sentences:  The Kh-23 became a development of the Kh-66 design with an improved propellant and new Delta-R1M guidance system.The main practical difference was that it was a line-of-sight radio-command weapon similar to the Bullpup, allowing it to be fired in level flight (unlike the Kh-66).The first ten were tested in early 1968, but significant delays were caused by problems with unreliable guidance which was eventually traced to the smoke generator which interfered with the antenna.