sentences:  The Kh-66 was effectively a heavy-warhead, beam-riding version of the K-8 (AA-3 'Anab') air-to-air missile rushed into service in Vietnam in 1968.The Kh-23 was an improved Kh-66 with command-guidance, similar to the AGM-12 Bullpup.Work on air-to-air missiles had started at the Kaliningrad Engineering Plant (then known as Plant #455, and later merged into Zvezda-Strela) in 1955.