first stage of the state acceptance trials (with the standard RP-6 and no FFARs) was postponed until January 1955, but by March of that year the aircraft was planned to be ready for the second stage, with the Sokol-M and FFARs. The same directive that changed the schedule also specified that the TRS-190 rocket be used as alternate armament. To allow for the increased weight of the rockets, lighter Nudelman-Rikhter NR-23 cannons replaced the N-37Ls. The first stage of the state acceptance trials of the Yak-120M were completed on schedule in January 1955; the second stage began in March and concluded in late April. However, the aircraft's performance was slightly short of the operational requirements, and the failure to complete the Sokol-M, coupled with a need for supersonic interceptors, forced abandonment of the Yak-120M. The prototype was used as a testbed for RD-9F engines under the designation